US2994025A - System for stopping elevators and the like - Google Patents

Description

y 1961 o. E. MITCHELL 2,994,025

SYSTEM FOR STOPPING ELEVATORS AND THE LIKE Filed May 51, 1957 3 Sheets-Sheet 1 FIG. 2

INVENTOR O. E MITCHELL By: 742 6 ATTORNEYS July 25, 1961 o. E. MITCHELL SYSTEM FOR STOPPING ELEVATORS AND THE LIKE 5 Sheets-Sheet 2 Filed May 31, 1957 FIG. 3

UR2 U'IM UHL DR'I

DHL DR INVENTOR frs O. E. MITCHELL ATTO URI

DR2 DIM DOWN 1 I 4 TS M July 25, 1961 o. E. MITCHELL SYSTEM FOR STOPPING ELEVATORS AND THE LIKE Filed May 31, 1957 3 Sheets-Sheet 3 FIG.5

INVENTOR O E. MITCHELL BY: @4 s 7/63! ATTORNEYS United States Patent 2,994,025 SYSTEM FOR STOP ING ELEVATORS AND THE LIKE I Omery Edward Mitchell, Toronto, Ontario, Canada, as-

signor, by mesne assignments, to 'Ihrnbull Elevator of Canada Limited, Toronto, Ontario, Canada, a corporation Filed May 31, 1957, Ser. No. 662,893

23 Claims. (Cl. 318-204) This invention relates to a system for bringing a moving apparatus to a predetermined lower speed, and is particularly concerned with bringing to a stop at a predetermined position or level a load conveyor such as an elevator car.

In many elevator car systems the elevator is brought to a stop from running speed by first decelerating the car from running speed to a constant and much lower landing speed, and with the car travelling at the landing speed it is possible to disconnect the driving motor and apply the brake when the car is a predetermined constant distance from the desired stopping position and obtain a fairly accurate stop.

An object of this invention is to provide an elevator system of. this kind having comfortable deceleration and improved stopping accuracy for a variety of load conditions without the addition of much costly equipment. The invention may be applied to the decelerating and stopping of other moving apparatus, but it will be discussed with reference to an elevator system since the application to other systems should involve little difficulty.

One feature of the invention is deceleration of an elevator car to landing speed in steps, the deceleration of the car being alternately arrested and restored, making possible more accurate control of the car over the period from running speed to stop, and thus more constant landing time under a variety of load conditions, than is possible with a freer deceleration to landing speed. The intervals during which the deceleration is arrested are conveniently initiated by means of a frequency filter which responds to the alternating voltage generated in the Wound rotor of an induction motor which drives the car, the frequency of this voltage varying inversely with the speed of the motor and car.

A very important feature of the invention is the stabilization of the speed of the car at the landing speed by having the frequency filter respond to the frequency generated when the landing speed is reached.

Another feature of the invention is the provision of means for regulating the rate of deceleration of the elevator. The rate of deceleration may be sensed by a derivative circuit responsive to the rate of increase of the voltage in the wound rotor of an elevator driving motor.

Another feature of the invention is the application of a substantially constant decelerating force during the final slide to stop from the landing speed. The control of deceleration to landing speed is conveniently achieved by controlled application of a brake to the driving motor, and the substantially constant decelerating force for the final slide to stop is conveniently obtained by disconnecting the motor and holding the brake pressure at or near the value that was required to hold the car at the landing speed.

Other objects, advantages and features of the invention will be apparent from the following description of a relatively simple embodiment illustrated in the accompanying drawings. In the drawings:

FIG. 1 is a schematic diagram of a known type of elevator;

the elevator of FIG. 1;

FIG. '3 is a schematic diagram of a control circuit for the elevator and embodying the present invention;

FIG. 4 is a vector diagram relating to part of the power circuit of FIG. 2; and

FIG. 5 is a graph showing the approximate speed of the elevator car at various stages of a typical ascent.

In the drawings relay contacts are given the same letters as the relays that operate them: for example, contacts URI and UR2 of FIG. 3 are respectively opened and closed by energization of the up relay UR.

Referring to FIG. 1, a three phase wound rotor induction motor 1 drives a brake pulley 2 and gear-reduction box 3 through a shaft 4. A brake shoe 5 is forced against the pulley 2 by a compression spring 6. The brake shoe can be retracted from contact with the pulley by energization of a brake magnet BM. A cable sheave 7 on the output shaft of gearbox 3 supports a hoist cable 8. Suspended from the cable 8 is a car 9 and counterweight 10. Thus the motor 1 and brake serve as means for controlling the speed of the elevator, the motor serving to apply accelerating forces when required, and the brake serving to apply retarding or decelerating forces. The brake illustrated is an electromagnetic friction brake, but of course other types of electromagnetic brakes could be used, for example, an eddy current or powdered iron type.

FIG. 2 is a schematic diagram of a suitable power circuit for the motor 1. The stator windings 11 of the motor are energized from three-phase lines L1, L2, L3 through contacts M1, M2 of a motor relay M (FIG. 3) and either through contacts U1, U2 of a relay U (FIG. 3) or contacts D1, D2 of a relay D, depending upon whether the car is to run up or down. In the rotor circuit, external resistors 12, :13, 14 are Y-connected via slip rings 15 across the rotor windings 16. Accelerator relays A, B, C (FIG. 3) may be operated in timed sequence to close contacts A1, A2, then contacts B1, B2 and then contacts 01, C2 so as progressively to decrease the external resistance in the rotor circuit to zero, causing the motor to accelerate to running speed, which is substantially synchronous speed.

In the stator circuit, there is provided an overhauling load sensing circuit 20. As indicated above, an overhauling condition exists when the combined weights of the car, its load, and the counterweight are such as to drive the motor 1 rather than be driven by it. The compensator 20 includes a resistor 21 in the line L1, and this resistor produces a small voltage drop which is amplified 'in magnitude by a transformer 22. The primary of another transformer 23 is connected from line L1 to either line L2 or L8, depending on the phase rotation of the source, to produce a voltage which lags the phase of the power component of current through resistor 21 by thirty degrees. A resistor 24 is connected across the secondary of transformer 23 and a portion of resistor 24 tapped at 24a is connected in series with the secondary of transformer 22. The resultant alternating voltage in the secondary circuit is rectified by a rectifier 25, smoothed by a capacitor 26 and applied to the coil of a relay OHL.

The vector diagram of FIG. 4 will assist in understanding what voltage is applied to the relay O'HL. The power component of current in line L1 (and hence through resistor 21) is shown for a typical load conditionv as the horizontal vector 1 The reference voltage tapped across resistor 24 is shown as the vector V The voltage V across the secondary of transformer 21 varies directly, in magnitude and phase, with the total current in the line L1. When the motor is hauling the maximum rated load for the elevator, this voltage may be represented by the vector V At a lower load, it may be represented by the vector V' When the motor is accelerating, it may be represented by the vector V In the condition of maximum overhauling, the voltage V may be represented by the vector V (In this condition the direction of the vector I will be reversed, the motor acting as a generator.) Under a lower overhauling condition, V may be represented by the vector V' The locus of the ends of these vectors V V' V V and V which represent the voltage V for a variety of load hauling conditions, is substantially a straight line. The difference between V and V is, after rectification, applied to the relay OHL, and this difference voltage increases in a substantially linear manner as the load changes from a maximum hauling to a maximum overhauling condition. At a predetermined overhauling condition, this voltage reaches the value V (FIG. 4) and the overhauling load relay OHL picks up. The tap 24a on resistor 24 may be adjusted to select the hauling condition at which relay OHL operates. Relay OHL will not operate during acceleration, regardless of load condition. The effect of relay OHL operating will be subsequently explained.

Referring to FIG. 3, a manual car switch CS in the elevator car is moved to either the up or down position to cause the car to run. Assume that the car is at the ground floor and that the switch is moved to the up position. Up relay UR is energized through a contact DRl and a limit switch UHL. Contact UR2 closes. Provided that the hall and car doors are closed, relay U is energized through hall door interlocks 30, car door interlock 31 and contacts UR3 and D3. Contacts U4 and U5 close, and motor relay M is now energized through them, causing power to be applied to the motor stator windings 11 (FIG. 2) through contacts M1, M2 and U1, U2.

Brake relay BR is energized through contacts M3 and UR4, causing the brake magnet coil BM to be energized through contacts BR]. and C3 and a resistor 32, The brake shoe 5 (FIG. 1) is pulled clear of pulley 2 and the car starts to move upwardly.

Accelerator relay A is also energized through contacts M3, BR2 and OHL1, and operates after a short time delay, closing contacts A1, A2 in the rotor circuit ('FIG. 2). A time of slide relay TS, hereinafter referred to, is energized through contact A3. Relay 13 becomes energized through contacts BR3 and A4, closing contacts B1, B2 in the rotor circuit. Relay C in turn is energized through contact B3, closing contacts C1, C2 in the rotor circuit. The car accelerates to running speed, as indicated at the left hand side of FIG. 5. Also contact C3 opens, cutting a portion of a resistor 33 via tap 33a, in series with the brake magnet coil BM to decrease the current through the brake magnet coil to a suitable value, so that the brake shoe can be applied quickly.

If there is an overhauling load condition in excess of the value determined by the setting of the tap 24a on resistor 24, contact OHL1 opens to deenergize relay A. This has no immediate effect, since relay B remains energized through contact B4. However contact A3 opens and relay TS becomes deenergized.

As the car ascends, decelerator switches UlM, UZM, U3M and USL are mechanically opened one after the other at predetermined distances before each floor level. These switches can be mounted on the car and be actuated by cams or other devices fixed in the hatch, or they can be mounted in a separate device whichv moves in synchronism with the car motion, as is well known. Switch U1M, which is the first decelerator switch to be opened as the car approaches a floor, recloses when the car is nearer floodlevel, but the other decelerator switches UZM, U3M and USL, which are successively closer to the floor level, remain open until the car starts to move away from the floor. However, the operation of these switches. at each floor level has no effect as long as the car switch CS is held in the up position.

The 'car switch must be returned 'to neutral position before the car reaches the floor at which it is desired to stop, but after the car has passed the preceding floor. Assume that the car switch is moved to neutral position in order to stop at the sixth floor. At a predetermined distance before the sixth floor level, the decelerator switch UlM is caused to open, deenergizing up relay UR. (Since switch UlM recloses when the car is nearer floor level the car may if desired be started up again by momentarily flipping the car switch CS to the up position.)

The opening of contact UR4 deenergizes brake relay BR, which causes contact BRl to open, deenergizing brake magnet BM, applying the shoe 5 of the electromechanical brake and causing the car to decelerate. The brake shoe pressure is applied gradually, since the electromagnetic discharge current of the coil BM circulates through rectifiers 34 and 35, and 36 and 37, and resistor 32.

Also the opening of contact BR3 deenergizes relays B and C, causing resistance to be introduced into the rotor circuit. If relay A is still energized, it will remain energized through contact A5. If, however, an overhauling load had caused contact OHL1 to deenergize relay A, it will remain deenergized even though contact OHL1 may close during the landing, since contact BR2 is open. With relay A deenergized, the driving torque of the motor is lower during slowdown than in the case where it is energized, and thus the driving effect of an overhauling load is compensated to some extent, and the heat dissipated by the brake is therefore reduced. The relay A is thus conditionable, by the circuit 20 which senses load hauling condition, to provide for deceleration with normal motor torque (contacts A1 and A2 closed) or with lower motor torque (contacts A1 and A2 open) when a predetermined amount of overhauling load exists. The sensing circuit 20, which responds to electrical conditions in the motor input circuit, conditions the relay A during operation at running speed, prior to the commencement of deceleration.

The deceleration is regulated by brake magnet current, which is controlled partially by a narrow-differential deceleration regulator 40. Any two slip-rings 15 of the motor 1 are connected to the primary of a transformer 41, while the output from the secondary is full wave rectified by rectifiers 42 and 43 to produce a direct voltage which increases in magnitude almost linearly as the motor slows down. The rectifiers supply a series-connected capacitor 44 and relay coil SR which due to the resistance of the latter, form a derivative circuit, the current through relay SR being substantially proportionate to the rate that the slip-ring voltage increases and thus to the deceleration. To discharge capacitor 44 quickly during travel of the elevator at full speed, a contact C4 of relay C is provided in series with a resistor 45 across the capacitor 44.

The current level required to operate relay SR may be adjusted by means of a conventional screw adjustment on the relay, to correspond to a desired deceleration. If the deceleration exceeds this desired value, the current through relay SR is sufficient to close a contact SR1, tapped at 33b to the resistor 33, causing a predetermined current to flow through the resistor 33 and brake magnet BM. The tap 33b on resistor 33 should be adjusted so that the brake shoes will not lift clear, but the brake shoe pressure should be appreciably reduced.

The reduction in brake shoe pressure immediately causes a reduction in deceleration, hence a smaller current through relay coil SR, and contact SR1 opens to increase the brake pressure. This deceleration regulation continues all during slowdown and tends to produce a constant average value of deceleration regardless of load condition. The derivative circuit thus constitutes means for sensing the deceleration, and the slowdown relay contact SR1 constitutes means operated by the sensing means for regulating the deceleration through the electromechanical brake.

A resistor 46 may be added across capacitor 43 to cause slowdown relay SR to regulate to a lower decelera tion as the motor approaches a slow speed: resistor 46 and relay coil SR act as a voltage divider, and as the motor slows down the voltage across the divider increases, and a smaller voltage dependent upon deceleration is therefore required to operate the relay and close contact SR1. But resistor 46 must be of sufficient resisttime that contact SR1 will not remain closed after the motor has stabilized at a slow landing speed under the inlicence of the filter now to be described.

The transformer 41 supplies input power to a frequency filter 50, the basic elements of which are capacitors 51, 52, 53, and the reactor 54, which elements constitute a so-called m-derived high-pass T-section filter. Capacitor 53 is connected by a tap 55a across part of a resistor 55, as is a capacitor 56 by a tap 55b. During the initial part of the slowdown, capacitors 53 and 56 are substantially or completely shorted out by decelerator switches U2M and D2M via tap 55c. Capacitors 51 and 52 and reactor 54 then constitute a simple T-section filter, with a critical pass frequency (above which all higher frequencies are substantially passed) approximately equal to half the line frequency.

The frequency of the motor slip-ring voltage increases linearly from zero frequency at synchronous speed to line frequency at standstill. If the car decelerates to half speed or less before switch U2M opens, the slipring frequency will exceed half the line frequency so that alternating voltage will begin to pass through the filter (point f in FIG. 5). This voltage is full-wave rectified by means of rectifiers 34, 35, 36 and 37, and produces a direct current in thebrake magnet coil BM. The resultant reduction in brake shoe pressure substantially arrests the deceleration so that the motor speed tends to stabilize at a speed corresponding to the critical pass frequency of the filter.

At a predetermined distance from the floor, switch U2M is caused to open. Capacitors 53 and 56 are now active, so that the filter is conditioned to have a higher critical pass frequency. Any alternating current which has been passing through the filter will now be cut off, full brake shoe pressure is resumed, and the deceleration of the car is restored, subject only to the action of the deceleration regulator 40.

If, before switch U3M opens, the motor decelerates to a speed such that the slip-ring frequency approaches the new critical pass frequency of the filter, current begins to flow through the filter and rectifier to the brake magnet BM (point in- FIG. 5) causing reduction in brake shoe pressure, substantial-1y stabilizing the motor at a speed such that the slip-ring frequency equals the critical pass frequency of the filter. This may be approximately one quarter of synchronous speed.

,Decelerator switch U3M is caused to open a short distance before floor level. Capacitor 56 is now disconnected from the filter circuit, so that capacitors 51, 52 and 53, and reactor 54 are active, the filter now being conditioned to have still another higher critical pass frequency, corresponding to the landing speed. Again, any current which has been flowing through the filter is out 01f, and the deceleration of the car is restored subject only to the control of the deceleration regulator 40. As the car approaches a speed such that the slipring frequency approaches the new critical pass frequency, current will again begin to flow through the filter (point 2, in FIG. 5) and the brake shoe pressure will be regulated to hold the car at landing speed. This can be approximately one sixth of the synchronous speed. Relay SR, being responsive to deceleration, will cease to operate when the car speed stabilizes.

The sharp cut-off characteristic of the filter can be spoiled to'any desired degree for each speed step by adjusting the taps on resistor 55. This results in a smoothing of the slowdown steps with some deterioraof the speed regulation. I

A short distance before floor level, the decelerator' switch USL is caused to open. Relays U and M are deenergized, disconnecting the supply to the motor stator. The sl-ilp ring voltage supplying current to the brake magnet BM is thus removed. Assuming that relay OHL operated during the run, due to a heavy overhauling load, relays A and TS are already deenergized, and the brake magnet currently rapidly decreases to zero. Thus full brake pressure is applied between the location of switch USL and floor level when stopping heavy overhauling loads.

Assuming however that relay OHL did not operate during the run, deenengization of relay M opens contact M3, deenengizing relay A, and contact A3 opens. This permits a capacitor 57 to discharge through a currentlimiting resistor 58 into relay coil TS, preventing relay TS from dropping out until shortly after the car has been brought to a stop. (In an elevator system a delay of about one-half a second in the dropping out of relay TS will normally be sufficient.) Thus contact TS1 remains closed, and the closing of contact M4 with the deenergization of relay M brings into operation a slide governor 60, now to be described, for maintaining brake magnet coil current until the car stops, and as will presently be seen the distance that the car slides from the position of switch USL is thus substantially equalized for all load conditions, and a comfortable stop is achieved.

A transformer 61, whose primary is connected to two of the lines L1, L2, supplies plate voltage to a pair of thyratrons V1 and V2, whose cathodes are tied to a tap 32a on the rmistor 32, with a return via resistor 32, brake magnet coil BM, and contacts M4 and TS1 to the transformer secondary winding that supplies the plates. The transformer 61 also supplies power to heaters H1 and B2 of the thyratrons V1 and V2. A rectifier 62 and capacitor 63 produce a direct-voltage negative bias which is applied to the shield grids of V1 and V2 by means of voltage divider resistors 64 and 65 and a current-limiting resistor 66. The ratio of resistances 64 and 65 is such that plate current begins to flow when the control grid bias is zero volts.

The control grids of the thyratrons are connected via a current-limiting resistor 67 to a capacitor 68. The capacitor 68 is in series with a now open contact M5 and a current-limiting resistor 69 across resistor 32. Prior to the opening of contact M5, the voltage on capacitor 68 was directly proportional to the brake magnet coil current through resistor 32, the capacitor thus sensing the force of the brake. The opening of contact M5 prevents the discharging of capacitor 68 except through a relatively large resistance 70. Hence the voltage on capacitor 68 constitutes a memory of the magnitude of brake magnet coil current which flowed during the period of landing speed just prior to removal of power from the motor stator.

Assume that the tap 32a on resistor 32 is set at the end of resistor 32 which is connected to resistor 33. The control grid voltage of the thyratrons V1 and V2 will be the same as the cathode voltage immediately after contact M5 opens. The brake magnet coil current will immediately begin to decay exponentially, causing a reduction in voltage drop across resistor 32. The control grids thus become positive with respect to the cathodes.

Plate current now begins to flow through contacts TS1 and M4, through thyratrons V1 and V2, resistor 32 and brake magnet coil BM. As soon as the brake magnet coil current is restored to the original value, as indicated by the voltage drop across resistor 32 becoming equal to the voltage across capacitor 68, the control grid voltage becomes zero and the thyratrons V1 and V2 cease to conduct. Thus, until shortly after the car has stopped (when time of slide relay TS drops out) the brake magnet coil current, and hence brake shoe pressure, is maintained at substantially the same value as was required to hold the motor at a steady landing speed. A short time after the car is brought to a standstill, contact TSl opens the plate circuit of the thyratrons, the current through brake magnet coil BM rapidly decreases to zero, and the brake is fully on.

' During the final slide to stop from landing speed, the deceleration under the control of the governor 60 will be substantially constant regardless of load or brake lining condition, since the only change in forces acting on the system from those which existed during the landing speed is the removal of forward motor torque,which is substantially constant. Consequently the stopping distance is substantially constant, resulting in good landing accuracy.

If the landing speed were the same for all loads, the substantially constant deceleration referred to in the preceding paragraph would ensure excellent landing accuracy. However with a practical filter 50 the landing speed does in fact vary somewhat with load. Consequently weaker final braking (more brake magnet coil current) may be desirable for greater hauling loads if the car is to stop exactly at floor level. Since the brake magnet coil current that is established to hold the elevator at landing speed will be greater for hauling loads than for overhauling ones, an increase of or so in brake magnet coil current during the final slide over that which existed at landing speed (no matter what the load) can have the desired effect. It is possible to maintain the brake magnet coil current during the slide to stop at any proportion, greater than unity, of the value of current that existed during landing speed by adjusting the tap 32a on resistor 32, since the thyratrons will operate to raise the voltage at the tap to that established on capacitor 68 during the operation at landing speed. Further, by increasing the resistance of resistor 65, it is possible to maintain the brake magnet coil current at the same value that existed during landing speed for any predetermined load, for example, balanced load. Balanced load is the load which, combined with the weight of the car, just balances the counterweight.

It will be seen that the frequency filter 50, as its critical frequency is altered by operation of the decelerator switches U2M, U3M, provides a control that can sequentially increase and reduce the brake magnet coil current to alternately arrest the deceleration and then allow further deceleration. During the intervals when the deceleration is arrested, the elevator car is held at substantially constant speed by balancing the accelerating and decelerating forces through regulation of brake pressure. These intervals are commenced by the speed of the motor 1, and hence of the car, reaching values corresponding to the critical frequencies of the filter, and are ended by operation of the decelerator switches which operate in sequence when the car is at predetermined locations and approaching the desired stopping level. Under some load conditions, the first one or two of the constant speed intervals or steps may be missed during the slowdown, due to the critical frequency of the filter being altered before the frequency at the slip rings has risen sufficiently to be passed by the filter, but by proper adjustment the carspeed should always stabilize at landing speed before switch USL opens.

The last of the constant speed intervals is the landing speed interval, which is terminated by operation of the decelerator switch USL. Normally this switch actuates governing means for restoring controlled deceleration during the final slide, the governing means consisting of the relays U and M which disconnect the motor, and the slide governor 60 which maintains the current to the brake magnet coil, thus providing a substantially constant brake force until the elevator stops. The force on the cable sheave due to car load and the weight of the car and counterweight may assist or oppose the brake, but theseforces remain unchanged during the final slide to stop.- The relay contact T81 is conditionable by the load condition sensingcircuit 20 to render the slide governor 8 ineffective, the final slide then occurring with the motor disconnected and the brake fully applied.

In case the car switch CS is not returned to neutral positionas the car approach% a terminal floor, the high speed mechanically operated limit switch UHL is caused to open a short distance past the point where switch UIM opens, deenergizing relay UR to produce a normal stopping sequence as if opening of switch UIM had caused relay UR to be deenergized.

The relay and switching sequence for a down trip is similar, except that the corresponding down relays and switches, identified by the letter D on the drawings, are substituted for the up relays and switches.

Automatic relevelling may be obtained, if desired, by short-circui-ting the contacts U4 and D4. Again assuming, for example, that the car is ascending and is to stop at the sixth floor, the switch USL opens when the car is, say, one inch below the floor, causing the motor 1 to be deenergized and the car to slide to a stop. However if the car stops more than one inch above the floor, the switch DSL will be closed and relays D and M will be energized through switch DSL, causing the car to run down until switch DSL reopens.

While a very simple type of elevator control, known as car switch with automatic landing, has been used for illustration, it is apparent that the invention may be applied to many types of elevator controls, and that many variations of the particular circuits shown may be used.

It will be seen that the filter 50 provides a simple means for ensuring that the braking is regulated at a landing speed, below running speed, at which the driving motor is deenergized, so that a much more accurate stopping of the elevator car is possible than would be the case if the motor were to be deenergized at the full running speed. Making the filter 50 adjustable to have a number of critical frequencies provides steps in the deceleration of the car to the landing speed, and the larger the number of steps the more constant the time of deceleration from running speed to stop under a variety of load conditions. It will be apparent that adjustment of the filter 50 can be effected by using an adjustable reactor rather than by varying the capacitance in the filter, but varying the capacitance is the preferred method. A novel control over deceleration is provided by the deceleration regulator 40. The slide governor 60, by maintaining brake magnet coil current after the driving motor has been deenergized, achieves uniformity in stopping distance regardless of load variations or brake lining condition, but failure of the electronic circuit of the slide governor does not disable the elevator.

What I claim as my invention is:

l. A system for bringing a moving apparatus from a running speed to a stop at a predetermined position, comprising means for driving the apparatus, retarding means for decelerating the apparatus against the force of the driving means, means for generating a signal having a frequency that varies with the speed of the apparatus, frequency filter means responsive to a signal frequency corresponding to a predetermined speed below the running speed to substantially arrest the deceleration, and decelerator means for restoring the deceleration by changing the response of the filter means to a signal frequency corresponding to a still lower speed.

2. A conveyor system comprising a Wound rotor induction motor for driving the conveyor and having rotor slip rings for delivering a signal whose frequency increases as the motor speed decreases, an electromagnetic brake for braking the motor and thus the conveyor and having a brake magnet coil that is energizable to control the braking, and a high-pass electrical frequency filter connected between the slip rings and the brake magnet coil whereby the brake magnet is energized through the filter when the signal frequency is above a predetermined value.

3. A system as claimed in claim 2, wherein means are provided [for altering the filter, thus altering said predetermined value of signal frequency.

4. A system as claimed in claim 3, wherein said altering means comprise a plurality of devices actuated sequentially as the conveyor approaches a desired stopping position to raise said predetermined value of signal fre quency and thus allow deceleration of the motor an conveyor.

5. A system for bringing an elevator from a running speed to a stop at a predetermined level, comprising a wound rotor induction motor for driving the elevator and having rotor slip rings for delivering an electrical voltage whose frequency increases as the motor speed decreases, an electromagnetic brake for braking the motor and thus the elevator and having a brake magnet coil that is energizable to reduce the braking, means for energizing the brake magnet coil, means actuated when the elevator is a predetermined distance from said level for deenergizing the brake magnet coil to decelerate the motor, a highpass electrical frequency filter and rectifier connected between the slip rings and the brake magnet coil for energizing the brake magnet coil when the frequency of said voltage exceeds a predetermined critical value thus substantially arresting the deceleration and bringing the motor to a substantially constant speed, means actuated when the elevator is a predetermined shorter distance [from said level for altering the filter to raise the critical frequency of the filter thus deenergizing the brake magnet coil and allowing further deceleration until the frequency of said voltage exceeds the raised critical frequency, when the motor is again brought to a substantially constant speed.

6. A system for bringing an elevator from a running speed to a stop at a predetermined level, comprising an electrical motor connectible to an electrical supply for driving the elevator, an electromagnetic brake for braking the motor and thus the elevator and having a brake magnet coil that is energizable by brake magnet coil current to reduce the braking, means for controlling the brake magnet coil current to produce a predetermined substantially constant elevator landing speed lower than the running speed, including means for sensing the brake magnet coil current value at said lower speed, and governing means for bringing the elevator from said landing speed to a stop, the governing means comprising means for disconnecting the motor from the supply and means controlled by the sensing means for maintaining the brake magnet coil current until the elevator stops at a constant value substantially equal to the value just prior to the disconnecting of the motor.

7. A system as claimed in claim 21, wherein the control means include governing means operable when the driving force of the driving means is removed to provide, for deceleration of the apparatus to a stop, a substantially constant force of the retarding means, the magnitude of such force being dependent upon the magnitude of the force of the retarding means just before the removal of the driving force of the driving means.

8. A system as claimed in claim 7, wherein the governing means hold the force of the retarding means at substantially the same value as existed just before the removal of the driving force of the driving means.

9. A system as claimed in claim 21, wherein additional decelerator means are provided for progressively adjusting the frequency filter means as the apparatus decelerates from the running speed to the landing speed, to regulate the retarding means in response to a plurality of signal frequencies of which said predetermined frequency is the last.

10. A system as claimed in claim 9, wherein at predetermined signal frequencies the frequency filter means substantially arrest the deceleration and the additional decelerator means then operate to adjust the frequency filter means and restore the deceleration.

11. A system as claimed in claim 9, wherein the ad- 10 ditional decelerator means and the first mentioned de celerator means comprise a plurality of control devices operable successively when the apparatus is at successive predetermined locations.

12. A system as claimed in claim 9, wherein the driving means comprise an induction motor and the signal generating means comprise a wound rotor of the induction motor.

13. A system as claimed in claim 12, wherein the retarding means comprise an electromagnetic brake for braking the motor and having a brake magnet coil that is energizable from the wound rotor of the motor through the filter means, the filter means comprising a high pass filter through which the brake is regulated when the frequency of the signal rises to said predetermined signal frequency corresponding to said predetermined lower speed.

14. A system as claimed in claim 13, wherein the control means include governing means operable when the driving force of the motor is removed to'provide, for deceleration of the apparatus to a stop, a substantially constant brake magnet coil current the magnitude of which is dependent upon the magnitude of brake magnet coil current just before the removal of the driving force of the motor.

15. A system as claimed in claim 23, wherein the driving means is a wound rotor induction motor having variable external rotor resistance, and the conditionable means increase the external resistance in response to the overhauling load.

16. A conveyor system comprising a wound rotor induction motor for driving the conveyor, retarding means for decelerating the conveyor and motor against the driving force of the motor, and means for regulating the deceleration of the conveyor and motor comprising a derivative circuit for deriving the rate of change of voltage generated in the wound rotor of the induction motor as the conveyor and motor speed is reduced by the retarding means, and means operable when said rate of change of voltage exceeds a predetermined value to control the retarding means in a manner to limit the deceleration.

17. A system for bringing a moving apparatus from a running speed to a stop at a predetermined position, comprising means for driving the apparatus, retarding means for decelerating the apparatus against the force of the driving means, and control means operable at a predetermined speed of the apparatus lower than the running speed to control the force of the retarding means so as substantially to balance the driving and retarding forces on the apparatus thus holding the apparatus at substantially said lower speed, the control means including governing means operable when the apparatus reaches a predetermined position to remove the force of the driving means and to maintain the force of the retarding means at substantially the same value as existed when the driving and retarding forces were substantially balanced so as to bring the apparatus to a stop from said lower speed with a deceleration substantially independent of the load on the apparatus.

18. A system for bringing a moving apparatus, for example, an elevator car, from a running speed to a predetermined lower speed, comprising means for driving the apparatus at the running speed, retarding means for reducing the speed of the apparatus against the force of the driving means, and control means for the retarding means comprising means for actuating the retarding means to decelerate the apparatus from its running speed despite the continuing driving force of the driving means, means for generating a signal having a frequency which varies with the speed of the apparatus as it decelerates, and frequency filter means between the signal generating means and the retarding means and responsive to a predetermined signal frequency corresponding to said predetermined lower speed to regulate the force of the retarding means so as substantially to balance the driving force of the driving means and stabilize the speed of the apparatus at saidpredetermined lower speed.

19. A system as claimed in claim 18, including means forregulating the deceleration of the apparatus from the running speed to said predetermined lower speed, the regulating means comprising derivative means for deriving the rate of change of speed of the apparatus during the deceleration and means operable when said rate of change exceeds a predetermined value to regulate the retarding means in a manner to limit said rate of change.

20. A system'as claimed in claim 18, wherein the driving means comprise a wound rotor induction motor, the retarding means comprise an electrically controllable brake, the generating means comprise the wound rotor of the induction motor and the frequency filter means comprise a high pass filter.

21. A decelerating system for bringing a moving apparatus, for example, an elevator car, from a running speed to -a predetermined lower landing speed and then to a stop at a predetermined position, comprising means for driving the apparatus, retarding means for decelerating the apparatus against the force of the driving means, and control means for the retarding means, the control means including means for generating a signal having a frequency which varies with the speed of the apparatus as it decelerates, frequency filter means between the signal generating means and the retarding means and responsive toa predetermined signal frequency corresponding to said predetermined lower speed to regulate the retarding means and stabilize the speed of the apparatus at said predetermined lower speed, and decelerator means for removing the driving force of the driving means when the apparatus is a predetermined distance from said predetermined position.

, 22. A decelerating system for bringing a moving apparatus, for example, an elevator car, from a running speed to a predetermined lower speed, comprising means for driving the apparatus, retarding means for decelerating the apparatus against the force of the driving means, and control means for the retarding means, the control means including means for generating a signal having a frequency and amplitude which vary with the speed of the apparatus as it decelerates, frequency filter means be tween the signal generating means and the retarding means and responsive to a predetermined signal frequency corresponding to said predetermined lower speed to regulate the retarding means and stabilize the speed of the ap- 12 paratus at said predetermined lower speed, and means for regulating the deceleration from the running speed to said predetermined lower speed, the regulating means comprising derivative means for sensing the rate of change of said amplitude and thus the deceleration, and means operated by the sensing means when said rate of change exceeds a predetermined value to regilate the retarding means and prevent the deceleration from exceeding a predetermined value.

23. A decelerating system for bringing a moving apparatus, for example, an elevator car, from a running speed to a predetermined lower speed, the apparatus being subject to a variety of load hauling conditions, the system comprising means for driving the apparatus, retarding means for decelerating the apparatus against the force of the driving means, control means for the retarding means, the control means including means for generating a signal having a frequency which varies with the speed of the apparatus as it decelerates, the control means also including frequency filter means between the signal-generating means and the retarding means and responsive to a predetermined signal frequency corresponding to said predetermined lower speed to regulate the retarding means and stabilize the speed of the apparatus at said predetermined lower speed, means for sensing an overhauling load condition when the apparatus is at running speed, and means conditionable by the sensing means to reduce the force of the driving means during deceleration of an overhauling load below the force exerted by the driving means during deceleration of a hauling load.

References Cited in the file of this patent UNITED STATES PATENTS 2,096,473 Stevens Oct. 19, 1937 2,099,576 Schiebler Nov. 16, 1937 2,232,257 Myles Feb. 18, 1951 2,487,891 Pinto Nov. 15, 1949 2,602,912 Landau July 8, 1952 2,746,567 Guttinger et a1 May 22, 1956 2,766,415 Schurr Oct. 9, 1956 2,767,367 Black Oct. 16, 1956 2,792,080 Dunlop May 14, 1957 2,821,672 Sichling et a1 Ian. 28, 1958 2,872,633 Schurr Feb. 3, 1959 FOREIGN PATENTS 744,160 Great Britain Feb. 1, 1956

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