CA1061403A - Induction motor brake circuit - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

This invention concerns an electrical drive arrange-ment in which a polyphase electric motor can be braked by connecting the capacitor across the motor windings. A
test system is provided to check that the capacitor is in a condition to effect braking, and the arrangement is such that the motor cannot be operated unless the test system indicates the capacitor is in a condition to brake the motor.

Description

` 1061403 This invention concerns an electrical drive arrange-ment, and more particularly relates to the dynamic braking of A.C. induction motors by capacitive self-excitation, either alone or advantageously together with other braking methods such as D.C. injection. Such braking systems are known to be effective particularly for use with polyphase A.C. motors;
our British Patent Specification 1231067, which issued May 5, 1971 to Dewhurst and Partner, discloses and claims one such system employing capacitors together with short circuiting of some of the stator windings of the motor and our British Patent Specification 1473327, which issued on May 11, 1977 also to Dewhurst and Partner, discloses another system employ-ing capacitors with D.C. injection. The present invention is suitable for use with the braking systems of our above men-tioned specifications, but is not exclusively suitable for use therewith.
The present invention provides an arrangement in which the state of the braking capacitor is monitored to determine whether it will be safe to operate the motor and subsequently rely upon the capacitor to perform its braking function.
According to the present invention there is pro-vided an electrical drive arrangement comprising an alternat-ing current induction motor, a capacitor, means for connect-ing the capacitor to the motor in such a manner as to cause braking thereof, means for testing that the capacitor is in a condition to effect said braking, and means to disable operation of the motor upon said caplcitor not being found b~ said testing means to be in a condition to effect said braking.
In the particular examples of the invention

- 2 -. 10~1403 described hereinafter,the capacitor is tested upon an operator attempting to start the motor. If the result of the test is satisfactory, the motor is permitted to be started, but otherwise the motor is disabled.
In order that the invention may be more fully understood and more readily carried with effect, two particular examples thereof will now be described by way of illustration with reference to the accompanying drawings in which Figure 1 is a circuit diagram of an electrical drive arrangement in accordance with the present invention.
Figure 2 is a circuit diagram of another example of an electrical drive arrangement in accordance with the present invention, Figure 3 is a circuit diagram of a phase sensitive detector of the arrangement of Figure 2, and Figure 4 illustrates various waveforms produced during operation of the arrangement of Figures 2 and 3, Figure 4A illustrates the rectified waveform on the bus bars 12, 13, Figure 4B illustrates the charging and discharging waveform of capacitor C1, - Figure ~C illustrates the waveform applied to terminal 2 of the PSD, and Figure 4D illustrates the output waveform of the transistor TR3.
Referring firstly to Figure 1, a three-phase A.C.
supply L1, L2, L3 is coupled to ~e stator winding terminals A, B, C of a three-phase induction motor through the contact pairs MC1, MC2, MC3 of a main contactor relay MC having ~061403 other contact pairs MC'~ and MC5 the function whereof will be explained hercinafter. The relay contact as illustrated in all of the figures of the drawings follow the convention that the contacts move towards the left-hand side, thus closing MC1 and opening MC4 for example, when the relay is energized.
The braking capacitor is designated C, and as shown, is connected to phases L2 and L3, and to stator terminals B and C, via contact pair MC4 of the main contactor relay MC.
These contacts are normally open during operation of the motor and close only when the capacitor braking action is initiated.
Coupled to the capacitor C is capacitor proving circuitry for determining whether the capacitor C will perform its braking function. The circuitry includes a capacitor prove relay CP having contact pair CP1 connected in the energization circuit of a relay RR having contact pairs RR1, RR2 and RR3 connected as shown. Relay RR has two functions, as will become more apparent hereafter, ihe first of which is to "remember" during the running time of the motor that thecapacitor C was tested to be satisfactory, `
and the second of which is to disconnect the capacitor after - proof of its condition has been obtained since it is con-sidered that the maintenance of a potential on the capacitor would be counter productive to safety considerations by virtue of the possibility that the capacitor might be damaged by the maintained potential.
The capacitor proving circuitry also includes diodes DI and D2 and resistors R1 and R2 connected as shown, the functions of which will become apparent from the _ ~ _ following e~plan~tiona, and the arrangement is completed by "start" pushbutton S1 and "stop" pushbutton S2 coupled between a D.C. supply and the main contactor relay MC as shown.
In operation of the illustrated circuitry, the "start" pushbutton S1 is depressed by the machine operat,or and main contactor relay MC is energized from the D.C.
supply. Contacts MC1, MC2 and MC3 close to connect the A.C. supply L1, L2, L3, to the motor terminals A, B, C
and the motor starts to run. Contacts MC4 of main contactor relayMC open and contacts MC5 close to establish a "potential" holding circuit for the main contactor relay MC which depends for its operation upon the condition of capacitor prove relay CP and of relay RR.
Assuming that the capacitor C is in good condition and that capacitor prove relay CP is therefore energized (as explained in the following) which causes its contacts CP1 to close, relay RR is then energized via normally closed S2, contacts MC5 now closed and contacts CP1 now closed.
The energization of relay RR closes its contacts RR2 thereby ,completing a holding circuit for relay MC, and also closes its own self-holding contacts RR3. At the same time contacts - RR1 open thereby disconnecting the capacitor prove circuit.
The motor can now run even if the "start" button S1 is opened, and in practice it is necessary only that the "start"
button S1 be held by the operator for some,what less than 100 m secs.
Had the capacitor C not been in good condition at the start of the above-described sequence, the relay CP
would not have been energized and the holding circuit for main contact relay MC would not have been completed. The motor would have stopped running as soon as "start" push-button S1 was released.
Considering the operation of capacitor prove relay CP, it will be seen that during the period when relay RR is not energized (i.e. the period immediately following depression of ~'start" pushbutton S1) the capacitor C receives a charge via the path including closed relay Contacts RRls resistor R1 which serves a current iimiting function, and diode D1 which sets the terminal of the capacitor C which is connected to terminal B of the motor at a relatively negative potential and the other capacitor terminal at a relatively-positive potential. Diode D2 is then reverse biassed. In the subsequent half cycle of the A.C. supply, the capacitor C is able to discharge from its positive terminal through the coil of relay CP to the junction point of diodes D1 and D2 and resistor R1 and, at this instant, this junction is held so that it cannot be more negative than the negative capacitor terminal. This discharge of the capacitor C
serves to "prove" the operative condition of the capacitor;
if a short circuit exists on the capacitor, or if it presents an unusually low impedance, the relay CP will not be energized. If an open circuit condition exists in the capacitor, then the relay CP cannot be energized. Otherwise, if the capacitor is in good condition, the relay CP will be energized.
The holding process on diode D2 results in the resistor R1 being connected directly across the two motor phase wires B and C and the power rating of R1 must ~e such as to accommodate this. By virtue of this arrangement, the possibility is precluded of a voltage doubling condition, which would result in unnecessarily high potentials being offered to the coil of relay CP, occurring as ~ e result of the combination of the potential sbored in the capacitor supplemented by the potentials at the motor terminals.
~ esistor R2 provides a discharge path for capacitor C after relay RR has been energized.
Another embodiment of an electrical dr-ve arrange-ment including apparatus for testing a braking capacitor will now be described with reference to Figures 2, 3 and ~.
The apparatus operates on the principle that when the braking capacitor is connected across the windings of the motor, the capacitor and the windings define a resonant circuit. The capacitor is tested by applying a test signal to the resonant circuit, and the phase of an output signal from the circuit is monitored to provide an indication of the condition of the braking capacitor. Any change in the -condition of the capacitor is manifested as a change in the phase of the output signal, which upon detection can be used to inhibit operation of the electric motor.
In the circuit arrangement shown in Figure 2, a polyphase electric motor having windings W1, W2, W3, is connected to a three-phase A.C. supply from terminals L1, L2, L3- The motor is provided with a braking capacitor C
- which can be selectively connected to the motor windings W
to brake the motor. As in the arrangement of Figure 1, the motor is switched on by operation of switch S1 and is switched off and braked by operation of switch S2. An electromechanical relay arrangement is provided which is arranged so that upon operation of the on switch S1, a single phase A.C. test signal derived across the terminals L1, L2, is applied to the resonant circuit comprising motor coils .

W1, W2 and braking capacitor C. A phase sensitive detector circuit P.S.D. is connected to the output of the resonant circuit to compare the relative phases of the input test signal and the output of the resonant circuit. If the phases of the signals accord with a predetermined allowable relationship, the relays are so arranged to switoh on the motor, but if the phases do not accord with the relationship, the motor is not switched on.
An A.C. signal for energizing the relays is derived from the phases L1 and L2 by means of a transfor~er T1, and is fed to the start and stop switches S1 and S2. Upon operation of the start switch S1, the relay energizing signal is fed to the coil Q1 of a relay P.P. which operates switches S3, S4, S5 and S6. Upon operation of switch S3, the relay energizing signal is fed to switch S7 which is a part of a relay P.O. that is controlled by the phase sensitive detector P.S.D. The arrangement is such that when the P.S.D.
indicates that the capacitor C is in a good order to effect motor braking, the relay P.O. is triggered so that switch S7 applies the relay energizing signal`to a coil Q2 of a further relay MC that controls switches S8 and S11. The relay MC
also has a self-holding switch S12. Operation of the relay - MC thus causes the three-phase supply to be connected through switches S9 - S11 to the motor windings W to start the motor.
Operation of the relay P.P. also causes a single phase A.C. test current to be applied to the resonant circuit comprising capacitor C and the windings W1, W2. The test current is derived from the terminals L1, L2 and is fed through switches S4, S5, resistors R1, R2 and switch S8.
The input to the resonant circuit is monitored by the P.S.D.

106~403 circuit; the voltage across resistor R2 being applied to terminals X and Y of the P.S.D. An output from the rcsonant circuit is applied to terminal Z of the P.S.D. through switch S6.
Thus to start the motor, an operator depresses the start switch S1, causing the relay P.P. to be operated such that an A.C. test signal is applied to the capacitor C and the windings W through the switches S4 and S5, and the relay energizing signal is applied to the switch 57. If the capacitor C is in good condition, the phase sensitive detector causes the relay PØ to operate in such a manner that switch S7 directs the relay energizing signal to the coil Q2 of relay MC which is thus energized. As a result, the switch S8, which was previously closed, is opened so as to disconnect the braking capacitor C from the windings W, and the previously open switches S9 - Sll are closed to connect the motor to the power supply termina]s L to start the motor.
The switch S12 holds the relay MC in its energized state and hence the motor starts and continues to rotate until the stop switch S2 is actuated, at which time, the relay MC is released to connect the braking capacitor across the windings W1, W2 and to disconnect the power supply from terminals L.
~ It will be appreciated that upon starting the motor, the P.S.D. and the relays must respond fast enough to operate the relay MC whilst the switch S1 is held instantaneously depressed by the operator. In practice, the switch is - typically held depressed for only a few hundred milliseconds.
A phase sensitive detector circuit having a fast enough ~, response for this purpose will now be described with reference to Figure 3.
' ~ _ 9 _ ,, The circuit has inputs X, Y, Z which correspond to those shown in Figure 2, the input test signal being applied to the circuit between terminals X and Y, while the output from the resonant circuit is applied to terminal Z. The single phase input signal applied across X and Y is fed to a four way rectifier D and then to bus bars 12, 13. A
transistor TRl is connected in series with a resistor R3 between the bus bars; switching on of the transistor being controlled by the output signal applied to terminal Z, which is connected to the transistor's base through diodes D3, D4 .
A slave transistor TR2 has its base connected to the collector of TRl to be ~witched on when TRl is switched on.
The transistors control a charging and discharging circuit for a capacitor Cl which, as will be explained here-inafter in more detail, is charged when the transistors are switched off, through a resistor R4 connected to the terminal X. The capacitor Cl is discharged when the transistors are ; switched on, and as a result the capacitor is charged and ; discharged sequentially in dependence upon the signal applied to the terminal Z. A transistor TR3 is connected between the bus bars 12, 13 with its base connected to the capacitor Cl to compare the relative phase of the rectified A.C. signal on the bus bars with the phase of the charging cycle of capacitor Cl. The transistor TR3 is arranged to fire a thyristor S.C.R. if the compared phases lie within a given relationship, the thyristor then causing energization of a coil Q3 of the relay PØ, *o actuate the switch S7 of Figure 2. The transistor TR3 is baissed to a switched-off state during positive-going half cycles of the signal applied to terminals X, Y through a current path between terminal X

and the base of the transistor, the path including a resistor Operation of the circuit of Figure 3 will now be described in more detail with reference to the waveforms shown in Figure 4.
As shown in Figure 4B, during positive-going half cycles of the input waveform appl.ied to terminals X and Y, the capacitor is charged such that its left hand terminal assumes a positive potential, the charging being effected A 10 - through resistor ~. When, however, the potential of the waveform applied to the terminal Z passes through zero, the transistors TR1 and TR2 are switched on, and the left hand terminal of the capacitor is connected via the conducting ~/
transistor T~ to the emitter of transistor TR3 and in consequence the right hand terminal takes up a negative potential with respect to the emitter of TR3, as shown at 14 in Figure 4B. As a result, transistor TR3 cannot conduct until the charge on condensor C1 has leaked away to zero as is indicated at 15 in Figure 4B. The time taken for the capacitor C1 to discharge to zero is a function of the phase difference between the signal applied across terminals X and Y, and the sig~al applied to terminal Z, all other factors - which could effect the discharging of the capacitor remaining substantially constant throughout each cycle of operation of the circuit. Thus, by determining whether the discharge time for capacitor C1 and hence the phase difference, exceeds a given value, the condition of the braking capacitor C
(Figure 2) can be established. This determination of the : discharge time is effected by the transistor TR3 which is arranged to be switched on only if the discharge of capacitOr .. . ,,, -- , . . ..

106~403 C1 extends in time from the positive going half cycle of the waveform applied to X, Y in which the discharge was initiated, into the next negative going half cycle of the waveform.
The output of the transistor TR3 is shown in Figure 4D. The positive going half cycle of the waveform applied to X, Y
corresponds to the half cycle numbered 1 of the rectified waveform applied to the bus bars 12, 13, and during this half cycle, the transistor TR3 is biassed off by a signal from X through the resistor R5. If, however, as is shown in Figure 4, the capacitor C1 has a charge remaining therein upon the occurrence of the next half cycle (2) of the rectified waveform of Figure 4A, the transistor is switched on until the charge on the capacitor C1 is dissipated, and hence the output of transistor TR3 is as shown in Figure 4D.
The output from transistor TR3 causes the thyristor S.C.R. to be fired, and consequently the relay PØ is operated indicating that the capacitor C of Figure 2 is in a condition to perform its braking function.
, The coil Q3, of relay PØ has a significant level of self inductance and, since no alternative path is provided for the inductive currents, they flow through the thyristor S.C.R. and maintain the thyristor in a fired condition between the periods during which transistor TR3 is conductive.
In the event of the braking capacitor C being short circuited, there will be less of a phase difference between the signals applied to the phase sensitive detector circuit of Figure 3, and the discharge of the capacitor C1 will not ex'end into the half cycle 2 of Figure 4. Hence the tran-sistor TR3 will not be switched on and the relay PØ will' not be energized. As a result relay MC will not be triggered i , and the motor will not start.
Conversely, in the event of the capacitor presenting an open circuit, the capacitor C would not provide a resonant circuit with the motor windings, and hence the relay P.O.
would not be operated to start the motor.
The sensitivit~- of the circuit can be altered by , ., 106~403 adjusting a variable resistor VR1 to adjust the rate of discharge of the capacitor Cl.
It is estimated that an average squirrel cage motor has an impedance when stalled that contains a power factor better than 0.4, and at this power factor, the displacement of voltage relative to current would be 66 . The dimensions of the braking capacitor C that is connected ~o the motor to effect braking are chosen to bring the power factor of the stalled motor to a value approaching unity. -Hence the phase sensitive detector circuit must be capable of detecting between power factors around 0.4 and unity for the circuit comprising the motor windings and the braking capacitor. We have found that the circuit arrangement described with reference to Figure 3 fulfils this requirement.
In some motor installations, it may prove necessary to calibrate the variable resistor VRl by the process of setting the resistor VRl to its maximum value, and with the capacitor C connected to the motor windings, operating the stop switch S22 and moving the resistor slider of VRl until the relay PØ operates. The setting of the resistor can then be checked by disconnecting the capacitor C from the motor, since if the resistor VRl is set correctly, the relay should fail to operate. Any apparently undue sensitivity of the variable resistor VRl can be expected to indicate that the capacitor C is presenting an inadequate capacitance to brake the motor.
Clearly many modifications and alterations could be made to the above described arrangemcnt; and for example the phase sensitive detector could be of other forms than that described. The P.S.D. could for example operate digitally.

_ 14 -Although, as above described, the invention con-templates that it would be sufficient to prove the condition of the braking capacitor only once, at the beginning, of each start to stop routine of the machine, in some circwnstances it could be desirable to interrogate the braking capacitor additionally at intervals in the running of the machine.
Such an arrangement could readily be accommoda~ed by the invention simply by incorporation of a timer circuit, for example, arranged to perform the capacitor interrogate routine at intervals as desired after the first interrogate routine performed on starting up the machine. Each additional interrogation of thecapacitor could be arranged to determine whether the machine was allowed to continue running or whether it was disconnected from its power supply.
Many other modifications and variations lying within the scope of the claims will be apparent to those skilled in the art.

_ 15 -

Claims (15)

WHAT WE CLAIM IS:

1. An electrical drive arrangement comprising an alternating current induction motor, a capacitor, means for connecting the capacitor to the motor in such a manner as to cause braking thereof, means for testing that the capacitor is in a condition to effect said braking, and means to disable operation of the motor upon said capacitor not being found by said testing means to be in a condition to effect said braking.

2. An arrangement in accordance with claim 1 wherein said testing means is adapted to be operated before start up of the motor.

3. An arrangement in accordance with claim 2 wherein said testing means comprises means for charging the capacitor, a relay, and means for discharging any charge accumulated in the capacitor through the relay to cause actuation thereof and thereby provide an indication of the condition of said capacitor.

4. An arrangement in accordance with claim 3 wherein said relay is arranged to operate a further relay for selectively connecting and disconnecting an electrical power supply to the motor.

5. An arrangement in accordance with claim 4 and including a stop switch for disconnecting an electrical supply from the further relay to cause deactuation thereof.

6. An arrangement in accordance with claim 5 wherein said braking capacitor is arranged to be connected and disconnected from the motor upon deactuation and actuation of said further relay respectively.

7. An arrangement in accordance with claim 2 wherein said testing means includes means for supplying a test signal to a circuit comprising the capacitor and a winding of the motor when the capacitor is connected thereto, and a phase sensitive detector for detecting the phase delay imparted to said test signal by said circuit whereby to provide an indication of the condition of said capacitor.

8. An arrangement in accordance with claim 7 wherein said circuit has an input to receive a periodic input signal, and said circuit has an output to provide an output signal therefrom, and wherein said phase sensitive detector comprises a test capacitor, means for charging the test capacitor by an amount dependent upon the relative phases of said input and output signals, means for subsequently discharging the test capacitor, and means for determining whether the test capacitor becomes discharged within a predetermined half cycle of said input waveform subsequent to the half cycle thereof in which said charging was initiated.

9. An arrangement in accordance with claim 8 including means for rectifying said input signal, and means for charging the capacitor from the rectified signal.

10. An arrangement in accordance with claim 9 and including semiconductor switching means arranged to cause charging of the test capacitor to an opposite polarity to the charging thereof from said rectified signal, said switching means being responsive to the voltage excursion of said output signal passing through a predetermined reference level.

11. An arrangement in accordance with claim 10 including an output relay, and further semiconductor switching means arranged to apply said rectified signal to the relay during a predetermined half cycle of said input waveform subsequent to the half cycle in which said charging was initiated and so long as said test capacitor continues to discharge.

12. An arrangement in accordance with claim 11 wherein said further semiconductor switching means comprises a transistor having its base connected to the test capacitor and its collector and emitter connected to said rectifying means, the base also being connected to receive the unrecti-fied input signal in such a manner that the transistor is switched off during the half cycles of the input signal in which said charging is initiated.

13. An arrangement according to claim 12 and including a thyristor connected to the output relay, said thyristor being arranged to be fired when said transistor is switched on.

14. An arrangement in accordance with claim 13 including a main relay for connecting and disconnecting the braking capacitor and a source of electrical power from the motor windings, said main relay being arranged to be actuated upon operation of the output relay.

15. An arrangement in accordance with claim 14 and including start and stop switching means for selectively applying an actuating signal to said main relay in dependence upon the condition of said output relay, and a pilot relay responsive to the actuating signal and arranged to apply the test sig-nal to said circuit.