US2063981A - Protective system for polyphase motors - Google Patents

Description

Dec. 15, 1936. H. c. BRUNNER PROTECTIVE SYSTEM FOR POLYPHASE MOTORS Filed May 19, 1934 INVENTOR Har/"y 65nm/'76m 'ATTRNEY WITN ESSES Patented Dee. is, 1936 PATENT o1=1=1c11a PROTECTIVE SYSTEM FOR POLYPHASE MOTORS Harry C; Brunner, Wilkinsburg, Pa., asslgnor to Westinghouse Electric & Manufacturing Company, Eastl Pittsburgh, Pa., a corporation of 4 Pennsylvania 13 Claims.

My invention relates to electric motors, and

particularly to thermal protective systems therefor.

An object oi my invention is to` provide a relatively simple and effective temperature control system to be applied to an electric motor to limit the temperature rise thereof.

Another object of my invention is to provide a motor-deenergizing means selectively responsive to frequent momentary peak overloads and to long-continued heavy\overloads. Another object of my invention is to provide a motor-temperstare-controlling means having different time delays in response to diierentkinds of overloads.

Other objects of my invention will either be evident from the following description oi' several embodiments of my invention, or will be specifically pointed out hereinafter. i

In practicing my invention, I provide a manually closable circuit-controlling switch biased to its open position, a thermally-actuable latch to hold the switch in closed position, and two heating elements in heat-transferring relation to the latch to aictuate it to release the switch, both of the heating'elements being permanently connected in circuit with the energizing winding of the motor and one of the heating elements being normally short circuited by la thermal switch mounted on the motor and operated in response to temperature rise thereof.

In a modified system embodying my invention I provide two double-pole switches oi' the same static switch constituting one element of my invention;

Fig. 4 is a view similar to Fig. 1 but showing a diierent type of motor;

Fig. 5 is a diagrammaticv view of a motor and a modiiied form of control system embodying my invention, and

Fig. 6 is a diagrammatic view of the modiiied form of control system as applied to another type .of motor.

Fig. 3 is a view in section through a thermos Application May 19, 1934, Serial No. 726,603

It is highly desirable in electric motors to properly protect the motor, and particularly the energizing windings thereof, against excessive temperature rise. This temperature rise may be caused by three different `kinds of overloads. First, by a number oi `momentary peak overloads, such as are encountered when a motor drives an appliance or device in which the load may varyM sharply and repeatedly; second, where for various reasons, a long-continued heavy overload occurs; and, third, in the event of extreme overloads, such as may occur in the case of a'locked rotor. It is further desirable, for obvious reasons, to maintain the motor in an energized and an operative condition for as long a time as may be possible with respect to the temperature of the energizing windings not exceeding a` maximum permissible value. It is, of course, obvious that in addition to the percent or amount of overload on the motor;

the length of time during which the overload is 2 on the motor is of interest, but it is further obvious that the effect of frequent momentary peak overloads will not be as great as will be the effect of very heavy continued overloads. Ito may even be possible that the frequency of the momentary peak overloads is such that the motor can continue in operation without excessive temperature rise but it is further obvious that in case of an extreme overload, such as is represented by locked motor conditions, the motor temperature will rise very quickly. In general, the current traversing the energizing windings ci an electric motor under locked rotor conditions is on the order of 6 to 8 times normal load, so that the energy lossin the motor windings with locked rotor, is from 36 to 64 times the normal copper loss.

Referring nrst to Fig. 1 of the drawing, I have there illustrated a motor Il which is to be understood as being illustrative of any kind of polyphase electric motor and which is provided with a frame I3, end or bearing brackets i5, and a rotor which is illustrated generally only by an end view of the shaft l1. In this particular embodiment oi the motor, as noted above, it is provided with polyphase windings, and reference to Fig. 5 of the drawing will show the three-phase delta-connected energizing windings i9, 2|, 23,

- as well as a rotor 25. I

The motor is energized from a suitable source of supply of electric energy comprising leads or circuits 26, 21 and 29, these circuits being connected -to the Junctions of the phase windings lI9. 2| and 2l through a circuit controller or switch al. Preferably 1 make this switch a manually reclosable switch or breaker biased to its open position and normally held in its closed position by a thermal element. For illustrative purposes, I have shown a pivotally mounted arm 33 which is manually actuable by a knob 35 and which may be actuated in a given direction, here shown as in a clockwise direction, by a spring 31, against a stop pin 39, to open the circuit ot the motor. The arm 33 is provided with a plate 4| of electric insulating material on which are mounted three contact segments 43, 45 and 41. Suitably iixed circuit terminals, two for each of the respective phase circuits, cooperate with the respective segments 43, 45 and 41 to close the circuit to the windings of the motor. f

A thermal latch, here shown as a bimetal strip 49, having a suitably shaped lug 5| at its free` end, cooperates with the arm 33 when the same is turned in a counter-clockwise direction to engage the substantially fixed contact members, to thereby hold the switch in closed position. When the thermal element 49 is heated it will turn in a clockwise direction to release the arm 33 which will then be moved quickly by the action of spring 31 to open the circuit of the motor-energizing windings.

The thermal element 49 is energized by a plurality of auxiliary heating elements 53 and 55, respectively, which are connected in series circuit with the motor energizing windings in either one of the circuits, or preferably in two different circuits, and I have shown heating'element 53 as being connected in series circuit in supply circuit conductor 29, while heating element 55 is connected in series circuit with supply circuit conductor 26.

I further provide means initially or normally short-circuiting heating element 55, and a den vice 51 for performing this function is shown as being .counted on the outside of motor frame |3 in Fig. 1 of the drawing and is further shown in section in Fig. 3 of the drawing.

This thermostatic switchl includes a suitable thin metal casing 59, the edges of whose open side are so shaped as to closely engage the outer surface of the frame |3 of the motor against which it may be secured by any suitable means.

A disc 6|, of electric-insulating material such^ as mica, is located against the inner face of the intermediate portion G3 of member 59, against which are secured a pair of fixed contact members 65 and 61, which are held in proper operative position by rivets 69, additional small insulating washers 1| being provided against the outer face portion 63 to properly insulate terminal members 13 from the metal member 59.

A bimetallic disc 15, which is preferably of the kind disclosed and claimed in Patent No. 1,448,240 to J. A. Spencer, is supported on a stud 11 which has screw threaded engagement with the central portion of part 63\and which may be locked in any desired adjusted position by a lock nut 19, as shown. The bimetal disc 15 is insulatedly supported on stud 11 by insulating washers 8| and 83 which surround one end portion of stud 11 of reduced diameter, metal washers and 81 cooperating with washers 8| and 83 to hold the disc loosely thereon, so that it can move from the position shown in Fig. 3 of the drawing,'where it is in engagement with the two fixed arcuate contact members 65 and 81, to a position where 'it is out of engagement therewith, and where it is dished in the opposite direction from that shown in Fig. 3 of the drawing. I have shown the outer end of stud 1T as hammered over against washer 81, but it is obvious that any other holding means may be employed.

Referring now to Fig. 2 of the drawing, I have there illustrated a motor 89 embodying the usual stator housing `9|, a stator 93, a stator winding 95, and a rotor 91 mounted on a shaft 99 and supported in the usual bearing brackets |0|. A thermostatic switch 51 is here shown as being ,mounted in any suitable manner, in direct heatreceiving relation to the stator winding 95, wherein it differs from the device 51 shown in Fig. l of the drawing, which may be mounted either on the housing I3 or on the stator of motor equivalent to stator 93 of Fig. 2 of the drawing.

Referring now to Fig. 4 of the drawing, I have there shown a circuit controller |03 which is substantially the same as circuit controller or switch 3| except that only two contact segments |05 and |51 are provided, since the motor which is to be controlled thereby has two energizing windings only, indicated at |09 and in Fig. 6 of the drawing, and energized from supply-circuit conductors'l and ||5. Insofar as the other details of the switchv are concerned, they are substantially the same as was hereinbefore described in connection with Fig. 1 of the drawing, and I also provide two small auxiliary heating elements ||1 and ||9 in heat transferring relation to a bimetal'latch 49, heating element ||1 being connected in series circuit with supplycircuit conductor ||5 and heating element ||9 being connected in series circuit with supply-circuit conductor ||3. A thermostatic switch 51 is again shown as being mounted on the frame of motor 2| and initially normally short-circuiting auxiliary heating element ||9.

Referring now to the modification shown in Fig. 5 of the drawing, I have there illustrated the use of two manually-closable two-pole thermal circuit breakers |2| and |23 for use with a threephase motor as already described. Each circuit breaker embodies the same elements as was hereinbefore described in connection with breaker |03 of Fig. 4 of the drawing, except that only a single auxiliary heater |25 and |26 is provided for the thermal latch. The cooperating xed and movable contact members in the two switches are congiected in circuit with the respective junctions of the phase windings so that lead |21 includes one break or cooperating set of contacts, lead |29 includes two such sets-one from each switch, While lead |3| includes one set only.

Referring to Fig. 6 of the drawing, I have there illustrated two manually-closable doublepole thermal switches |3| and |33-each embodying a thermal latch and a single auxiliary heater in heat-transferring relation to the latch.

In this system each of the supply circuit leads includes two series-connected breaks. Each switch includes auxiliary heating elements |35 and |31, respectively.

Auxiliary heater i 25 of Fig. 5 is initially shortcircuited by a thermalswitch |39 located in heat-receiving relation to the motor and a similar thermal switch is provided in the system shown in Fig. 6 to cooperate with auxiliary heater |35.

Referring to the auxiliary heaters provided to actuate the thermal latch 49,1 prefer to make the current-carrying capacity of the normally shortcircuited heating element, (that is, either heating element 55, ||9, |25 or |35) less than the normal full load current capacity of the motor. For illustrative purposes I may make the heater 55 of 7.5 amperes capacity if the full load current in each of the three supply-circuit conductors of Fig. 1 is 10 amperes. I do not, however, desire to be strictly limited to this percentage relation since it may vary with the character of the load on which the motor is to operate. As has already ben set forth, this heating element is initially short-circuited and is, therefore, normally ineffective.

Referring now to heating elements 53, H1, |26 and |31, these are made of a somewhat higher current-carrying capacity than the normal full load current of the motor energizing conductors and, as has already been set forth, these heaters are continuously in circuit.

It will be obvious, ilrst of all, that the effect of heater 53 on'the thermal latch 49 Will be negligible so long as the motor load and current does not exceed the normal full load value. Itis further obvious that the heatingeffect of heating elemerit 53 on the thermal latch is also negligible in case of moderate overloads, say on the order of 125% or even 150% normal load. It is also ob.-

vious vthat in the case of very heavy overloads such as may occur under locked' rotor conditions,

where the current is on the order of 6 to 8 times normal full load current, the energyl loss in resistor 53 is very greatly increased, so that it will quickly transfer heat tothe thermal llatch and cause it to move in such a direction as-to cause the switch toopen. The same comments apply to the auxiliary heaters |I1, |26 and |31.

L'et itbe assumed that instead of an extremely momentary p'eak overload, and not until its temperature has reached a predetermined value corresponding to a temperature of the rotor just short of the danger value-will it open to energize or to make effective heating element 55.. As soon as this low-capacity heating element is energized it will transmit an amount of heat to the thermal latch which will cause it to move in such a direction as to effect .opening of the switch. The same comments apply to the auxiliary heaters H9, and |35.

It is, therefore, obvious that while one of the plurality of auxiliary heating elements energizing a thermal lamb has a short-time delay under certain operating conditions of the motor, the other auxiliary heating element has a much longer time-delay under other predetermined operating conditions of the motor.

The device and system embodying my inventionthus distinguish between the two different kinds of overloads and,` since the respective ele- `ments are proportioned for the respective dif. ferentoperating conditions, they will be properly operative with safety to themselves and to the motor at all times and will effect deenergization of the motor at the proper time to limit the temperaure rise ofthe motor to a safe value, irrespective of whether this temperature rise was caused by ashort-time very heavy overload condition or by arelatively long-timesubjection to.

intermittent peak or momentary overloads.

Idesire it to be understood that the illustration of the circuit-breaker is general only but'wish to mined maximum value.

point' oumhat I have found a manually. losam thermally latched circuit breaker' to be a relatively inexpensive piece of apparatus as compared to electromagnetic'circuit-breakers which have been so frequently used heretofore in thermal protective systems appliedto electric motors.

I further prefer to use a snap-acting disc of the kind hereinbefore' described which has a relatively large temperature differential of operation. A disc of lthis kind which will open when the temperature of the motor is on the order ofy 95c C., may not reclose until a temperature on the order of 50 C. has been reached. It is'to be noted, however, that even though thermal switch '51 or any one of the other equivalent switches may have been operated to its open position and 'may have eiecteda shut down of the motor, it

is possible to restart the motor as soon as ,the thermal latch has cooledsuificiently 'to hold the switch in closed position. The motor may thus continue in operation after a brief shut down,

' particularly if it is possible to reduce the load on the motor until it has cooled down, it being well f operated in accordance therewith when the tem-A perature rise of the motor reaches a predeter- Various -fu'rther'modiilcations may be made in the device and system embodying my invention without departing vfrom the spirit and scope thereof, and I desire, therefore, that only such limitations shall be placed' thereon as are imposed by the prior art or appended claims.

I claim as my invention:

1. A thermal protective -system for motor having an energizing Winding, comprising circuit-interrupting means for the energizing vn'nding biased to open position, thermallyactuable latching means normally .holding the circuit-interrupting means closed, a plurality of heatingelements in heat-transferring relation to the latching' means,v and means responsive tothe temperature of a selected part of said motor, for governing the circuit connections of one of said are set -forth in the an electric i heating elements to cause opening of the circuit interrupting Vmeans selectively responsive tov different characters of motor overload.

2. A system as described in claim l in which said responsive means includes a thermal switch directly controlled by the temperature of a selected motor part for initially rendering ineffective one of the heating elements.

3. Av thermal protective system for an motor having an energizing Winding, comprising circuit-interrupting means for the energizing winding biased to open position, thermallyactuable latching means normally holdingthe circuit-interrupting means closed, a long time delay thermal energizing means and a short time delay thermal energizingwmeans for the latchin'g means controlled in accordance with the load on themotor to cause/opening 'of the circuit inter' rupting means in accordance with intermittent short-time overloads and with vlong continued heavy overloads, said long time delay thermal electric energizing means including means thermguy..

associated with the motor to initially render it ineffective.

4. A thermal protective system for an electric motor having an energizing winding, said system comprising circuit-interrupting means for the energizing winding biased to open position, thermally-actuable latching means normally holding the circuit-interrupting means closed, a plurality of heating elements in heat-transferring relation to the latching means and connected in series circuit with the energizing Winding to cause opening oi the circuit-interrupting means under predetermined different load conditions and a thermally-actuable switch controlled by the temperature of the motor for initially rendering ineffective one of the heating elements.

5. A protective system for an electric motor having an energizing winding, comprising a switch in the circuit of the energizing Winding biased to open position, a thermally-actuable latch normally holding the switch in closed position, and a pair of heating elements in heattransferring relation to the latch and means responsive to the temperature of a selected part of said moto-r for governing the circuit connections of one of said heating elements to cause selective actuation of the latch in accordance with intermittent short-time overloads and with long-continued heavy overloads.

6. A protective system for an electric motor having an energizing winding, comprising switching means for governing the circuit of said winding, two actuating means for said switching means, and means responsive to the temperature of a selected part of said motor for governing the operation of one of said actuating means to thereby effect opening of the switching means respectively in accordance with diierent characters of motor overloads.

7. A protective system for an electric motor having an energizing winding, comprising a switch, a thermal latch for holding the switch in closed position, a plurality of motor-current energized heaters of different current-carrying capacities and having dierent time lags of operation, and means responsive to the temperature of a selected part of said motor for governing the connections of one oi said heaters for selectively energizing said latch to open the motor circuit under diierent operating conditions.

8. A thermal protective system for an electric motor having an energizing winding, comprising a switch biased to open position, a thermally actuable latch normally holding the switch in closed position, and a plurality of heating elements in heat-transferring relation to the latch, said heating elements being connected to be traversed by the current traversing the motor energizing winding, and a thermal switch subjected to the temperature of the motor, initially short-circuiting one of said heating elements.

9. A thermal protective system for an electric motor having an energizing winding, comprising a switch in circuit with the energizing winding and biased to open position, a thermallyactuable latch normally holding une switch in closed position, a plurality of heating elements in heattransferring relation to the latch and connected in series circuit relation with the energizing winding, and a thermal s witch controlled in accordance with the motor temperature normally short-circuiting one of said heating elements to cause a relatively long-time delay in the opening of the switch under predetermined motor-load conditions. s

10. A protective system for an electric motor having an energizing winding, comprising a switch in circuit with the energizing winding and biased to open position, a thermally-actuable latch holding the switch in closed position, an initiallyineiective heatingelement controlling the latch, means controlledby motor temperature to render the heatingelement effective and responsive to motor current, and a continuously effective heating element responsive to motor current and having a relatively short-time lag of operation under motor-overload operating conditions.

11. A thermal protective system for an electric motor having an energizing winding, comprising a manually-closable switch for the energizing winding biased to open position, a thermallyactuable latch normally holding the switch in closed position, and means for thermally energizing the latch to cause opening o1' the switch, said means including a heating element in heattransferring relation to the latch and connected in series circuit with the energizing winding and a thermal switch normally short-circuiting said heating element, mounted in heat-receiving relation relatively to the motor and operative at a certain temperature of the motor to remove said short-circuit on the heating element.

12.- A thermal protective system for an electric motor having an energizing winding, comprising a manually-reclosable switch in circuit with the energizing winding, means biasing the switch to its open position, a blmetal latch normally holding the switch in closed position, a heating element in heat-transmitting relation to the latch in circuit with the energizing winding, thermallyactuable means normally short-cirouiting said heating element, said thermally-actuable means being mounted on the motor and adapted to remove the short circuit on the heating element at a certain motor temperature.

13. A system as set forth in claim 3 in which the long-time delay thermal energizing means includes a heating element in series with the energizing winding and having a current-carrying capacity less than that of the normal motor load and a thermal switch directly controlled by the temperature of the motor and initially rendering said thermal energizing means ineffective, and the short-time delay thermal energizing means includes a heating element in series circuit with the energizing winding and having a current-carrying capacity greater than that of the normal motor load.

HARRY C. BRUNNER.

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