US2521120A

US2521120A - Overload protector

Info

Publication number: US2521120A
Application number: US715477A
Authority: US
Inventors: Marshall C Harrold
Original assignee: Motors Liquidation Co
Current assignee: Motors Liquidation Co
Priority date: 1946-12-11
Filing date: 1946-12-11
Publication date: 1950-09-05
Anticipated expiration: 1967-09-05

Description

s P 1950 M. c. HARROLD 2,521,120

OVERLOAD PROTECTOR Filed Dec. 11, 1946 66 B/METAL I45 u I44 I 2 LI VENTO:

BY CM Patented Sept. 5, 1950 UNITED STATES PATENT OFFICE corporation of Delaware Application December 11, 1946, Serial No. 715,477

6 Claims. (Cl. 318-2'21) This invention relates to electric apparatus and more particularly to overload protectors for electrio motors.

Where motor overload protectors are self-reing it is customary to set the protector to trip at a sufiiciently low current flow to properly protect the motor windings for one or several resettings and yet sufficiently high to permit a generous amount of overload. Should stalling conditiOnS T eavy overloads continue for extended '10 periods of time the protectors will reset repeatedly. The customary setting permits each successive how of current upon each resetting to increase the temperature of the windings so that eventually the temperature becomes high enough to damage the insulation.

It is an object of my invention to provide an overload protector in which the settings are sufficiently high to obtain full output of the motor and in which the heating of the motor is re-' duced as the protector continues to trip.

It is another object of my invention to provide an overload protector in which the closed period of the protector is reduced or the open period of the protector is prolonged or both, as 335 the stalling or overload condition continues.

It is another object of my invention to use the rise in temperature of the starting control to control the reduction of the proportion or closed time to open time of the protector as stalling or overload conditions continue.

These objects are attained by placing the thermal element of the overload protector in heat exchange relation with the electromagnetic starting control so that as the protector continues to trip and reset and the motor and starting control rise in temperature, the proportion of closed to open periods of the protector will be reduced. Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompany- Referring now more particularly to Fig. 1, there is shown, for the purpose of illustrating one application of my invention, a capacitor start and run motor control system including an electric motor 20 having a. main winding 22 and a phase winding 24. The common junction of the main 2 and

phase windings

22 and 24 is connected by a switch 26 to the supply conductor 28. The other end of the main winding 22 is connected by the conductor 30 to the starting relay electromagnet coil 32 which in turn is connected by the

conductors

34 and 36 to the actuating bimetal 38 of the motor overload protector 40. The actuating biinetal 38 carries at its free end a, contact which is adapted to make contact with a stationary contact 42. The stop screw 44 limits the extreme open circuit position of the 'bimetal 38. A compression spring 46 extends between the free end of the bimetal 38 and an anchorage 48 to cause the bimetal 38 and its contact to open and close with a snap action. The bimetal 38 may be completely self-heated or it may be both selfheated and externally heated by the electric heator 50 connected between the stationary contact and the second supply conductor 52. I

The other end of the phase winding '24 is connected by the conductor 54 to a permanent capacitor 56 which in turn is connected by the

conductors

58 and 60 to the conductor 36 which through the overload protector 40 connects with the supply conductor 52. This same end of the phase winding 24 is connected by the conductor 62 to a stationary contact 64 mounted upon the supporting frame 66 which supports the electromagnetic coil 32. The armature 6B of the electromagnet 32 is connected to a leai spring 10 carrying a movable contact adapted to make contact with the stationary contact when the electromagnet 32 isenergized and the armature 68 is lifted. I This closes thecircuit through the conductor 12 to the startingcapacitor which connects through the conductor 16, and the

conductors

60 and 36 with the overloadprotector 40 which in turn is connected to the supply conductor 52. As soon as the motor 21] starts and attains sufiicient speed the current flow through the electromagnet 32 becomes sufiiciently small so that the armature 68 drops and separates the movable contact from the stationary contact 64 to disconnect the starting capacitor ID from the phase winding circuit.

Should an overload occur, the bimetal 38 will be heated by the current flow through it and the heater 50 and will bow upwardly to lift the movable contact from the stationary contact 42 to disconnect the system from the supply conductor 52. The upward movement of the bimetal so will be stopped by the stop screw 44. The position of the stop screw '44 will determine the temperature to which the bimetal 38 must cool before it will return to the closed circuit position shown in Fig. l. The setting of the stop screw depends upon the application of the m tor 28. For some applications a long off period is permissible but for many others the off period should be as short as possible so that operation can be resumed with a minimum of delay.

Under the latter condition it will often happen that if there is a continued heavy overload or a stalling condition in which the motor is prevented from rotation, the current flow will be so heavy as to cause the overload protector to trip within a very short time. Since the protector is self-resetting it will reset regardless of the continued cause of the tripping and at each resetting a considerable amount of current will flow through the windings of the motor 28. Each time the protector resets current flows through the motor dissipating a certain amount of energy in the motor to heat the motor until the protector again trips. If this is permitted to continue for a sufficiently long period of time the motor windings will become sufiiciently hot to damage the insulation.

According to my invention, instead of anchoring the bimetal 38 upon any conventional support, I fasten the end of the bimetal 38 directly to theU-shaped support 65 for the electromagnet 32 by some suitable fastening means so that the anchored end of the bimetal 38 is in direct heat exchange relation with the electromagnet 32. Since the electromagnet 32 heats up in the same way as the electric motor under continued stall or overload conditions, the bimetal 38 will be heated from the heat generated by the electromagnet 32 in proportion to the heating of the motor 23 during each starting period. This heating by the electromagnet 32 will be in addition to the self-heating and/ or external heating usually provided as, for example, shown in Fig. 1. With this arrangement the electromagnet 32 is heated and becomes progressively hotter because of the heavy current fiow during continued stalling and overload conditions occurring during the starting. period. The bimetal 38 will derive heat from the electromagnet to trip at a lower current value and to remain open for a longer period before the resetting temperature is reached. Thus by this arrangement the tripping is accelerated and the resetting is retarded.

In Fig. 2 is illustrated another form of the invention in which the same motor, motor circuit and starting control are used and bear the same reference characters. The overload protector, however, is difierent and includes an external heater I50 connected at one end to the supply conductor I52 and at the other end to the anchorage of the actuating bimetal I38. The free end of the bimetal I38 carries a movable contact which is adapted to contact the stationary contact I42 connected by the conductor I31 with a conductor I extending to one end of the electromagnet coil 32. The reset stop screw I 44 is mounted upon the end of the bimetal strip I45 which is anchored to and is in heat exchange relation with the frame 66 of the electromagnet coil 32 and derives heat therefrom with the rise in temperature of the electromagnet 32.

A toggle spring I46 extends between the free end of the bimetal I38 and a support I48 which may be mounted upon the frame 68. The toggle spring I46 causes the bimetal I38 to move to open and closed positions with a snap action. As mentioned in connection with Fig. 1, the position of the stop screw I44 determines the temperature at which the bimetal I38 will reset. If the screw I44 is moved closer to the stationary contact I42 the bimetal I38 will reclose at a higher temperature. The bimetal I45, however, is so arranged that as it rises in temperature due to the heating of the electromagnet 32 it will curl upwardly to move the stop screw I44 away from the stationary contact I 42 so that it will take longer for the bimetal I38 to cool to the lower temperature it must reach before it is reset. The longer the stalling or overload condition continues the higher will be the temperature of the electromagnet 32. Thus as the stalling or overload condition continues, the stop screw I44 will be moved further away from the stationary contact I42 to prolong the open circuit period of the overload and to reduce the proportion of closed time to open time of the overload protector so that the temperature of the motor is prevented from reaching dangerous limits.

By either of these arrangements, the heating effect of the current during protector controlled overload and stalling conditions is reduced as the condition continues so that the temperature of the motor will be safely limited. While I have shown the invention as applied to a capacitor start and run motor, it may be applied to other types of motors requiring a starting control, such as, for example, split phase motors.

While the form of embodiment of the invention as herein disclosed, constitutes a preferred form, it is to be understood that other forms might be adopted, as may come within the scope of the claims which follow.

What is claimed is as follows:

1. In combination, an electric motor having main and phase windings, an electrically operated starting control for controlling the energization of the phase winding, a thermal element in heat exchange relation with said starting control, self-resetting overload protector for deenergizing said electric motor upon an overload, said overload protector including'a resetting stop connected to and operated by said thermal element for varying the reclosing of the protector in accordance with the temperature of the starting control.

2. In combination, an electric motor having an electromagnetic starting control, a thermal element in heat exchange relation with said starting control, a self-resetting overload protector for deenergizing said electric motor upon an overload, said overload protector including a resetting stop connected to and operated by said thermal element for varying the reclosing setting of the protector.

3. In combination, an electric motor having an electromagnetic starting control, said starting control including a metal magnetic circuit portion, a metal thermal motor in direct metal-tometal contact with said metal magnetic circuit portion and in metallic heat exchange relation therewith, a self-resetting overload protector for deenergizing said electric motor upon an overload, said overload protector including a resetting stop connected to andoperated by said thermal motor for varying the reclosing setting of the protector.

4. In combination, an electric motor having main and phase windings, an electromagnetic starting control for controlling the energization of the phase winding, said starting control including an electromagnet coil and a metal frame of magnetic material fixed to and rigidly connected to said coil, a thermal element mounted directly upon said frame directly adjacent said coil in direct metal-to-metal contact with and in direct metallic heat exchange relation with said frame, means for heating said thermal element in response to motor current, and selfresetting means connected in series with both the main and phase windings and operated by said thermal element for completely deenergizing said motor upon an overload.

5. In combination, an electric motor having main and phase windings, an electromagnetic starting control for controlling the energization of the phase winding, said starting control in cluding an electromagnet coil and a metal frame of magnetic material fixed to and rigidly connected to said coil, a self-resetting overload protector connected in series with both the main and phase windings for completely deenergizing said electric motor upon an overload, said protector including metal thermal means mounted direct- 1y upon said metal frame directly adjacent said coil in direct metal-to-metal contact with and in direct metallic heat exchange relation with said metal frame for reducing the proportion of time the protector is closed whenever unsuccessful starting attempts continue.

6. In combination, an electric motor having main and phase windings, an electromagnetic starting control for controlling the energization of the phase winding, said starting control including an electromagnet coil and a metal frame of magnetic material fixed to and rigidly connected to said coil, a self-resetting overload pro- REFERENCES CITED 'lhe following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,491,396 Hall Apr. 22, 1924 1,637,042 Hutt July 26, 1927 1,763,197 Speiser June 10, 1930 1,785,852 Wilms et a1 Dec. 23, 1930 2,037,118 Chubbuck Apr. 14, 1936 2,175,032 Shaefer Oct. 3, 1939 2,255,437 Pearce Sept. 9, 1941 2,299,669 Werner Oct. 20, 1942 2,311,048 Harrold Feb. 16, 1943 2,379,602 Stickel July 3, 1945 2,436,909 Werner Mar. 2, 1948