SOLID STATE MOTOR OVERLOAD PROTECTION SYSTEM

• US 3,585,451 A
• Filed: 12/24/1969
• Issued: 06/15/1971
• Est. Priority Date: 12/24/1969
• Status: Expired due to Term

• Current Assignee: York International Corporation (Johnson Controls International plc)
• Original Assignee: Arthur R. Day
• Inventors: Day, Arthur R.
• Application Number:
  US04/887,857
• Time in Patent Office: 538 Days
• US Class Current: 361/22
• CPC Class Codes: F25B 2600/026 by controlling unloaders
• CPC Class Codes: F25B 49/005 of safety devices F25B49/02 and F25B49/04 take precedence
• CPC Class Codes: F25B 49/025 Motor control arrangements
• CPC Class Codes: H02H 3/025 Disconnection after limiting, e.g. when limiting is not sufficient or for facilitating disconnection
• CPC Class Codes: H02H 7/08 for dynamo-electric motors
• CPC Class Codes: H02H 7/0857 by lowering the mechanical load of the motor

Abstract

A solid state overload protection system for protecting a polyphase electric motor which drives a reciprocating- or positive-displacement type compressor of the type having mechanical load reducing means is disclosed. The system samples the peak current drawn by each phase of the electric motor and the temperature of the motor windings. During overload (overcurrent or overtemperature) situations, a mechanical load reducer is latchably operated to reduce the motor loading. If the overload condition persists (e.g. for 6 seconds) after the load reduction has occurred the current to the motor is interrupted. Otherwise, the motor is reloaded after a longer period (of, e.g., 6 1/2 minutes).

Claims

1. In a refrigeration system of the type employing an electric motor driven compressor having mechanical load reducer, the improvement in the protection system for the electric motor comprising:

means for sampling a first motor overload condition and for providing a signal when an overload condition exists;

means, coupled to said overload signal producing means, for controlling the mechanical load reducer to latchably reduce the mechanical load of the compressor in response to the commencement of the overload signal;

a timer unit coupled to said mechanical load reducer controlling means for producing at least an early and a late delay signal in response to the latchably reducing of the mechanical load by said load reducing control means, said early delay signal occurring a short interval after the reduction of the mechanical load and said late delay signal occurring after a further period of time;

means responsive to the presence of both said early delay signal and said overload signal for deenergizing the motor; and

means responsive to said late delay signal for operating said load reducer control means to unlatch the reduction of the mechanical load to the compressor;

whereby a continuous or frequently reoccurring overload may releasably deenergize the electric motor while an infrequent and temporary overload may decrease the mechanical load on the electric motor to allow for correction of the overload situation.

2. The improvement in a refrigeration electric compressor motor protection system as defined in claim 1, wherein:

a second means for sampling a second motor overload condition and for providing a second signal when an overload condition exists is provided;

said means for controlling the mechanical load reducer is also coupled to said second overload signal producing means and responds to the commencement of either and both of the first and second overload signals to latchably reduce the mechanical load of the compressor;

said timer unit also produces an intermediate delay signal, in response to the latchably reducing of the mechanical load which intermediate delay signal occurs after the early signal and before the late signal;

said deenergizing means is also coupled to said second sampling means for deenergizing the motor when the second overload signal and the intermediate delay signal are present.

3. The improvement in a refrigeration electric compressor motor protection system as defined in Claim 2, wherein:

said timer unit also produces additional sampling signals after the early delay signal and before the late delay signal for causing said deenergizing means to deenergize the Motor should the first motor overload signal be present at the occurrence of these additional signals.

4. The improvement in a refrigeration electric compressor motor protection system as defined in claim 3, wherein:

said timer unit also produces second sampling signals periodically after said intermediate delay signal and before said late delay signal for causing said deenergizing means to deenergize the motor should the second motor overload signal be present at the occurrence of these second additional signals.

5. The method of protecting an electric motor of the type used to drive a compressor with means for reducing its mechanical load from electrical overload comprising the steps of:

sensing a load on the motor, developing a load representative signal therefrom, comparing that signal with a standard, producing an overload signal when that load representative signal exceeds the standard;

reducing the mechanical load which the compressor presents to the motor in response to the overload signal;

producing at least two delayed signals in response to the overload signal, an early time delay signal which occurs after a short time delay period and a later time delay signal which occurs after a long time delay period;

interrupting the current supplied to the motor if at the time of the early delay signal, the overload signal is still present;

reloading the compressor in response to the later time delay signal;

whereby persistant and frequently reoccurring sensed overloads will cause the current to be interrupted but short time and infrequent overloads will cause the mechanical load to be reduced to allow for correction of the short time and infrequent overload situations.

6. An overload protection system for a compressor motor comprising:

means for sampling a motor load condition and for developing a signal representative of that condition, a mechanical load reducer for the compressor;

means, coupled to said load reducer and to said sampling and load signal developing means, for latchably operating said load reducer in response to load representative signal indicating a preselected magnitude of overload;

a timer coupled to said load reducer operating means for producing at least two time delayed signals in response to the operation of the load reducer by said operating means, one of said delay signals being delayed by one period of time and the other being delayed in time by another longer period of time, from the operation of the load reducer;

means, coupled to said sampling and load representative signal producing means and to said timer, for deenergizing the compressor motor in response to the one time delay signal if the load representative signal represents a motor overload above a predetermined level when the one signal occurs and not deenergizing the compressor if it does not; and

means, coupled to said timer and said load reducer operating means for causing said load reducer operating means to stop the operation of said load reducer to reload the compressor motor in response to the occurrence of another delay signal, whereby small and short duration overloads may be counteracted without deenergization of the motor but large and persistant overloads will cause deenergization and the motor is protected.

7. The overload protection system for a compressor motor as defined in Claim 6, wherein:

said sampling and load representative signal producing means samples current drawn by the motor and produces a voltage signal representative of an overcurrent overload situation, a temperature sensor is provided associated with the windings of the compressor motor;

means, coupled to said temperature sensor and to said load reducer operating means are provided, for developing a signal representative of an overtemperature condition in the motor windings, and for causing said load reducer operating means to latchably reduce the load of the compressor in response to the overteMperature signal.

8. The overload protection system as defined in claim 7 employed to protect a polyphase compressor motor, wherein:

said current sampling and overcurrent signal producing means, includes;

means for sampling the current drawn by each phase winding of the polyphase motor and for developing therefrom a DC voltage signal whose amplitude is proportional to the maximum instantaneous current being drawn by any one of the phase windings, and a comparator for comparing the DC voltage signal of said sampling means with a reference and for producing the overcurrent signal when the DC voltage signal reaches or exceeds a predetermined relationship with said reference signals.

9. The overload protection system as defined in claim 7, wherein:

said latchably operating means for said load reducer includes a Flip-Flop having a '"'"''"'"''"'"''"'"'set'"'"''"'"''"'"''"'"' and '"'"''"'"''"'"''"'"'reset'"'"''"'"''"'"''"'"' input and a '"'"''"'"''"'"''"'"'1'"'"''"'"''"'"''"'"' and '"'"''"'"''"'"''"'"'0'"'"''"'"''"'"''"'"' output which '"'"''"'"''"'"''"'"'1'"'"''"'"''"'"''"'"' output is coupled to said load reducer and said '"'"''"'"''"'"''"'"'set'"'"''"'"''"'"''"'"' input is coupled to both said overtemperature signal producing means and to said overcurrent signal producing means to have said Flip-Flop '"'"''"'"''"'"''"'"'set'"'"''"'"''"'"''"'"' by either or both of the overtemperature on the overcurrent signals;

said timer includes;

a unijunction transistor timing circuit connected to the '"'"''"'"''"'"''"'"'1'"'"''"'"''"'"''"'"' output of said Flip-Flop for producing the delay signal one period of time after the '"'"''"'"''"'"''"'"'1'"'"''"'"''"'"''"'"' output signal is produced by setting said Flip-Flop; and

a first delay circuit coupled to said unijunction transistor timing circuit for activation by the one of the delay signals to an intermediate delay signal occurring in time between the one and the other delay signals; and

said deenergizing means includes;

an '"'"''"'"''"'"''"'"'AND'"'"''"'"''"'"''"'"' gate whose inputs are connected to the overtemperature signal producing means to produce a command signal when both of said signals are present;

means responsive to the command signal to deenergize the motor.

10. The overload protection system as defined in claim 9, wherein:

said timer further includes a second delay circuit coupled to said first delay circuit for activation by the intermediate delay signal to produce, after a further time delay, the other delay signal which is coupled to the '"'"''"'"''"'"''"'"'reset'"'"''"'"''"'"''"'"' input of said Flip-Flop to reset it and thereby reload the compressor;

said deenergizing means includes;

a second '"'"''"'"''"'"''"'"'AND'"'"''"'"''"'"''"'"' gate which has two inputs one of which is connected to said unijunction transistor timing circuit of said timer and the other of which is connected to the overcurrent signal producing means, to develop a second command signal output in response to the presence of both the one delay signal and the overcurrent signal, said means responsive to the command signal to deenergize the motor, and means for reenergizing the motor in response to a reset signal;

the system includes a reset switch for producing a reset command which is coupled to both the '"'"''"'"''"'"''"'"'reset'"'"''"'"''"'"''"'"' input of said Flip-Flop to reset said Flip-Flop and also to said reenergizing means for reenergizing the motor.

11. The overload protection system as defined in claim 6, wherein:

said timer also includes means for producing additional sampling signals at times between the one and other delayed signal so as to periodically sample therebetween the motor load condition and to deenergize the compressor motor should the sensed overload exceed the predetermined level at occurrence of the additional sampling signals.