An Improved Method for Selecting Thermal Overloads to Provide Half Voltage Protection

A condition of half normal voltage imposed on a refrigerator compressor will result in overheating of the motor if the ~ondition persists for an extended time. The line current and heating of the compressor shell are similar to the conditions which exist under peak loading. A thermal overload which allows compressor operation under peak conditions and still prevents overheating when half normal voltage is imposed must be selected carefully. This paper will describe an application technique which has been successfully used to select a proper rating in a systematic manner using existing appli•cation curves in a unique way. The paper will illustrate this technique with a theoretical example. INTRODUCTION There are infrequent occurances in the field when the line voltage drops to a point significantly below the level of which a refrigerator is designed to operate. This would be due to an extreme malfunction of the power distribution system caused by a lightning storm for example. The mechanisms of such a malfunction and number of occurances are beyond the scope of this paper. The fact is that there is a critical range of voltage which will cause overheating of many refrigerator compressor motors, to the point of permanently damaging the insulation system if the thermal protector is not sized properly to function in that range. This critical range of voltage extends from the point at which the motor is self-protected due to its own impedance, to the point just below that at which the current starting relay picks up and allows the compressor to run. This range of voltage is typically 40% to 70% of the 53 normal operating line voltage. An example of this range for a llSV compressor is shown in Figure 1. 0 45V 75V LINE VOLTAGE