A compressor is generally considered to be seized if it hums but will not start, and draws locked rotor current for several seconds as measured with a clamp-on ammeter (Figure SP-9-2). This can be caused by a mechanical or electrical problem within the compressor or by an external electrical problem. Normally, locked rotor current lasts for a fraction of a second when the motor first starts. The locked rotor current can be four to six times the normal running current.
Many compressors are replaced unnecessarily because they are incorrectly judged to be seized. Before condemning the compressor, be sure that the mechanical refrigeration system or electrical system conditions listed below have been considered.
• Unequaiized system pressures, especially in units using permanent split capacitor (PSC) compressor motors.
• Low supply voltage. If the supply voltage is within ±10% of the motor’s nameplate rating, but a single-phase P SC motor fails to start, the installation of a start capacitor and start relay may be required to increase the motor starting torque to correct the problem.
• Compressor contactor not making good contact on all poles.
• Defective (open) start relay.
• Start or run capacitor open or weak.
If a single-phase PSC compressor motor will not start, it may be a result of low voltage, improper pressure equalization, or a weak run capacitor. Also, the compressor motor may have open, shorted, or grounded windings. After you have determined that none of the above conditions are the cause for the motor not starting, you should attempt to start the motor with a temporary capacitance boost (Figure SP-9-3). The procedure for capacitance boosting a PSC compressor motor is provided in the detailed procedure given at the end of this section, and is briefly described here.
Capacitance boosting involves momentarily connecting a start capacitor approved by the compressor manufacturer (at least 88 to 108 MFD, 440-volt) across the unit’s existing run capacitor when power to the compressor is turned on. Since the start capacitor is in parallel with the run capacitor, the values of the capacitors combine, providing increased capacitance and increased torque to start the PSC motor. The compressor motor should continue running at full speed with the start capacitor removed from the circuit. Once started, allow the compressor to run for about ten minutes, then turn off the power to the unit.
Allow the system pressures to equalize, then try starting the compressor again without the temporary start capacitor. If the motor does not start after several attempts, a start kit should be installed. Start kits designed for use with most compressors are available from HVAC distributors.
Start kits include all the components and hardware needed to complete an installation, including a start capacitor and a start relay. Start kits that use a resistive device called a thermistor, instead of a start capacitor and start relay, are also available. The operation of both the start relay/start capacitor circuit and the start thermistor circuit are discussed in more detail later in this section.
Three-phase motors can sometimes be started or unstuck by temporarily reversing or interchanging any two of the leads. After the motor has started, always turn off the power, then reconnect the compressor leads as shown on the unit wiring diagram. Failure to wire the compressor per the wiring diagram will result in the compressor crankshaft rotating in the opposite direction. This may cause improper compressor operation and may result in damage to the compressor.
