Electrical Considerations

Always be sure that the electric power source matches the motor power requirements. Check operating voltage, frequency, and phase. External field wiring to a motor should be done in accordance with the latest edition o f the National Electrical Code (NEC) and local code requirements.

Operating voltages applied to compressor motors must be maintained within ± 10% of the voltage value given on the compressor/compressor motor nameplate. If the operating voltage falls outside these limits, the system should be turned off until the problem is corrected. Voltage tolerances used for single-phase and three-phase compressor motors are:
• Single-Voltage Rated Motors — The input supply voltage should be within +10% of the motor’s nameplate voltage. For example, a motor with a nameplate single voltage rating of 230 volts should have an input voltage that ranges
between 207 volts and 253 voits (±10% of 230 volts).
• Dual-Voltage Rated Motors — The input supply voltage should be within ±10% of the motor’s nameplate voltage. For example, a motor with a nameplate dual voltage rating of 208/230 volts should have an input voltage that ranges between 187 volts (—10% o f 208 volts) and 253 volts ( + 10% of 230 volts).
• Three-Phase Motors — These motors should never be used where a voltage imbalance greater than 2% exists. Failure to meet these basic considerations will damage motor windings.

A voltage imbalance of more than 2% between any two legs of the supply voltage applied to a three-phase compressor motor must be corrected. A small imbalance in the input voltage results in a considerable amount of heat being generated in the motor windings. With only a 5% imbalance, the winding temperature can increase as much as 50% over the safe level. The procedure for he measurement o f three-phase input voltage and the calculation of the voltage imbalance in a three-phase system is provided in Service Procedure SP-7.

Current imbalance between any two legs of a three-phase system may not exceed 10%. Voltage imbalance will always produce current imbalance, but a current imbalance may occur without a voltage imbalance. This can happen when an electrical terminal, contact, etc. becomes loose or corroded, causing a high resistance in the leg. Since current follows the path of least resistance, the current in the other two legs will increase, causing more heat to be generated in those two windings. The procedure used to check current imbalance is included in the detailed procedure at the end of this section.


Categories: Service Procedures | Leave a comment