The most frequent concern about high current with a three-phase motor is high no-load current. But the broad issue of high no-load current isn’t the only three-phase motor issue to which plants should pay heed: High current with load and lower-than-expected no-load current are potential areas of concern, too. Let’s explore the sources of all of these.
High no-load current: Motor not rewound
One situation in which higher-than-expected no-load current can occur is with reconditioned motors. Although some motors with no-load currents above or below the guidelines may still be satisfactory, motors with no-load current outside of these ranges warrant further analysis.
When no-load current is high or low, consider the actual test operating voltage versus the motor’s rated voltage. If the applied voltage is not within 10% of the motor’s rated voltage, then the no-load current can be much higher or lower than expected. For example, test-operating a motor rated 200 volts on a 240-volt supply system is almost certain to result in relatively high no-load current.
A misconnection also can cause unusually high or low no-load current. For example, consider a 12-lead single voltage motor intended to be connected parallel-delta (Figure 1) for 460 volts. Because most 12-lead motors are dual-voltage, the motor could be mistakenly connected series-delta (Figure 2) for operation at 460 volts, which would result in exceptionally low no-load current. The solution is to connect the motor for parallel delta.
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