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 shown in Table 1 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.
Another possible issue with a 6-lead or 12-lead wye-delta motor is misconnection with a delta instead of a wye. Although most of these motors operate with the delta connection, some use the wye connection. If that is the case and the motor is operated with the delta connection, the no-load current will probably far exceed the rated current.
A good practice with 6- or 12-lead wye-delta motors is to test-operate them in the wye mode and check no-load current. If the current is exceptionally low, reconnect for delta and repeat the no-load test. If, however, the no-load current appears normal in the wye connection, verify the correct connection for the application. If the connection should be delta, there is an error in the winding.
Motor with no nameplate
A less-common scenario is a motor without a nameplate that has a relatively high no-load current for an assumed power rating. In this case, first determine the frame size from a NEMA dimensions chart by measuring the width from bolt-hole center to bolt-hole center of the motor feet (front to back and side to side) as well as the height from the base to the shaft center.
Next, determine the number of poles. At 60 Hz, the number of poles is equal to 7,200 divided by the no-load rpm, rounded to the nearest integer. For example, if the no-load speed is 1,798 rpm, the motor has 4 poles (7,200/1,798 = 4.004 = 4).
The frame size and number of poles/speed can be used to closely estimate the motor power rating. If the new estimated power rating differs from the assumed power value, use the new rating to evaluate the no-load current.
High no-load current: Rewound motor
The most common causes of high no-load current after a motor is rewound are incorrect winding data and stator core damage.
Incorrect winding data that increases the magnetic flux levels as compared with design levels will result in increased no-load current. Although the effect on full-load current is not nearly as significant, the higher flux levels also will result in higher starting current, which can trip protective devices. In extreme cases, this can cause contactors to blow open or weld closed. Service centers can usually confirm that the winding data is correct by following good practices, such as comparing the data to a winding database or using a computer program to calculate and evaluate winding magnetic flux densities.
Service centers can also evaluate the condition of a stator core by performing a core test that evaluates core loss (watts per pound) and hot-spot versus ambient temperature rise. The results can be compared with industry norms or cores of similar motors.
High current with load
Probable causes of high current with load include mechanical overload, excessively high magnetic flux densities and, less frequently, an open rotor. An error in winding data that results in lower-than-design-level magnetic flux also can cause high current with load.