Industrial Refrigeration Compressor Index Page
Troubleshooting Electrical Stator
Amperage Unbalance
Is It Due to the Motor, the Line, or Both?
When you encounter a situation of amperage unbalance in the three supply lines to meter, here's how to tell if the fault is in the motor, the line, or in both.
In the following examples, the three supply lines are designated A, 8, and C - the three motor terminals as T-1, T-2, and T-3.
The following 3 sets of amperage readings are required:
Then tabulate these 9 amp readings as follows:
| A-1 | B-1 | C-1 |
| A-1 | B-1 | C-1 |
| A-1 | B-1 | C-1 |
Total each column vertically (A-1 + A-2 + A-3, etc.). These figures indicate the unbalance effect of the power supply. Total each line horizontally(A-1 + B-1 + C-1, etc.). These figures give the unbalance effect produced by the motor.
For example, assume the following readings:
| Amps | Amps | Amps | |
| First reading | A-1=49 | B-2=44 | C-3=39 |
| Second reading | A-2=39 | B-3=39 | C-1=44 |
| Third reading | A-3=44 | B-1=48 | C-2=44 |
Then tabulate and total per table one.
| A | B | C | Totals | |
| 1. | 49 | 48 | 44 | 141 |
| 2. | 39 | 44 | 44 | 127 |
| 3. | 44 | 39 | 39 | 122 |
| 132 | 313 | 127 |
In this example, we would conclude that:
- Power supply amperage variations are within acceptable limits
- Motor amperage variations are unacceptable. The motor should be replaced.
Let's assume that the readings had been as follows:
| Amps | Amps | Amps | |
| First reading | A-1=49 | B-2=44 | C-3=39 |
| Second reading | A-2=44 | B-3=39 | C-1=44 |
| Third reading | A-3=46 | B-1=44 | C-2=44 |
Then,
| A | B | C | Totals | |
| 1. | 49 | 44 | 44 | 137 |
| 2. | 44 | 44 | 44 | 132 |
| 3. | 48 | 39 | 39 | 126 |
| 141 | 127 | 127 |
In this situation, the power supply is the major cause for the unbalance. If the cause of the unbalance is the power supply, it can be located and corrected, the chances are that the motor unbalance will decrease somewhat due to the external effects.
So before condemning the motor, which is easy to do, run through this exercise to see where the major source of unbalance lies.
And remember that each NCE rewound motor is checked with an extremely sensitive double bridge. Motors with variations of more than 3% between phases are rejected. Each motor is run in our plant and amperage measured and recorded of each phase.
If an NCE motor shows large amperage variations where connected in the field, the chances are very high that there is an unbalance in the power supply.
