How to Troubleshoot Variable Speed Electric Motors

Variable speed drives on electric motors control the rotational speed of the motor by varying the voltage frequency. Due to their sensitivity, these mechanisms are subject to malfunction. There are three basic components to a variable speed drive for an electric motor: a rectifier circuit unit, a DC circuit unit and an inverter circuit unit. These units convert AC power to DC power to a form of AC power that an alternating current motor can use. Testing one or more of these units with a digital volt-ohm meter can help you narrow down the source of the problem.

Instructions

1

Do a visual inspection of the controller displays and wiring connections. Check for any loose connections and retighten.

2

Check incoming three-phase power and make sure there is no imbalance of the line voltage. Look for a deviation of less than 5 percent. Check for overvoltage or undervoltage, overcurrent trips or overload of the motor.

3

Use a classic method to find the source of the problem by splitting the circuit in half, then isolating the problem by checking for the presence or absence of a signal at distinct points.

4

Follow a stochastic troubleshooting process if you are a more advanced technician with good existing knowledge of the variable speed electric motor in question. Make educated guesses based on random tests to pinpoint the source of the problem. Do this so as to avoid taking the system apart.

5

Make sure the capacitors have been discharged before handling them. Check the input rectifier for a diode drop, using a digital volt-ohm meter. First test the positive bus with the negative lead, then check each incoming phase with the positive lead; if the reading indicates "open" for the positive, then the charge resistor is open and thus failed. Switch to the negative bus using the positive lead and the negative lead on the incoming phases. Check that the negative bus does not read as short or open. Then place both leads on the bus, one on the positive and the other on the negative. You should see "charging," not short.

6

Look for power problems mentioned above; if there is no problem, but the system does not run, you may have these issues instead: an improper connection, incorrect programming or a bad circuit board.