A Motor Control Circuit ( Part 3 )

A motor control circuit (Part 1), (Part 2)

Checking Pushbuttons (Start and Stop)

Check from L1, to the side of the coil connected to L2 trough the overload contacts. If 240 volts is measured, the control circuit is complete and the contacts of the magnetic starter should be closed. See figure below.

Checking for Defective Coil

Measure the voltage from L1 to the side of the coil being supplied from L1. If a voltage reading is not obtained, the coil is bad and must be replaced. See figure below.

Checking Pushbuttons with Ohmmeter

Pushbuttons can be check for proper continuity with an ohmmeter. The stop buttons have normally closed contacts and resistance when read. Place one of the ohmmeter probes on one side of the stop button and the other probe on the other side of the stop button. If the contacts are closed, the ohmmeter needle will peg. See figure bellow.

A Motor Control Circuit ( Part 2 )

A motor control circuit (Part 1), (Part 3)

Checking Fuses

See figure below. Measure voltage from the line side of L1, to the load side of L2. If 240 volts is read, the fuse in L2 is good. If no voltage is measured, the fuses in L2 is bad. Measure from the line side of L2 to the load side of L1. If 240 volts is read, the fuse in L1 is good. Next, measure from the line  side of L1 to the load side of L3. The voltmeter will measure 240 volts if the if the fuse in L3 is good.

 

Checking the Magnetic Starter

When it has been determined that the supply power is available to the line and load side of the fuses and the fuses are good, the control circuit must be checked. See figure below. The first step is to measure the voltage to L1 and L2. If 240 volts is measured between L1 and L2, the control circuit voltage to the controls is correct.

Leave the probe on L1, and check the line and the load side of the overload contacts until the coil (C). This check will verify if any one of the overload contacts is open in the control circuit from L2. If the overload contacts are open, reset the overload and check for the correct voltage. An overload condition on the motor windings can trip open the overload contacts.

If the inlets and outlets on the motor enclosure are blanked with lint or dirt, this condition can create an overload. A stuck bearing on a motor or driven load can cause the motor windings to be overloaded. NOTE: if the overload contacts are not open, check for a loose connection or broken wire. Loose connections are common causes of trouble in motor control circuits. A visual inspection will sometimes detect loose, discolored, or burned wires, or other malfunctions.

A Motor Control Circuit ( Part 1 )

A motor control circuit (Part 2), (Part3)

Troubleshooting a motor control circuit should always be done beginning with the most obvious components. By checking the most obvious components first, the trouble is often quickly found and may be easily rectified. A basic procedure to follow when troubleshooting a motor control circuit is:

1. Check supply voltage.
2. Check all terminal connections.
3. Check fuses.
4. Check magnetic starter.
5. Check push buttons.
6. Check coil.

Checking Supply Voltage

Supply voltage is checked by using a voltmeter. See figure below. Measure the phase-to-phase voltage from supply side, L1 to L2, L1 to L3, and L2 to L3. Read the phase-to-phase voltage. For example, the phase-to-phase voltage may be 48 volts or 240 volts.