How To Determine Coil Resistance Of CA7 AC Or DC Contactors

Question:

What is the resistance on my contactor coil?

Answer:

With an Ohm meter, the contactor coil resistance can be measured using the A1-A2 terminals of the coil. But how do you know if the measurement is correct? In a troubleshooting situation it may be necessary to measure the coil resistance from the contactor coil. When measuring this resistance, a zero (a short circuit) or an infinity (an open circuit) is not what you want to see on your meter.

The resistance charts below for Series CA7 Contactors are not printed in the Sprecher & Schuh catalog, but may be helpful in these troubleshooting situations. If the coil resistance does not meet these specifications, there may be a connection or other problem. Contact technical support for assistance.

For Electronic DC Contactors CA7-9E..55E, the input power (current) is controlled and adjusted by an electronic circuit. The input impedance (A1-A2) of the electronic coil (110-125VDC) is dependent on the input voltage, on the pick-up or holding phase, and on the frame size of the contactor. These resistance measurements vary during pick-up and are constant during hold-in. The varying pick-up values are created by an input impedance, and this makes it difficult to determine.

Coil resistance is stated in ohms. For coils used with CA7-60D thru CA7-97D contactors, the coil resistance value is measured when the coil is in its shelf state (de-energized). The coils used with CA7-60 thru CA7-97 contactors use a 'two-stage' coil that has two windings instead of one. The coil resistance, when the coil is fully energized, will be much higher because both windings will be pulled into the coil circuit.

For additional information please contact your nearest authorized distributor, sales representative, or call our customer service or technical support lines.