If the normally closed contacts between terminals 1 and 2 are dirty or arched and pitted, there is a chance the ohmmeter may read a resistance greater than zero but less than infinity. For example, if the ohmmeter reads 200 ohms, the potential relay should still be discarded because of dirty or pitted contacts.
The more the current leads the voltage wave, the more phase shift there will be, and the motor will have more starting power or torque. Without the start capacitor, the motor will usually lock its rotor and draw locked rotor amps (LRA). This will cause the motor’s overload to open and the motor will short-cycle on the overload. This short-cycling is detrimental to motor windings, starting relays, and capacitors because of its overheating effect.
If the contacts are stuck in the closed position, the relay would have to be checked with a voltmeter in the running mode, since the contacts between 1 and 2 are normally closed when not in operation. Once the motor is up and running, use a voltmeter to measure the voltage between terminals 1 and 2. A voltage reading of zero would prove that the contacts are not opening. (See Figure 3.) Also, a high amp draw from the start capacitor and start winding circuit still being energized is a telltale sign that the contacts have not opened.
If the ohm reads infinity on the (R x 100) scale, the relay coil is opened. The relay should be discarded and a new one installed. An opened relay coil will prevent the contacts between 1 and 2 from opening. This is caused from no magnetism in the iron core that the coil is wrapped around. This, again, will cause high amp draws. Always use the model number of the old relay when ordering a new one, or cross-reference it with a different manufacturer’s relay.
Tomczyk is a professor of HVAC at Ferris State University, Big Rapids, MI, and the author of Troubleshooting and Servicing Modern Air Conditioning & Refrigeration Systems, published by ESCO Press. To order, call 800-726-9696. Tomczyk can be reached at email@example.com (e-mail).
Publication date: 11/04/2002