Friday, July 3, 2009

Solid State Relay

I recently talked about Potential and Current relays. Now I would like to discuss with you the function of solid state relays. With the advancement of solid state controls, another type of current type control called a PTC (positive temperature coefficient) control is used in the starting circuit of small fractional horse power motors. Certain ceramic materials (PTC) increase their resistance as they heat up when current passes through them.

The device is used to drop out the start winding and the start capacitor when the resistance increases. The device has the capacity to cover a range of horse powers.
Below is a PTC device.

In applications where an exact replacement current relay cannot be found, or in an emergency, a solid state relay can be installed in its place to get the unit on line. These units are based on a time factor to open the start circuit. Generally the rotors speed will be about 60 to 80% of its full load speed. Solid state relays can be considered normally closed at room temperatures, though they do not have actual contacts, so at room temperature the resistance is very low. When the compressor is started, current flows through the thermister into the start winding capacitor. The current flow generates heat that causes the thermister resistance to increase. Within a second or two, the resistance in the thermister increases so high that the current in the start winding (capacitor) is reduced to milliamps. This effectively opens the start circuit and the motor runs in its inductive run mode. A small current continues to flow through the thermister maintaining its higher temperature and higher resistance as long as the motor is running. When the motor stops running the thermister cools, reducing the resistance, and preparing the relay for the next start sequence.

Permanent split capacitor motors do not need a starting relay, because their run capacitor provides the phase shift needed to begin and maintain rotation. However, after some years of operation, PSC motors may have some difficulty starting. To help these compressors in starting, a solid state relay with capacitor has been made available to assist with the starting mode. Once the rotor of the compressor reaches about 65% using this device, the relay opens as previously described. There are several different models of thermister-based start assist devices available to meet the needs of fractional and integral motor horse power PSC compressors up to 10 HP in size.

Roger J. Desrosiers

About the Author: Roger is a contributing faculty member of
He has over 40 years experience in Air Conditioning and Refrigeration. He is also a member of R.S.E.S., CM, The Association of Energy Engineers, Certified Energy Manager, ASHRAE, Certified Pipe Fitter,
United Association and is 608 Universal Certified.