Electric Motor Controls

Motor Controls

Electric motors can be made with several different designs, but most have some parts in common. First they need something to get them started. This is done with relays, contactors and motor starters. Relays consist of a coil that opens or closes one or more sets of contacts when activated. There are three types of starting relays used in HVAC. They each have a different way of removing the start winding from the circuit after the motor is running near full speed. Current relays are made of a very low resistance coil and a set of normally open contacts. They are used on fractional horsepower motors that require low starting torque. When locked rotor amperage (LRA) is applied the contacts close and energize the start winding. Once the motor is running at full-load amperage the contacts open and deenergize the start winding. The Potential relay works off of the back EMF from the motors start winding. They have normally closed contacts which open to remove the start winding from the circuit. On many permanent split-capacitor motors that are having trouble starting, a hard start kit may be added. A hard start kit only consists of a potential relay and a start capacitor. Solid state relays are also used on fractional horsepower motors. They are made of a PTC device which increases its resistance with an increase in temperature. With enough of an increase they produce enough resistance to stop current flow thru the start winding out of the circuit. Contactors are basically just larger relays which can be used to start larger motors. Contactors have moveable and stationary contacts and can be rebuilt if necessary. They may have as many as six contacts in several different configurations. They also come in different sizes and can be used on 24v circuits all the way up to 460v circuits.

Motor starters are contactors with overload protection built in. Some contactors can actually be converted to starters. Since circuit breakers only protect the...