Tech Page: Relays

The relay is one of the most common components found in the HVACR field today. From furnaces to condensers to control panels, relays are omnipresent. In addition to controlling motors, valves, and switches, there are a number of other applications for this electrical workhorse of a control. Today we will discuss the parts of a relay, its operation, and the various types of relays.


Electromagnet — A wire coiled around a soft iron core. The coil becomes magnetized as electrical current flows through it.

Armature — The part of the relay that moves when the coil is magnetized.

Contacts — The part of the relay that carries current. There are stationary contacts and those attached to the armature, which moves.

Normally open contacts — Contacts that close once current is applied.

Normally closed contacts — Contacts that open once current is applied.

Replay spring — Keeps the armature from completing the circuit when there is no applied voltage.


SPST — single-pole, single-throw

SPDT — single-pole, double-throw

DPST — double-pole, single-throw

DPDT — double-pole, double-throw

The pole refers to the armature. A single-pole has one contact attached. A double-pole has two contacts attached to the armature.

The throw indicates the position of the contact attached to the armature. With a single-throw, one position completes the circuit. With a double-throw, two different positions energize the circuit.


Thermal relay — The thermal relay incorporates a bimetal blade with a resistance heater wrapped around it. When power is supplied to the heater, it warms the bimetal blade, which initiates the switch action.

Electromagnetic relay — When the coil of the relay is energized, it becomes magnetized and moves the armature to either open or close the contacts. When the current stops, the spring returns the armature to its normal position.

Potential relay — This electromagnetic relay drops the start windings out of a high torque electrical motor circuit.

Current relay — This can be either an electromagnetic or thermal relay. It drops the start windings out of a low torque electrical motor circuit.

Electronic relay — Incorporates triacs, diacs, and solid-state transistors to perform switching action.

Current-sensing relay — Monitors current in a circuit. An increase in current initiates a switching action by the relay.

Time delay relay — An internal timer is initiated when power to the relay is either made or cut off. When the timed sequence is complete, the relay reverts back to the normal position.

Sequencing relays — A thermal-type relay used in electrical heating equipment to stage the start-up of electric heat elements.

Overload relays — Interrupts power if there is an increase in current above the rating of the relay. Either mounted on the side of a contactor or incorporated as one of the components in a motor starter.

Compressor lockout relay — This relay locks out the compressor in the event a safety device, such as a pressure switch, takes the compressor off-line in an abnormal condition. The relay must be reset in order for the system to start again.

Hot wire relays — A thermal-type of relay used to drop out the start windings in split-phase motors.

Fan centers — This control, which incorporates a transformer and relay, sequences and assures proper blower motor speeds in the heating and cooling operation of air handlers and furnaces.

Internal motor overload relay — This relay is located inside the windings of a motor. It protects the motor from excessive current draw and temperature.


Contactors — A type of relay designed to switch greater current and voltage than a standard relay. Contactor parts are heavy duty and are built to withstand repeated on/off operation.

Motor starter — Built to handle heavy-duty current and voltage like the contactor, the starter incorporates overload heaters to protect the load.

Rothacker is a director of For questions or comments on the Tech Page, contact Rothacker at (e-mail).

Publication date: 10/21/2002

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