Basics of Motor starters

A motor starter is designed to provide power to motors. The motor starter is made up of a contactor with an overload relay attached physically and electrically to it . The function of the overload relay can be summarized as follows.

  • Overload relays are designed to meet the special protection needs of motor control circuits.

  • They allow harmless temporary overloads that occur when a motor starts.

  • The overload relay will trip and disconnect power to the motor if an overload condition persists.

  • Overload relays can be reset after the overload condition has been corrected.

From the above three phase magnetic flowmeter.

  • When the START button is pressed coil M is energized closing all normally open M contacts.

  • The M contacts in series with the motor close to complete the current path to the motor. These contacts are part of the power circuit and must be designed to handle the full load current of the motor.

  • Control contact M (across START button) closes to seal in the coil circuit when the START button is released. This contact is part of the control circuit and, as such, is only required to handle the small amount of current needed to energize the coil.

  • An overload (OL) relay is provided to protect the motor against current overloads. The normally closed relay contact OL opens automatically when an overload current is sensed to de-energize the M coil and stop the motor.

Read also: Electric motor starters