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Motor protection switch: function, structure, use

Motor protection switch serve as a protective device for electric motors. In practice, motor protection switches are often used as an alternative to device and line protection.

Synonym (s): circuit breaker, motor protection, motor protection relay

Engine protection explained by Lovato

Motor protection switches are manufactured for three-phase motors, among other things. The currents must be monitored. This is done either thermally-mechanically (bimetal), thermally-electrically (PTC) or electronically (current measurement). Incorrect connections to the power supply without a motor protection relay lead to damage due to overload or failure of a conductor.

In principle, motor protection switches should protect electric motors from overload. There are also various additional aids. Undervoltage releases can also be integrated or auxiliary contacts such as indicator lights can be installed.

Function and structure

Example motor protection switch + main switch function

The aspects to be monitored for the engine are: Overload, No start or failure an outer conductor (with three-phase current). The common and most commonly used motor protection switches are 3-pole and are switched on and off manually by hand.

Motor protection switches have a thermal release, usually a bimetal, to protect the motor winding from overheating (overload). The thermal release of the motor protection switch is set to the rated current “In” of the motor. In addition, most switches are still with one electromagnetic release Mistake. This protects against a short circuit; However, this feature is not mandatory for labeling as a motor protection switch.

Some motor protection switches where the main contacts have isolating properties can also be used as main switches if the handle can be locked in the "0" position.

Remote switch-off is possible with accessories such as undervoltage or shunt releases.

Motor protection switches consist of the following individual parts:

  1. Switch lock: is used to switch the main contacts on and off
  2. Main contact system: conducts the current with low loss through low contact resistance
  3. Electromagnetic overcurrent release: switches when a current value is exceeded (short-circuit range)
  4. Thermal overcurrent release: operational overload through bimetal release
  5. Arcing chamber: cooling of the main contacts
  6. Drop anchor: only included with current-limiting circuit breakers
  7. Differential trip slide: only included if phase failure protection is received

The right motor protection switch

The choice of the right motor protection switch depends on the rated current of the electric motor to be connected.
If a motor protection switch is to take over overload protection and short-circuit protection for the line and the motor, this must be in accordance with the standard DIN VDE 0113 be installed at the beginning of the motor supply line. However, this function is not absolutely necessary. There are also other US standards for motor protection switches.

Other protective functions

Function and structure of motor protection switches - presented in an understandable way by the "electrician".

In addition to the motor protection relay, which belongs to the category of monitoring the power supply to the electric motor, there is another category: direct monitoring of the motor temperature. Which includes:

  • Full engine protection
  • Bimetal switch
  • PTC thermistor

Both protective functions are sometimes combined with each other, e.g. when it comes to load runs.

Manufacturer of motor protection switches / relays

  • Lovato Electric
  • Eaton Electric
  • OEZ Czech Republic
  • OJ Sursum
  • Schneider Electric
  • Elektra
  • Avolta Elektro

Sample relay

  • BUZE motor protection switch
  • ABL circuit breakers
  • TeSys
  • SM1E from OEZ