Evaluating low and medium voltage motor starting devices

Voltage fluctuations not only affect performance, it can also put the safety of electrical devices and users at risk. That’s where motor starting devices come into play. They ensure electrical equipment gives optimum performance and lasts lifetime despite worst case line voltage and load conditions.


Every time we switch on electrical equipment, all we wish and pray is that it will work properly. But switching loads can be quite a headache even with the best of devices.

The reason? Voltage fluctuations not only affect performance, it can also put the safety of electrical devices and users at risk. That’s where motor starting devices come into play. They ensure electrical equipment gives optimum performance and lasts lifetime despite worst case line voltage and load conditions.

What is a motor starting device?

All motors must have a control device to start and stop the motor called “motor controller”. In Art 100, the National Electrical Code (NEC) defines a controller as “a device or group of devices that serves to govern, in some predetermined manner, the electric power delivered to the apparatus to which it is connected.” A NEC definition in Art.430.2 states, “A controller is any switch or device that is normally used to start and stop a motor by making and breaking the motor circuit current.”

Simply put, motor controller is the actual device that energizes and de-energizes the circuit to the motor so that it can start and stop.The starting mechanism that energizes the circuit to an induction motor is called the “starter”.When a motor is energized, the starting device’s role is to supply the motor with sufficient current to provide adequate starting torque under worst case line voltage and load conditions.

Low voltage motor-starting contactor:

A most commonly used device, the low voltage motor-starting contactor is designed for best possible performance and lifetime even in worst line voltage and load conditions.It plays part in reliable operation and protection of the motor and the personnel using the motor. Defined as “a two-state (on-off) device for repeatedly establishing and interrupting an electric power circuit”, a contactor obtains interruption by introducing a gap or very large impedance.

Contactors are different from circuit breakers, their function is not to interrupt short-circuit current.  Circuits of a motor require separate protection for short-circuit.Use of contactors is typically referred to as magnetic control for they are closed magnetically via their control coils.

Small motors can use low voltage motor-starting contactors in the form of fractional-horsepower, manual control switches.All the designing and manufacturing of motor starting contactors and switches in the US is required as per following norms: UL 508, NEMA ICS-1 and NEMA ICS-2.

Low voltage motor-starting contactors are capable of interrupting operating overloads but not short circuits or faults beyond operating overloads.On the basis of interrupting medium and their ability to interrupt currents, low voltage manual and magnetic controllers have been classified in the following three categories:

  • Class A: Oil-immersed manual, magnetic or AC air-break and vacuum break controllers meant for service on up to 600 V.
  • Class B: Magnetic or DC air-break manual controllers which are meant for service on up to 600 V.
  • Class V: Vacuum-break magnetic controllers for service on up to 1500 V.

Rated by NEMA, low voltage contactors are designed from smallest size (00) to the largest (9) for various duty applications.

Control Method

Contactors are controlled through the use of maintained-contact device which is referred to as two-wire control.For added advantage of allowing the contactor to open and remain open even if the line voltage fails, momentary contact devices are used in the control of contactors. This is referred to as three-wire control.In case of a power failure, the three-wire control protects motors from under-voltage and also prevents inadvertent re-energization.

Medium Voltage Contactors: The Basics

Using vacuum for interruption, medium voltage contactors provide safe and reliable protection even in most demanding operating conditions.They are load breaking devices with a limited short circuit making and breaking capacity.

The Classification:

Class E: The controllers in this class include medium voltage air-break, vacuum, or oil-immersed controllers.Controllers in Class E have been divided into two (Class E1 and Class E2) categories:

  • Class E1: Controllers in this category make use of their contacts to start or stop a motor. Contacts are also employed to interrupt a short circuit or fault which is beyond the operating overloads.
  • Class E2: Controllers in this category make use of their contacts to start or stop a motor and employ fuses to interrupt short circuits and faults which in excess to the operating overloads.

 Jeson Pitt is an online marketing manager for D&F Liquidators.

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