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The Most Common LV/MV Motor Starting Devices (on photo: Allen Bradley contactor)
The Most Common LV/MV Motor Starting Devices (on photo: Allen Bradley contactor)

Two-state ON-OFF device

The most common motor starting device is the low voltage motor-starting contactor. A contactor is defined as “a two-state ON-OFF device for repeatedly establishing and interrupting an electric power circuit.” Contactors are designed for optimum performance and lifetime when switching loads; they are not designed for interrupting short-circuit currents and therefore motor circuits require separate short-circuit protection.

Because contactors are closed magnetically via their control coils, the use of contactors is typically referred to as magnetic control.

For small motors, typically fractional-horsepower, manual control switches are also available. Motor starting contactors and switches in the United States are typically designed and manufactured per NEMA ICS-1, NEMA ICS-2 and UL 508.

A controller is defined as “a device or group of devices that serves to govern, in some pre-determined manner, the electric power delivered to the apparatus to which it is connected.”

Motor starting contactors are available as integral units with externally-operable switching means, defined as a combination controller.

A starter is defined as: “a form of electric motor controller that includes the switching means necessary to start and stop a motor in combination with suitable overload protection”. A combination starter,which includes the motor switching contactor as well as overload protection and an integral disconnecting device, is a type of combination controller.

Examples of LV contactors used for motor starting
Examples of LV contactors used for motor starting


Low Voltage manual and magnetic controllers are classified as Class A, B, or V according to their interrupting medium and their ability to interrupt currents:

Class A: Class A controllers are AC air-break, vacuum break, or oil-immersed manual or magnetic controllers for service on 600 V or less. They are capable of interrupting operating overloads but not short circuits or faults beyond operating overloads.

Class B: Class B controllers are DC air-break manual or magnetic controllers for service on 600 V or less. They are capable of interrupting operating overloads but not short circuits or faults beyond operating overloads.

Class V: Class V controllers are AC vacuum-break magnetic controllers for service on 1500 V or less, and  capable of interrupting operating overloads but not short circuit or faults beyond operating overloads.

Low voltage NEMA-rated contactors are designated in sizes 00 (smallest) through 9 (largest) for various duty applications per [5]. Figure 1 shows a NEMA-rated low voltage contactor along with a manual motor starting switch, a starter, and a combination starter.

Figure 1 - a.) Motor starting contactor, b.) Manual motor starter, c.) Motor starter with contactor and overload relay, d.) Combination starter with magnetic-only circuit breaker, contactor, thermal overload relay and pilot devices
Figure 1 – a.) Motor starting contactor, b.) Manual motor starter, c.) Motor starter with contactor and overload relay, d.) Combination starter with magnetic-only circuit breaker, contactor, thermal overload relay and pilot devices

Control of contactors using maintained-contact devices is referred to astwo-wire control. Use of momentarycontact devices in the control of contactors is referred to as three-wire control

Three-wire control has the advantage of allowing the contactor to open and remain open if the line voltage should fail. This arrangement is typical to provide undervoltage protection for motors and prevent inadvertent re-energization after a power failure.

Two-wire and three-wire control are shown in figure 2 below.

Low voltage contactor control (full-voltage non-reversing control shown): a.) Contactor nomenclature, b.) Two-wire control, c.) Three wire control
Figure 2 – Low voltage contactor control (full-voltage non-reversing control shown): a.) Contactor nomenclature, b.) Two-wire control, c.) Three wire control

Medium voltage contactors are typically use vacuum as the interrupting means. Unlike a circuit breaker, a medium voltage vacuum contactor is specifically designed for long life in load-interrupting duty rather than for short-circuit interrupting duty.

However, unlike their low voltage counterparts a medium voltage contactor may be able to interrupt short-circuit currents beyond operating overloads.

Vacuum 3-pole contactor with electromagnetic operating mechanism for medium-voltage switchgear
Vacuum 3-pole contactor with electromagnetic operating mechanism for medium-voltage switchgear; by SIEMENS (photo credit: directindustry.com)

Medium voltage air-break, vacuum, or oil-immersed controllers are classified as class E. Class E controllers are further divided into class E1 and E2 as follows:

Class E1: Class E1 controllers employ their contacts for both starting and stopping the motor and interrupting short circuits or faults exceeding operating overloads.

Class E2: Class E2 controllers employ their contacts for starting and stopping the motor and employ fuses for short circuits or faults exceeding operating overloads.

Above 7200 V, motor control is generally accomplished using circuit breakers.


Motor starter explained in details (VIDEO)

Cant see this video? Click here to watch it on Youtube.

Reference: AC Motors, motor control and motor protection – Bill Brown, P.E., Square D Engineering Services

About Author //

author-pic

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

4 Comments


  1. KULDIP
    Oct 03, 2015

    PLS HELP ME ,HOW TO DOWNLOAD MULTIPLE FILE AT 1 TIME, OR HOW TO DOWNLOAD FOLDER, PLS IMPROVE THIS. I REQUEST U


  2. Francisco Alvarez
    May 12, 2015

    Good article.


  3. Zeeshan Akhtar
    Apr 11, 2015

    Nice work man.


  4. Budi Hermanto
    Feb 24, 2015

    Dear Edvard,

    Could you help me to give a formula to choose the mccb size & determine setpoint over load relay for protection motor 380Vac, 3 phase?
    Im very appreciate your feedback and thank you so much.

    Regards,

    Budi Hermanto

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