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Starting and switching motors

Selection criteria

Electrical motors must be accelerated from rest up to the operating speed with a starting device. In the case of variable speed drives, the motor controller must also manage the motor speed during operation.

Low voltage switchgear and controlgear application guide
Low voltage switchgear and controlgear application guide (photo credit:

The motor and method of starting selected depend on the load torque, the desired starting characteristic (starting current, acceleration) and on the characteristic of the supply.

See also Section 1.7 (Three-phase asynchronous motors) with respect to the characteristic properties of induction motors as the most frequently used motors.

Main criteria for the selection of the starting method

When making the decision whether to use a:

  • Direct-on-line starter
  • Electromechanical starter for the starting with reduced current or
  • Electronic motor control devices (soft starters, inverters)

The following items should be taken into account to find a suitable solution from the points of view of application and productivity:

  • How high is the torque required to start the load?
  • Can transmission components such as belts, gearboxes or chains be damaged by the high starting torque with direct starting?
  • Does the plant require gentle and continuous acceleration or are torque peaks permissible?
  • Are there any restrictions with respect to supply loading?
  • Do technologically more complex products offer additional functions for optimisation of the application (for example pre-warning functions of motor protection relays, mirror contacts for safety controllers, communication links etc.)?
  • In addition to starting, are features of controlled coasting to a stop or braking to be taken into account?
  • In addition to starting, are aspects of speed control to be taken into account once the motor has started (for example from process engineering or energy saving perspectives)?

The selection of suitable starting methods is a critical factor in achieving optimum economic efficiency in every motor control application. Table 3.1 1 (not presented here) provides guidance with respect to the various methods for starting squirrel-cage induction motors.

Kind of motor Starting procedures for squirrel-cage standard motors compared (typical values) Special squirrel-cage motors

Title:Low voltage switchgear and controlgear application guide – NHP Electrical Engineering Products Pty Ltd
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Low voltage switchgear and controlgear application guide
Low voltage switchgear and controlgear application guide

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  1. Jose luis romero
    Dec 31, 2019

    Hola,soy jose luis romero y encontré su informacion en Internet y me suscribí a su página, la cual me pareció muy interesante e ilustrativa y quisiera saber si ustedes tienen instructivos y diagramas en español no importa que estén a la venta, ya que me encanta hacer trabajos de electricidad, quiero conocer mas acerca de sus productos, gracias

  2. Jovan Milošević
    Mar 31, 2018

    Very useful! Thank you!

    • Jovan Milošević
      Jan 31, 2020

      I downloaded and read “Low voltage switchgear and controlgear application guide” and found interesting note about right and wrong star-delta connection of three phase induction motors. In this guide, it’s recommended connection U1-V2; V1-W2 and W1-U2 because of lower transient currents peaks in clockwise rotation during switching from star to delta connection (page 3-74, figure 3.38). This is something I saw for the first time and it’s opposite of many recommendations I saw in many manuals, handbooks, etc, which is U1-W2; V1-U2 and W1-V2. Moeller’s Wiring manual gives as an example U1-V2; V1-W2 and W1-U2 case. Schneider electric also in it’s manuals and product data sheets (for RE8YG31BUTQ) give U1-V2; V1-W2 and W1-U2 solution but many others like Quora give the wrong example which also rotates in clockwise but produce bigger transient voltage between connection points of motor. This produce bigger currents from net and sometimes protecting devices reacts in manner of disconnecting motor.

      • Greg Robinson
        May 06, 2023

        I teach electrical trades and most teachers are unaware of this. Both technical manuals put out by Telemechanique and Sprecher and Scheh state this. The starter must first be supplied with the correct phase sequence. Then the connections you have stated must be followed. If not premature burnout of the star contractor will occur. The other Delta connection still works, it’s just that the surge currents and voltages during transition from star to delta are less with the recommended connection.

  3. Ghulam Qadir Dar
    Mar 30, 2018

    Very interesting and informative topics. It adds my knowledge.

  4. Gopal
    Oct 06, 2015

    I am mad about EEP.

  5. Obinna
    Feb 25, 2015

    This is a great site with lots of interesting stuff for electrical engineers.

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