Part 2: Overview of motor starting methods

Electric motors as work horses of the industry are extensively used since decades. The topic of motor starting is therefore well known and various starting methods had been developed. This article presents an overview of most common motor starting methods.

Which solution to choose? The selection of suitable starting method depends on following factors:

– Short circuit capacity of the supply grid (i.e. grid impedance and voltage drop)

– Max. allowed voltage drop on motor terminals during start up

– Min. starting torque for safe acceleration (and synchronization)

– Max. starting torque not exceeding allowed max. shaft torque

– Frequency of start-up (how often is required to start the motor)

– Load torque and inertia

– Application requirements

– Economical aspects

Last time in the introduction the main advantages and drawbacks of direct on-line (across the line) start [1] were described. And the list of drawbacks was the longer one…

motor DOL start - pros and cons
Figure 1: Pros and cons of direct on-line motor start [1]

Therefore, let’s look at other – more sophisticated – alternatives to start an electric motor.

Reactor start

Purpose of starting reactor is to limit the starting (“inrush”) current. However, as the starting current is reduced, the starting torque is dropping with the square of the current.

motor starting methods - reactor start

Advantages:

– lower starting current

– relatively low cost solution

Disadvantages:

– additional reactor with breaker

– starting torque reduced quadratic with starting current

Autotransformer start

With this method the starting torque reduces only proportionally with starting current. In addition, autotransformer may have several taps for smoother start-up.

autotransformer motor start

Advantages:

– lower starting current

– starting torque reduced linearly with starting current (not quadratic)

– more freedom than reactor start (might have several taps)

Disadvantages:

– more expensive than reactor start

Capacitor start

Capacitor is often used in combination with reactor or auto-transformer. It can also be used for direct on-line start. The purpose is to support the grid voltage during start. Attention shall be paid to possible resonances.

motor starting methods - capacitor start

Advantages:

– creating strong grid (increased short circuit capacity)

Disadvantages:

– space requirements

– capacitor might be source of overvoltages

Combination of reactor and capacitor

It is very common to combine the starting capacitor a reactor together as an improved solution with more degree of freedom.

motor starting methods: reactor and capacitor motor start

Advantages:

– lower starting current

– stronger grid with smaller voltage drop

Disadvantages:

– additional reactor, capacitor and breakers

– starting torque reduced quadratic with starting current

– space requirements

– capacitor as potential source of overvoltages

Frequency controlled start (VFD)

VFD motor start is the most elegant method of starting. The solution provides a wide range of benefits are stated below. It is usually the most expensive solution as well.

motor starting methods: VFD soft start

Advantages:

– lowest starting current

– motor in synchronism with supply during entire start-up

– no limitation of starting torque

– full torque from zero speed

– elimination of severe torque oscillations

– minimized mechanical stress of shaft components

– fastest acceleration (if required)

– more freedom to optimize motor design

– adjustable speed ramps

– one VFD can soft start multiple motors (even different types)

– motor protection covered by VFD during start-up

Disadvantages:

– costly solution if VFD used only for one motor and only for starting purpose

– possible impact on motor (insulation, harmonic heating) – can be eliminated by selecting suitable technology and correct system design

For more details on VFD motor start please refer to a dedicated article [2].

Motor starting methods: Summary

There are multiple methods how to start an electric motor. This article provides an overview of most common motor starting methods with their strengths and weaknesses. Only squirrel cage asynchronous machines and electrically excited synchronous machines are considered.

References

[1] Motor starting methods: Direct on-line start, https://mb-drive-services.com/direct-on-line-start/

[2] Motor start with a VFD, https://mb-drive-services.com/vfd-motor-start/

[3] Medium voltage AC drives, https://new.abb.com/drives/medium-voltage-ac-drives

[4] Motors and generators, https://new.abb.com/motors-generators

motor direct on-line start
variable speed motors and critical speeds

1 Comment

VFD as soft starter - MB Drive Services · December 12, 2020 at 10:38 pm

[…] other starting methods listed in [2], the VFD starts the motor in a frequency controlled manner. The inverter output frequency ramps up […]

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