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Comparison of Soft Starting and Frequency Converter Motor Starting

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Soft starting

A soft starter is, as you would expect, a device which ensures a soft starting of a motor. A softstarter has different characteristics to the other starting methods. It has thyristors in the main circuit, and the motor voltage is regulated with a printed circuit board.

Comparison of Soft Starting and Frequency Converter Motor Starting
Comparison of Soft Starting and Frequency Converter Motor Starting (on photo: VFDs installed on fan and pump motors; credit: crockett-facilities.com)

The softstarter makes use of the fact that when the motor voltage is low during start, the starting current and starting torque is also low.

Motor soft starter
Motor soft starter

Advantages

Soft starters are based on semiconductors. Via a power circuit and a control circuit, these semi-conductors reduce the initial motor voltage.

This results in lower motor torque.

During the starting process, the soft starter gradually increases the motor voltage, thereby allowing the motor to accelerate the load to rated speed without causing high torque or current peaks.

Soft-starting curve - Synchronous speed - Full load torque (left) and Full load current (right)
Soft-starting curve – Synchronous speed – Full load torque (left) and Full load current (right)

Soft starters can also be used to control how processes are stopped. Soft starters are less expensive than frequency converters.

Another feature of the softstarter is the softstop function, which is very useful when stopping pumps where the problem is water hammering in the pipe system at direct stop as for star-delta starter and direct-on-line starter.


Drawbacks

They do, however, share the same problem as frequency converters: they may inject harmonic currents into the system, and this can disrupt other processes. (Read more about this)

Voltage ramp for soft starter. Run-up time is around 1 sec.
Voltage ramp for soft starter. Run-up time is around 1 sec.

The starting method also supplies a reduced voltage to the motor during start-up.

The soft starter starts up the motor at reduced voltage, and the voltage is then ramped up to its full value. The voltage is reduced in the soft starter via phase angle. In connection with this starting method current pulses will not occur. Run-up time and locked-rotor current (starting current) can be set.


Electric Motor Soft Start 60HP (VIDEO)

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Part 2


Frequency converter starting

Frequency converters are designed for continuous feeding of motors, but they can also be used for start-up only.

The frequency converter is sometimes also called VSD (Variable Speed Drive), VFD (Variable Frequency Drive) or simply Drives, which is probably the most common name.

The drive consists primarily of two parts, one which converts AC (50 or 60 Hz) to DC and the second part which converts the DC back to AC, but now with a variable frequency of 0-250 Hz. As the speed of the motor depends on the frequency this makes it possible to control the speed of the motor by changing the output frequency from the drive and this is a big advantage if there is a need for speed regulation during a continuous run.

As stated above, in many applications a drive is still only used for starting and stopping the motor, despite the fact that there is no need for speed regulation during a normal run. Of course this will create a need for much more expensive starting equipment than necessary.

By controlling the frequency, the rated motor torque is available at a low speed and the starting current is low, between 0.5 and 1.0 times the rated motor current, maximum 1.5 x In.

Another available feature is softstop, which is very useful, for example when stopping pumps where the problem is water hammering in the pipe systems at direct stop. The softstop function is also useful when stopping conveyor belts from transporting fragile material that can be damaged when the belts stop too quickly.

It is very common to install a filter together with the drive in order to reduce the levels of emission and harmonics generated.

Frequency converter and its line diagram
Frequency converter and its line diagram

Advantages

The frequency converter makes it possible to use low starting current because the motor can produce rated torque at rated current from zero to full speed. Frequency converters are becoming cheaper all the time.

As a result, they are increasingly being used in applications where soft starters would previously have been used.

Frequency converter curve – Synchronous speed – Full load torque (left) and Full load current (right)
Frequency converter curve – Synchronous speed – Full load torque (left) and Full load current (right)

Drawbacks

Even so, frequency converters are still more expensive than soft starters in most cases; and like soft starters, they also inject harmonic currents into the network.


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Edvard Csanyi

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