When should I use a Softstarter versus a Variable Speed Drive?

What is a Softstarter?

A softstarter is, as you would expect, a device that ensures a soft starting of a motor.  By reducing inrush currents and limiting torque, softstarters protect your equipment, extend the life of your motor, and reduce motor heating. During start-up, softstarters provide gentle acceleration up to full speed, saving energy and reducing emissions.

The Advantages of Soft starters

Soft starters are based on semiconductors, via a power circuit and control circuit, these semi-conductors reduce the initial motor voltage.  This results in lower motor torque and during the starting process, the soft starter gradually increases the motor voltage, thereby allowing the motor to accelerate the load to the rated speed without causing high torque or current peaks.  Softstarters can also be used to control how processes are stopped, by using the voltage control in reverse and having a controlled ramp down facility. They are also less expensive than variable speed drives.

The Drawbacks of Softstarters

Soft starters share some of the same problems as frequency converters: they may inject harmonic currents into the system, which can disrupt other processes, however most modern-day units have either an integral by-pass option, or an integral bypass relay to bring in a separate, shorting by-pass contactor .


The starting method supplies a reduced voltage to the motor during start-up, as voltage is proportional to torque the current limit, ramp time and voltage ramp settings have to be set to allow enough torque to accelerate the motor and not stall. The soft starter starts up the motor at a 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.


The 2 main disadvantages being

  1. Reduced torque/power during the motor starting period
  2. The motor can only run at the fixed frequency (50Hz standard) speed.

Starting with a Variable Speed Drive

Variable Speed Drives ( or Frequency convertors) are designed for the continuous mains supply to motors, but they can also be used for start-up only.  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, it’s possible to control the speed of the motor by changing the output frequency from the drive. 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, even though 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.

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.

The Drawbacks to Starting with a VSD

Even so, VSDs  are still more expensive than softstarters in most cases; and like soft starters, they also inject harmonic currents into the network.  Although manufacturer catalogue’s try to make it easy as possible, there are many variables associated with the selection and rating of the optimum electric motor and AC converter for a Variable Speed Drive (VSD) application. In many cases, it requires considerable experience to get the selection right.

The reason for its difficulty is that there is always an engineering trade-off between the following:

  • The need to build-in a margin of safety into the selection procedure
  • The need to keep the initial cost to a minimum, by selecting the optimum type and size of motor and converter for each application

Your Checklist for Decision Making

The following checklist covers most of the factors that need to be considered when deciding the equipment is best suited for you:

  • The nature of the application
  • Maximum torque and power requirements and how these change with speed
  • Starting torque requirements
  • The speed range – minimum and maximum speed
  • Acceleration and deceleration requirements (Is braking necessary?)
  • Compatibility with the mains supply voltage
  • Environmental conditions where the converter and motor are required to operate, ambient temperature, altitude, humidity, water, chemicals, dust, etc.
  • Ventilation and cooling for the converter and motor
  • Direction (uni-directional or bi-directional)
  • Accuracy of the speed control
  • Dynamic response (speed and torque response requirements)
  • Speed regulation requirements with changes in load, temperature, supply voltage
  • The duty cycle, including the number of starts and stops per hour
  • Overall power factor of the drive system and its effect on the mains supply
  • EMI and harmonics in the mains power supply, in the motor, and motor cable
  • Are EMI filters required??
  • Earthing, shielding, and surge protection requirements
  • Torque pulsations in the rotor shaft
  • Control method – manual, automatic, analogue, digital, communications
  • Control and communications interfaces required for the plant control system
  • Indications required
  • Reliability requirements are dedicated standby unit required
  • Protection features, in-built and external features required
  • Power and control cable requirements
  • Parameter settings, local or remote programming
  • Maintenance, spares and repair considerations
  • Cost of the alternative systems, taking into consideration the capital cost, performance advantages, energy savings, efficiency, or process improvements
  • Noise due to the harmonics in the motor
  • Mechanical resonance at certain motor speed


If you’re looking for more information and guidance, please contact our engineers who are always happy to offer support.

Contact our team on: www.quantum-controls.co.uk/ +44 (0)1661 835 566 [email protected]

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