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Soft starters & Variable Frequency Drives


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  • 2 weeks later...

Hi there Milliamp,


It seems your question regarding the differences between soft starters and variable frequency drives has gone un-answered for quite some time!


The short answer to your question is that soft starters are used to accelerate a motor from zero speed to full speed over a user defined period of time. Variable frequency drives on the other hand can control the speed at which a motor operates, either below or above its synchronous speed.


Unfortunately time is short at present so I'll limit my response to the above-mentioned.


It is possible other forum members will add more.




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  • 4 weeks later...

A soft starter reduces the voltage applied to the motor during start, but does not alter the frequency.

A variable speed drive alters both the voltage and the frequency.


An induction motor is a pseudosynchronous device, that is it tries to run at the frequency of the supply. If the supply frequency is kept constant, but the voltage is reduced, that accelerating torque field remains at the supply frequency, but it's magnitude is reduced by the square of the voltage reduction. This will cause a reduced accelerating torque during start, resulting in a more gradual acceleration to full speed. It is important that the voltage is high enough to enable the motor to accelerate the load to the rated speed. If the voltage is too low, the motor will accelerate to the speed where the shaft torque produced is equal to the load torque and will remain at tht speed until either the voltage is changed, the load is reduced, or the protection operates. Induction motors must not be operated under these conditions for any period of time as they will be damaged.


If the frequency of the supply is altered, the motor will run at the new frequency. i.e. halve the frequency applied to the motor and it will run at half speed. Because the difference between the supply frequency and motor speed is very low, there is no damage to the motor. The voltage must be changed with the frequency in order to prevent the flux in the iron from being too high and causing motor heating. This can be done by keeping the ratio between voltage and frequency constant. (constant v/f ratio)


A soft starter reduces the voltage applied to the motor, reducing the start torque and current. A drive alters the frequency applied to the motor, altering it's operating speed.


Best regards,

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