Motor Soft Start Circuit Using Pulse Width Modulation

High wattage motors such pump motors or other forms of extreme industrialized motors often obtain massive current at the time of their initial power switch ON, which usually impacts the associated fuses and switches adversely causing these to either damage of or degrade overtime. In order to remedy the condition a soft start circuit becomes highly crucial.
Although the above designs are extremely helpful, these kinds of could very well be considered somewhat low tech with their strategy.

In this post we are going to see how the method could also be applied implementing a much advanced PWM based motor soft start controller circuit.

The concept here is to apply a steadily incrementing PWM to a motor anytime it’s turned on, this steps enables the motor to attain a linearly expanding speed from zero to maximum within a established time frame, which can be variable.
PWM generator circuit for developing soft start in motors, and pumps
With reference to the figure above, the formation of the linearly incrementing PWM is obtained by using two 555 IC, built in their regular PWM mode.

I have by now talked about the concept elaborately in one of my before articles discussing the best way to use IC 555 for generating PWM.


PWM generator circuit for implementing soft start in motors, and pumps


As could be observed in the diagram, the configuration employs two 555 ICs, IC1 being wired prefer as astable, while IC2 as a comparator.

IC1 generates the needed clock signals at a particular frequency (dependent upon the values of R1 and C2) that could be put on to pin#2 of the IC2.

IC2 utilizes the clock signal to obtain triangle waves across its pin#7, to make certain that all these is usually as compared to the potential available from its control voltage pin#5.

Pin#5 acquires the needed control voltage by means of an NPN emitter follower stage created using the help of T2 as well as the associated components.

When power is switched ON, T2 is raised on with a ramping or a gradually increasing voltage at its base via R9, and owing to the proportionate charging of C5.

This ramping potential is appropriately duplicated across the emitter of T2 with regards to the supply voltage at its collector, meaning the base data is transformed into a slowly increasing potential starting from zero to almost the supply voltage level.

This ramping voltage at pin#5 of IC 2 is quickly as compared to presented triangle wave across pin#7 of IC2, which is certainly translated into a linearly incrementing PWM at pin#3 of IC2.

The linearly incrementing method of the PWMs goes on until C5 is completely imposed and the base of T2 reaches a stable voltage level.

The above design looks after the PWM generation whenever power is switched ON.

In order to implement the PWM soft start impact, the output from pin#3 of IC2 is required to be used on a triac power driver circuit, since shown below:


The above picture shows how the switch ON soft start PWM control may be enforced on heavy motors for the planned purpose.



In the picture above we see how triac driver isolators with zero crossing detector can be employed for driving the motors with the linearly incrementing PWMs for carrying out a soft start impact.

The above process effectively deals with the start ON overcurrent situating on single phase motors.

However when a 3 phase motor is utilized, the following strategy should be considered for executing the suggested 3 phase soft start on motors.