The post explains an effective PWM motor soft start circuit which can be used for enabling heavy motors with a soft start and thus prevent the equipment from drawing dangerous high currents.

Why a Soft Start

High wattage motors such pump motors or other forms of heavy industrial motors tend to draw huge current during their initial power switch ON, which in turn impacts the associated fuses and switches adversely causing these to either blow of or degrade overtime. In order to remedy the situation a soft start circuit becomes highly imperative.

In a few of my previous articles we discussed regarding a related topic, which you may learn comprehensively through the following posts:

Soft start circuit for pump motors

Soft start circuit for refrigerators

Although the above designs are quite useful, these may be considered slightly low tech with their approach.

In this article we'll see how the process may be implemented using a much sophisticated PWM based motor soft start controller circuit.

Using PWM Concept

The idea here is to apply a gradually incrementing PWM to a motor each time it's switched ON, this action allows the motor to attain a linearly increasing speed from zero to maximum within a stipulated period of time, which may be adjustable.

Example circuit for a variable 48V motor controller with soft start

How it Works

Referring to the figure above, the production of the linearly incrementing PWM is achieved with the help of two 555 IC, configured in their standard PWM mode.

I have already discussed the concept elaborately in one of my earlier articles explaining how to use IC 555 for generating PWM.

As may be witnessed in the diagram, the configuration employs two 555 ICs, IC1 being wired like as astable, while IC2 as a comparator.

IC1 generates the required clock signals at a given frequency (determined by the values of R1 and C2) which is applied to pin#2 of the IC2.

IC2 utilizes the clock signal to generate triangle waves across its pin#7, so that these may be compared with the potential available at its control voltage pin#5.

Pin#5 acquires the required control voltage via an NPN emitter follower stage made with the help of T2 and the associated components.

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

This ramping potential is appropriately duplicated across the emitter of T2 with respect to the supply voltage at its collector, meaning the base data is converted into a gradually increasing potential ranging from zero to almost the supply voltage level.

This ramping voltage at pin#5 of IC 2 is instantly compared with available triangle wave across pin#7 of IC2, which is translated into a linearly incrementing PWM at pin#3 of IC2.

The linearly incrementing process of the PWMs goes on until C5 is fully charged and the base of T2 attains a stable voltage  level.

The above design takes care of the PWM generation each time power is switched ON.

Video Clip:

The following video shows a practical test result of the above PWM circuit implemented on a 24V DC motor. The video shows the PWM pot adjustment response of the circuit on the motor, and also an additional battery indicator LED response while the motor is switched ON and OFF.

Integrating a zero Crossing Triac  Controller

In order to implement the PWM motor soft start circuit effect, the output from pin#3 of IC2 is required to be applied to a triac power driver circuit, as shown below:

The above image shows how the switch ON soft start PWM control may be implemented on heavy motors for the intended purpose.

In the image above we see how triac driver isolators with zero crossing detector can be employed for driving the motors with the linearly incrementing PWMs for executing a soft start effect.

The above concept effectively takes care of the start ON overcurrent situating on single phase motors.

However in case a 3 phase motor is used, the following idea can be used for implementing the proposed 3 phase soft start on motors.

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