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.
Note: Please use a Darlington BC547 configuration at pin#5 of IC2 instead of a single BC547. This will produce a more effective response compared to a single BC547
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.
I am using a12 v dc 1800 watt Nipon Denso geared car starter motor to pre spin 24 foot rotor blades on my gyrocopter are there any off the shelf 12v soft start controllers that are capable of doing this or will i need to get one specially designed ?
I think you will have to get it specially designed. Or you can build and customize the following concept for your application, it should be able to provide you with the intended results:
https://www.homemade-circuits.com/wp-content/uploads/2019/04/variable-soft-start.jpg
I should have mentioned I do not need speed control just soft start as the starter motor does not have sufficent power to get the rotors up to full speed { the starter motor runs out of puff at around 120 rotor RPM} after which the pre spin is turned off then the forward air speed through the rotors will bring them up to flying speed. Is there a circuit that will just give a gradual increase in motor speed without having to turn a potentiometer { speed control not needed }
OK, in that case you can try the following simplified design:
https://www.homemade-circuits.com/wp-content/uploads/2023/10/555-soft-start-circuit.jpg
The 470uF capacitor determines the slow start length.
Would any of these circuits work for a 2HP 220VAC pump motor?
The above circuits are for DC motors. For AC motors the following concept might work:
https://www.homemade-circuits.com/adding-soft-start-to-water-pump-motors/
I’m new to using MOSFET and your article is very helpful.
I am looking at powering a treadmill Brushed 4 HP 90 VDC 12 AMP @ 5200 RPM motor from a 80 V, 6 Ah lithium battery with a IRF3710PbF MOSFET. But I’m worried about the inrush power when the controller signal is 100% at start.
Therefore it seems your double 555 design would solve the issue to ramp up (limit) the power to the motor.
Do you see any issue using your laid out circuit for my application?
Thanking you in advance for your help and guidance.
I see no issues with the above soft start pwm concepts and it can be perfectly used for your application
Awesome! Thank you for your quick feedback.
Will return with the result once put together.
No problem, all the best to you.
Hi
I have a PWM modulated clutch in my car. It is electro magnetic in operation and is a sealed unit and there is zero documentation.
I suspect the clutch engages to 100% in an instant rather than smoothly ramping up. I wish to build a circuit as a diagnosis tool to prove or disprove my theory.
So, its a 12V power supply and the supply to the clutch is PWM, presumably by the controller PCB. I have no details about the frequency or any other data about the power supply. My suspicions are that this controller is at fault in the modulation phase.
So, I need to instal a simple, and temporary circuit just before the clutch to slowly ramp up the current say over a one or two second period.
I guess my problem is that I need to soft start and already PWM’c current supply. Grateful for any ideas.
Hi, you can probably try the following circuit for implementing the intended soft start on your clutch device. Please test the circuit with a DC lamp first to verify its working.
Woo Hoo!…..
Many thanks..
I’ll give it a try in the next few weeks.
I am glad to help. Let me know if you have any problems.
Hello Swagatam. Great article and thank you for your insights! I would like to make or buy a soft-starting device for our Leeson 24V, 3/4 HP, 29A 1800 RPM motor. This motor drives hold plate fridge compressor on a boat. I would like this PWM soft starting device to activate and come up to full motor speed at 5 to 10 seconds. I have some EE beginners skils and can build a circuit but I would also consider a well built reliable device I could buy. What would be your suggestion? Thank you! Serge
Thank you Serge, glad you liked it! Making a practical prototype can be difficult for me due to lack of time, I am sorry about it. Nevertheless the circuit has been tested by me, so you can feel free to build it yourself and check how it responds. If you need any further assistance let me know through comments I’ll try to solve them as soon as possible.