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PWM Motor Soft Start Circuit to Prevent High Consumption during Power Switch ON

PWM Motor Soft Start Circuit to Prevent High Consumption during Power Switch ON

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

Dc motor soft start with speed control

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.

SHARING IS CARING!


About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!



125 thoughts on “PWM Motor Soft Start Circuit to Prevent High Consumption during Power Switch ON”


  1. Howdy, Friend! Interested to Learn Circuit Designing? Let's Start Discussing below!
  2. Hallo Mr.Swagatam,
    thanks for your inspirational article.

    I would like to ask you whether it is possible to use an readymade module as a PWM?

    Can I use this module for soft start of 3 phase and 1 phase motor? Or will it be better to build a PWM circuit according to the diagram in the article?

    Thanks
    Michal

  3. Dear swag
    Will this circuit work with 5vdc? what modfications will i do on the circuit to power it with 5v DC instead of 12Dc thanks

    • Hi Matthias, It will work with 5V also without any change in the design, you may only have to increase the value of C5 to 220uF or may be more!

  4. Hello Swag! I was wondering about the start capacitor for single phase motors, is it already implemented or is some type of work around involved to get to work properly? I wanted to implement a soft start circuit for my drill press, as when it starts it really jerks hard on the pulleys and belt, and I believe over time this will be harmful to the drill press and components….it is a 1/3 HP, 7 Amp, single phase, AC induction motor with a start capacitor…if so which circuit would suit to my needs the best? And I would like to thank you for the professional help in advance, as there is no cooler electronics site on the web as yours! You do a great service for hobbyists and professional’s alike, and I thank you for that Swag!! Sincerely, Michael Sakach- electronics hobbyist, and Swagatam follower!!

    • Thank you Michael, it’s always a pleasure helping interested hobbyists like you!

      As far as I know single phase induction motors are not self starting, and needs an external force to initiate rotation, so may be your motor already has a capacitor for enabling the initial pickup for the motor.

      To control initial start up speed I would recommend trying a phase-chopped triac based circuit, similar to our home dimmer switches but with a heavy duty triac.

      You can experiment this by manually controlling a series connected dimmer switch with your motor. If you are able to control the speed with this devices then you can either implement the slow start manually each time you start your machine, or automate it using one of the designs presented in the following article:

      https://www.homemade-circuits.com/adding-soft-start-to-water-pump-motors/

      I would soon merge this article with the above one since both basically have the same purpose

      Let me know if you need any further help:)

  5. Hello Sir,
    I think my circuit is not working properly because when i connect a filament bulb there’s no gradual changes on the light intensity.
    i thoroughly checked my connections and it is exactly the same with the 2nd circuit above.
    may i ask Sir if the T1, T2 or the R8 (pot) connected the right way?

    Please bear with me for asking too much Sir, just want to make it works for the benefit of my old man.

    Thanks,
    Amor

  6. Good day Sir Swagatam,

    I made the 2nd circuit Sir but its not working properly
    when i apply power to the circuit the motor never gaining its full speed
    i mean the speed of the motor remain in the soft rotation
    but when i turn the pot (R1, R8) that’s the only time the motor will gain its max speed.

    I’m using a 12v cpu fan to test the circuit and using the same power source both for circuit and fan.
    what do you think i am missing Sir?

    Thanks,
    Amor

    • Hi Amor, A CPU fan works on BLDC principle, and I don’t think a BLDC fan would suit the second circuit.
      Alternatively you may try replacing the BC547 with two BC547s joined in Darlington form, I think this could help to solve the issue

      • Thank you very much for addressing our concerns Sir,
        instead of using two bc547 what specific darlington transistor i could use inplace of bc547?

          • Greetings!

            Hello Sir, i’ve tried your recommendation but there’s no changes happens.
            when i set the pot to low then the motor remain in a slow rotation and if i set to pot to higher then the motor will start and remain at its constant high speed the same happens when i use a 12v led strips.

            • Hi Amor, first confirm whether your circuit is correctly working or not by replacing the motor with a filament bulb (rated to work with the existing DC supply). If the bulb illumination varies uniformly, then that would would indicate the motor is not suitable to work with normal PWM application, and might require special ICs or drivers for the operation.

  7. Hello Swag. Thanks for your dedication to this page.

    How do you go about calculating Rin. You suggested 4k7 but the datasheets suggest that the resistor should follow the Ift of the optocoupler chosen. The voltage from the PWM circuit should be around 11V, so if I had to choose 10mA as If doesn’t this give for a 1k1?

    • You are welcome Ronald.
      In electronics resistor values are never critical unless it is related to time delays or frequency. For LED current limiting the parameters are not so critical.
      The switch ON specification of the internal triac of the opto is also not so critical, that implies within a certain margin the switching can be implemented normally.
      With a 4k7 resistor also the LED switching will be optimal enough to trigger the triac, with a 1.1K it will be a little sharper, but in both the cases the trigger will happen normally.

  8. Hello Swag,
    I was just wondering how to modify this schematic to work also as soft start / soft stop for led strips. What I figured out is something using a 3rd 555 to generate a delay for power supply switch off and a NAND to control R9 (ON state -> OFF state).
    Have you got any other suggestion please?

    • Hello Astercaster, No external circuits will be required for the mentioned modifications. You can simply connect the LED strip with the mosfet in the second diagram and get the require results. The LED will fade-in and fade-out each time the ON/OFF switch is operated.

  9. Good day Sir Swagatam,

    how could i integrate the above 2nd circuit in a electric rickshaw unit that has a 3 speed and a reverse functionality? does the soft start also affect the reverse mode?

    it is okay to charge the battery (48v ) from ac outlet while the soft start circuit permanently connected?

    • Good Day Amor,

      Your motor’s negative is at the moment connected directly with battery negative, that will need to be removed and connected with the shown mosfet’s drain pin.

      after this the circuit and the 48V supply will need to be configured with a DPDT switch such that both the motor and the circuit are switched ON simultaneously each time the switch-ON operation is conducted.

      • Hello Sir Swagatam,

        Should i put a large heat sink for the irf540?
        what is the wattage of the 100 ohm resistor at the base of the mosfet?

        Thanks,
        AMOR

  10. Hi Sir Swagatam, we have an electric tricycle with a 500W 48v differential motor with 3 speed switch and powered by a 48v 20Ah battery.

    Would you please recommend a suitable soft start circuit for this kind of motor ( differential )
    if ever, should i power it separately or just tap it to one of the 12v from the 48v battery supply?

    • Hi Amor, although I do not know much about differential motors, I believe the second circuit from the above article will be able to implement the required slow start to your motor perfectly, you can give it a try…

      • Thank you Sir Swagatam, is there any replacement for the mosfet IRF540? what about F9540N?
        about the watt ratings of the resistors, are they all 1/4 watt?

        • Hi, F9540N is same as IRF9540N which is the P version of the IRF540, it won’t work since the soft start action would be simply reversed…it may work if a NPN BJT is used as the buffer in the middle

  11. Hello. Very interested in your PWM generating circuit. Your motor driver circuits with the triac and the SCR are for AC motors. Like your correspondent who wants to soft start a car starter motor, I require to soft start the starter on a motorcycle. Both of us need a DC driver circuit providing rather large currents. Can you help please.

    • Could you please specify the motor specifications, is it a DC motor?? If it is then you can easily drive it by usingg a TIP35 transistor with pin#3 of IC2 in the first diagram, and by attaching motor between the positive battery supply and collector of the transistor…the emitter will connect with the ground of the motorcycle or the battery negative, and the base of the BJT can be connected to pin#3 through a 220 ohm 1 watt resistor

  12. Hi Swagatam, glad to come across your site. Thanks for posting all good ideas. I have a question on the above PWM Motor SoftStarter circuit.

    Can I replace FKPF12N60 with BT139 / BTA16/600B etc. ???

  13. hii
    can u please tell how much apprpx. current can be reduced if we use it with single phase induction motor 3 hp runnig amp at full load is 15amp.

    • the current will start from minimum and then settle to the specified normal value (15 amp) with this soft start circuit, however the above design needs some corrections , I'll try to do it soon.

  14. Hallo. I have two questions.

    Would be possible use the 3 phase softstart use in Europe? There is 3 phase voltage 3x 400V.

    And, I'm not sure, what is PWM input in left side of the circuit. Is possible use a switch?

    Regards,
    Mike

    • Hello, yes the design can be used for any 3 phase induction motor in any country, the PWM input is derived from the first circuit..and this must be set correctly for achieving the soft start.

  15. T1 is for ensuring that the PWM do not get affected with a fluctuating DC supply to the circuit….if your DC is constant you can remove the T1 and the associated parts…

  16. Swagatam, I have been reading your design for a PWM-based soft start for a motor. I really like the 2 x 555 design (which, I guess, could be implemented with a 556 dual timer chip). What puzzles me is the interface between this PWM circuit and the mains electricity. The output of the PWM circuit is asynchronous with the mains, but the 3061 forces synchrony with the mains (50Hz in my case) through the zero-crossing detector. You didn't give the frequency of the oscillator, but I presume it is faster than the mains. I don't doubt that you have tested it, but I can't see how it works. As my motor is a universal motor (in a table saw), and would run quite happily on DC, would it not be a better idea to rectify the mains for the motor, then use the PWM to switch an IGBT on the earth-side of the motor? That way, the mains frequency doesn't really come into the question. I guess it would need a transistor between the timers and the IGBT – even though the gate is insulated.

  17. Hello, i am using your circuit to soft start a engine starter (car), but it is going from 0 to 12V really quickly, changing R1 and R3 to 1M went from 1ms to 32ms of rising time, but this is too much low for me yet, i need it to be around 2-3s of rising time. Do you know what i could do ?
    Really thanks in advance.

    • Hi, R1 is not related to PWM adjustment, you should try increasing the value of C5 for getting lengthier soft starts…if possible use a Darlington for T2 for a better response

  18. Hi mr. Swatagam

    Could you pls. give the parts spec. for the 110v " Hot line switching circuit" the one with the SCRs
    or if i could use the 220v circuit for 120v line what would be changed for that?
    (Benchsaw 110v 12amp)

    Your input is very well appreciated.
    Roger

  19. Hi, could you tell me what the value and wattage of R1 + 2 should be in the third circuit? Also the amp rating of D1 + 2. I will be using the circuit to soft start a 1 1/2 HP pool pump, 120 VAC 12.5 FLA.

  20. sir, what are the voltage ratings of the three 0.01microfarad in ckt 3. we used 25v capacitor but it burnt out. we replace it with 0.01microfarad 200v capacitor, but there is no output voltage when it is connected to the load. if it has no load, the output voltage is 230v balance. what do you think is the problem in your ckt..? why is it that the voltage is zero when connected to the load. thanks.

    • ecnamay,
      the voltage rating of the capacitor should be obviously high, since you are working with mains voltage. it should be 400V in the input is 230V…

      You might have done something seriously wrong or or may your parts are of low quality and that's why not working.

      the circuit idea is perfect and will work immediately but only if it's done by proper understanding.

  21. sir if you are going to run a motor using this design, what motor will you use? in terms of its power. .
    and also sir, if we used 0.5hp induction motor, do we have to change the ratings of the components (e.g. resistors/ capacitors)? or do we have to modify the circuit.

    • good day ecnemay, all DC motors will work with the the first circuit after a transistor driver stage and AC motors with the second and the third circuits.

      power of the motor will depend on the rating of the power devices used

  22. good day sir, we checked again the voltages of the three wires connected to the motor, it all turned out 210V. the tester we used in which it reads 7V is damaged.sorry 🙂

  23. good day sir. why is it that the output voltage that is to be supplied to the motor is only 7volts, the 3 wires connected to the motor are all 7 volts.

    • good day ecnemay,

      are you referring to the third circuit?

      did you connect mains 3 phase across the shown points in the third circuit? please elaborate on this?

  24. good day sir..based on my research, there s already a triac inside the optocoupler, so why use another triac that is connected to pin4? thanks sir.

  25. thank you sir i will make as soon as possible and tell you the result…sir i would like to ask what is the process in the triac power driver? what is the use of the triac and optocouplet that it can create a soft start in motor?.. thanks sir

    • the opto coupler acts like a buffer in between the PWM source and the triac and makes sure that the triac is driven perfectly by considering and adjusting many parameters which relieves the user from designing precision external triac driver stage and related protection circuitry.

  26. good day sir., i would like to ask if you have a complete calculation of the ckt i want to how did you arrive with those components im an electrical student i only have little knowledge of electronics. thank you sir

    • the subject can be quite extensive, not possible to explain in few sentences, because there could be strings of explanations one relating to the other and will need to be too elaborate.

  27. Mr Majumdar can i use the above soft starter circuit to an underwater pump 30hp?the voltage is 400V 50hz 3 phase..is there any problem with the starting current?

  28. Hello Swagatam, first of all BIG THX for your awesome work!
    I am going to build a 1-phase-to-3-phase AC motor inverter. It will use a separate opto-isolated IGBT driver on 20 VDC. The diagram above could be used as the initial logic circuit to supply the driver with PWM signal. I am wondering how would your diagrams change if I intend to use 5V logic circuit instead of 12V. Any suggestions?

    • Thank you Julio, the first circuit can work with any voltage between 5V and 15V, so there's nothing that would need to be changed for a 5V operation, if possible use the CMOS (7)555 ICs instead of the indicated analogue 555 ICs, for getting perfect results.

    • in fact I am thinking about using a 556 IC. It's two 555 timers in one package, isolated, only sharing Vcc and GND. I have to check if there's a CMOS version of that. Thank you.

    • I'm a noob, but maybe it will help others wondering like me:
      I checked on my parts supplier website. They have two kinds of 555 and 556 ICs – 4.5-18V 0.5MHz or 2-18V 2.7MHz. While not specified (on their webpage), those second ones are CMOS. Seemed like an overkill at first for 20KHz, but now it makes perfect sense. Thank you again.

  29. Hello sir, i'm interested in using this design on an old monarch lathe. It's a large machine running 230v three phase power, would this design work in that application?

  30. Sir I have some doubt please clarify it. In our college while performing the experiment we use auto transformer as a starter by adjusting there tappings we can start the ac motor smoothly. Disadvantage of auto transformer is its more costier. While seeing this circuit I hope that its cost less while comparing to auto transformer so that I planned to implement it. By pwm technique we need to adjust the pulse given to a motor at the starting and finally width of a pulse becomes maximum so that motor starts smoothly as per your first circuit how can I vary the pulse in a first circuit I need to use a pot for controlling the reference voltage?
    2)sir please give detail information about optocoupler I didn't know any think about this please share any other website link about optocoupler.I need a detail information about the optocoupler u mentioned here.please help me sir. I am new to electronics but I have a eagerness to do it

    • Vijay, you can vary the PWM manually by replacing R9 with a 1K pot, and by eliminating C5.

      Opto-couplers are devices which enable isolating two stages in a circuit where the two stages may be at extreme levels and needs to be completely isolated….for example in SMPs the output DC is controlled by the input AC stage through a feedback from the DC stage to the Mains AC stage through an optocoupler.

      the MOC series which is shown in the above article is a more advanced opto, since it has a feature called zero crossing detector….you can check the datasheet of this part for a detailed info.

  31. I understand the first circuit sir.please give the information about a second circuit sir.I am planning to implement it on my college lab as my mini project. Please reply sir

    • the second circuit is meant for AC operated motors, the PWM from the first circuit is fed to pin1/2 of the optocoupler in order to implement the soft start over the connected AC motor on the right side with the triac stage

    • it looks like it can used like an inverter AC, but it will need to be thoroughly tested first and experimented so that the results become perfectly verified.

      you will need the assistance of an expert engineer for implementing the tests.

  32. thanks for the reply Swagatam. so can i use this circuit with my ac? i am also trying to make stabilizer for ac as those available in market are expensive. which of your stabilizer designs should i consider and what should be the auto transformer rating for the said purpose? plz also let me know that can we convert normal ac into inverter ac?
    ac for air conditioner

  33. thanks for the reply swagatam. so can i use this circuit with my ac? i am also trying to make stablizer for my ac as those available in markets are very expensive. which of ur stabilizer designs will work with that and what rating of auto transformer should be made for said purpose?

  34. hi Swagatam. i want to know that the circuit mentioned has any affect on electricity consumption? as motor needs more current in starting phase? if yes then can this circuit be used with air conditioners compressor?

    • Hi Muhammad, yes the current will be initially low and will gradually increase as the PWMs widens so this will enable a slow start or a soft start to the motor.

      the time delay can be altered by altering the value of C5 appropriately

  35. sir i saw your article for 1watt led driver Transformerless circuit.
    https://homemade-circuits.com/2012/03/how-to-make-simplest-1-watt-led-driver.html
    i this you used 1uf/400v PPC capacitor for producing 350mA current to drive 1 watt led.

    but in your 2nd article for Calculating Capacitor Current in Transformerless Power Supplies.

    https://homemade-circuits.com/2015/01/calculating-capacitor-current-in.html

    you said 1uf/400v PPC capacitor can produce approx 69mA current.
    then how it is possible 1uf/400v PPC capacitor can produce 350mA current to glow 1watt led.. if 69mA current is used to glow LED then led glow very dim.
    plz explain me. i cant understand

    • Yes that's right, the proposed circuit will light up the LEDs at 70mA……….to counter this you can use 4 LeDs (1 watt) in series to produce an equivalent light of a 1 watt LED….4 LEDs will mean more money spent on the LEDs, but since it will not demand the need of heatsinks and special aluminum back PCB, so the cost will balance out ultimately

    • sir previous i glow 3 led of 1watt in series through 12V 500mA local ac adapter. but the cost of this adapter is approx 80rs.. sir can u give me 12V or 18V 500mA ac adapter circuit.? which i can build easly at home at cheap price..
      thnkx in advance.

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