• Skip to main content
  • Skip to primary sidebar

Homemade Circuit Projects

Need circuit help? Post them in the comments! I've answered over 50,000!

Blog | Categories | About | Contact | Calculators-online
You are here: Home / Motor Controller / AC Motor Speed Controller Circuits using Back EMF

AC Motor Speed Controller Circuits using Back EMF

Last Updated on January 2, 2024 by Swagatam 49 Comments

In this post I have explained a few circuit diagrams for building AC motor speed controller circuits using back EMF concept.

Working Principle of AC Motors

All household appliances like electric drills, sanders, sewing machines, and food mixers utilize AC electric motors of the series-wound type.

While in operation, these motors generate a back-electromotive force (back-emf) that is directly proportional to their speed.

The effective voltage applied to these motors equals the actual supplied voltage minus the back-emf.

This intriguing fact inherently endows these motors with a self-regulating capability regarding their speed.

An increase in the motor's workload naturally leads to a reduction in speed and back-emf, consequently boosting the effective applied voltage and ultimately causing the motor's speed to revert to its initial value.

Using Triac

Most AC motors are typically engineered to offer single-speed functionality. To impart variable speed control to these motors, one can readily employ Triac phase-controlled circuits.

An example of such a circuit is illustrated in the following figure. This particular circuit proves especially advantageous for managing lightly loaded appliances like food mixers and sewing machines.

AC motor speed control using triac

Nonetheless, it's important to note that the range of control provided by this setup is somewhat limited.

In contrast, electric drills and sanders contend with substantial fluctuations in load, rendering them less compatible with the above AC motor speed controller circuit.

Using SCR

For these high-load appliances, the preferable choice is the AC variable speed regulator circuit depicted in the following figure.

AC motor speed control using SCR

In this configuration, an SCR serves as the control element, delivering half-wave power to the motor (resulting in a reduction of around 20% in available speed/power).

During the off half-cycles, the SCR senses the motor's back-emf, thereby facilitating automatic adjustment of the subsequent gating pulse, which aids in achieving some degree of speed regulation.

The network composed of R1, RV1, and D1 allows for a phase adjustment of 90 degrees, ensuring that all motor power pulses maintain a minimum duration of 90 degrees and deliver substantial torque.

At lower speeds, the circuit enters a 'skip cycling' mode, where power pulses are intermittently supplied to suit the motor's load conditions.

Delivering High Torque

This type of AC motor speed controller circuit excels in delivering substantial torque at low speeds but may exhibit some motor 'chattering.'

Similar to the prior circuit, it's essential to recognize that it offers only a limited range of control.

Closed Loop AC Motor Speed Controller using Back EMF

The next article presented below explains a very simple closed loop AC motor speed controller circuit that may be used for controlling single phase AC motor speeds.

The circuit is very cheap and uses ordinary electronic components for the required implementations.

The main feature of the circuit is that it’s a closed loop type, that means the speed or the torque of the motor can never get affected by the load or the speed of the motor in this circuit, on the contrary the torque is indirectly proportional to the magnitude of the speed.

Circuit Operation:

Referring the circuit diagram of the proposed single phase closed loop AC motor controller, the involved operations may be understood through the following points:

For the positive half cycles of the input AC, the capacitor C2 is charged through the resistor R1 and the diode D1.

AC 220 V torque compensated motor speed controller circuit

The charging of C2 persists until the voltage across this capacitor becomes equivalent to the simulating zener voltage of the configuration.

The circuit wired around transistor T1 effectively simulates the operation of a zener diode.

The inclusion of the pot P1 makes it possible to adjust the voltage of this “zener diode”. Precisely speaking, the voltage developed across T1 is literally determined by the ratio between resistors R3 and R2 + P1.

The voltage across the resistor R4 is always maintained equal to the 0.6 volts that’s equal to the required conducting voltage of T1’s base emitter voltage.

Therefore it means that the above explained zener voltage should be equal to the value that may be acquired by solving the expression:

(P1 + R2 + R3 / R3) × 0.6

Parts List for the above closed loop AC motor speed controller circuit

  • R1 = 39K,
  • R2 = 12K,
  • R3 = 22K,
  • R4 = 68K,
  • P1 = 220K,
  • All diodes = 1N4007,
  • C1 = 0.1/400V,
  • C2 = 100uF/35V,
  • T1 = BC547B,
  • SCR = C106
  • L1 = 30 turns of 25 SWG wire over a 3mm ferrite rod or 40 uH/5 watt

How the Load is Positioned for a Special Reason

A careful investigation reveals that the motor or the load is not introduced at the usual position; rather it’s wired up just after the SCR, at its cathode.

This causes an interesting feature to be introduced with this circuit.

The above special position of the motor within the circuit makes the firing time of the SCR dependant on the potential difference between the back EMF of the motor and the “zener voltage” of the circuit.

That simply means that the more the motor is loaded, the quicker the SCR fires.

The procedure quite simulate a closed loop type of functioning where the feedback s received in the form of back EMF generated by the motor itself.

However the circuit is associated with a slight drawback. The adoption of an SCR means the circuit can handle only 180 degrees of phase control and the motor cannot be controlled throughout the speed range but only for 50% of it.

Another disadvantage associated due to the inexpensive nature of the circuit is that the motor tends to produce hiccups at lower speeds, however as the speed is increased this issue completely disappears.

The Function of L1 and C1

L1 and C1 are included for checking the high frequency RFs generated due to the rapid phase chopping by the SCR.

Need less to say the device (SCR) must be mounted on a suitable heatsink for optimal results.

Back EMF Drill Speed Controller Circuit

This circuit is mainly used to control the steady speed of smaller series wound motors, as found in several electric hand drills, etc.

The torque and the speed is controlled by P1 potentiometer. This potentiometer configuration specifies how minutely the triac could be triggered.

drill speed controller circuit using back EMF

When the speed of the motor drops just under the preset value (with load connected), then the motor 's back EMF decreases.

As a result, voltage around through R1, P1, and C5 rises so that the triac is activated earlier and motor speed tends to increase. A certain proportion of speed stability is achieved in this manner.

You'll also like:

  • 1.  Bluetooth Motor Controller Circuit
  • 2.  Constant Torque Motor Speed Controller Circuit
  • 3.  Incubator Reverse Forward Motor Controller Circuit
  • 4.  High Current Sensorless BLDC Motor Controller using Back EMF
  • 5.  3 Phase Brushless (BLDC) Motor Driver Circuit
  • 6.  Push Button Fan Regulator Circuit with Display

About Swagatam

I am an electronics engineer with over 15 years of hands-on experience. I am passionate about inventing, designing electronic circuits and PCBs, and helping hobbyists bring their projects to life. That is why I founded homemade-circuits.com, a website where I share innovative circuit ideas and tutorials. Have a circuit related question? Leave a comment.... I guarantee a reply!

Previous Post: « How to Generate Electricity from Sea Water – 2 Simple Methods
Next Post: GSM Based Cell Phone Remote Control Switch Circuit »

Reader Interactions

Comments

  1. Swagatam says

    November 18, 2024 at 7:57 am

    Hi, that design looks like a standard fan dimmer circuit to me. I am not sure how it can work with a back EMF concept?
    You are right, the 68k looks too high, you can probably reduce it to 10k or 15k and check the results.
    A similar tested design has been discussed in the following post, you can check it out:
    https://www.homemade-circuits.com/simple-ceiling-fan-regulator-circuit/

    Reply
  2. Виталий says

    October 8, 2024 at 5:10 pm

    Последняя схема, на симисторе, у меня не заработала.

    Reply
    • Swagatam says

      October 8, 2024 at 5:14 pm

      please try the following design and let me know:
      " rel="ugc">comment image
      Make sure to add a 47 ohm resistor in series with the diac to safeguard it from surge currents.

      Reply
  3. Ian Kidd says

    April 22, 2024 at 6:17 pm

    What role does C6 play in the firing time of the TRIAC

    Reply
    • Swagatam says

      April 22, 2024 at 6:55 pm

      It converts each single firing pulse for the diac/triac into small bursts of pulses.

      Reply
      • Ian Kidd says

        April 24, 2024 at 7:01 am

        thanks

        Reply
      • Gelu says

        March 19, 2025 at 6:53 pm

        Buna ziua domnule.in locul lui p1 se poate pune un termistor pentru a lucra în funcție de temperatura?Când e rece motorul să meargă aproape cu viteza maxima,iar la căldură să se reducă turația.Multumesc.

        Reply
        • Swagatam says

          March 20, 2025 at 7:56 am

          Hi Gelu,
          P1 can be replaced with an appropriately rated thermistor but it might not control the motor with full range, because a pot can be varied from zero to max, which might not be achievable with a thermistor.

          Reply
          • Gelu says

            March 20, 2025 at 9:41 am

            Buna ziua domnule.Multu.esc mult pentru răspuns,chiar daca nu se oprește total motorul nu e o problema,am vrut doar să știu dacă merge un termistor in locul lui p1,ca sa nu fac degeaba circuitul.Fiti binecuvântat.

            Reply
            • Swagatam says

              March 20, 2025 at 11:28 am

              No problem Gelu, yes P1 can be replaced with a thermistor, if accuracy is not important.
              However, please be warned that the last two circuits were not tested by me, they were contributed by external authors.

              Reply
              • Gelu says

                March 20, 2025 at 5:30 pm

                îmi asum riscul și dacă sunt probleme vă anunț.Multumesc.

                Reply
                • Swagatam says

                  March 20, 2025 at 5:37 pm

                  Sure, if you face any problems please feel free to ask me here!

                  Reply
  4. Jacques FRADIN says

    December 12, 2022 at 9:22 pm

    Bonjour, que faudrait -il faire pour un controle du moteur en surintensité afin de couper le triacs fonction que l’on trouve dans les verins de portails? MERCI

    Reply
  5. Norberto says

    June 11, 2021 at 7:44 pm

    Dear I
    try to install vfd for motors of neither small workshop at home
    1.5 kw motors approx, electric
    saw etc I use diac and triac bt139 800, but my question is with the harmonics
    The reason is that my workshop is at the back of my house and the start-ups are strong because of the house lines
    I have two sets of diac / triac, and I cannot find filters for these applications, I
    would put a resistor and capacitor in parallel to the load in each case.
    What value do you recommend? Voltage
    220 volts at 50 Hz
    Norberto

    Reply
    • Swagatam says

      June 12, 2021 at 8:13 am

      Hello, you can add a RC snubber, across the triac, but I think a snubber across a triac is required for protecting the triac from high voltage spikes, it is mostly not for harmonics. For harmonics you can try adding a 1uF/400V capacitor right across the motor wires and see if that helps

      Reply
  6. Ibrahim says

    May 5, 2021 at 9:31 am

    Sir, how could I simulate this circuit? I mean what software should I use?

    Reply
    • Swagatam says

      May 5, 2021 at 10:51 am

      Ibrahim, I never use simulator for my circuits, so can’t help much with this subject

      Reply
    • Bwengye Duncan says

      September 16, 2021 at 5:29 pm

      you can use use proteus

      Reply
  7. Gabriele says

    February 11, 2021 at 4:53 pm

    Avrei un problema da risolvere
    e’ possibile avere un circuito di protezione da cortocircuito e limitatore di potenza
    su un alimentatore variabile da 300V DC.
    Grazie per il VS aiuto
    Gabriele

    Reply
  8. Elias Novaes says

    January 15, 2021 at 8:38 pm

    Hello Friend! Does this project work on a 1.4 hp 90v engine?

    Reply
    • Swagatam says

      January 15, 2021 at 8:50 pm

      Hello friend, it will work if the motor is a series wound motor…

      Reply
  9. Paul says

    August 3, 2020 at 3:07 am

    The resistor values in the parts list don’t match the circuit diagram.

    Reply
    • Swagatam says

      August 3, 2020 at 9:21 am

      I have corrected the parts list now so that they match with the diagram.

      Reply
  10. stephen adams says

    July 26, 2020 at 3:59 pm

    The diagram says voltage controlled by “P1, R3 and R4” Should this not be P1, R3 and R2 as these are the inputs to the base of the transistor?
    R4 is the current limiter for the the Triac.

    Reply
    • Swagatam says

      July 26, 2020 at 6:43 pm

      You are right, thank you for pointing out the mistake, I have corrected it now in the article.

      Reply
  11. Muhammad Nauman says

    May 29, 2020 at 11:46 pm

    I used a triac based dimmer to control ac 220v fan speed but motor creates hum noise in low speed. Is there any other circuit to control speed without hum noise?

    Reply
    • Swagatam says

      May 30, 2020 at 9:42 am

      use inductor capacitor filter as shown in the last diagrams of this article:

      https://www.homemade-circuits.com/how-to-make-simplest-triac-flasher/

      Reply
  12. Ernesto says

    May 14, 2019 at 11:47 pm

    Can this circuit be used with a capacitor start motor?

    Reply
    • Swagatam says

      May 15, 2019 at 7:46 am

      Yes it can be tried…

      Reply
  13. Svetoslav says

    September 12, 2018 at 10:18 am

    Hi
    does this cirquit work with 220v or is designed for 110v thanks

    Reply
    • Swagatam says

      September 12, 2018 at 10:39 am

      Hi, it can be used for both 220V and 120V inputs

      Reply
  14. Arnold says

    September 5, 2018 at 9:57 am

    What maximum power of the motor it can control and what components should be used for the speed controller for motor 1000 – 1200 watt, 115 v,

    Reply
    • Swagatam says

      September 5, 2018 at 1:54 pm

      upto 300 watts can be controlled with the shown set up, for higher wattage you may have to upgrade the SCR with a higher value, and also reduce the gate resistor value appropriately

      Reply
  15. Dhanasekar Manickam says

    June 12, 2017 at 8:08 am

    Does it control motor speed precisely without a use of tachogenerator?

    Reply
  16. Arkhantos says

    July 30, 2016 at 9:00 pm

    Do you have the inductance value of L1? May i use a toroidal inductor instead?
    By the way, amazing blog you have, and i really appreciate that you answer!

    Reply
    • Swagatam says

      July 31, 2016 at 5:19 am

      L1/C1 are not crucial, they are placed for PFC….to reduce RF interference in the atmosphere.

      still if you want to include them , you can try 200 turns of 24 SWG magnet wire on any iron core such as an iron bolt.

      I am glad you liked my website…please keep reading.

      Reply
  17. Arkhantos says

    July 25, 2016 at 12:22 am

    Will this circuit provide a stable frequency of revolution (RPM) even under load?
    Meaning the motor will keep spinning at the same speed even when a load is aplied?

    Reply
    • Swagatam says

      July 25, 2016 at 3:09 am

      yes it will as long as long as the input voltage does not fluctuate….

      Reply
    • Arkhantos says

      July 25, 2016 at 8:23 am

      Thanks!
      Im thinking to use it to control a washing machine universal motor in order to make a belt sander.
      Will a 10amp thyristor be enought?
      Also, what power dissipation capability will be needed in the resistors?

      Reply
    • Swagatam says

      July 25, 2016 at 3:00 pm

      The design was originally intended to control drill machines, so probably any similar motor can be controlled with this circuit, although the capability is restricted to 180 degrees only….The resistors can be all 1/4 watt rated, a 10amp SCR will do if the motor max consumption does not exceed this value.

      Reply
  18. pathak says

    February 4, 2015 at 8:06 am

    Sir , How AC supply to motor is completed? Means Both the half cycles. My another Q is can I connect Transfomer pri to control 230 ac v , so that sec volage automaticaly get controlled. Sir Expecting your reply. my email [email protected]
    Thanks, & Regards,

    Reply
    • Swagatam says

      February 4, 2015 at 2:46 pm

      Pathak, the above circuit will control only one half cycles of the AC and therefore will be able to provide a 180 degree phase control only….for full control you can make any regular dimmer switch circuit….the transformer can be also controlled by using a dimmer switch circuit in by connecting it series with the trafo primary

      Reply
  19. Swagatam says

    December 19, 2013 at 5:02 am

    at what voltage??

    Reply
  20. Francisco Santander says

    November 29, 2013 at 3:47 am

    what kind of motor (HP or Wattage) can you aplly this circuit to?

    Reply
    • Swagatam says

      November 29, 2013 at 5:47 am

      you can use any wattage AC motor provided the SCr is rated appropriately for handling the current.

      Reply
  21. kamran says

    October 21, 2013 at 12:11 pm

    P1 = VR1

    Reply
    • Swagatam says

      October 22, 2013 at 5:10 am

      yes.

      Reply
  22. S M OMAR Faruk says

    September 3, 2013 at 4:13 am

    I would like to ask, What is this P1?

    Reply
    • Swagatam says

      September 3, 2013 at 11:44 am

      Where's P1?

      Reply

Need Help? Please Leave a Comment! We value your input—Kindly keep it relevant to the above topic! Cancel reply

Your email address will not be published. Required fields are marked *

Primary Sidebar

Subscribe to New Circuit Ideas

Categories

  • Arduino Projects (87)
  • Audio and Amplifier Projects (132)
  • Automation Projects (17)
  • Automobile Electronics (101)
  • Battery Charger Circuits (82)
  • Datasheets and Components (102)
  • Electronics Theory (143)
  • Free Energy (37)
  • Games and Sports Projects (11)
  • Grid and 3-Phase (19)
  • Health related Projects (25)
  • Home Electrical Circuits (12)
  • Indicator Circuits (14)
  • Inverter Circuits (87)
  • Lamps and Lights (142)
  • Meters and Testers (69)
  • Mini Projects (46)
  • Motor Controller (64)
  • Oscillator Circuits (27)
  • Pets and Pests (15)
  • Power Supply Circuits (108)
  • Remote Control Circuits (50)
  • Security and Alarm (64)
  • Sensors and Detectors (100)
  • Solar Controller Circuits (59)
  • Temperature Controllers (42)
  • Timer and Delay Relay (49)
  • Transmitter Circuits (29)
  • Voltage Control and Protection (37)
  • Water Controller (36)

Calculators

  • Battery Back up Time Calculator
  • Capacitance Reactance Calculator
  • IC 555 Astable Calculator
  • IC 555 Monostable Calculator
  • Inductance Calculator
  • LC Resonance Calculator
  • LM317, LM338, LM396 Calculator
  • Ohm’s Law Calculator
  • Phase Angle Phase Shift Calculator
  • Power Factor (PF) Calculator
  • Reactance Calculator
  • Transistor Astable Calculator
  • Transistor base Resistor Calculator
  • Voltage Divider Calculator
  • Wire Current Calculator
  • Zener Diode Calculator
  • Filter Capacitor Calculator
  • Buck Converter Calculator
  • Boost Converter Calculator
  • Solar Panel, Inverter, Battery Calculator
  • Wire Current Calculator
  • SMPS Transformer Calculator
  • IC SG3525, SG3524 Calculator
  • Inverter LC Filter Calculator

People Also Search

555 Circuits | 741 Circuits | LM324 Circuits | LM338 Circuits | 4017 Circuits | Ultrasonic Projects | SMPS Projects | Christmas Projects | MOSFETs | Radio Circuits | Laser Circuits | PIR Projects |

Recent Comments

  • Swagatam on Simple Delay Timer Circuits Explained
  • Swagatam on The Role of Inductor Coil in SMPS
  • Swagatam on 7 Modified Sine Wave Inverter Circuits Explored – 100W to 3kVA
  • Swagatam on 7 Modified Sine Wave Inverter Circuits Explored – 100W to 3kVA
  • Victor on 7 Modified Sine Wave Inverter Circuits Explored – 100W to 3kVA

Company

  • Privacy Policy
  • Cookie Policy
  • About Me
  • Contact
  • Disclaimer
  • Copyright
  • Videos
  • Sitemap

Social Profiles

  • Twitter
  • YouTube
  • Instagram
  • Pinterest
  • My Facebook-Page
  • Quora
  • Stack Exchange
  • Linkedin
  • © 2025 · Swagatam Innovations