The post discusses a single phase variable frequency drive circuit or a VFD circuit for controlling AC motor sped without affecting their operational specifications.
What is a VFD
Motors and other similar inductive loads specifically do not "like" operating with frequencies that might be not within their manufacturing specs, and tend to become a lot inefficient if forced to under such abnormal conditions.
For example a motor specified for operating with 60Hz may not be recommended to work with frequencies of 50 Hz or other ranges.
Doing so can produce undesirable results such as heating up of the motor, lower or higher than the required speeds and abnormally high consumption making things very inefficient and lower life degradation of the connected device.
However operating motors under different input frequency conditions often becomes a compulsion and under such situations a VFD or a variable frequency Drive circuit can become very handy.
A VFD is a device which allows the user to control the speed of an AC motor by adjusting the frequency and voltage of the input supply as per the motor specifications.
This also means that a VFD allows us to operate any AC motor through any available grid AC supply regardless of its voltage and frequency specs, by suitably customizing the VFD frequency and voltage as per the motor specifications.
This is normally done using the given control in the form of a variable knob scaled with different frequency calibration.
Making a VFD at home may sound to be a difficult proposition, however a look at the design suggested below shows that after all it's not so difficult to build this very useful device (designed by me).
Circuit Operation
The circuit can be fundamentally divided into two stages: The half brige driver stage and the PWM logic generator stage.
The half bridge driver stage uses the half bridge driver IC IR2110 which single handedly takes care of the high voltage motor drive stage incorporating two high side and low side mosfets respectively.
The driver IC thus forms the heart of the circuit yet require just a few components for implementing this crucial function.
The above IC however would need a high logic and a low logic in frequencies for driving the connected load at the desired specific frequency.
These hi and lo input logic signals become the operating data for the driver IC and must include signals for determine the specified frequency as well as PWMs in phase with the mains AC.
The above info are created by another stage comprising a couple of 555 ICs and a decade counter. IC 4017.
The two 555 ICs are responsible for generating the modified sine wave PWMs corresponding to the full wave AC sample derived from a stepped down bridge rectifier output.
The IC4017 functions as a totem pole output logic generator whose alternating frequency rate becomes the MAIN frequency determine parameter of the circuit.
This determining frequency is plucked from pin#3 of IC1which also feeds the IC2 triggering pin out and for creating the modified PWMs at pin#3 of IC2.
The modified sine wave PWMs are scanned at the outputs of the 4017 IC before feeding the IR2110 in order to superimpose exact "print" of the modified PWMs at the output of the half bridge driver and ultimately for the motor which is being operated.
Cx and the 180k pot values should be appropriately selected or adjusted in order to provide the correct specified frequency for the motor.
The high voltage at the drain of the high side mosfet must also be calculated appropriately and derived by rectifying the available mains voltage AC after suitably stepping it up or stepping it down as per the motor specs.
The above settings will determine the correct volts per Hertz (V/Hz) for the particular motor.
The supply voltage for both the stages can be made into a common line, same for the ground connection.
TR1 is a stepped down 0-12V/100mA transformer which provides the circuits with the required operating supply voltages.
The PWM Controller Circuit
You will have to integrate the outputs from the IC 4017 from the above diagram to the HIN and LIN inputs of the following diagram, appropriately. Also, connect the indicated 1N4148 diodes in the above diagram with the low side MOSFET gates as shown in the below diagram.
The Full Bridge Motor Driver
Update:
The above discussed simple single VFD design can be further simplified and improved by using a self oscillatory full bridge IC IRS2453, as shown below:
Here the IC 4017 is completely eliminated since the ful bridge driver is equipped with its own oscillator stage, and therefore no external triggering is required for this IC.
Being a full bridge design the output control to the motor has a full range of zero to maximum speed adjustment.
The pot at pin#5 of IC 2 can be used for controlling the speed and the torque of the motor through PWM method.
For V/Hz speed control the Rt/Ct associated with the IRS2453 and R1 associated with IC1 can be respectively tweaked (manually) for getting appropriate results.
Simplifying Even More
If you find the full bridge section overwhelming, you can replace it with a P, N-MOSFET based full bridge circuit as shown below. This variable frequency driver uses the same concept except the full-bridge driver section which employs P-channel MOSFETs at the high side and N-channel MOSFETS on the low side.
Although the configuration may look inefficient due to the involvement of P-channel MOSFETs (due to their high RDSon rating), the use of many parallel P-MOSFETs might look like an effective approach for solving the low RDSon issue.
Here 3 MOSFETs are used in parallel for the P-channel devices to ensure minimized heating of the devices, on par with the N-channel counterparts.
how are VFD’s applied to BLDC motors and what are the limits of “overspeed” based on frequency limits?
Thanks
Doug
Hi, Can this very useful circuit be used to drive a syncronous 220-240VAC motor?
I think it can be used to drive the specified motor.
Hi, I have couple of questions…
I would like to control a 1/2 HP single phase pump. The pump uses a permanent capacitor. Could I use a two phase VFD drive eliminating the need for the capacitor?
I would need to know the phase shift that the capacitor introduces.
I would also like to use a solar panel to drive the pump, varying the speed (freq.) and hence power according radiation…. the panel outputs 9.85 Amps @ 45.2 Volts (@ max radiation). I guess I would need a step up transformer to bring the voltage up to 220 V.
Hi, Sorry, I am not an expert with motors, so I am not sure whether a capacitor start motor can be controlled with a 2 phase VFD or not, or whether the capacitor can be removed or not.
If you want to use a solar panel, then certainly you will need to use an inverter to convert the solar panel DC into a 220 AC for the motor
hello, the circuits are tested? I did a simulation of the last schematic and it doesn’t work, included with the optocouplers to drive the mosfets
Hello, you must do the simulation in a step wise manner, first try simulating the 555 section, then the 4017 section.
The 555 and the 4017 sections are perfectly tested OK, but the mosfet section has problems, which might need some corrections. I have added the necessary warning message with the diagram.
can updated circuit with irs2453 be directly used for 250-300 v. or it need to modify as stated in comments using 4N25 optocoupler
If you are using IRS2453 IC with 4 N channel mosfets then you can easily use 300 v directly with the mosfets, no optocouplers will be required.
Hi Swagatam,
thank you very much for this interesting article!
I’d like to build a VFD for a 240V universal motor of about 1 kW; it might peak at 40A during startup and at low speeds/high loads and I was not able to find suitable mosfets, since I’m also concerned that back emf can reach more than 600V.
Do you have any suggestion for suitable mosfets P/N (or mosfets configuration), and do you believe I should use some snubber network or the like?
I’d like very much to hear any suggestion from you, thanks!
Thank you LCU,
For the P channel you can try FQP4P40 mosfets and IRF740 for the N channel. However, there seems to be a mistake in the last diagram.
The source voltage for the P channel mosfets is 310V, and the gate voltage is 12V which cannot work. For switching OFF the P channel effectively, its gate voltage must be same as its source voltage.
Therefore we may have to modify the H bridge section in the last diagram through opto couplers for effective switching of the P channel mosfet as shown in the following example design:
https://www.homemade-circuits.com/wp-content/uploads/2021/02/4047-solar-transformerless-inverter-compressed.jpg
FQP4P40 has a max Vgs of 30V or -30V. When the optocoupler shorts the gate to ground you will have around 0V at the gate and 310V at the source. Would this not make Vgs -310V and destroy the mosfet?
You are right! I think this can be rectified by adding a 15V zener diode across the gate/source pins of the P channel mosfets.
whats the function of bc 557 in the last circuit ? can it or anything else be removed to simplify even more it ?
BC557 allows the IC2 555 to work with a constant current which ensures the PWMs are always consistent regardless of Dc supply variations
Dear Sir,
I wish to make your last drawing…..using 8 mosfet…it is simple from other…So pl give mosfets number…8 nos..
Hello Anilkumar, the upper mosfets can be any 400 V P-channel mosfets, and the low side mosfets can be any 400 V mosfets.
Sir,
I wants single phase 230v 50 hz 2800rpm circuit…is it possible for 2800rpm pump…
Yes that’s possible by adjusting the output frequency to 50 Hz.
Sir,
I will try this method Surely…..
Sure Anil, you can try it.
in the last circuit you changed again to the 4017 ic or is it worng ?
Nothing wrong, it is correct.
is it 10-500hz in 50hz line right ? in 60hz will be 12-600hz ? can’t it go lower like 3-6hz ? what modification it would need for that ?
Frequency can be adjusted by adjusting the R1 pot in the last circuit. It can be any value between 10 and 500 Hz.
and can it be lower than 10 ?
yes it can be lower than 10 Hz
can i reduce this pot to have less range like using 50k or 10k pot wil give what range ?
what pot can give 0-100hz ?
You can try a 100k pot
Hello swagatam
my fridge is 200watts . But because of high surge / starting current the inverter trips. What is the best solution to run the fridge? super capacitor , car battery , change to invreter compressor etc?
Hello JK, The only feasible way is to either to adjust the trip setting to higher level, r to replace the inverter with a higher power inverter. If existing inverter power can be upgraded by adding more number of external batteries then that is also a good option. Super capacitors might not be capable of handling this issue unless many of them are used.
hi sir, i have a problem similar to this project would you help me to answer my question? i want to make a vfd by using igbt instead of mosfet. in all full bridge circuits with mosfets we have both n & p channel mosfets but on internet there are only n-channel igbt s. for making a full bridge with igbt should i use only four n-channel igbt ? ( do igbts have p-channel or they are only in n-channel?).many thanks
Shapur, it depends on the driver IC. If the driver IC is built for driving only n channel deice then p channel cannot be used, and vice versa.
Hello Swagatam,
is this circuit able to run a single phase induction motor capacitor start-capacitor run type?
If not, is there a possibility to make this kind of speed control for this motor type?
Regards.
Hello Bursac, yes according to me the output can be used for operating a capacitor-start type motor, provided the output AC is filtered with some large non-polar capacitors.
I”m a student of University college
I have a project in a single phase variable frequency driver
what is the most accurate circuit diagram here .
The last circuit is the easiest one, but it will need to be built and verified stage-wise and is recommended only for experts who are well versed with electronic circuits, and exactly know how to proceed with the steps, with the help of an oscilloscope.
Also, please note that the various stages involved in the diagram are tested individually for other applications, but the whole unit as one has not been tested yet by me.
Your circuit are very interesting.
I would like to vary the speed of 1.5 HP induction motor.
Is it possible to have a complete circuit diagram including all the components value.
Thank you, I have updated the parts in the last diagram clearly. All the resistors are 1/4 watt, and all the non-polar capacitors are ceramic type.
MOSFETs will be as per the input voltage rating
Hello Sir,
Is this possible to start single phase induction motor without start capacitor using above circuit?
Regards
Manoj
Hello Manoj, I don’t think that’s possible with any circuit
i need full circuit diagram of Single Phase Variable Frequency Drive VFD Circuit
thanks
I am Dip. Elec. Engr.
I want to run variable frequency device for my sewing machine.
I want to use IGBT
which circuit I should use?
You can use the last circuit!