Single Phase Variable Frequency Drive VFD Circuit

In this post I have explained 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 to Calculate Single Phase VFD Parameters

Input Power Calculations:

Power Factor (PF):
This is all about measuring how efficiently we are using AC power. It is extremely important for making sure we size the Variable Frequency Drive (VFD) correctly.

Apparent Power (S):
This represents the total power that’s supplied to the VFD, and it includes both real power and reactive power.
Formula: S = V * I

Real Power (P):
This is the actual power that the motor uses to do its work.
Formula: P = V * I * PF

Reactive Power (Q):
This is the power that bounces back and forth between the source and the load without doing any useful work.
Formula: Q = √(S² - P²)

Output Power Calculations:

Motor Power (P_motor):
This refers to the mechanical power that comes out of the motor.
Formula: P_motor = (Torque * Speed) / 9550 (for horsepower).

The constant 9550 is an important number we use in the formula that connects power, torque, and rotational speed when we’re working with the SI system of units.

Formula:

Power (kW) = Torque (Nm) * Speed (rpm) / 9550

What is its Importance

So, what is so important with the constant 9550? It comes from how we convert between different units of power (in kilowatts), torque (in Newton-meters), and rotational speed (in revolutions per minute). This number helps us to tackle the differences in these units, making sure that our equation works correctly.

• Power is measured in watts (W) or kilowatts (kW).
• Torque is measured in Newton-meters (Nm).
• Rotational speed is measured in revolutions per minute (rpm).

To get to that constant 9550, we need to think about a few things:

• Converting power from kW to W.
• Changing rotational speed from rpm to radians per second.
• Using the relationship that says power equals torque times angular velocity (power = torque * angular velocity).

When we do these conversions and simplify everything, we end up with that handy constant 9550 in our formula.

Remember:

This formula is specifically designed for the SI unit system. If you happen to be using different units, for example like horsepower or foot-pounds, then you will need to use a different conversion factor to make it work.

Motor Efficiency (η):
This is a measure of how well the motor converts input power into output power.
Formula: η = P_motor / P_input

VFD Sizing:

VFD Power Rating:
The power rating of the VFD needs to be higher than the apparent power of the motor's, so that it can handle any potential overloads without an issue.

Input Current:
We can calculate the input current based on the VFD's power rating and the input voltage specifications.

Output Current:
We also need to figure out the output current based on the power rating of the motor and output voltage.

Motor Speed Control:

Motor Speed (N):
The speed of an induction motor is directly connected to the frequency of the voltage we apply.
Formula: N = (120 * f) / P (where f is frequency and P is the number of poles)