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 basically as the name suggests s device which can be used for operating an inductive load with the any desired frequency which might be most suitable for it as per its company specs.
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).
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 Half-Bridge Driver Circuit
The PWM Controller Circuit
Correction in the above Finalized Design
A careful investigation reveals that the diode feeds from IC2 in the above diagram is wrongly connected with the LIN/HIN inputs of the mosfet driver IC.
For a correct implementation of the VFD operation and the intended controls, the diodes must be integrated with the gates of the respective MOSFETs as indicated in the following corrected version:
The finalized integrated design of the above circuits may be witnessed in the following diagram, as drawn by Mr. Vuleek Unteeluv
R1 is for frequency adjustment, and R8 for Voltage Adjustment, both of these controls could be used for optimizing the V/Hz ratio for a particular motor, manually.
Note: Pin#16 of IC is shown configured with a PNP transistor stage which ensures that the IC 4017 keeps the mosfets powered only as long as its pin#14 is supplied with the cock pulses. This arrangement makes sure that under no circumstance the output of IC 4017 hangs and keeps one of the mosfets permanently ON and the other OFF, however if you feel the PNP stage is not crucial you may remove it and connect pin#16 directly with the supply positive.
Important: Make sure to connect the SD pin of the half-bridge driver IC with the negative or the ground line, otherwise the IC will not work.
Simplified and Improved Design
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.
The circuit above needs a small correction. It seems the low side PWM control arrangement using BC547 transistors will invert the PWM phase synchronization. Therefore an additional BC547 stage must be employed to correct the synchronization process, as shown below: