In this post we discuss the making of a simple 3 phase induction motor speed controller circuit, which can be also applied for a single phase induction motor or literally for any type of AC motor.
When it comes to controlling the speed of induction motors, normally matrix converters are employed, involving many complex stages such as LC filters, bi-directional arrays of switches (using IGBTs) etc.
All these for ultimately achieving a chopped AC signal whose duty cycle could be adjusted using a complex microcontroller circuit, finally providing the required motor speed control.
However according to me the same can be accomplished through a much simpler concept using the advanced zero crossing detector opto coupler ICs, a power triac and a PWM circuit.
Thanks to the MOC series of optocouplers which has made triac control circuits extremely safe and easy to configure, and allow a hassle free PWM integration for the intended controls.
In one of my earlier posts I discussed a simple PWM soft start motor controller circuit which implemented the MOC3063 IC for providing an effective soft start on the connected motor.
Here too we use an identical method for enforcing the proposed 3 phase induction motor speed controller circuit, the following image shows how this can be done:
In the figure we can see three identical MOC opto coupler stages configured in their standard triac regulator mode, and the input side integrated with a simple IC 555 PWM circuit.
The 3 MOC circuits are configured for handling the 3 phase AC input and delivering the same to the attached induction motor.
The PWM input at the isolated LED control side of the opto determines the chopping ratio of the 3 phase AC input which is being processed by the MOC ICS.
That implies, by adjusting the PWM pot associated with the 555 IC one can effectively control the speed of the induction motor.
Output at its pin#3 comes with a varying duty cycle which in turn switches the output triacs accordingly, resulting in either increasing the AC RMS value or decreasing the same.
Increasing the RMS through wider PWMs enables acquiring a higher speed on the motor, while decreasing the AC RMS through narrower PWMs produces an opposite effect, that is it causes the motor to proportionately slow down.
The above features are implemented with a lot of precision and safety since the ICs are assigned with many internal sophisticated features, specifically intended for driving triacs and heavy inductive loads such as inductions motors, solenoids, valves, contactors, solid state relays etc.
The IC also ensures a perfectly isolated operation for the DC stage which allows the user to make the adjustments without the fear of an electric shock.
The principle can be also efficiently used for controlling single phase motor speed, by employing a single MOC IC instead of 3.
The design is actually based on time proportional triac drive theory. The existing upper IC555 PWM circuit may be adjusted to produce a 50% duty cycle at much higher frequency while the lower new PWM circuit may be used for controlling the speed of the induction through the associated pot. This 555 IC is recommended to have relatively lower frequency than the upper IC 555 circuit. This may be done by increasing the pin#6/2 capacitor to around 100nF.
Using Matrix Converter
A 3 phase induction motor can be also controlled through a matrix converter circuit as shown below.
Possibly, the above concept using MOC IC could be modified as shown below using additional bi-directional IGBT switches, user discretion is advised before trying this out:
Using a Full Bridge IGBT control Circuit
If the setting up procedures of the above triac based design look daunting to you, the following full-bridge PWM based induction motor speed control could be tried:
The circuit shown in the above figure utilizes a single chip full-bridge driver IC IRS2330 (latest version is 6EDL04I06NT) which has all the features in-built in order to satisfy a safe and a perfect 3 phase motor operation.
The IC only needs a synchronized 3 phase logic input across its HIN/LIN pinouts for generating the required 3 phase oscillating output, which finally is used for operating the full bridge IGBT network and the connected 3 phase motor.
The PWM injection is implemented through 3 separate half bridge NPN/PNP drivers stages, controlled with a PWM feed from an IC 555 PWM generator as seen in our previous designs. This PWM level may be ultimately used for controlling the speed of the induction motor.
The following image shows the 3 phase processor using a single IC 4049, which is applied with 3 phase inputs from the mains supply, and this is appropriately dimensioned aross the N1---N3 NOT gates for feeding the HIN/LIN inputs of the IC IRS2330
If you have any further doubts regarding this 3 phase induction motor speed controller design, you are most welcome to post the same through your comments.