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Automatic Torque Optimizer in Induction Motors

In this article we discuss a circuit design which will help to
optimize the torque of an induction motor by analyzing its current

The idea was requested by Mr. Mike:

The Request

Yes that’s right Swagatam!
Can you help me in providing that
automatic torque control/PWM control circuit? which will make sure that
the motor speed is never affected regardless of the load
The Circuit Design

The design is specifically intended for electric vehicles
which are designed to work with induction motors, and therefore here an
inverter is included for operating the induction motor from a battery.

The proposed automatic torque optimizer circuit for induction motor
can be witnessed in the above diagram. Since it’s designed for an
electric vehicle, an inverter circuit is included, and built using an IC
The IC 555 along with the associated mosfets and transformer forms a decent inverter circuit
for driving the specified single phase induction motor from a 12V or a
24V battery. For a 24V battery the IC section will need to be stepped
down to 12V through a suitable voltage regulator stage.
Coming back to the actual design, here we need to make sure that the
induction motor connected with the transformer initiates with a lower
speed and begins gaining momentum, speed and torque as it gets loaded.
Basically to implement this, a PWM becomes the best technique and in this design too we take the advantage of the IC 555’s built in PWM optimization feature.
As we all know that the pin#5 of the IC 555 forms the control voltage
input of the IC, which responds to a varying voltage to adjust the
pulse width level at its pin#3, meaning for higher potential levels at
pin#5, the pulse width at pin#3 gets wider and for lower potentials at
pin#5, the pulse width at pin#3 gets narrower.
In order to translate the load specification into a varying voltage
at pin#5, we need a circuit stage capable of converting the rising load
on the induction motor into a proportionately rising potential
difference at pin#5 of the IC 555
This is done by introducing a current sensing resistor Rx, which transforms the rising current drawn by the load into a proportionately rising potential difference across itself.
This potential difference is sensed by the BC547 and transferred the
data to the connected LED, which is actually the LED inside an LED/LDR opto coupler made at home manually.
As the LED brightness increases in response to a rising current
consumption by an attached load, the LDR resistance proportionately goes
The LDR can be seen forming a part of the potential divider network
across an Opamp’s non-inverting input, therefore when the LDR resistance
falls, the potential at pin#3 of the opamp rises, which in turn causes a
correspondingly rising voltage at the output of the opamp.
This happens because the opamp is configured as a voltage follower
circuit meaning the voltage data at its pin#3 will be exactly replicated
at its output pin#6 and in an amplified manner.
This correspondingly rising voltage at pin#6 of the opamp in response
to the rising load on the induction motor feeds a rising potential at
pin#5 of the IC555. This in turn causes the initial narrower PWM at
pin#3 of IC 555 to become wider. When this happens, the inverter mosfets
begin conducting more current to the transformer enabling
proportionately higher power to the induction motor, and the process
enables the load to operate with more power and with optimal
Conversely as soon as the load is reduced, the current through Rx is
also reduced which lowers the LED brightness, and the opamps output
potential drops correspondingly, which finally causes the IC 555 to
narrow down its PWM for the mosfets and reduce the power input to the
The above explained torque optimizer circuit for induction motors is
intended for electric vehicles, however if you are interested to operate
an ordinary high power DC motor such as a tread mill motor,
in that case the transformer section could be simply eliminated, and
the motor could be directly connected as indicated in the following

I am sure you would be having many concerned questions, so please do
not hesitate to put them forth through your valuable comments. All your
related queries will be answered at an earliest

One thought on “Automatic Torque Optimizer in Induction Motors

  1. Thanks Uday, yes the principle could be implemented for 3 phase motors also but the design could get much complex and elaborate than the above shown design

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