Constant Torque Motor Speed Controller Circuit

The post explains a DC Motor controller which features a constant torque compensation for enabling the motor to run at a consistent speed irrespective of the load on it.

Drawback of Ordinary Speed Controllers

One drawback of the majority of simple speed controllers is they only provide the motor with a predetermined constant voltage. As a result the speed doesn't remain constant and varies with the load on the motor, due to absence of torque compensation.

For example in a model train, with simple controllers the speed of the train gradually decreases for the climbing gradients and accelerates while heading downhill.

Hence for model trains the pot control adjustment to keep up a selected motor speed likewise deviates depending on the load that the engine may be tugging.

The constant torque motor speed controller circuit explained in this article gets rid of this issue by tracking the motor speed and maintaining it constant for a predetermined control setting, no matter what the load may be on the motor.

The circuit can be applied in most of the models which uses a DC permanent magnet motor.

Calculating the Back EMF Factor

The voltage across the motor terminals comprises of a couple of factors, the back e.m.f. produced by the motor, and the voltage dropped across the armature resistance.

The back e.m.f. generated by the motor winding is normally proportional to the motor speed, which means that the motor speed could be monitored by measuring this back emf content. But, the main issue is to isolate and detect the back e.m.f. from the armature resistance voltage.

Supposing a separate resistor is attached in series with the motor then, considering that a common single current passes through this resistor and also through the armature resistance, the voltage drop across the two series resistors could well be equivalent to the drop across the armature resistance.

Actually, it can be assumed that when these two resistance values are identical then the two voltage magnitudes across each of the resistors will also be similar. With this data, it may be possible to deduct the voltage drop of R3 from the motor voltage, and get the required back e.m.f value for the processing.

Processing Back EMF for Constant Torque

The proposed circuit continuously monitors the back e.m.f. and accordingly regulates the motor current to ensure that, for an assigned pot control setting, the back e.m.f., along with the motor speed is maintained at a constant torque.

To be able to make the circuit description easier it is deemed that P2 is adjusted and held to its center position, and the resistor R3 is selected as an equivalent to the resistance value of the motor armature.

Calculating Motor Voltage

The motor voltage can be calculated by adding the back e.m.f. Va with the voltage dropped across the motor internal resistance Vr.

Considering that R3 drops a voltage Vr, the output voltage Vo will be equal to Va + 2 V.

The voltage at the inverting input (-) of IC1 will be Va + Vr, and that at non-inverting input (+) will be Vi + (Va + 2Vr - Vi) / 2

Since the above two voltage magnitudes are supposed to be equal, we organize the above equation as:

Va + Vr = Vi + (Va + 2Vr - Vi) / 2

Simplifying this equation provides Va = Vi.

The above equation indicates that the back e.m.f. of the motor is consistently held at the same level as the control voltage. This allows the motor to work with a constant speed and torque for any specified setting of the P1 speed adjustment.

P2 is included to compensate the difference level that may exist between the R3 resistance and the armature resistance. It executes this by adjusting the magnitude of positive feedback on the non-inverting input op amp.

The op amp LM3140 basically compares the voltage developed across the motor armature with the back emf equivalent across the motor and regulates the base potential of the T1 2N3055.

T1 being configured as an emitter follower regulates the speed of the motor in accordance with its base potential. It increases the voltage across the motor when a higher back emf is detected by the op amp, resulting in an increase in the motor speed, and vice versa.

T1 should be mounted over a suitable heatsink for proper functioning.

How to Set Up the Circuit

Setting up of the constant torque motor speed controller circuit is done by adjusting P2 with the motor with varying load until the motor achieves a constant torque regardless of the loading conditions.

When the circuit is applied for model trains, care must be taken not to turn P2 too much towards P1 which might result in the model train slowing down, and conversely P2 must not be turned too much in the opposite direction, which might result in the train speed actually getting faster while climbing an uphill gradient.