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DC Motor Speed Controller Circuit Explained

Updated on May 9, 2013

About the Circuit

Simple DC operated motors with a permanent magnetic stator behave as an independently energized motor.

The speed of an ideal motor with an infinitely low internal resistance is in direct proportion to the voltage applied, irrespective of the torque.

The motor thus runs at a speed at which its reverse electromotive force (e.m.f) equals the supply voltage.

The reverse e.m.f. is directly proportional to the force of the (constant) magnetic field, and the motor speed. In theory, therefore, the motor speed can be held constant with a constant supply voltage.

The speed reduction observed in practice arises from the voltage drop across the internal resistance, Ri, of the armature winding. Thus, when the motor is loaded, its current consumption, and hence Vm, increases, reducing the effective supply voltage.

This effect can be eliminated by l means of Rf compensation, which essentially entails measuring the motor’s current consumption, relating this to the motor’s instantaneous drop A across Rl, and increasing the I supply voltage accordingly. In fact, this calls for a voltage source with negative output impedance, since it caters for a higher output voltage when the load is increased.

The basic set-up of the supply required here is shown in diagram. The load current is measured as the drop across sensing resistor R3. The DC transfer function of this amplifier is written as U2:U1+ILR2R3/R1 which accounts for the negative output impedance because then Rout = -RzRs/ R1 For optimum results, this impedance must be kept about equal to that of the motor. Figure shows the practical circuit of the motor driver based on a power operational amplifier. The Type Ll65 from SGS can supply up to 3 A at a maximum supply voltage of 36lL and is therefore eminently suitable for the present application. Capacitors C1 and Cz suppress noise on the reverse e.m.f. from the motor.

Due care should be taken, however, in so extending the circuit, because this readily leads to instability.

The motor itself already forms a fairly complex load, since the revolving rotor winding is mainly inductive, and the rotor itself represents a fairly large capacitance. Noise suppression components such as R4 and Ca add to the complexity of the load and may result in control instability which becomes, manifest in the motor’s tendency to alternately reverse its direction at a relatively low rate.

Also, the response to a fast change in the torque may be impaired and high frequency oscillation may occur (notice- able as excessive heating of IC1 and or R4). When the circuit was tested with a small PCB drill, best results were obtained by omitting R4-C3 and including Cz. If the motor has a noise suppression network, Cz must be omitted, and Rs added to protect the opamp inputs against too high differential voltages as a result of commutation voltage peaks. Clearly, D1 and D2 have been included with this in mind.

Preset P1 is adjusted until the motor remains stable. Over- compensation of the motor will give rise to apparently uncontrolled movement. The adjustment of P1 should be carried out when the motor has not yet reached its normal operating i temperature, because its self- heating gives rise to an increase in the internal resistance. The use of a symmetrical supply (1-18V max.) enables two quadrant operation of the motor (cw/ccw rotation), which can then be used to power model trains and the like.

The motor is halted when P2 is set to the centre position. The ground rail may be connected to the negative supply rail if only one l direction of revolution is required (PCB drills).

The maximum supply is then 36 YL making a greater voltage available for the motor, so that 24 V types can be controlled also although it is not possible to completely halt these.

The motor can be protected against overloading by selecting a supply voltage that causes the opamp to clip when it outputs the maximum motor current.

Finally, IC1 is capable of supplying considerable current, and must; therefore, be Ht- ted with a fairly large heatsink. The quiescent current of the circuit is about 50 mA.

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