Speed Control of a Shunt Motor by Field Adjustment

Author: E.E. Kimberly

Just as the voltage of a shunt generator may be controlled by varying the resistance of its field circuit, so may the speed of a shunt motor be controlled by the same means. Refer again to equation (11-3), and let it be assumed'that a shunt motor is running at 2000 rpm. If the field-circuit resistance be increased, the main-field flux will be decreased and E₀will be proportionately decreased. To balance the equation, Ia must increase. An increase in Ia produces an increase in torque. The increase in torque causes the speed to increase to a new value, which just requires the new torque to maintain it. Thus, an increase in the shunt-field resistance of a shunt motor causes its speed to increase.

Because of excess mechanical stresses and also because of commutation limitations, it is not advisable to operate a constant-speed motor at more than 15 per cent above its rated speed. With the field-circuit resistance at its minimum, it is usually possible to obtain a speed as low as 90 per cent of the rated speed at full load. However, the speed of many shunt motors cannot be reduced to rated value at no load, even by removing the external rheostat resistance entirely.