Basic Motor Theory (14)

Motor Characteristics

Motor Operation

As previously stated, a conductor moving through a magnetic field due to the motor action also generates a voltage which is in opposition to the applied voltage. This is the back EMF. Then for motor action the voltage equation is:

V = E + IA RA = K1 Flux S + IA RA

where:
V = applied or terminal voltage
E = back EMF
IA = armature current
RA = armature circuit resistance’s
K1 = machine constants
Flux = flux per pole
S = speed

When comparing this equation with the voltage equation of a generator, it can be seen that in a generator the generated voltage is higher than the terminal voltage while in a motor the opposite is true. Therefore, as long as the generated voltage is less than the terminal voltage, a machine operates as a motor and takes power from the electrical side, but when the generated voltage becomes greater than the terminal voltage, the machine becomes a generator, supplies electric power, and requires mechanical energy to keep operating.
The back or counter EMF acts as a control for the amount of current needed for each mechanical load. When the mechanical load is increased, the first effect is a reduction in speed. But a reduction in speed also causes a reduction in back EMF, thus making available an increased voltage for current flow in the armature. Therefore, the current increases which in turn increases the torque. Because of this action, a very slight decrease in speed is sufficient to meet the increased torque demand. Also, the input power is regulated to the amount required for supplying the motor losses and output.
Speed Torque Curves
Speed torque curves for the three forms of excitation are shown in Figure 25. In a shunt excited motor, the change in speed is slight and, therefore, it is considered a constant speed motor. Also, the field flux is nearly constant in a shunt motor and the torque varies almost directly with armature current.
In a series motor the drop in speed with increased torque is much greater. This is due to the fact that the field flux increases with increased current, thus tending to prevent the reduction in back EMF that is being caused by the reduction in speed. The field flux varies in a series motor and the torque varies as the square of the armature current until saturation is reached. Upon reaching saturation, the curve tends to approach the straight line trend of the shunt motor. The no load speed of a series motor is usually too high for safety and, therefore, it should never be operated without sufficient load.
A compound motor has a speed torque characteristic which lies between a shunt and series motor.
Speed Regulation
Speed regulation is the change in speed with the change in load torque, other conditions being constant. A motor has good regulation if the change between the no load speed and full load speed is small.
Percent Speed Regulation = (SNL – SFL) / SFL x 100 A shunt motor has good speed regulation while a series motor has poor speed regulation. For some applications such as cranes or hoists, the series motor has an advantage since it results in the more deliberate movement of heavier loads. Also, the slowing down of the series motor is better for heavy starting loads. However, for many applications the shunt motor is preferred.
Motor Starting
When the armature is not rotating, the back EMF is zero and the total applied voltage is available for sending current through the armature. Since the armature resistance is low, an enormous current would flow if voltage were applied under this condition. Therefore, it is necessary to insert an additional resistance in series with the armature until a satisfactory speed is reached where the back EMF will take over to limit the current input.