Field Excitation - h1011v4

Starting a Synchronous Motor

AC Motor Types Summary

Electrical Science Volume 4 of 4
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AC MOTOR TYPES AC Motors Field Excitation For a constant load, the power factor of a synchronous motor can be varied from a leading value to a lagging value by adjusting the DC field excitation (Figure 9). Field excitation can be adjusted so that PF = 1 (Figure 9a). With a constant load on the motor, when the field excitation is increased, the counter EMF (V_G) increases. The result is a change in phase between stator current (I) and terminal voltage (V_t), so that the motor operates at a leading power factor (Figure 9b). V_p in Figure 9 is the voltage drop in the stator winding’s due to the impedance of the windings and is 90^o out of phase with the stator current. If we reduce field excitation, the motor will operate at a lagging power factor (Figure 9c). Note that torque angle, a, also varies as field excitation is adjusted to change power factor. Figure 9 Synchronous Motor Field Excitation Synchronous motors are used to accommodate large loads and to improve the power factor of transformers in large industrial complexes. ES-12 Page 14 Rev. 0

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