Motor Operation

The subject of this book is turbogenerators. These units seldom operate as a motor. (One such example is when the main generator is used for a short period of time as a motor fed from a variable speed converter. The purpose of this operation is for starting its own prime-mover combustion turbine). However, this section presents an introductory discussion of the synchronous machine, and thus the motor mode of operation is also covered. If a breaking torque is applied to the shaft, the rotor starts falling behind the revolving-armature-induced magnetomotive force (mmf) (F_s). In order to maintain the required magnetizing mmf (F_r) the armature current changes. If the machine is in the underexcited mode, the condition motor in Figure 1.24a represents the new phasor diagram.

On the other hand, if the machine is overexcited, the new phasor diagram is represented by motor in Figure 1.24b. The active power consumed from the network under these conditions is given by

Active power = V₁ × I₁ × cos ϕ₁ (per phase)

If the breaking torque is increased, a limit is reached in which the rotor cannot keep up with the revolving field. The machine then stalls. This is known as “falling out of step,” “pulling out of step,” or “slipping poles.” The maximum torque limit is reached when the angle δ equals π/2 electrical. The convention is to define δ as negative for motor operation and positive for generator operation. The torque is also a function of the magnitude of φ_r and φ_f. When overexcited, the value of φ_f is larger than in the underexcited condition. Therefore synchronous motors are capable of greater mechanical output when overexcited. Likewise, underexcited operation is more prone to result in an “out-of-step” situation.