Saturday, February 9, 2013

Control of the Active and Reactive Power of a Synchronous Motor

Control of the Active and Reactive Power of a Synchronous Motor

Control of the Active and Reactive Power of a Synchronous Motor

The active power

Pm = P = 3VIac.t = ɷrotTload

of a synchronous motor connected to a large power system (V = const) can be varied by varying the load torque applied to its shaft (T1oad = VAr). As the load torque is increased, the power of a synchronous motor also increases, leading to a large angle θ and to a reduction in the margin of stability, π/2 – θ . If a synchronous motor is not to lose its margin of stability with increasing active power, one has to increase the field current at the same time. Large­ size synchronous motors are equipped with suitable apparatus which maintains the necessary margin of stability in response to a change in the active power of the motor.

The reactive power

Q = 3VI sin φ

of a synchronous motor connected to a large power system (V = coast) at constant active power P is adjusted by varying the field current If . When It is less than some limiting value, If,lim (P), the reactive power of the motor is inductive in its effect, QL = 3VIreac,L . When It is greater than some limiting value, If > If,lim , the reactive power of the motor is capacitive in its effect, QC = 3VIreac, C .

As a rule, synchronous motors are operated at an excitation level which results in a capacitive reactive power, so that the inductive reactive power of the induction motors connected to the same system is balanced out and the system is relieved of reactive current.

A case of practical importance is the use of a synchronous motor as a synchronous condenser which is an adjustable capacitive element (see Fig 18, P = 0). Synchronous condensers serve to improve the power factor of the system (see Sec. 2.20).