Starting
The interaction of the main field produced by the rotor and the armature current of the stator will produce a net average torque to drive the synchronous motor only when the rotor is revolving at speed n in synchronism with the line frequency f; n=120 f/p, p=poles. The motor must be started by developing other than synchronous torques. Practically, the motor is equipped with an induction-motor-type squirrelcage winding on the rotor, in the form of a damper winding, in order to start the motor. The motor is started on the damper windings with the field winding short-circuited, or terminated in a resistor, to attenuate the high “transformer”-induced voltages. When the motor reaches the lowest slip speed, nearly synchronous speed, the field current is applied to the field winding, and the rotor poles accelerate and pull into step with the synchronously rotating air-gap magnetic field. The damper windings see zero slip and carry no further current, unless the rotor oscillates with respect to the synchronous speed. Starting curves for a synchronous motor are shown in Fig. 5.3.
The damper winding is designed for high starting torque, as compared to an induction motor of the same rating. The closed field winding contributes to the starting torque in the manner of a three-phase induction motor with a one-phase rotor. The field winding produces positive torque to half speed, then negative torque to full speed, accounting for the anomaly at half speed. The maximum and minimum torque excursion at the anomaly is reduced by the resistance in the closed field winding circuit during starting. The effect is increased by the design of the damper
winding. The velocity of the rotor during the synchronizing phase, after field current is applied, is shown in Fig. 5.4. The rotor is assumed running at 0.05 pu slip on the damper winding. The undulation in speed, curve 1, is the effect of the poles attempting to synchronize the rotor just by reluctance torque. The added effect of the field current is shown by curve 2, and the resultant by curve 3. The effect of the reluctance torque of
curve 1 is not dependent on pole polarity. The synchronizing torque of curve 2, with the field current applied, is pole polarity dependent; the poles want to match the air-gap field in the forward torque direction. Curve a shows a successful synchronization. Curve b shows the condition of too much load or inertia to synchronize. The method used to start a synchronous motor depends upon two factors: the required torque to start the load and the maximum starting current permitted from the line. Basically, the motor is started by using the damper windings to develop asynchronous torque or by using an auxiliary motor to bring the unloaded motor up to synchronous speed.Solid-state converters have also been used to bring up to speed large several-hundred-MVA synchronous otor/generators for pumped storage plants. Techniques for asynchronous starting on the damper windings are the same as for squirrel-cage induction motors of equivalent rating. Across-the-line starting provides the maximum starting torque, but requires the maximum line current. The blocked-rotor kVA of synchronous motors as a function of pole number is shown in Fig. 5.5. If the ac line to the motor supplies other loads, the short-circuit kVA of the line must be at least 6 to 10 times the blocked rotor kVA of the motor to limit the line-voltage dip on starting. The starting and pulling torques for three general classes of synchronous motors are shown in Fig. 5.6. The torques is shown for rated voltage; for across-the-line starting, the values will be reduced to (pu). Reduced-voltage starting is used where the full starting torque of the motor is not required and/or the ac line cannot tolerate the full starting current. The starter includes a three-phase open-delta or three-winding autotransformer, which can be set to apply 50, 65, or 80% of line voltage to the motor on the first step. The corresponding torque is reduced to
25, 42, or 64%. The starter switches the motor to full voltage when it has
reached nearly synchronous speed, and then applies the field excitation to synchronize the motor. ANSI C50.11 limits the number of starts for a synchronous motor, under its design conditions of Wk2, load torque, nominal voltage, and starting method, to the following:
1. Two starts in succession, coasting to rest between starts, with the
motor initially at ambient temperature, or
2. One start with the motor initially at a temperature not exceeding its
rated load operating temperature. If additional starts are required, it is recommended that none be made until all conditions affecting operation have been thoroughly investigated and the apparatus examined for evidence of excessive
heating. It should be recognized that the number of starts should be
kept to a minimum since the life of the motor is affected by the number
of starts
No comments:
Post a Comment