Synchronous Motor PART-1

A synchronous motor is a machine that transforms electric power into mechanical power. The average speed of normal operation is exactly proportional to the frequency of the system to which it is connected.

Unless otherwise stated, it is generally understood that a synchronous motor has field poles excited with direct current
.

Types

The synchronous motor is built with one set of ac polyphase distributed windings, designated the armature, which is usually on the stator and is connected to the ac supply system. The configuration of the opposite member, usually the rotor, determines the type of synchronous motor. Motors with dc excited field windings on silent-pole or round rotors, rated 200 to 100,000 hp and larger, are the dominant industrial type. In the brushless synchronous motor, the excitation (field current) is through shaft-mounted rectifiers from an ac exciter. In the slip-ring synchronous motor, the excitation is supplied from a shaft-mounted exciter or a separate dc power supply. Synchronous-induction motors rated below 5 hp, usually supplied from adjustable-speed drive inverters, are designed with a different reluctance across the air gap in the direct and quadrature axis to develop reluctance torque. The motors have no excitation source for synchronous operation. Synchronous motors employing a permanent-magnetic field excitation and driven by a transistor inverter from a dc source are termed brushless dc motors.

Theory of Operation


The operation of the dc separately excited synchronous motor can be explained in terms of the air-gap magnetic-field model, the circuit model, or the phasor diagram model of Fig. 5.1. In the magnetic-field model of Fig. 5.1a, the stator windings are assumed to be connected to a polyphase source, so that the winding currents produce a rotating wave of current density Ja and radial armature reaction field Ba. The rotor carrying the main field poles is rotating in synchronism with these waves. The excited field poles produce a rotating wave of field Bd. The net magnetic field Bt is the spatial sum of Ba and Bd; it induces an air-gap voltage Vag in the stator windings, nearly equal to the source voltage Vt. The current-density
Distribution Ja is shown for the current Ia in phase with the voltage Vt , and (in this case) pf=1. The electromagnetic torque acting between the rotor and the stator is produced by the interaction of the main field Bd and the stator current density Ja, as a J×B force on each unit volume of stator conductor. The force on the conductors is to the left (_ф) the reaction force on the rotor is to the right (+ф) and in the direction of rotation.




The operation of the synchronous motor can be represented by the circuit model of Fig. 5.1b. The motor is characterized by its synchronous reactance xd and the excitation voltage Ed behind xd. The model neglects saliency (poles), saturation, and armature resistance, and is suitable for first-order analysis, but not for calculation of specific operating points, losses, field current, and starting.

The phasor diagram of 5.1c is drawn for the field model and circuit model previously described. The phasor diagram neglects saliency and armature resistance. The phasors correspond to the waves in the field model. The terminal voltage Vt is generated by the field Bt ; the excitation voltage Ed is generated by the main field Bd; the voltage drop jIaxd is generated by the armature reaction field Ba; and the current Ia is the aggregate of the current-density wave Ja. The power angle d is that between Vt and Ed, or between Bi and Bd. The excitation voltage Ed, in pu, is equal to the field current Ifd , in pu, on the air-gap line of the noload (open-circuit) saturation curve of the machine.


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;





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.

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