The Synchronous Motor
The three-phase synchronous motor consists of the following components:
• A laminated stator core with a three-phase armature winding
• A revolving field with an amortisseur winding and slip rings
• Brushes and brush holders
• Two end shields housing the bearings that support the rotor shaft
The stator core and windings of a synchronous motor are similar in construction to those of a three-phase squirrel-cage induction motor or a wound-rotor induction motor. The leads for the stator windings are marked T1, T2, and T3. These leads end in a terminal box, which normally is mounted on the side of the motor frame.
The rotor has salient field poles. The poles are connected to give alternate polarity. There must be the same number of rotor field poles as stator field poles. The field circuit leads are brought out to two slip rings mounted on the rotor shaft. A squirrel-cage (amortisseur) winding is provided as a means of starting the motor. The synchronous motor is not self-starting without this auxiliary winding.
Figure 17–1 shows a rotor having salient poles and an amortisseur winding. The amortisseur winding consists of copper bars embedded in the laminated metal structure of each pole face. The copper bars of this special squirrel-cage winding are brazed to rings mounted on each end of the rotor.
Carbon brushes, mounted in brush holders, contact the two slip rings. The terminals of the field circuit are brought out from the brush holders to a second terminal box, which is mounted on the motor frame. The two leads for the field circuit are marked F1 and F2.
The rated three-phase voltage is applied to the stator windings, resulting in a rotating magnetic field. This field travels at synchronous speed. The speed is determined by the same factors that govern the synchronous speed of induction motors. The synchronous speed is found using the following equation:
The rotating magnetic field set up by the stator windings cuts across the amortisseur (squirrel-cage) winding of the rotor. The amortisseur winding is a squirrel-cage winding very similar to the type A winding in a squirrel-cage induction motor. Due to its relatively high resistance, it provides low starting current and high starting torque per ampere of starting current. Voltages and currents are induced in the bars of this winding. A magnetic field is set up in the squirrel-cage windings. This field reacts with the stator field and causes rotation of the rotor. The speed of the rotor increases until it is just below the synchro- nous speed of the stator field. In other words, there is a slight slip of the rotor behind the magnetic field set up by the stator windings. When the synchronous motor is started as an induction motor with the amortisseur windings, the rotor accelerates to about 85% to 97% of the synchronous speed.
The field circuit is excited from an outside direct-current source. Magnetic poles of fixed polarity are set up in the rotor field cores. The fixed magnetic poles of the rotor are attracted to unlike poles of the rotating magnetic field. (This field was set up by the stator windings.)
Figure 17–2 shows how the rotor field poles lock with unlike poles of the stator field. As a result, the rotor speed becomes the same as the speed of the stator field. This speed is the synchronous speed. When the rotor begins turning at synchronous speed, there is no longer any cutting action across the bars of the amortisseur winding. At this point there is no induced voltage in the amortisseur winding, and it therefore has no effect on the operation of the motor.
DC FIELD EXCITATION
The direct current for synchronous motors is obtained from a dc exciter circuit. Such a circuit may supply field excitation to several ac machines. A dc generator may be coupled directly to the synchronous motor shaft. Other installations may use electronic rectifiers to supply the dc excitation current.
The dc connections to a synchronous motor are shown in Figure 17–3. A field rheostat controls the current in the separately excited field circuit. When the field switch is open, the field discharge resistor is connected directly across the field winding.
The Brushless Exciter
Most large synchronous motors use an exciter that does not depend on brushes and slip rings. This is accomplished by adding a separate small alternator of the
armature type on the shaft of the synchronous motor. The armature rotates between wound electromagnets. The dc excitation current is connected to the wound stationary magnets (Figure 17-4). The amount of voltage induced in the armature can be controlled by varying the amount of dc current supplied to the electromagnets. The output voltage of the armature is connected to a three-phase bridge rectifier mounted on the rotor shaft (Figure 17-5). The three-phase bridge rectifier converts the three-phase alternating current produced in the armature to direct current before it is applied to the rotor of the synchronous motor. Because the three-phase armature and rectifier are contained on the rotor shaft, they all turn together and no brushes or slip rings are needed to provide excitation for the rotor of the large synchronous motor. The rotor of a synchronous motor with a brushless exciter is shown in Figure 17–6.
The dc field circuit is never energized when the synchronous motor is started because the rapidly rotating field produces an alternating torque on the stationary rotor poles. In general, a synchronous motor is started by connecting its armature winding (stator) to the ac line and its field winding (rotor) to a field discharge resistor. The motor is started as an induction motor.
At the instant of startup, the rotating stator field cuts the turns of the dc field coils many times per second. The stator field turns at the synchronous speed and induces a high voltage in the field windings. This voltage may reach 1500 V. This means that the field circuits must be well insulated and enclosed to protect personnel. The field discharge resistor is connected across the field windings so that the energy in the field circuit is spent in the resistor. This arrangement also reduces the voltage at the field terminals, although it is still high enough to be a shock hazard.
Achieving Synchronous Speed
Once the motor accelerates to nearly 95% of synchronous speed, the field circuit is energized from the dc source. The field discharge resistor is then disconnected. The rotor will pull into synchronism with the revolving armature (stator) flux. Thus, the motor will operate at a constant speed. If the load has a high inertia and is hard to start, special automatic equipment is required to apply the field. To ensure that there is a successful transition from induction motor operation to synchronous operation, the field must be applied at the best position of the rotor slip cycle.
To shut down the motor, the field circuit is deenergized by opening the field discharge switch. As the field flux collapses, a voltage is induced in the field windings. This voltage may be large enough to damage the insulation of the windings. To prevent such a high volt- age, the field discharge resistor is connected across the field circuit. As a result, the energy stored in the magnetic field is spent in the resistor and a lower voltage is induced in the field circuit.