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Synchronous motors are so named because of their ability to operate at synchronous speed. They are able to operate at the speed of the rotating magnetic field because they are not induction motors. They exhibit other characteristics that make them different than squirrel cage or wound rotor induction motors. Some of these characteristics are:
• They can operate at synchronous speed.
• They operate at a constant speed from no load to full load. Synchronous motors will either operate at synchronous speed or they will stall and stop running.
• They can produce a leading power factor.
• They are sometimes operated without load to help correct plant power factor. In this mode of operation, they are called synchronous condensers.
• The rotor must be excited with an external source of direct current.
• They contain a special squirrel cage winding called the amortisseur winding that's used to start the motor.
Starting a Synchronous Motor
A special squirrel cage winding, called the amortisseur winding, is used to start a synchronous motor. The rotor of a synchronous motor is shown in Ill. 1.
The amortisseur winding is very similar to a type A squirrel cage winding. It provides good starting torque and a relatively low starting current. Once the synchronous motor has accelerated to a speed close to that of the rotating magnetic field, the rotor is excited by connecting it to a source of direct current. Exciting the rotor causes pole pieces wound in the rotor to become electromagnets. These electromagnets lock with the rotating magnetic field of the stator and the motor runs at synchronous speed. A synchronous motor should never be started with excitation applied to the rotor. The magnetic field of the pole pieces will be alternately attracted and repelled by the rotating magnetic field, resulting in no torque being produced in either direction.
High induced voltage, however, may damage the rotor windings and other components connected in the rotor circuit. The excitation current should be connected to the rotor only after it has accelerated to a speed that's close to synchronous speed.
There are several ways in which excitation current can be supplied to the rotor of a synchronous motor, such as slip rings, a brushless exciter, and a DC generator.
Small synchronous motors generally contain two slip rings on the rotor shaft. A set of brushes are used to supply direct current to the rotor (Ill. 2). If manual starting is employed, an operator will manually excite the rotor after it has accelerated close to synchronous speed. During this acceleration process, a high voltage can be induced into the windings of the rotor. A resistor, called the field discharge resistor, is connected in parallel with the rotor winding. Its function is to limit the amount of induced voltage when the motor is started and limit the amount of induced voltage caused by the collapsing magnetic field when the motor is stopped and the excitation current is disconnected. A switch called the field discharge switch is used to connect the excitation current to the rotor. The switch is so designed that when it's closed it will make connection to the direct current power supply before it breaks connection with the field discharge resistor. When the switch is opened, it will make connection to the field discharge resistor before it breaks connection with the direct current power supply. This permits the field discharge resistor to always be connected to the rotor when DC excitation isn't being applied to the rotor.
The Brushless Exciter
A second method of supplying excitation current to the rotor is with a brushless exciter. The brushless exciter has an advantage in that there are no brushes or slip rings to wear. The brushless exciter is basically a small three-phase alternator winding and three-phase rectifier located on the shaft of the rotor. Refer to the photo graph in Ill. 1. At the back of the rotor a small winding can be seen. This is the winding of the brush less exciter. Electromagnets are placed on either side of the winding (Ill. 3). A three-phase rectifier and fuses are also located on the rotor shaft. The rectifier converts the three-phase alternating current produced in the alternator winding into direct current before it's supplied to the rotor winding (Ill. 4). The amount of excitation current supplied to the rotor winding is controlled by the amount of direct current sup plied to the electromagnets. The output voltage of the alternator winding is controlled by the flux density of the pole pieces.
Direct Current Generator
Another method of supplying excitation current is with the use of a self-excited direct current generator mounted on the rotor shaft. The amount of excitation current is adjusted by controlling the field current of the generator. The output of the armature supplies the excitation current for the rotor. Since the generator is self-excited, it does not require an external source of direct current. Although that's an advantage over sup plying the excitation current through slip rings or with a brushless exciter, the generator does contain a commutator and brushes. The generator generally requires more maintenance than the other methods.
Automatic Starting for Synchronous Motors
Synchronous motors can be automatically started as well as manually started. One of the advantages of a synchronous motor is that it provides good starting torque with a relatively low starting current. Many large motors are capable of being started directly across the line because of this feature. If the power company won't permit across the line starting, synchronous motors can also employ autotransformer starting, reactor starting, or wye-delta starting. Regardless of the method employed to connect the stator winding to the power line, the main part of automatic control for a synchronous motor lies in connecting excitation current to the rotor at the proper time. The method employed is determined by the manner in which excitation is applied to the rotor.
In the case of manual excitation, the field discharge switch is used. Brushless exciter circuits often employ electronic devices for sensing the rotor speed in order to connect DC excitation to the rotor at the proper time. If a direct current generator is employed to provide excitation current, a special field contactor, out-of-step relay, and polarized field frequency relay are generally used.
The Field Contactor
The field contactor looks very similar to a common three pole contactor (Ill. 5). This isn't a standard contactor, however. The field contactor contains a DC coil and is energized by the excitation current of the rotor.
The field contactor serves the same function as the field discharge switch discussed previously. The two outside contacts connect and disconnect the excitation current to the rotor circuit. The middle contact connects and disconnects the field discharge resistor at the proper time.
The out-of-step relay is actually a timer that contains a current-operated coil instead of a voltage-operated coil.
The coil is connected in series with the field discharge resistor. The timer can be pneumatic, dashpot, or electronic. A dashpot type of out-of-step relay is shown in Ill. 6. The function of the out-of-step relay is to disconnect the motor from the power line in the event that the rotor isn't excited within a certain length of time. Large synchronous motors can be damaged by excessive starting current if the rotor isn't excited within a short time.
The Polarized Field Frequency Relay
The polarized field frequency relay (Ill. 7) is responsible for sensing the speed of the rotor and con trolling the operation of the field contactor. The polarized field frequency relay (PFR) is used in conjunction with a reactor. The reactor is connected in the rotor circuit of the synchronous motor. The polarized field frequency relay contains two separate coils, one DC and one AC (Ill. 8). Coil A is the DC coil and is connected to the source of direct current excitation. Its function is to polarize the magnetic core material of the relay. Coil B is the AC coil. This coil is connected in parallel with the reactor (Ill. 9). To understand the operation of the circuit, first consider the path of magnetic flux taken if only the DC coil of the PFR is energized (Ill. 10). Note that the flux path is through the cross bar, not the ends, of the relay. Since the flux does not reach the ends of the pole piece, the armature isn't attracted and the contact remains closed.
When the synchronous motor is started, however, the rotating magnetic field of the stator induces an AC voltage into the rotor winding. A current path exists through the reactor, field discharge resistor, and coil of the out-of-step relay. Since the induced voltage is 60 hertz at the instant of starting, the inductive reactance of the reactor causes a major part of the rotor cur rent to flow through the AC coil of the polarized field frequency relay. Since alternating current is flowing through the AC coil of the PFR, each half cycle the flux produced in the AC coil opposes the flux produced by the DC coil. This causes the DC flux to be diverted to the ends of the pole pieces where it's combined with the AC flux, resulting in a strong enough flux to attract the armature, opening the normally closed contact (Ill. 11).
In this type of control, a direct current generator is used to supply the excitation current for the rotor. When power is first applied to the stator winding, the rotor isn't turning and the DC generator isn't producing an output voltage. The rotating magnetic field, however, induces a high voltage into the rotor windings, supplying a large amount of current for the AC coil of the polarized field frequency relay. As the rotor begins to turn, the DC generator begins to produce a voltage, supplying power for the DC coil of the PFR. The combined flux of the two coils will cause the normally closed PFR contact to open before the field relay can energize. As the rotor speed increases, less AC voltage is induced in the rotor circuit, and the frequency decreases in proportion to rotor speed. As the frequency decreases, the inductive reactance of the reactor becomes less, causing more current to flow through the reactor and less to the AC coil. The AC coil of the PFR produces less and less flux as rotor speed increases. When the rotor reaches about 90% of the synchronous speed, the AC flux can no longer maintain the current path through the PFR armature, and the DC flux returns to the path, as shown in Ill. 10. When the armature drops away, it re closes the PFR contact and connects the coil of the field relay to the line. When the field relay energizes, direct current is connected to the rotor circuit and the field discharge resistor and out-of-step relay are disconnected from the line.
Power Factor Correction
As stated previously, synchronous motors can be made to produce a leading power factor. A synchronous motor can be made to produce a leading power factor by over-exciting the rotor. If the rotor is under-excited, the motor will have a lagging power factor similar to a squirrel cage or wound rotor induction motor. The reason for this is that when the DC excitation current is too low, part of the AC current supplied to the stator winding is used to magnetize the iron in the motor.
Normal excitation is achieved when the amount of excitation current is sufficient to magnetize the iron core of the motor and no alternating current is required.
There are two conditions that will indicate when normal excitation has been achieved:
1. The current supplied to the motor will drop to its lowest level.
2. The power factor will be 100% or unity.
If more than normal excitation current is supplied, over-excitation occurs. In this condition, the DC excitation current over-magnetizes the iron of the motor, and part of the AC line current is used to de-magnetize the iron. The de-magnetizing process causes the AC line current to lead the voltage in the same manner as a capacitor.
Due to their starting characteristics and ability to correct power factor, synchronous motors are generally employed where large horsepower motors are needed.
They often provide the power for pumps, compressors, centrifuges, and large grinders. A 2500 horsepower synchronous motor used to drive a water circulating pump is shown in Ill. 12.
1. What is a synchronous motor called when it's operated without load and used for power factor correction?
2. What is an amortisseur winding and what function does it serve?
3. Should the excitation current be applied to the rotor of a synchronous motor before it's started?
4. What is the function of a field discharge resistor?
5. What controls the output voltage of the alternator when a brushless exciter is used to supply the excitation current of the rotor?
6. What is the purpose of the DC coil on a polarized field frequency relay?
7. What is the purpose of an out-of-step relay?
8. Why is it possible for a synchronous motor to operate at the speed of the rotating magnetic field?
9. Name two factors that indicate when normal excitation current is being applied to the motor.
10. How can a synchronous motor be made to produce a leading power factor?
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