Synchronous Motor:Procedure for Starting a Synchronous Motor

Procedure for Starting a Synchronous Motor

While starting a modern synchronous motor provided with damper windings, following procedure is adopted.

1. First, main field winding is short-circuited.

2. Reduced voltage with the help of auto-transformers is applied across stator terminals. The motor starts up.

3. When it reaches a steady speed (as judged by its sound), a weak d.c. excitation is applied by removing the short-circuit on the main field winding. If excitation is sufficient, then the machine will be pulled into synchronism.

4. Full supply voltage is applied across stator terminals by cutting out the auto-transformers.

5. The motor may be operated at any desired power factor by changing the d.c. excitation.

Comparison Between Synchronous and Induction Motors

1. For a given frequency, the synchronous motor runs at a constant average speed whatever the load, while the speed of an induction motor falls somewhat with increase in load.

2. The synchronous motor can be operated over a wide range of power factors, both lagging and leading, but induction motor always runs with a lagging p.f. which may become very low at light loads.

3. A synchronous motor is inherently not self-starting.

4. The changes in applied voltage do not affect synchronous motor torque as much as they affect the induction motor torque. The breakdown torque of a synchronous motor varies approximately as the first power of applied voltage whereas that of an induction motor depends on the square of this voltage.

5. A d.c. excitation is required by synchronous motor but not by induction motor.

6. Synchronous motors are usually more costly and complicated than induction motors, but they are particularly attractive for low-speed drives (below 300 r.p.m.) because their power factor can always be adjusted to 1.0 and their efficiency is high. However, induction motors are excellent for speeds above 600 r.p.m.

7. Synchronous motors can be run at ultra-low speeds by using high power electronic converters which generate very low frequencies. Such motors of 10 MW range are used for driving crushers, rotary kilns and variable-speed ball mills etc.

Synchronous Motor Applications

Synchronous motors find extensive application for the following classes of service :

1. Power factor correction

2. Constant-speed, constant-load drives

3. Voltage regulation

(a) Power factor correction

Overexcited synchronous motors having leading power factor are widely used for improving power factor of those power systems which employ a large number of induction motors (Fig. 38.49) and other devices having lagging p.f. such as welders and flourescent lights etc.

(b) Constant-speed applications

Because of their high efficiency and high-speed, synchronous motors (above 600 r.p.m.) are well-suited for loads where constant speed is required such as centrifugal pumps, belt-driven reciprocating compressors, blowers, line shafts, rubber and paper mills etc.

Low-speed synchronous motors (below 600 r.p.m.) are used for drives such as centrifugal and screw-type pumps, ball and tube mills, vacuum pumps, chippers and metal rolling mills etc.

(c) Voltage regulation

The voltage at the end of a long transmission line varies greatly especially when large inductive loads are present. When an inductive load is disconnected suddenly, voltage tends to rise considerably above its normal value because of the line capacitance. By installing a synchronous motor with a field regulator (for varying its excitation), this voltage rise can be controlled.

When line voltage decreases due to inductive load, motor excitation is increased, thereby raising its p.f. which compensates for the line drop. If, on the other hand, line voltage rises due to line capacitive effect, motor excitation is decreased, thereby making its p.f. lagging which helps to maintain the line voltage at its normal value.

QUESTIONS AND ANSWERS ON SYNCHRONOUS MOTORS

Q. 1. Does change in excitation affect the synchronous motor speed ? Ans. No.

Q. 2. The power factor ? Ans. Yes.

Q. 3. How ?

Ans. When over-excited, synchronous motor has leading power factor. However, when underexcited, it has lagging power factor.

Q. 4. For what service are synchronous motors especially suited ? Ans. For high voltage service.

Q. 5. Which has more efficiency; synchronous or induction motor ? Ans. Synchronous motor.

Q. 6. Mention some specific applications of synchronous motor ?

Ans. 1. constant speed load service 2. reciprocating compressor drives

3. power factor correction 4. voltage regulation of transmission lines.

Q. 7. What is a synchronous capacitor ?

Ans. An overexcited synchronous motor is called synchronous capacitor, because, like a capacitor, it takes a leading current.

Q. 8. What are the causes of faulty starting of a synchronous motor ? Ans. It could be due to the following causes :

1. voltage may be too low – at least half voltage is required for starting

2. there may be open-circuit in one phase – due to which motor may heat up

3. static friction may be large – either due to high belt tension or too tight bearings

4. stator windings may be incorrectly connected

5. field excitation may be too strong.

Q. 9. What could be the reasons if a synchronous motor fails to start ? Ans. It is usually due to the following reasons :

1. voltage may be too low

2. some faulty connection in auxiliary apparatus

3. too much starting load

4. open-circuit in one phase or short-circuit

5. field excitation may be excessive.

Q. 10. A synchronous motor starts as usual but fails to develop its full torque. What could it be due to ?

Ans.

1. exciter voltage may be too low

2. field spool may be reversed

3. there may be either open-circuit or short-circuit in the field.

Q. 11. Will the motor start with the field excited ? Ans. No.

Q. 12. Under which conditions a synchronous motor will fail to pull into step ? Ans. 1. no field excitation 2. excessive load 3. excessive load inertia

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