UNIT9

E3106/09/16

ELECTRICAL MACHINERY & CONTROL                           

UNIT 9

 

ALTERNATING CURRENT (AC) MACHINERY (PART V)

 

 

             

                            OBJECTIVES

 

 

 

GENERAL OBJECTIVE

 

To interpret and apply the concept of synchronous motors

 

 

SPECIFIC OBJECTIVE

 

By the end of this unit, you would be able to:

 

• explain the basic principle of synchronous motors
• describe the approaches starting of synchronous motors
• draw characteristics of three types exciting synchronous motors

 

 

 

 

                            INPUT

 

 

 

9. INTRODUCTION TO SYNCHRONOUS MOTORS

 

 

Synchronous motors are synchronous machines used to convert electric power to mechanical power. As the name implies, synchronous motors run in synchronism with revolving field. The speed of rotation is therefore tied to the frequency of the source. Because the frequency is fixed, the motor speed stays constant, irrespective of the load or voltage of the three-phase line.

 

However, synchronous motors are used not so much because they run at constant speed but because they possess other unique electrical properties.

 

 

 

 

Most synchronous motors are rated between 150 kW (200 hp) and 15 MW (20 000 hp) and turn at speeds ranging from 150 to 1800 r/min.

 

 

 

 

 

 

 

 

 

 

 

 

9.1              BASIC PRINCIPLE OF SYNCHRONOUS MOTORS

 

 

 

Figure 9.1: Two pole synchronous motor

To understand the basic concept of synchronous motor, Figure 9.1 which shows a two-pole synchronous motor. The field current IF of the motor produces a steady-state magnetic field BR. A three-phase set of voltages is applied to the stator of the machine, which produces a three-phase current flow in the windings.

 

A three-phase set of currents in an armature winding produces a uniform rotating magnetic field BS. Therefore, there are two magnetic fields present in the machine, and the rotor field will tend to line up with the stator field, just as two bar magnets will tend to line up if placed near to each other. Since the stator magnetic field is rotating the rotor magnetic field (and the rotor itself) will constantly try to catch up.

 

The larger the angle between the two magnetic fields (up to a certain maximum) the greater the torque on the rotor of the machine. The basic principle of synchronous motor operation is that the rotor “chases” the rotating stator magnetic field around in a circle never quite catching up with it.

  

 

 

 

 

 

 

 

 

 

Test your UNDERSTANDING before you continue to the next input

ACTIVITY 9A
 

 

 

 

 

 

1. Fill in the blanks with the answers given below.

    electric power                 mechanical power                         synchronous machines

     field                    voltage                          frequency                          load

 

Synchronous motors are  ……………….    …………….  used to convert …………..  …………… to …………….  …………….. . As the name implies, synchronous motors run in synchronism with revolving ………… . The speed of rotation is therefore tied to the frequency of the source. Because the ………….  is fixed, the motor speed stays constant, irrespective of the ………..  or ……………  of the three-phase line.

 

2. Describe the basic principle of synchronous motor by using a suitable figure. 

 

 

 

 

 

 

 

FEEDBACK TO ACTIVITY 9A

 

 

1. synchronous machines , electric power , mechanical power, field, frequency, load, voltage.

 

2. Refer the note 9.1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                            INPUT

 

 

 

2. STARTING SYNCHRONOUS MOTORS

 

 

A synchronous motor cannot start by itself; consequently the rotor is usually equipped with a squirrel-cage winding so that it can start up as an induction motor. To understand the nature of the starting problem, we will see the Table 9.1. This figure shows a 60 Hz synchronous motor at the moment power is applied to its stator windings. The rotor of the motor is stationary and therefore the magnetic field BR is stationary. The stator magnetic field BS is starting to sweep around the motor at synchronous speed.

 

Table 9.1: Starting Problem in Synchronous Motor

	Machine at t = 0s When BR and BS are exactly lined up. By the induced-torque equation tind = kBR x BS the induced torque on shaft of the rotor is zero.
	Machine at time t = In such a short time, the rotor has barely moved but the stator magnetic field now points to the left. By the induced-torque equation, the torque on the shaft of the rotor is now counterclockwise.
	Machine at time t = At that point BR and BS point in opposite directions and tind again equals zero.
	Machine at time t = The stator magnetic field now points to the right and the resulting torque is clockwise.
	Machine at timet = The stator magnetic field is again  lined up with the rotor magnetic field, and tind = 0. During one electrical cycle, the torque was first counter-clockwise and then clockwise and the average torque over the complete cycle was zero.

 

 

 

 

 

 

 

 

 

 

 

 

 

can a synchronous motor be started??

 

 

 

             

 

 

 

Three basics approaches can be used to safely start a synchronous motor:

1. Reduce the speed of the stator magnetic field to a low enough value that the rotor can accelerate and lock in with it during one half-cycle of the magnetic field’s rotation. This can be done by reducing the frequency of the applied electric power.

 

2. Use an external prime  mover to accelerate the synchronous motor up to synchronous speed, go through the paralleling procedure, and bring the machine on the line as a generator. Then turning off or disconnecting the prime mover will make the synchronous machine a motor.

 

3. Use damper windings or amortisseur windings. Amortisseur windings are special bars laid into notches carved in the face of a synchronous motor’s rotor and then shortened out on each end by a large shorting ring.

 

 

 

 

 

 

 

 

 

 

 

Test your UNDERSTANDING before you continue to the next input

ACTIVITY 9B
 

 

 

 

 

 

9.3.               Why can’t a synchronous motor starts by itself?

9.4.              What techniques are available to start a synchronous motor?

 

 

 

             

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

FEEDBACK TO ACTIVITY 9B

 

 

3. A synchronous motor has no net starting torque and so cannot starts by itself.

 

4. The techniques are available to start a synchronous motor are :
1. reducing electrical frequency
2. using an external prime mover
3. using amortisseur windings or damper windings

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

              INPUT

 

 

 

 

3. CHARACTERITICS OF THREE TYPES EXCITING SYNCHRONOUS MOTORS

 

 

A synchronous motor is said to have normal excitation when its (back emf) Eb = V (voltage supply). If field excitation is such that Eb < V, the motor is said to be under-excited. On the other hand if D.C field excitation is such that  Eb > V, then motor is said to be over excited. These characteristics can understand by using Table 9.2.

 

Table 9.2: Three Types Exciting Synchronous Motor

Eb              : Back emf ER              : Total voltage (Eb + V) Ia              : Armature current Eb ER V Ia q f a	Normal Excitation Magnitude back emf Eb is equal with V. Armature current Ia is lagging V in phase angle f. It has a lagging power factor.
Eb ER Ia V q f a	Under Excitation Magnitude of the back emf Eb is smaller than V. Armature current Ia is lagging V in phase angle f. It has a lagging power factor.
Eb ER V Ia f q	Over Excitation Magnitude back emf Eb is greater than V. Armature current Ia is leading V in phase angle f. It has a leading power factor.

Test your UNDERSTANDING before you continue to the next input

ACTIVITY 9C
 

 

 

 

 

 

5. What are the differences between normal excitation and over excitation?

 

6. Explain the under excitation by using the vector diagram.

 

 

 

 

 

             

 

 

 

 

 

 

 

 

 

 

 

 

FEEDBACK TO ACTIVITY 9C

 

 

9.5.             

Normal Excitation	Over Excitation
Magnitude back emf Eb is equal with V. Armature current Ia is lagging V in phase angle f. It has a lagging power factor.	Magnitude back emf Eb is greater than V. Armature current Ia is leading V in phase angle f. It has a leading power factor.

 

9.6.

Eb ER Ia V q f a         Under Excitation Magnitude of the back emf Eb is smaller than V. Armature current Ia is lagging V in phase angle f. It has a lagging power factor.

 

 

 

 

 

 

 

 

 

 

SELF-ASSESMENT

 

If you face any problem, discuss it with your lecturer

You are approaching success. TRY all the questions ini this self-assessment section and check your answers with those given in the feedback on Self-Assessment given on the next page.

 

 

 

Question 9-1

 

A              From the figure below,

(i)              How to produce a uniform rotating magnetic field BS?

(ii)               What will be happen to magnetic field in rotor?

(iii)               How do you know the torque will increase in this machine?

 

Question 9-2

 

A              These are the techniques to start a synchronous motor reduce the speed of the

stator magnetic field, use an external prime mover and use damper windings or amortisseur windings. Explain the second approach.

 

 

Question 9-3

 

A              A synchronous motor has 12kV voltage supply which is back emf for this motor is 10kV. The armature current is lagging the voltage supply. This motor connected to the load. From this statement, can you:

              (a)              explain the characteristic of the excitation of this motor

              (b)              sketch the vector diagram for this excitation

 

B              Figure is a vector diagram for over excitation motor synchronous, if the load of this motor is increase, what will be happened to the motor.

Eb

ER

V

Ia

f

q

             

 

 

 

 

 

 

 

 

 

 

 

 

 

FEEDBACK TO SELF-ASSESMENT

 

 

 

Question 9-1

 

A              (i)              A three-phase set of currents in an armature winding produces a uniform

rotating magnetic field BS.

2. the rotor field will tend to line up with the stator field.
3. The larger the angle between the two magnetic fields (up to a certain maximum) the greater the torque on the rotor of the machine.

 

Question 9-2

 

A              Use an external prime  mover to accelerate the synchronous motor up to

synchronous speed, go through the paralleling procedure, and bring the machine on the line as a generator. Then turning off or disconnecting the prime mover will make the synchronous machine a motor.

 

Question 9-3

 

A              (a)               under excitation

*               Magnitude of the back emf Eb is smaller than V.

*              Armature current Ia is lagging V in phase angle f.

Eb

ER

Ia

V

q

f

a

*              It has a lagging power factor.

              (b)              

                           

 

B              *              power factor improves and approach unity.

              *              the armature current also increase.