1 school of electrical systems engineering abd rahim 2008 eet421 power electronic drives – 2)...

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ABD RAHIMABD RAHIM 2008 2008

EET421Power Electronic Drives

– 2) synchronous motor

Abdul Rahim Abdul Razak

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Synchronous motors have the characteristic of constant speed between no load and full load. Their speed ns (synchronous speed) only depends on AC line frequency f, and the # of motor poles per phase, P.

Figure 1: Revolving-field synchronous motor.

2) SYNCHRONOUS MOTORS

P

f120n s

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Example 1:

1) What pole number would be needed for a synchronous motor to run at a speed of 300 rpm from a 60Hz supply?

24p

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Advantages…

Synchronous motors have the following advantages over non-synchronous motors:

• Speed is independent of the load, provided an adequate field current is applied.

• Accurate control in speed and position (stepper motors).

• They will hold their position when a DC current is applied to both the stator and the rotor windings.

• Their power factor can be adjusted to unity by using a proper field current relative to the load. Also, a "capacitive" power factor, (current phase leads voltage phase), can be obtained by increasing this current slightly, which can help achieve a better power factor correction for the whole installation.

• Their construction allows for increased electrical efficiency when a low speed is required (as in ball mills and similar apparatus).

• More efficient compared to induction motor (large industrial scale)

SYNCHRONOUS MOTORS

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They are capable of correcting the low power factor of an inductive loads when they are operated under certain conditions (overexcited).

Often used to drive DC generators or as replacement of a capacitor banks to improve power factor .

Synchronous motors are designed in sizes up to thousands of horsepower. They may be designed as either single-phase or multiphase machines.

SYNCHRONOUS MOTORS

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Examples

1) Brushless permanent magnet DC motor – Toyota Prius, Honda Civic Hybrid car

2) Stepper motor – printer, floppy drive etc

3) Slow speed AC synchronous motor – cement factory

4) Switched reluctance motor. – washing machine, electric car

SYNCHRONOUS MOTORS

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SYNCHRONOUS MOTORS

Figure 2 :

Equivalent circuit of a synchronous motor

KVL Equivalent equation :

or

Steady states conditions…

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SYNCHRONOUS MOTORS

Torque equation : or

Where δ is the load angle.

Figure 3 :

Equivalent phasor diagram of a synchronous motor

Steady states conditions…

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SYNCHRONOUS MOTORS

Figure 4 :

Torque-speed characteristic of a synchronous motor

• Loads are basically constant-Speed devices.

• Terminal voltage and the system frequency will be constant regardless of the amount of power drawn by the motor.

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• The steady-states speed of the motor is constant from no load all the way up to the maximum torque that the motor can supply (called the pullout torque).

• Thus the speed regulation SR, of this motor is 0 percent.• The torque equation can be stated as:

• Thus, The maximum or pullout torque Tmax, occurs when δ =90º. • The maximum power can be produce is :

• Normal full-load torques are much less than that, however. In fact, it would be indicated at 1/3 rd of the pullout torque .

• when the torque on the shaft of a synchronous motor exceeds the pullout torque, the rotor can no longer remain locked to the stator and net magnetic fields. Instead, the motor will slows down and at last will vibrate severely.

• The loss of synchronization after the pullout torque is exceeded is known as slipping poles.

Sm

Aind X

EV3

sin

S

A

X

EV3P max

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SYNCHRONOUS MOTORS

When the field current IF, changes … how does it affect the synchronous motor?

1) Consider a synchronous motor operating at lagging power factor

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SYNCHRONOUS MOTORS

2) Increasing IF will will result magnitude EA to increase. But it does not effect the real power supplied from to the motor (power will only increase when torque increased). Thus, P=constant.

3) Input power to motor is given by:

or

4) So the distance proportional to power on the phasor diagram (EA sin δ and IA

cos θ) must therefore

be constant. Thus, when

EA to increase, it will only

slide along the constant

power line (EA1 – EA4).

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SYNCHRONOUS MOTORS

6) When value of EA increased, magnitude of IA will slightly decrease then increased. But it can only do so within the constant power line (IA1 – IA4).

Note:

At low EA, the armature current is lagging, and the motor is an inductive load (consuming reactive power Q). As the field current is increased, the armature current eventually lines up with VØ, and the motor looks purely resistive. As the field current is increased further, the armature current becomes leading, and the motor becomes a capacitive load. It is now acting like a capacitor-resistor combination. Consuming negative reactive power Q or, alternatively, supplying reactive power Q to the system.

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SYNCHRONOUS MOTORS

The V curves…

Therefore, by controlling the field curent IF of a synchronous motor, the reactive power consumed or supplied to by the motor can be controlled.

The several V curves drowns represent a different motor power levels. For each curve, the minimum armature current occurs at unity power factor (when only real power is being supplied to the motor)

For field currents less than the value giving minimum IA, the armature current is lagging. Consuming Q. For field currents greater than the value giving the minimum IA., the armature current is leading, supplying Q to the power system as a capacitor would.

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SYNCHRONOUS MOTORS

Underexcited overexcited

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Example 2:

A 208V 45-kVA, 0.8-PF-leading, Δ- connected, 60Hz synchronous motor has a synchronous reactance of 2.5Ω and a negligible armature resistance. Its friction and windage losses are 1.5 kW, and its core losses are 1.0 kW. Initially the shaft is supplying a 15HP load with initial power factor of 0.85PF lagging. The field current IF at these conditions is 4.0 A.

a) Sketch the initial phasor diagram of this motor, and find the values lA and EA. 25.8L-31.8 deg 182L-17.5V

b) lf the motor's flux is increased by 25 percent, sketch the new phasor diagram of the motor. What are values lA and EA and the power factor of the motor now? 227.5L-13.9 22.5L13.2 leading

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Example 3:

Develop a table showing the speed of magnetic field rotation in ac machines of 2, 4, 6, 8, 10, 12, and 14 poles operating at frequencies of 50, 60, and 400 Hz.

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Example 4:

At a location X, it is necessary to supply 300 kW of 60-Hz power. But, the only power sources available at the site is 50 Hz. It is decided to generate the power by means of a motor-generator setconsisting of a synchronous motor driving a synchronous generator. How many poles should each of the two machines have in order to convert 50-Hz power to 60-Hz power? 10 -12

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Synchronous motor & power factor correction

The following figure shows a large power system whose output is connected through a transmission line to an industrial plant at a distant point. The industrial plant shown consists of three loads. Two of the loads are induction motors with lagging power factors, and the third load is a synchronous motor with a variable power factor.

What does the ability to set the power factor of one of the loads do for the power system?

Example 5:The power system in Figure 5-39 operates at 480V. Load 1 is an induction rllt,,rconsuming1 00k W at 0.78P Fl agging,a nd load2 is an inductionm otorc onriunur l200 kW at 0.8 PF lagging.L oad 3 is a synchronousm otorw hose real poworr .r Isumotionis 150k W.a. lf thes ynchronoums otori s adjustedto operatea t 0.85P Fl aggingw, lrirli r;t l r , ,transmissionli nec urrenti n thiss ystem?b. lf the synchronoums otori s adjustedto operatea t 0,85P Fl eadingw, hrrtr

At a locatio

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Synchronous motor & power factor correction

Example 5:

The power system in Figure 5-39 operates at 480V. Load 1 is an induction motor consuming 100kW at 0.78PF lagging, and load 2 is an induction motor consuming 200kW at 0.8PF lagging. Load 3 is a synchronous motor whose real power consumption is

150kW.

a. lf the synchronous motor is adjusted to operate at 0.85PF lagging, what is the transmission line current in this system? 667A

b. lf the synchronous motor is adjusted to operate at 0,85PF leading, what is the transmission line current in this system? 566A

c. Assume that transmission line losses PLL given as ; PLL = 3IL2RL

how do the transmission losses compare in the two cases? 28% less

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How does it works?..

Assume that the application of three-phase AC power to the stator causes a rotating magnetic field to be set up around the rotor.

The rotor is energized with DC (it acts like a bar magnet). The strong rotating magnetic field attracts the strong rotor field activated by the dc. This results in a strong turning force on the rotor shaft. The rotor is therefore able to turn a load as it rotates in step with the rotating magnetic field. It works this way once it’s started.

However, one of the disadvantages of a synchronous motor is that it cannot be started from a standstill by applying three-phase ac power to the stator… Why ??

SYNCHRONOUS MOTORS

N

S

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Because ..

…when ac is applied to the stator, a high-speed rotating magnetic field appears immediately. This rotating field rushes past the rotor poles so quickly that the rotor does not have a chance to get started.

Since the field is rotating at synchronous speed, the motor must be accelerated before it can pull into synchronism.

Therefore, separate starting means must be employed.

A synchronous motor in its purest form has no starting torque. It has torque only when it is running at synchronous speed.

SYNCHRONOUS MOTORS

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Starting Methods :

1) Variable AC frequency

2) Mechanical drive - Turn the rotor into Synchronize speed

3) DC motor drive – coupled on the common shaft

4) Embedded Squirrel cage winding on rotor poles.

SYNCHRONOUS MOTORS – drive & control

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1) Variable AC frequency


Note : Electronic speed control – constrain:-

Modern solid state electronics has able to increase the options for speed control. By changing the 50 or 60 Hz line frequency to higher or lower values, the synchronous speed of the motor can be changed.

However, decreasing the frequency of the AC current fed to the motor also decreases reactance Xs which increases the stator current.

This may cause the stator magnetic circuit to saturate with disastrous results. Thus in practice, the voltage to the motor needs to be decreased when frequency is decreased.

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Example 6:

If a 60-Hz synchronous motor is to be operated at 50 Hz, will its synchronous reactance be the same as at 60 Hz, or will it change? (Hint: Think about the derivation of XS .)


SOLUTION:

The synchronous reactance Xs represents the effects of the armature reaction

voltage Estat and the armature self-inductance Ls. The Estat is caused by the

armature magnetic field Bs , and the amount of voltage is directly proportional to the speed with which the magnetic field sweeps over the stator surface. The higher the frequency, the faster Bs sweeps over the stator, and the higher the armature reaction voltage Estat is.

Therefore, the armature reaction voltage is directly proportional to frequency (EA ~ f). Similarly, the reactance of the armature self-inductance is directly proportional to frequency so the total synchronous reactance Xs is directly proportional to frequency (XS ~ f),. If the frequency is changed from 60 Hz to 50 Hz, the synchronous reactance will be decreased by a factor of 5/6 as well.

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Example 7:

What voltage should be used to allow a 420V, 60Hz, 4-pole synchronous motor to be used on a 50Hz supply? 350v

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Self starting method..

A squirrel-cage type of winding is added to the rotor of a synchronous motor to cause it to start. The squirrel cage is shown as the outer part of the rotor in figure 4-7.

It is so named because it is shaped and looks something like a turnable squirrel cage. Simply, the windings are heavy copper bars shorted together by copper rings.

4) Embedded Squirrel cage winding on rotor poles.

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Self starting process..

Fleming′s Right Hand Rule Also known as the Generator Rule this is a way of determining the direction of the induced emf of a conductor moving in a magnetic field.

Fleming′s Left Hand RuleAlso known as the Motor Rule this is a way of determining the direction of a force on a current carrying conductor in a magnetic field.

SYNCHRONOUS MOTORS

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Starting process…

A low voltage is induced in these shorted windings by the rotating three-phase stator field. Because of the short circuit, a relatively large current flows (induced emf) in the squirrel cage.

This causes a magnetic field that interacts with the rotating field of the stator. Because of the interaction (left hand rule), the rotor begins to turn, following the stator field; the motor starts. We will run into squirrel cages again in other applications, in more detail.

To start a practical synchronous motor, the stator is energized, but the dc supply to the rotor field is not yet energized. The squirrel-cage windings bring the rotor to near synchronous speed. At that point, the dc field is energized. This locks the rotor in step with the rotating stator field. Full torque is developed, and the load is driven.

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Starting process…

A mechanical switching device that operates on centrifugal force is often used to apply dc to the rotor as synchronous speed is reached.

The practical synchronous motor has the disadvantage of requiring a dc exciter voltage for the rotor. This voltage may be obtained either externally or internally, depending on the design of the motor.

Constant speed

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Questions to ponder…

1. What requirement is the synchronous motor specifically designed to meet?

2. What is the speed regulation of a synchronous motor?

3. When would a synchronous motor be used eventhough its constant speed characteristic is not needed?

4. Why can't a synchronous motor start by itself?

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

6.What happens to a synchronous motor as its field current is varied?

7. A synchronous motor is operating at a fixed load. Once the field current is increased, the armature current falls, was the motor initially operating at a lagging or a leading Power factor?

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The end on synchronous motor

1 school of electrical systems engineering abd rahim 2008 eet421 power electronic drives – 2)...

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