Machines; Synchronous Motors

Question 1

What rate does a rotor turn in a Synchronous motor?

Answer 1

Same speed as the rotating magnetic field of the stator (synchronous speed)

Question 2

In order to rotate synchronous Motors require a magnetic field in the rotor, how can this field be supplied?

Answer 2

Brushed DC excitation
Brushless excitation
Permanent magnet

Question 3

What is the starting current and torque like on a synchronous motor?

Answer 3

Low starting current per horsepower and high starting torque

Question 4

What is the speed regulation like on a synchronous motor?

Answer 4

They operate at a constant speed from no load to full load and maintain maximum efficiency

Question 5

What is a synchronous motor called when it’s operated at no load and used for power factor correction only

Answer 5

Synchronous condenser

Question 6

What kind of synchronous motors do not require an excitation system for the rotor?

Answer 6

Synchronous motors with Permanent magnet Pole

Question 7

What is one disadvantage of using a permanent magnet pool synchronous motor?

Answer 7

Cannot alter the power factor

Question 8

What advantages does a synchronous motor have over a squirrel cage induction motor?

Answer 8

1 can be operated at a leading power factor
2 gives constant speed from no load to full load
3 operate at higher efficiency especially when operating at Unity power factor

Question 9

What are some disadvantages of a synchronous motor compared to an induction motor?

Answer 9

1 requires DC excitation supplied from a separate source to run
2 may I followed of synchronism or stop rotating if overloaded.
3 small starting torque, synchronous motor needs assistance to get to operating speed
4 doesn’t work well in applications that require frequent starting or Direction changes

Question 10

What are the rotor and stator like in a synchronous motor construction compared to AC induction motor?

Answer 10

Stator is similar to the stator of an AC rotating field alternator. Stator supports the Armature windings which are connected to the supply voltage.

Question 11

Where are the amortisseur windings in a synchronous motor located?

Answer 11

Within the rotor

Question 12

What sort of Supply is applied to the Armature windings of a synchronous motor?

Answer 12

Three-phase AC Supply to produce a rotating magnetic field

Question 13

Where are the field windings in a synchronous motor?

Answer 13

On the rotor

Question 14

How many numbers of poles and field windings make up the rotor?

Answer 14

As many as there are poles and Armature windings on the stator

Question 15

What mechanical connection device allows DC voltage to develop the rotor magnetic fields?

Answer 15

Slip rings and brushes

Question 16

Why is DC supply to the rotor?

Answer 16

To develop a constant magnetic field that interacts with the rotating magnetic field of the stator

Question 17

What type of construction are most synchronous motor rotors?

Answer 17

Salient Pole

Question 18

What does it mean to be doubly excited?

Answer 18

Synchronous motors have to electrical inputs the stator and the rotor

Question 19

What kind of supply is used for the stator and what kind of supply is used for the rotor?

Answer 19

Stator= 3 phase AC
Rotor= DC

Question 20

When a synchronous motor is a two-pole machine how many pole pairs would you expect to see?

Answer 20

1 pole pair for each phase. (Ie. 6)

Question 21

What stator connection is most commonly found for synchronous Motors?

Answer 21

Wye connected but they can also be Delta connected

Question 22

What direction is each phase winding wound on the laminated steel cores?

Answer 22

All in the same direction

Question 23

How does the magnetic field rotate through the stator?

Answer 23

Each phase is at a maximum 120 electrical degrees apart.
The magnetic field produced by the field coils in one particular phase depends on the magnitude and direction of current through that phase.

From one instant to the next the magnetic field smoothly shift its position and rotates as the current Rises and falls at different times in the windings

Question 24

When the Wye connected point is the South Pole in a given phase that phase is that a max

Answer 24

** see figure 4 pg. 8 030401e**

Question 25

What is the speed of the stator's rotating magnetic field dependant on?

Answer 25

The frequency of the supply voltage. directly proportional

Question 26

What is the relationship between rotational frequency and the number of magnetic poles produced in the stator?

Answer 26

Inversely proportional to each other. The greater the number of magnetic poles produced in the stator the slower the speed of the stator's rotating magnetic field

Question 27

What is the equation for synchronous speed?

Answer 27

N(s) = (120 × f) ÷ p

Question 28

How do you control the intensity of the magnetic field produced by the rotor field windings?

Answer 28

Vary the field excitation

Question 29

Why do you start a synchronous motor with little or no load?

Answer 29

So the rotor can build inertia enough to rotate. Too much counter torque wood prevent this

Question 30

What are the three common methods to start a synchronous motor?

Answer 30

1 use a shaft mounted induction motor 2 use the load from the synchronous motor as a starting motor 3 use an armortisseur on the rotor

Question 31

What is the purpose of the amortisseur windings?

Answer 31

The voltage induced into these findings from the stators rotating magnetic field produces current in the windings which produces a flux that interacts with the stator flux this produces torque

Question 32

What's the risky thing that happens drink start process?

Answer 32

Rotating stator magnetic field cuts the turns of the DC winding which induces High unwanted voltages in the DC field windings. This can damage the winding insulation slip Rings brushes and other components

Question 33

How do we reduce The Unwanted High voltages across the DC windings during the start process?

Answer 33

Connect a field discharge resistor across the field using two normally closed contacts of a synchronizing switch

Question 34

What type of switch is used in the field discharge system of a synchronous motor?

Answer 34

Make before break Style

Question 35

What is the relationship between the supply voltage applied to the stator Armature and the counter electromotive Force induced in the Armature?

Answer 35

Cemf opposes the supply voltage by 180 degrees

Question 36

True or false Supply phase voltage is fixed

Answer 36

True

Question 37

True or false cemf generated voltage (E(generated)) is fixed

Answer 37

False it can be varied by changing the field excitation

Question 38

What is induced voltage or cemf due to?

Answer 38

Field flux

Question 39

What is the value of current in the stator windings determined by?

Answer 39

The resultant voltage applied to the stator winding impedance

Question 40

What is the value of impedance?

Answer 40

The phasor some of: Synchronous reactance (due to inductance and armature reaction of the coils) and resistance (due to the resistance of the copper in the coils)

Question 41

What is the torque angle?

Answer 41

The angle between the centre of a stator pole and the centre of a corresponding rotor Pole

Question 42

What affects the torque angle?

Answer 42

Increased load

Question 43

What does a torque angle on a phasor diagram illustrate?

Answer 43

That generated voltage is not 180 degrees out of phase with phase voltage. When the motor is under load it legs the 180 degree reference line by the torque angle value

Question 44

What is the angle between the stator and rotor poles called?

Answer 44

Torque angle

Question 45

What is pull in torque

Answer 45

The torque the rotor must develop to pull into synchronism once the DC windings have been energized. It must be greater than the counter torque of the load

Question 46

What is pull out torque

Answer 46

The value of the load counter torque required to rip the rotor out of synchronism. The point where the load is sufficient to break the magnetic attraction between the two fields

Question 47

What degree angle represents pull out torque?

Answer 47

90° for cylindrical Motors | 60 to 70 degrees for Salient pole Motors

Question 48

At what load would you find Unity power factor on the synchronous motor?

Answer 48

Rated load

Question 49

At what load would you find a legging power factor on a synchronous motor?

Answer 49

Overload

Question 50

At what load would you find a leading power factor on a synchronous motor?

Answer 50

Light load

Question 51

At what load would you find no losses on a synchronous motor?

Answer 51

No load

Question 52

What effect does changing the DC excitation have on torque angle?

Answer 52

Changing DC excitation is mainly to change power factor and its effect on torque angle is generally small enough to be ignored

Question 53

What is the excitation of a motor when it results in a lagging power factor (Iarm lagging Vphase)?

Answer 53

Under excited

Question 54

What is the excitation of a motor when it results in a leading power factor (Iarm leading Vphase)?

Answer 54

Over-excited

Question 55

What is a rotary capacitor?

Answer 55

Another word for synchronous condenser or a synchronous motor operating add a leading power factor for power factor correction purposes only

Question 56

What two values are compared to produce a v curve?

Answer 56

Armature current and DC field current of a synchronous motor

Question 57

What loading would cause a deeper Vee curve?

Answer 57

No load

Question 58

When referencing a v curve as field current increases what happens to the torque angle?

Answer 58

It becomes less lagging and more leading

Question 59

Why is a full load vee curve more shallow?

Answer 59

Armature current is on the x axis, and more Armature current flows to produce the power to drive the load. (I(arm) values stay higher as field current values increase)

Question 60

What values does a bell curve compare?

Answer 60

Power factor and field current

Question 61

At what point on a bell curve does a synchronous motor reach normal excitation?

Answer 61

At the peak or Unity power factor

Question 62

At what point on a bell curve does a synchronous motor reach normal excitation?

Answer 62

At the peak or Unity power factor

Question 63

As field current is increased from Unity power factor what happens to the torque angle?

Answer 63

Becomes more leading

Question 64

Which would take more field current to produce Unity power factor: a motor operating with a full load or with no load?

Answer 64

A motor operating with full load would take Moorefield current to reach Unity power factor