Powerpoint - Three Phase Motor Theory

Question 1

Eyebolt and shackle is used for lifting _____

Answer 1

motors

Question 2

Name two types of three phase motors;

1. Squirrel cage induction
2. _____ rotor induction

Answer 2

wound

Question 3

Squirrel cage rotor: advantages;

____ starting torque

no starting torque

high ______ torque

easily ______

Answer 3

high, running, reversed

Question 4

Wound rotor types:

wye wound and _____ wound

Answer 4

delta wound

Question 5

Synchronous motor: characteristics;

will operate at the speed of the rotating ______ field from full to no load, DC is applied to rotor from an independent ______, ____ rings on rotor are required, DC will apply to rotor when motor speed is ___% as the rotor wing magnetic field

Answer 5

magnetic, source, slip rings, 95%

Question 6

Motor Protection Device:

The most common cause of motor failure is ________, the excessive generation of heat can be caused by a variety of problems such as loss of refrigerant, defective start relays,excessive loads

Answer 6

overheating

Question 7

Motor Protection Device:

Purpose is to disconnect power to motor before ______ occurs, Types of motor protection devices are ____ duty, ____ duty, and auto reset

Answer 7

failure, pilot duty, line duty

Question 8

Two major classifications of motor overload protection are internal and _______

Answer 8

external

Question 9

Overload devices can operate on temperature, current or a combo of temp and

Answer 9

current

Question 10

Overload device based on temperature:

Max temp rise of motor windings is ___C, it senses the motor ______ temp only, set to open at 200F and close at 150F

Answer 10

40C, winding

Question 11

Overload device based on current:

Max current draw of a motor on start is ___ times the FLA

Answer 11

6 times

Question 12

Name three 3-phase motor starting methods;

1. Across the line
2. ____ winding start
3. ______ voltage start

Answer 12

1. across the line
2. part winding start
3. reduced voltage start

Question 13

Across the line start has a high in rush current of about ____%, no ____ control cct needed, has as little as ___ wiring terminals

Answer 13

300%, start, 3

Question 14

Part winding start is to reduce ______ current, ____ contactors are required, time delay of 0.5 to 1.5 seconds from motor starting to fully engage all windings

Answer 14

inrush, two,

Question 15

Part-Winding start dual voltage motor:

Most of these motors have __ leads, can be used for two different ______,

Answer 15

9, voltages,

Question 16

Purpose of reduced voltage?

Method to do so? primary resistor starter or _____________ starter

Answer 16

lower inrush current and lower mechanical stress

primary resistor starter or auto transformer

Question 17

Primary Resistor Starter:

widely used for small ___ application, voltage apply to the motor winding down to 70% when motors start, full voltage occurs when motor speed is up to ___%

Answer 17

80%

Question 18

Primary Resistor: Advantages;

1. closed transition starting - motor not disconnected from line during acceleration
2. Smooth acceleration - terminal voltage increases with speed. As the motor accelerates, the voltage drop across the starting resistor decreases
3. Standard motors are used - no need for a special _____

Answer 18

motor

Question 19

Primary Resistor Starter: Disadvantages:

1. heat may be difficult to _______
2. line voltage start and starting torque is difficult to _______

Answer 19

dissipate, adjust

Question 20

Name three starting methods for a three phase motor?

Answer 20

Across the line start

Part-winding start

Reduced voltage start

Question 21

Across the line start: characteristics’

• Motor has as little as __ wiring terminals.
• High inrush _____ (about 300%).
• No control _______ needed

Answer 21

3

current

circuit

Question 22

Part-winding start: characteristics;

• To reduce ______ current
• two contractors are required
• time delay of 0.5 to __ second from motor starting to fully engage all windings is typical
• stators can have 6 or __ terminals

Answer 22

• inrush
• 0.5-1.5 second
• 6 or 9 terminals

Question 23

Part-winding start dual voltage motor: characteristics’

• Most of these motors have __ leads.
• can be used for two different ________
• different jumper connection required for different

Answer 23

• 9
• voltages
• voltages

Question 24

Reduced Voltage: Purpose;

• Lower inrush current by lowering applied _______
• lower mechanical _____ on belts and couplings
• Method used is a primary resistor starter or ____________

Answer 24

• voltage
• stress
• autotransformer

Question 25

Primary Resistor Starter:

• Widely used for small ___ application
• Full voltage occurs when motor speed is up to ___%
• Nema specifies resistor should be suitable for 5 second start each 80-90 seconds for an hour

Answer 25

• hp

- 80%

Question 26

Primary Resistor Starter: advantages;

• closed transition starting: motor not disconnected from line during _________
• Smooth acceleration: terminal voltage increases with speed. As the motor accelerates, the voltage drop across the starting resistor _______
• Standard Motors are used - no need for special ______

Answer 26

• acceleration
• decreases
• motor

Question 27

Primary Resistor Starter: disadvantages;

• power loss may be significant and heat is difficult to _______
• percent of line voltage start and starting torque is difficult to ______

Answer 27

• dissipate

- adjust

Question 28

Autotransformer starter:

• use an autotransformer to reduce the motor starting ________
• the motor starts at 50 -__% of the line voltage
• biggest torque-per-amp of line current due to the __________ effect
• more often used with big ___ motors

Answer 28

• voltage
• 80%
• transformer
• hp

Question 29

Autotransformer Starter: Advantages;

• produces the max torque per amp of line _____
• maximum reduction of ______ current
• no need for special ______

Answer 29

• current
• inrush
• motor

Question 30

Autotransformer: disadvantages;

• More ________ than other starters in smaller hp ratings
• not suitable when starting is _______ or acceleration is long

Answer 30

• expensive

- frequent