Powerpoint - Three Phase Motor Theory Flashcards
Eyebolt and shackle is used for lifting _____
motors
Name two types of three phase motors;
- Squirrel cage induction
- _____ rotor induction
wound
Squirrel cage rotor: advantages;
____ starting torque
no starting torque
high ______ torque
easily ______
high, running, reversed
Wound rotor types:
wye wound and _____ wound
delta wound
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
magnetic, source, slip rings, 95%
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
overheating
Motor Protection Device:
Purpose is to disconnect power to motor before ______ occurs, Types of motor protection devices are ____ duty, ____ duty, and auto reset
failure, pilot duty, line duty
Two major classifications of motor overload protection are internal and _______
external
Overload devices can operate on temperature, current or a combo of temp and
current
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
40C, winding
Overload device based on current:
Max current draw of a motor on start is ___ times the FLA
6 times
Name three 3-phase motor starting methods;
- Across the line
- ____ winding start
- ______ voltage start
- across the line
- part winding start
- reduced voltage start
Across the line start has a high in rush current of about ____%, no ____ control cct needed, has as little as ___ wiring terminals
300%, start, 3
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
inrush, two,
Part-Winding start dual voltage motor:
Most of these motors have __ leads, can be used for two different ______,
9, voltages,
Purpose of reduced voltage?
Method to do so? primary resistor starter or _____________ starter
lower inrush current and lower mechanical stress
primary resistor starter or auto transformer
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 ___%
80%
Primary Resistor: Advantages;
- closed transition starting - motor not disconnected from line during acceleration
- Smooth acceleration - terminal voltage increases with speed. As the motor accelerates, the voltage drop across the starting resistor decreases
- Standard motors are used - no need for a special _____
motor
Primary Resistor Starter: Disadvantages:
- heat may be difficult to _______
- line voltage start and starting torque is difficult to _______
dissipate, adjust
Name three starting methods for a three phase motor?
Across the line start
Part-winding start
Reduced voltage start
Across the line start: characteristics’
- Motor has as little as __ wiring terminals.
- High inrush _____ (about 300%).
- No control _______ needed
3
current
circuit
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
- inrush
- 0.5-1.5 second
- 6 or 9 terminals
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
- 9
- voltages
- voltages
Reduced Voltage: Purpose;
- Lower inrush current by lowering applied _______
- lower mechanical _____ on belts and couplings
- Method used is a primary resistor starter or ____________
- voltage
- stress
- autotransformer
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
- hp
- 80%
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 ______
- acceleration
- decreases
- motor
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 ______
- dissipate
- adjust
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
- voltage
- 80%
- transformer
- hp
Autotransformer Starter: Advantages;
- produces the max torque per amp of line _____
- maximum reduction of ______ current
- no need for special ______
- current
- inrush
- motor
Autotransformer: disadvantages;
- More ________ than other starters in smaller hp ratings
- not suitable when starting is _______ or acceleration is long
- expensive
- frequent
