Introduction to 3-Phase Induction Motors Flashcards
What is an electrical motor?
A device which converts electrical power into mechanical power.
How is rotational motion produced?
An imbalance between driving and restricting forces.
When will a motor run at a constant speed?
When the motor torque and load counter torque are balanced.
What happens when a motor’s load increases?
When counter torque is increased, the motor will draw more current to increase torque and achieve balance.
The higher torque requirement represents a lower motor speed.
What is torque commonly referred to? How is it calculated?
The moment of force.
T (torque) = F (force) * r (radius)
What are two methods to measure a motor’s torque?
A prony brake.
A dynamometer.
How is mechanical power output of a motor calculated?
It is the product of torque and rotational frequency (speed) in radians per second;
or simply:
P (power) = [T (torque) * n (speed *rpm*)]/9.55
9.55 is the constant which converts rpm into rad/sec.
Will a motor always draw it’s full rated power + losses?
No, A motor only delivers the torque that a load requires. Therefore, if a load requires only half torque, then the motor only draws (*roughly*) half power.
*There are small speed changes which are neglected*.
What is signified by a motor’s nameplate power rating?
Maximum output mechanical power. (pre - service factor)
What are the four main losses associated with motors?
Electrical:
Iron losses (Hysteresis, Eddy Currents)
Copper losses (I2R)
Mechanical:
Friction
Windage
What are the three main parts of a motor?
Frame
Stator
Rotor
What is the front and back end of a motor frame called?
End bells.
When would a drip proof or ODP motor frame be used?
In dry, indoor, non hazardous environments.
Why are slot windings better than salient pole windings?
They facilitate the use of standard parts and cancel some harmonics produced by the salient poles.
How are concentric windings different from lap windings?
Concentric windings (varying pitch) have coils for each phase/pole, becoming progressively larger centered on the smallest coil.
Lap windings (identical pitch) are all similar and “lapped” over one another.
How are the stator windings of large motors always connected?
Delta.
How is the effect fo metal fatigue in the stator reduced?
The windings are wrapped, then dipped in liquid varnish or resin and baked to solidify the coils.
What is synchronous speed?
The speed at which the magnetic field rotates around the stator windings.
It is constant.
What are the two types of rotors used in induction motors?
Squirrel cage.
Wound rotor motor.
Why are SCIM rotor bars skewed?
It prevents the stator and rotor teeth from magnetic locking (torque pulsations or cogging) and creates a uniform speed.
Makes motor operation smoother and quieter.
Are rotor laminations usually the same size as stator laminations?
No they are usually thicker.
What is the difference between synchronous speed and rotor speed defined as?
Slip speed.
We need this speed difference for induced current to flow.
What is a typical slip speed of a WRM?
2-5% of synchronous speed.
In a 2 pole motor, how many electrical cycles are needed for one mechanical rotation (of the stator field)?
One electrical cycle.
What is magnetomotive force?
The movement of electric charges which creates magnetic flux.
Using a left hand conductor rule, what does the thumb indicate?
Direction of current flow.
What is the basis of the left hand coil rule?
With a hitchhiker hand, If current flows from wrist to fingertip, the thumb indicates flux (north to south).
Regardless of left hand generation or right hand motor rules, what is indicated by your:
- Thumb?
- Forefinger?
- Center finger?
- Thrust.
- Field direction (N to S).
- Current flow.
Which force is responsible for motor rotation?
Lorentz Force.
When load counter torque is increased, how is a motor’s transformer action affected?
Secondary current increases, along with slip speed.
As slip speed increases, greater voltage and current are induced in the rotor.
Primary current also increases.
Which value best reflects the power factor of the rotor circuit?
The stator current.
What are the two categories of motor frame?
Fractional and integral horseppower.
What are the standard fractional horsepower frames?
42, 48 and 56.
What are the NEMA standard maximum variations in voltage and frequency of a motor?
Voltage may not exceed +/- 10%;
Frequency may not exceed +/- 5%; and
Combined variation may not exceed +/- 10%.
What is a problem cuased by low voltage on a motor?
The low voltage requires higher current for the same electrical power input, possibly tripping overload devices.
is rotor frequency different thatn rotational frequency?
Yes, the rotor frequency is the frequency of the current within the rotor, whereas rotational frequency is the speed which the rotor is spinning.
What is a typical slip speed?
2-8% of synchronous speed.
How many poles do most motors have?
4 poles.
1800 rpm synchronous speed.
For the same horsepower size, which motor is physically larger?
High speed.
Low speed.
The low speed motor is larger as more iron is required to produce the needed torque.
When are engineers likely to specify a belt or gear drive connected to a motor?
When load speeds of less than 1150 rpm are required.
What insulation class allows the most heat? Which are the most common?
Class H.
Class B and F.
Why is a NEMA starting code (kVA/hP) used?
To estimate locked rotor current, or starting current.
When is a motor automatically considered to be continuous duty?
When duty is not listed on the nameplate.
What are the primary characteristics of the following rotor designs?
- A
- B
- C
- D
- Normal starting torque, high starting current - can handle brief overloads
- Normal starting torque, low starting current
- High starting torque, low starting current - designed for starting heavy loads
- High starting torque, low starting current - High inertia starts; Poor speed reg.
A motor with a service factor of 1.15 or greater allows a current that is _______, when:
- Rated voltage and frequency are applied.
- Line voltages are balanced.
- Ambient temp is less than or equal to 40 degrees C.
125% of rated.
Can a maximized service factor shorten motor life?
Yes, if excessively overloaded.
If looking at a NEMA motor and the the junction box is on the left, which end faces you? Which direction will the shaft spin?
The back end (NEMA)/ drive end (IEC).
Clockwise.
