Motors Flashcards
Sync Speed of 2 pole motor?
3600RPM
Sync Speed of 4 pole motor?
1800
Sync Speed of 6 pole motor?
1200
Sync Speed of 8 pole motor?
900
Formula for sync speed?
Frequency x 120/ number of poles
A four pole motor is operating at 1655, what is the slip speed?
145
What are the six rotor parameters affected by slip?
Induced voltage, impedance of rotor, frequency of motor, current of rotor, reactance of rotor
What is rotor parameter is constant?
resistance
When is rotor voltage at a maximum?
When the rotor is stalled (blocked rotor)
What is the rotor frequency of a stalled motor?
60 hz
A rotor is found to have a pf of .7. A few moments later, the same rotor has a pf of .92. What does this mean?
The rotor has sped up
As load is added to a rotor, what will happen to the torque produced?
Increase
What is breakdown torque?
the max torque that a motor can produce. Any more and it would stall
What percent of sync speed to NEMA A and B motors produce breakdown torque?
70-80%
At what power factor does a rotor produce breakdown torque?
.707
What can be said about resistance and reactance at breakdown torque
they are equal
What is the inrush current in a typical NEMA A motor?
600% of full load current
A NEMA B rotor has ___ inductance than a NEMA A rotor
more
What type of NEMA motor is considered a double squirrel cage?
C
What rotor design has highest starting torque?
D
What rotor design has the highest full load speed?
A
What rotor design in the most efficient?
A
Which rotor design in the most common?
B
Which rotor design produces 275% of breakdown torque?
D
As a WRM is accelerated, the resistance of the rotor ___
decreases
What are two disadvantages of a wound rotor motor?
Expensive and hard to maintain
Can rotor speed ever be the same as sync speed?
No
Why can’t rotor speed be the same as sync speed?
Because then the magnetic fields would be in sync and no lines of force would be cut, therefore, no voltage would be induced
The speed difference between sync speed and rotor speed is known as ___
slip
The slip speed ___ as load is added
increases
A motor with low percent speed regulation is more efficient than one with high speed regulation. True or False?
True
What are three quantities that are DIRECTLY proportional to slip?
Induced voltage, frequency, and reactance
As a squirrel-cage motor accelerates from standstill to normal running speed, active rotor current:
increase then decreases
In North America, a motor running at 90% of sync speed has a rotor frequency of ____
6 hz
Mechanical power is the product of what two mechanical quantities?
Speed and torque
Is windage a mechanical or an electrical loss?
mechanical
What is the formula for output power of a motor?
Torque times speed divided by 9.55
Which rotor design produces the highest starting current?
A
Which motor has the highest breakdown torque?
A
Which design of motor has the lowest slip speed?
A
Which rotor design in the most common?
B
What is the starting torque of a type A motor?
150%
What is the breakdown torque of a type A motor?
200%
What is the starting current of a type A motor?
600%
Even though a type A motor is the most efficient, it isn’t the most common. Why?
Very high inrush current
How do we lower the inrush current in a type B motor compared to a Type A?
The rotor bars are embedded closer to the core, leading to a higher reactance (therefore, higher impedance and lower current)
What is the starting torque of a type B motor?
150%
What is the starting current of a type B motor?
500%
What is the breakdown torque of a type B motor?
200%
What is the starting current of a Type C motor?
500%
What is the starting torque of a Type C motor?
225%
When does a type C motor produce max torque?
at the moment of start
What is the starting current of a type D motor?
400%
What is the starting torque of a type D motor?
275%
When does a type D motor produce max torque?
at the instant of start
Which type of motor is the least efficient?
Type D
When is the speed control of a WRM most suitable?
from 0 RPM to Full load speed
Which motor is the best?
WRM
Why isn’t a WRM very common?
High cost and hard to maintain
The starting torque of a WRM can be adjusted by _____
by adjusting the resistance
How do we lower inrush current in a WRM?
connecting the resistors from the control circuit
What is speed regulation?
Percentage change from no load to full load
What is the formula for speed regulation?
no load - full load divided by full load x 100
What is the formula for rotor voltage?
Ebr x slip
What is the formula for rotor frequency?
Fbr x slip
What is the formula for rotor current?
Ibr x slip
What is the formula for active rotor current
Ir x pf
What is the formula for rotor reactance?
Xbr x slip
What is the formula for rotor impedance?
Pythagorean theorum
What is the formula for rotor power factor?
R/Z
When does breakdown torque occur in both words and as a number?
When reactance = resistance. .707 power factor
Are type A and B rotors suitable for high inertia loads?
Fuck no
Describe a type C motor
Double squirrel cage. The outer bars are small to cause higher resistance. The inner bars are large to cause high resistance
What is the disadvantage of a type C motor?
doesn’t dissipate heat very well
An ohmmeter indicates three groups of three. What kind of motor is this?
Delta
An ohmmeter indicates one group of three. What kind of motor is this?
Wye
An ohmmeter indicates three groups of two. What kind of motor is this?
Wye
What is the first step in a lead ID for a delta motor?
Find three groups of three
What is the second step in a lead ID for a delta motor?
Find centre of each group
What is the third step in a lead Id for a delta motor?
Label T1 permanently
What is the fourth step in a lead ID for a delta motor?
Flux shower
What is the first step in a lead ID for a wye motor?
Find one group of three and three groups of two
What is the second step in a lead ID for a wye motor?
Label centre points
What is the third step in a lead ID for a wye motor?
Connect 15 volts across centre points to find our proper leads
What is the fourth step in a lead ID for a wye motor?
Find polarities. Jumper our leads and then apply 15V across circuit. Measure from the centre to the end and if we get an additive polarity, it is correct. If we don’t, flip the leads.
What is phase A on a IEC motor?
U
What is phase B on an IEC motor?
V
What is phase C on an IEC motor?
W
What does TENV stand for?
totally enclosed non ventilated
What is windage?
Energy lost due to air movement produced by motor
Where do friction losses occur?
in the bearings
What does a prony brake do?
Measures counter torque (and torque)
What is a real life example of a prony brake?
The dial on a stationary bike
How do we adjust the countertorque of a prony brake?
tighten or loosen the screw
What are the three main parts of a motor?
frame, stator, rotor
Torque is the product of what two quantities?
Force and radius
What are two kinds of stator windings?
lap and concentric
What are the main parts of a stator?
laminated slots, conductors, conductor insolation, protective sheets
What are the main parts of a frame?
frame, end bells, bearings, junction box
Are lap windings the same size or different?
same
are concentric windings the same size or different?d
different
Do lap or concentric windings have varying pitch?
concentric
Slip speed is between ____ and _____ percent
2 and 5
What are the two major components of a stator?
core and windings
What is magnemotive force?
movement of charge caused by a magnetic flux
When current is zero, flux is ___
zero
Torque is called ____ force
Lorentz
What finger points towards Lorentz force
Thumb (torque)
Where is the fan likely located on a motor?
On the front
If a conductor is forced down through a magnetic field, where is the most force on it?
on the bottom
Current always flows from ___ to ____ outside a magnet
North to South
Current always flows from ___ to ____ inside a magnet
South to North
What is duty?
amount of time that a motor can run without a cooling period
The drive end is called the ___ on a NEMA motor
back
What are the four main classes of insulation?
A, B, C, D
Which insulation class can be used for the hottest temperatures?
H
What values are found on the nameplate?
The full load values
Large motors are ALWAYS connected in ___
delta
When is the speed control most practical for a WRM?
from half load to full load
Should you hook up 208 volts to a 230 volt motor?
No
As we add resistance to a WRM, speed ____
decreases
When we hook up less than rated voltage to a motor, current _____
increase
What does a motor do?
converts electrical power into mechanical power
How does a motor work?
mutual induction
What is the advantage of windings in slots of windings on poles?
more flexible manufacturing
What does the frame do?
It holds the stator and its end brackets with their bearings
What does the stator do?
It holds the stationary iron with its slots into which windings are inserted
What do we have to make sure of when we replace a motor in an existing installation?
the same frame
For NEMA motors, is the shaft on the front or the back?
back
Looking down at a motor with the shaft pointing away from you, which side is the junction box on?
the right side
The front end of a motor is referred to as what in IEC?
drive end
1 hp = ____ W
746
Is a motor where the name plate voltage is 230/460 suitable for connection to a 208 volt system?
fuck no
What is the voltage variations?
10%
What is the standard ambient temperature for motors?
40 degrees celsius
What conditions have to be met for us to be able to multiply the power by the service factor?
Ambient temp of 40 or less, line voltages balanced, proper voltage and frequency applied
Can we apply the service factor under non ideal conditions?
No
Many three phase motors are dual voltage, true or false?
true
What voltage are dual voltage motors usually designed for?
240/480
What do terminal designations indicate?
polarity
What do we connect T1 to?
L1
How many windings do single voltage motors have?
Three
How do we connect a dual voltage motor if we want to use the lower voltage?
in parallel
How do we connect a dual voltage motor if we want to use the higher voltage?
In series
How many leads do dual voltage motors have?
12, but only nine are brought out to the JB
How can we change the direction of a motor?
swap any two leads
How many windings do dual voltage motors have?
Three
An ohmmeter reveals three groups of three. What kind of motor is this?
Delta
