Motors Flashcards
What happens to slip speed as load is added
The slip speed increases
Do motors with higher full load rotor speeds have higher or lower percent speed regulation?
Lower speed regulation %
Do motors with higher full load rotor speeds have higher or lower percent efficiency?
Higher efficiency
Which rotor parameter stays constant
Resistance
Which rotor parameter is proportional to rotor speed?
Power factor of the rotor
Pf(r)
Which five rotor parameters increase with slip speed?
EZfIX Voltage Impedance Frequency Current Reactance
What is the formula to find voltage of the rotor
Er = Ebr x s
Which rotor parameters are directly proportional to slip speed
Frequency, reactance and voltage
What does I r(R) represent
Active, in phase, horizontal or resistive current in the rotor
What is breakdown or pullout torque
The point at which max torque is reached and any added load causes the motor to stall.
State the formulas to find breakdown torque
Rr = Xr Ir(R) = Ir(x) pf(r) = .707 or cos45* 0theta(r) = 45*
If mechanical load is reduced slip speed will do what
Decrease
If mechanical load is increased then what will happen to %slip?
Increase
As slip speed increases, what will happen to rotor speed?
Rotor speed decreases
If mechanical load is increased what will happen to the impedance of the rotor (Zr)?
Z(r) increases
Within normal range, as load is increased, what will happen to rotor torque
Torque increases
As load is decreased what will happen to rotor voltage
Deceases
Slip goes does down as load decreases, voltage is proportional to slip
As load is increased what will happen to reactance of the rotor
Rotor reactance will increase
As slip is decreased what will happen to the power factor of the rotor?
Power factor will increase
As load is increased what will happen to rotor resistance
It will stay the same because it is conctant
As load is increased from no load to full load, the stator current will do what
Increase
As load increase, motor current will do what
Increase
Is motor current directly proportional to power and torque?
No, but it will increase with them
Is torque directly proportional to slip?
Yes
Is mechanical power output directly proportional to slip
Yes
What causes torque in a motor
The interaction of the rotor in the stator fields
What is another name for breakdown torque?
Pullout torque
Both nema A and B design rotors produce a max torque of what percent of sync speed?
75-80%
The rotor winding ms of a WRM are brought out and connected to what?
Slip rings
The rotor current of a WRM is carried through what?
Brushes to external resistors
Is the resistance of the WRM rotor circuit high or low at start ?
High
What happens to the external resistors of a WRM once it has started?
They are bypassed so that the rotor runs with a low resistance at full load speed.
What are the best starting characteristics for a motor?
High resistance (R) and low inductance(L)
What are the best running chracteristics for a motor?
Low resistance (R) and low inductance(L)
Can you change the speed of a WRM by adjusting the resistance of the rotor? Explain
Yes, the greater the resistance the lower the speed
At what speed range can a WRM be used for speed control?
Half load to full load… torque is unstable at lower speeds
Which rotor produces the best starting torque with the lowest starting current
Wound Rotor Induction Motor
What are some disadvantages to a WRM?
Higher cost
Larger frame size
More maintenance
Additional equipment ( resistors and controller)
Define synchronous speed
The speed of rotating magnetic field in the stator
Nsync = 120f/N
Define slip speed
The difference between rotor speed and sync speed
Ns = nsync - nr
Define slip
The ratio of slip speed to sync speed
s = ns /nsync
When is stator frequency the same as the rotor frequency?
At the instant of start
Blocked rotor
When is rotor reactance at max?
At the instant of start (blocked rotor)
Xr = rotor
When is rotor impedance at max?
When rotor reaches slip speed
What is proportional to the rotor power factor?
Rotor speed is proportional to Pf
What is proportional to active rotor current?
Torque
Ir(R) = active rotor current Ir(R) = Ir x Pfr
When does breakdown torque (max torque) occur?
When the active current equals the reactive current.
List four disadvantages of WRIM?
Higher cost
Larger frame
Requires aux equipment
Extra maintenance
Lost four losses that contribute to inefficiency in a SCIM?
Hysteresis
Eddy currents
Windage
Friction
What are the iec names for t1 T2v and t3?
U1, V1,W1
How many leads does a dual voltage motor have ?
12 leads in total, but only 9 at the terminal box
Which terminals are T1,T4, and T7 typically associated with ?
Phase A
How many leads does a 9 lead motor have?
12 leads. But only 9 at the terminal box
True or false
The high voltage dual voltage delta motor is connected in parallel
False
Dual voltage delta is connected in series
Is the high voltage wye connection of a dual voltage motor wired in series or parallel?
Series
How many groups of windings in a 3 lead motor
Three groups
How many groups of leads are in a 6 lead wye delta motor?
3 groups of 2
Is name plate data of a wye delta motor for the wye or delta connection ?
It’s is for the delta connection
Based on running condition
This motor starts in wye and runs in delta
True or false
A part winding motor has two or more sets of 3 phase windings
True.
Once started all winding are connected in parallel for running operation
Which motor has one group of 3 leads and 3 groups of two leads?
Dual voltage 9 lead wye motor
True or false
A 9 lead delta connected motor had 3 groups of 2 leads
False
A 9 lead delta connected motor has 3 groups of 3 leads
True or false
A delta connected 9 lead motor is wired in series for a high voltage connection.
True
Rotor resistance can be increased by (increasing/decreasing) the thickness of the rotor bars
Decreasing
Rotor inductance can be (increased/decreased) by imbedding the rotor bars in the rotor
Increased
Which rotor design is the most efficient?
NEMA A
Which rotor design produces the highest starting current
NEMA A
Which rotor design is the most common
NEMA B
Which rotor design produces the lowest starting current
NEMA D
Which rotor design is the least efficient
NEMA D
Which rotor designs produces a breakdown torque of 200% of full load
NEMA A And B
Which rotor designs produces starting torque 150% of full load
NEMA A And B
Which rotor design is made of a double squirrel cage
NEMA C
Which rotor produces the maximum starting torque
NEMA D
When do NEMA D motors produce breakdown torque?
At start
Which rotor design produces the lowest full load slip
NEMA A
