AC Machines Flashcards
What is an AC machine?
A device that converts AC mechanical energy into electrical energy
What are the three basic types of AC motors?
SCIM - squirrel cage induction motor
WRIM - wound rotor induction motor
Synchronous motor
In an AC generator the windings are placed on the ______
Stator
How do you change direction of rotation of a three phase motor?
Change any two stator leads
What is synchronous speed?
The rotational speed of the magnetic field
Rate of the stator field rotation
How is the synchronous speed determined?
By the number of stator poles (poles per phase) and the frequency of applied voltage
How can you determine the direction of rotation?
Make an electrical connection prior to a mechanical connection, energize, and confirm direction. Bump Test
Use a phase rotation meter (not common)
How is the rotation speed of the magnetic field determined?
Frequency
How can we control the rotational speed of the magnetic field?
Control the frequency with a VFD
What is the formula for finding synchronous speed?
S = 120 x f / P
P is number of poles per phase.
What are the two principles necessary for the operation of a SCIM magnetic field?
A stator magnetic field
A rotor magnetic field
T or F: rotor bars of a SCIM have high resistance?
False. Very low resistance
What is rotor slip?
The difference between synchronous speed and rotor speed.
If sync speed and rotor speed were the exact same there would be no lines of flux being cut
T or F: rotor speed can never be equal to synchronous speed in a motor?
True. If rotor speed and sync speed matched exactly, there would be no cutting of magnetic flux.
How do you calculate rotor slip?
Slip % = sync speed - rotor speed / sync speed
or Slip % = no load - full load / no load
Slip amount = sync speed - rotor speed
How do you find rotor frequency with sync speed?
F = P x S / 120
P is number of stator poles
S speed in RPM’s
As rotor accelerates to full speed, what happens to the frequency of the rotor? (Both SCIM and WRIM)
Decreases
What is the code rule for marking of equipment (nameplate)
2-100
What is the formula to reflect the horsepower and torque relation
HP = (torque x speed) / constant
What is the ‘constant’ number for foot pounds?
5252
What does the nameplate RPM reflect?
The revolutions per minute the motor will run at, at full load. Motor will run faster as light or no load, and will run slower at overload
When a nameplate states the motor type as induction, how do you tell the difference between a SCIM and a wound rotor?
The wound rotor induction motor will have three visible slip rings on the rotor shaft
Frame charts are available to determine exact frame sizing, but what is a general rule of thumb for sizing frames?
Centerline shaft height above the bottom of the base is the first two frame digits divided by 4.
Frame 143T: 14 divided by 4 = 3.5inches. The shaft height would be 3.5 inches above the motor base.
Which NEMA design code is most common?
B
What are the three factors that determine torque?
Strength of stator magnetic field, strength of rotor magnetic field, and the phase angle difference between the stator and rotor magnetic fields
What is the formula to find Torque?
T = HP x 5252 / RPM
At the instant of start up, what is the slip? What is the rotor RPM?
Slip is at 100%, rotor is at 0 RPM
At the instant of start up, voltage is at ____
Maximum.
At the instant of start up, current will be ____
Very high, it will create a strong magnetic field around rotor bars (conductors). Very high current around rotor bars will cause high current in stator, causing inrush current.
Rotor frequency is proportional to what?
Directly proportional to the slip.
When rotor is not rotating but the stator is energized, the frequency will be the same as the supply. Once the rotor starts turning, the frequency in the rotor decreases
When will rotor frequency match the supply frequency?
When the rotor is not turning but the stator is energized
When the rotor is turning at full speed, what will the frequency be?
Very low, only a couple of Hz
Phase windings are placed how many degrees apart in the stator?
120°
How does an induced voltage motor operate?
The stator windings are connected to the supply voltage, when power is applied a current flows through the windings causing an expanding electromagnetic field which cuts across the rotor bars (conductors). When these conductors are passing through a magnetic field from the windings, emf is induced in the conductor, causing current flow in the conductors. The current in the stator and rotor bars are constantly changing direction (AC), causing the rotor to become an electromagnet.
What is another term for locked rotor torque?
Starting torque
What is starting torque?
The amount of torque developed when rated voltage and frequency is applied to the stator but the rotor has not started turning (very brief moment)
What amount of locked rotor torque is produced in a NEMA B?
About 150% of its full-load torque
What is full-load torque?
The torque developed when a motor is operating at rated speed (slip speed)
What is another term for starting current?
Locked rotor current
Starting current or locked rotor current is ____ the amount of full load current
Six times
What is the maximum slip % of a NEMA B motor?
5%
What section of the code covers motors?
Section 28
How do you calculate speed regulation?
%Speed Reg = Sync - rotor speed / rotor speed
Or no load speed - full load speed / full load speed
How do you improve power factor of a SCIM?
Add more load, install PF correction capacitor
How would you increase rotor RPM on a SCIM?
Increase frequency, decrease load.
In a SCIM, how many electrical degrees apart are the single phase windings?
120°
What are the three main parts that make up a SCIM?
Stator, rotor, end bells/caps
What are the three principles of the magnetic field for the operation of a three phase motor?
- Voltages are 120° out of phase from each other
- Voltages change polarity at regular and equal intervals
- The arrangement of stator windings. Matching poles must be directly across from each other.
When a motor states that it is a 6 pole motor, what does that mean?
6 poles per phase
How can you change the direction of the magnetic field (change direction of the motor)?
Switch any two stator leads (switch any two lines from supply to the motor terminals)
As mechanical load is applied to motor shaft of an induction motor, what happens to the speed?
The rotor speed will decrease. More load means more induced voltage and currents in rotor bars to increase - causing greater torque and reduced speed.
In an induction motor, as voltage in the stator increases and current in the rotor increases, what happens to torque?
Increases.
Vice versa. Voltage and current decrease, torque decreases, speed increases
How do you find rotor frequency at slip speed with slip percent?
Fr = Slip % x Source Frequency
Fr is Frequency of rotor
When supplied voltage is reduced by half, rotor current and voltage is also reduced by half, what is the rotor torque?
Rotor torque will be 25% of its original value.
In an induction SCIM, what percent slip is typically breakdown torque?
20%
What is the difference in PF between no load and full load of an induction motor?
At no load PF will be very low, the phase angle will be very large.
At full load, PF will be very high, around 85-90% lagging, and phase angle will be very small.
What is the formula for efficiency of an induction motor?
Eff = output / input x 100
What is the formula to find output of an induction motor?
Pout = Pin - copper losses and stray power losses
Copper losses = I^2xR
What are the advantages of a SCIM?
- Excellent speed reg, small percent slip - ideal for constant speed applications
- Simpler design, little maintenance required
- No brushes or slip rings - ideal for hazardous and explosive locations
What is the main difference between the SCIM/WRIM induction motors and the Synchronous motor?
SCIM and WRIM use magnetic induction to power the rotor, the SYNC motor uses an external DC source to power the rotor.
Which code rule outlines marking of motors?
2-404 Marking of motors
What are the types of motor enclosures?
TEFC: totally enclosed fan cooled
TE: totally enclosed
DP or ODP: drip proof or open drip proof
WP: weatherproof
XP: explosion proof
What are the types of bearings in a motor?
Bushing
Ball bearing
Roller bearing
In a dual voltage motor, how are the windings connected for 1. High voltage, and 2. Low voltage?
High voltage the windings are connected in series
Low voltage the windings are connected in parallel
For a dual voltage motor connected in Wye, what is the distinction between a 9 lead and a 12 lead drawing?
A 9 lead will be connected in the middle, a 12 lead will not be connected in the middle.
What type of connection would be needed for a wye - delta starter?
12 lead Wye
What happens if a line on a three phase motor becomes open?
Single phasing. If open occurs while running, motor will continue to run at reduced capacity. Motor will not be able to restart if there is an open because there will not be enough torque
When a motor is single phasing, how many degrees out of phase are the stator and rotor currents?
90° rotor current will lag stator current.
What is frame type C?
Flange mounted motor with threaded bolts
What is type D frame?
Flange mounted motor, non threaded bolts.
What is type H frame?
Base can be mounted in either 56, 143T, or 145T positions.
What is type J frame?
Mounts specifically jet pumps, has a 56C face.
What is type JM frame?
Pump shaft is designed for mechanical seal
What is JP type frame?
Pump shaft is designed for a packing type seal
What is type S frame?
Short shaft. Not intended to be used with belt drives. Couples directly to the load.
What is the T frame?
Made after 1964
What is type U frame?
Manufactured between 1952 and 1964
What is type Y frame?
Indicates motor has a special mounting configuration. Does not specify what mounting configuration is, just that it’s special
What is type Z frame?
Indicates that the shaft is special in some irregular way. Could be larger I diameter, threaded, longer etc.
On the nameplate, what does Max Temp indicate?
The rise in ambient temperature the motor will exhibit when operating at continuous full load.
What are the four classes of insulation ratings?
A, B, F, and H
What is the temperature rating for class A insulation?
105°C
What is the temperature rating for class B insulation?
130°C
What is the temperature rating for class F insulation?
155°C
What is the temperature rating for class H insulation?
180°C
What is insulation rating based off of?
The temperature of the hottest part of the motor and the ambient temperature rating.
What is the rotor circuit referred to in the CEC?
Secondary circuit. Section 28
When is a WRIM necessary?
When strong starting torque and range of speed control is needed.
Used for cranes, elevators, large pumps, large compressors.
Which motor has the highest starting torque?
WRIM
What are some advantage of the WRIM?
Variable torque, starting current can be controlled, smooth acceleration, power factor can be controlled by resistance banks, windings sustain higher inrush current than rotor bars
What is the power factor at optimal torque of a WRIM?
70.7%
On a WHIM, how many electrical degrees apart are the sine waves?
120° out of phase with each other
How is the induced voltage of a SCIM determined?
- Strength of stator magnetic field
- # of turns being cut
- Speed of cutting action
When is maximum torque developed in an induction motor?
When the stator and rotor flux are in phase with each other.
What is starting current/locked rotor current/inrush current determined by?
Type of rotor bars, HP rating, and applied voltage
T or F: For any given value of slip, the torque output of an induction motor will vary as the square is applied voltage.
True
T or F: synchronous motors can operate at synchronous speed.
True
What are synchronous condensers?
A synchronous motor used to correct power factor
What is the starter winding of a synchronous motor called?
Amortisseur winding
Squirrel cage winding
Which motor maintains a constant speed from no load to full load?
Synchronous motor
Are field discharge resistors connected in parallel or series to the rotor winding of a synchronous motor?
Parallel
Why are field discharge resistors connected in parallel on a synchronous motor?
To reduce induced voltage
Which SCIM rotor design has the highest resistance?
Design D, type A bars.
A double squirrel cage starting torque can be as high as ____% rated full load torque
250
At no load, how much is the current lagging the applied voltage?
90°
At no load, power factor is ____
Close to 0 because there is no VARS
What happens when a motor is single phasing?
If running: Rotor will continue to turn at reduced speed, rotor will have high reactance and low resistance, torque will decrease, current will be higher than normal, will cause damage to stator windings.
If not running: voltage induced will not be high enough to start motor, current will still flow which will cause damage.
On a WRIM, as load increases, rotor frequency _____
Increases
What is the formula to find rotor RPM when given the full load Hz?
Hz x 60 x 2 / poles
Hz: hz at full load (usually very small amount)
60: seconds per minute
2: number of pulses of one sine wave per phase
What is the purpose of the field discharge resistor?
Helps to reduce the starting voltage across the windings
What are the connections of M2, and M3 of a WRIM?
M2 is connected to the middle slip ring, M3 is connected close to the rotor windings
The amount of current flow in the rotor of a WRIM, is determined by
Amount of induced voltage and total impedence
I = E/Z
Why is torque higher per amp in a WRIM than a SCIM?
Because of the resistance bank, rotor and stator current are closer to being in phase with each other.
one factor to determine torque is the phase angle difference of stator and rotor flux
What will happen to a WRIM if no resistance is connected?
It will not start. No current flow.
How is speed controlled in a WRIM?
By removing resistance. Leaving a small resistance will allow the motor to continue to run at a slow speed because stator and rotor flux is reduced.
What is the armortisseur winding?
The set of squirrel cage rotor bars between the stator and the rotor windings
What is the purpose of the armortisseur winding when the motor is running?
To dampen torque fluctuations when a load changes. Keeps rotor and stator at sync speed
What is breakdown torque?
The greatest amount of torque that the motor can incur.
200-250%
What are the start and run characteristics of a WRIM?
Start: high resistance, low inductance, low starting current, high torque
Run: low resistance, low inductance, high current, fair torque, very efficient
What is pullout torque?
When the load on the motor is too great, the rotor can be pulled out of sync from the magnetic field. The force pulling the magnets apart is greater than the strength of the magnetic attraction.
What is the pullout torque amount of a synchronous motor?
150-200%
T or F: a synchronous motor can be started from both an AC or DC source?
F. Only DC source.
Ex) DC generator.
What is the benefit of using a synchronous condenser over a capacitor bank?
Capacitor bank cannot be easily controlled. A synchronous condenser only has to have the excitation current changed to cause an alteration of power factor.
What type of motor is a selsyn motor?
A self synchronous motor.
What are selsyn motors used for?
To provide angular feedback, and to provide position control
Do selsyn motors operate on single phase or three phase?
Single phase AC
What are the three single phase motors?
- Resistance start induction run
- Capacitor start induction run
- Capacitor start capacitor run / permanent slip capacitor
How do split phase motors operate?
They split the current through two separate windings to simulate a two-phase power system
What are the three factors that cause a magnetic field to rotate?
- Voltages change polarity at equal intervals
- Voltages are 120° out of phase from each other
- The arrangement of windings around the inside of the motor
When is voltage induced at its maximum?
At the instant of start up
When frequency and inductance decrease, what happens to power factor and torque?
Increase
When frequency and inductance increase, what happens to power factor and torque?
Decreases
What two types of current is required for a synchronous motor
Power current, field excitation current
What are the three types of excitation in a synchronous motor?
Under excited, normally excited, over excited
What are the two main applications for a capacitor in an AC circuit?
PF correction, increase starting torque in a single phase motor
In a capacitor start motor, the capacitor is placed in series or parallel with the start winding and centrifugal switch?
Series
At what percent of rated speed can the start winding be disconnected from the circuit?
75%
How is the centrifugal switch connected in a circuit?
In series between the start and run winding
What is the purpose of the centrifugal switch?
To disconnect the starting winding from the running winding at 75% rated speed
What is the advantage of a capacitor start motor over a split phase motor?
Better starting torque
What is the disadvantage of a capacitor start motor over a split phase motor?
Increased cost due to the capacitor
How much torque can a capacitor start motor provide in comparison to a split phase motor?
1.5 times more torque. Starting torque can be 200-350% of full load torque
How do you reverse a capacitor start motor?
Reverse the start or run winding connections, preferably the start winding
What are the properties of the run winding?
Many turns of larger wire, low resistance, high inductive reactance
What are the properties of the start windings?
Fewer turns of smaller wire, high resistance, low inductive reactance
Start windings are placed at a ____ angle in reference to the run windings
90°
Should the run or start winding be placed closer to the top of the stator core?
Start winding
What are the start and run windings numbered as?
Run winding: T1 and T4
Start winding: T5 and T8
How would you connect the terminals to have a split phase motor run clockwise?
Connect T5 of the start winding to T1 of the run winding
How would you connect the terminals of a split phase motor to have it run counter clockwise?
Connect start winding terminal T5 to terminal T2 or T4 of the run winding.
