Line Y-1 Flashcards
What is the given name to the stationary part of the motor?
Stator
What is the given name to the rotating part of the motor?
Rotor
What is the purpose of the laminations in the cores of the rotor and stator?
To reduce Eddy currents
Why are the bars of the squirrel cage slightly skewed so that they are not parallel with the slots in the stator?
To make the motor run smoothly
What is the main purpose of the end bells on a motor?
To house the bearings that support the rotor shaft and to keep it aligned with the stator
What two ways are motor enclosures classified?
The way they protect the motor from the environment
The way they are cooled
What name is given to the machine with an enclosure that does not prevent the free exchange of air between the inside and the outside of the machine?
Open machine
What name is given to the machine with an enclosure that prevents the free exchange of air between the inside and the outside of the machine?
Totally enclosed machine
The windings of a three-phase winding is made up of three single-phase windings placed _________ electrical degrees apart from each other
120
What are the two purposes of the stator core?
Support the stator windings
Provide a low-reluctance path for the magnetic circuit
What does the frame designation tell about the motor?
The physical dimensions of the motor in NEMA standards
Does the horsepower rating of the motor refer to input power or to output power?
Output power, at rated speed
What does the service factor of the motor tell about the motor?
The service factor, when multiplied by the horsepower rating of the motor, gives the maximum power by the motor may be required to supply continuously without damaging the motor
What would an electrician use the insulation system designation for when installing a motor?
To determine the insulation (temperature) type of the supply conductors
What does the ambient temperature rating refer to?
The highest temperature allowed for the cooling medium that contacts the heated parts of the motor
What units is the speed of the motor rated?
Revolutions per minute (RPM)
For a motor to operate successfully under running conditions at full load, by what maximum percentage may the frequency of the supply vary?
It should not vary more than 5% of the rating on the nameplate
For a motor to operate successfully under running conditions at full load, what is the maximum variation from rated voltage allowed?
It should not vary more than 10% of the rating on the nameplate
What relationship is expressed by the design letter on a motor nameplate?
The relationship between motor torque and motor speed
Define the term “full-load current”
The full-load current is what the motor will draw when it operates at rated voltage and supplies rated horsepower
The flux created by any winding will be at a maximum when the current through that winding is at a __________
Maximum
When the current through a winding reverses, the flux direction created by that winding __________
Reverses
What effect does increasing the frequency of the supply have on the speed of the rotating magnetic field?
Increasing the frequency of the supply increases the speed of the rotating magnetic field.
What effect does increasing the number of poles have on the speed of the rotating magnetic field?
Increasing the number of poles in the machine decreases the speed of the rotating magnetic field
Define the term synchronous speed
The synchronous speed is the speed of the rotating magnetic field.
Since there is no electrical connection between the stator and the rotor, how is current made to flow in the rotor of a squirrel-cage induction motor?
Through electromagnetic induction
Define the term slip speed
Slip speed is the difference between synchronous speed and rotor speed.
Define the term percent slip
Percent slip is the slip speed expressed as a percentage of the synchronous speed.
Define the term speed regulation
Speed regulation is the difference between no-load speed and full-load speed of the rotor
How is the direction of rotation reversed in a squirrel-cage induction motor?
By reversing the direction of the rotating field. This is accomplished by interchanging any two of the stator line connections to the source
Define the term full-load torque
The full-load torque of a motor is the torque necessary to produce rated horsepower at rated full-load speed.
Define the term breakdown torque
The breakdown torque of a motor is the maximum torque the motor will produce
Define the term locked rotor torque
The locked rotor torque of a motor is the maximum torque the motor will develop at rest for all angular positions of the rotor with rated voltage and frequency applied to the stator
At what rotor speed will the stator current be the highest?
When the rotor is at a standstill (100% slip)
Which part of the linear-induction motor actually moves?
The stator
Relative to the direction of the moving magnetic field, in what direction does the linear-induction motor move?
The stator travels in the direction opposite that of the moving field.
How does the rotor of a wound-rotor induction motor differ from that of a squirrel-cage induction motor?
It contains a three-phase winding rather than a squirrel-cage design. The rotor may be either wye- or delta-connected, with the leads brought out so that external resistance may be placed in the rotor circuit
What is the purpose of the three slip rings and their corresponding brushes on the wound-rotor motor?
The slip rings and brushes provide an electrical connection between the moving rotor and the external rotor leads
How are the three rotor terminals designated?
M1, M2 and M3
What is the purpose of the spring on the brush holders?
The spring on each brush holder applies force on the brush to keep it in contact with the slip ring
What is the purpose of the pigtail or shunt attached to each brush?
It makes the electrical connection between the brushes and the external rotor leads.
What two additional pieces of information are normally supplied on the nameplate of a wound-rotor induction motor that you would not find on a squirrel-cage induction motor?
rotor voltage and rotor current
A wound-rotor motor is designed mainly to be a variable- _____________ motor
Torque
With respect to locked rotor reactance, what value of rotor resistance will give maximum starting torque?
The resistance should be approximately equal to the locked rotor reactance.
Torque is directly proportional to rotor current. Why is it, then, that increasing the resistance of the rotor circuit increases the torque output of the motor?
Increasing the resistance of the rotor circuit improves its power factor and brings the rotor current more in phase with the stator flux
With the secondary terminals M1, M2 and M3 short-circuited, will the starting torque of the wound-rotor motor be high or low?
Low
Why is the efficiency of the wound-rotor motor poor when resistance is left in the rotor circuit?
The resistors dissipate energy in the form of heat.
To decrease the speed of a wound-rotor motor, the resistance is:
Added to the rotor circuit
Why is it not practical to control the speed of a wound-rotor motor below 50% slip?
The speed of the motor becomes too unstable beyond 50% slip.
What does the term duty cycle mean when applied to the secondary resistors?
The amount of time the resistors are expected to carry current with respect to the amount of time they are not
How is the direction of rotation reversed on a wound-rotor motor?
By interchanging any two of the line leads to the stator
State three advantages a wound-rotor motor has over a squirrel-cage induction motor.
High-starting torque with low current
Smooth acceleration under heavy load
No abnormal heating during starting
State three disadvantages a wound-rotor motor has over a squirrel-cage induction motor.
Large physical size
Initial and maintaining costs higher
More complicated control
What is the name of the squirrel-cage winding placed near the surface of the field poles in a synchronous motor?
Amortisseur winding (or damping winding)
What are the two functions of amortisseur winding in a synchronous motor?
The amortisseur winding’s main purpose is to dampen any momentary speed fluctuations that may occur when the mechanical load on the motor is suddenly changed.
Its secondary purpose on some synchronous motors is to start the motor and accelerate it up to speed
Is the rotor field winding fed with alternating current or direct current?
Direct current
How is the field excitation supplied to the rotor?
Through slip rings and brushes
How are the field terminals of the synchronous motors identified?
F1 and F2
When are external converters normally used to supply the field excitation to a synchronous motor?
When the DC supply is required for other loads in the plant
How is the level of DC excitation controlled in an external converter?
By placing a rheostat in series with the DC supply
Where is a DC exciter for a synchronous motor normally mounted?
On the same shaft as the synchronous motor
How is the level of DC excitation controlled in a built-in exciter?
By varying the exciter’s voltage output, by using a rheostat in series with the shunt field of the exciter
What provides the DC excitation in a synchronous motor with a brushless exciter?
A three-phase alternator and rectifier assembly mounted on the rotor of the synchronous motor
How is the level of DC excitation controlled on a synchronous motor equipped with a brushless exciter?
By varying the field current to the brushless exciter
Why is the synchronous motor not self-starting?
The rotating stator field is travelling too fast for the rotor to accelerate and lock in step.The field moving past the rotor produces constant torque reversals. This results in a net starting torque of zero
At what point in the starting process is the DC excitation applied to the field?
When the rotor is close to synchronous speed. The percentage of slip varies with the mechanical load on the motor
During acceleration and deceleration of the rotor, why is the field short-circuited or connected across a discharge resistor?
To prevent a high voltage from being induced in it
Can the direction of rotation of a synchronous motor be reversed by reversing the polarity of the DC field supply?
No. The direction of rotation is reversed by reversing the direction of rotation of the stator field. This is done by interchanging any two of the supply leads to the stator
What type of induction motor is sometimes used to start synchronous motors that have large starting torque requirements?
Wound-rotor induction motor
Why is no current induced in the amortisseur winding when the rotor is rotating at synchronous speed?
Because there is no relative motion between the stator field and the amortisseur winding at synchronous speed
Why is it a problem for the synchronous motor to turn at lower-than-synchronous speed for extended periods of time?
The amortisseur winding may overheat
Define the term pull-in torque
Pull-in torque is the torque required to pull the rotor into synchronism with the rotating stator field once the DC excitation has been applied
What is the term used to describe the angle between the centre of the stator poles and the centre of the rotor poles?
Torque angle
Define the term pull-out torque
The maximum sustained torque that the motor can develop at synchronous speed
What happens if the pull-out torque is exceeded?
The motor will pull out of synchronism and slow down
As load is added to a synchronous motor, what happens to the power factor of the motor?
It becomes less leading (or more lagging)
What controls the magnitude of the current flowing in the stator windings?
The resultant of the applied voltage to the stator and the voltage induced in the stator by the rotor field
How does the torque angle of the rotor affect the stator currents?
When the torque angle changes, the angle of the voltage induced in the stator by the rotor also changes.This results in a new resultant emf (ER) in the stator. The new resultant voltage causes a new current to flow in the stator
What happens to the power factor of a synchronous motor when the field current is increased under a fixed load?
Increasing the field current with a fixed load will cause the power factor to become more leading (or less lagging)
When a synchronous motor is operated over or under-excited, does this change the in-phase component of the stator current?
No
At approximately what percentage of full-load speed do the centrifugal switch contacts open?
75%
Describe the constructional difference between the main and auxiliary windings.
The main winding is made of many turns of larger wire and is placed lower in the stator slots to give it a lower resistance and higher inductance.
The auxiliary winding is made of fewer turns of smaller wire. It is placed near the surface of the stator slots to give it a higher resistance and lower inductance
What is the physical displacement between the main and auxiliary windings of a two-pole split-phase motor?
In a two-pole motor, it is 90 mechanical and electrical degrees.
How is the direction of rotation reversed in a split-phase motor?
By reversing the direction of current through the auxiliary winding
Why does a standard split-phase motor (without a starting capacitor) have a low starting torque compared to a capacitor-start, split-phase motor?
The standard split-phase motor has a phase shift between the main and auxiliary winding of 30–50 electrical degrees. The capacitor-start, split-phase motor has a phase shift of 90 electrical degrees, which corresponds better with the physical displacement of the poles in the motor
Which type of split-phase motor combines good starting torque with good overload capacity?
The capacitor-start, capacitor-run split-phase motor
Which type of split-phase motor operates at synchronous speed?
The reluctance motor
State the relationship between load and speed for a series motor.
The speed decreases as load is added.
How does the rotor part of the series motor receive its energy?
Through the brushes and commutator
How is the direction of rotation reversed in a series motor?
By interchanging the two armature leads, or the two series field leads
What may happen if a large series motor is operated with no load connected?
The motor may run away
What two variables affect the counter emf generated in the armature?
The speed of the armature and the strength of the series field
What two variables affect the torque developed by the rotor of the series motor?
The current through the armature and the strength of the series field
Will an AC series motor operate on DC?
The AC series motor will operate satisfactorily from DC and up to 60 hertz AC.
Why does the direction of rotation not change each time the alternating current changes direction?
Both the field polarity and the current through the armature change at the same time, resulting in no torque reversal
Why are the stator and rotor cores of the universal motor laminated?
To reduce eddy-current losses
What are the two types of windings in the stator of a shaded-pole motor?
Exciter winding, shading coil
How do the two stator windings receive their energy from the source?
The exciter winding receives its energy by conduction. The shading coil receives its energy by electromagnetic induction
In what direction does the field move across the pole face in a shaded-pole motor?
From the unshaded portion to the shaded portion
How is the direction of rotation of a shaded-pole face motor reversed?
By removing the stator and turning it end-for-end before replacing it over the rotor
Is the starting torque of a shaded-pole motor high or low compared to a split-phase motor?
Low
At what speed does the rotor of a hysteresis motor rotate?
Synchronous speed
From what type of steel is the rotor of a hysteresis motor rotor made? Why is this the case?
Hard steel; because of its large hysteresis loss (retains permanent magnetism)
