I4 Learning Task 5 thru 10 Flashcards
How does the rotor of a wound-rotor induction motor differ form that of a squirrel-cage induction motor?
It contains a three-phase winding rather than a squirrel-cage design.
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
When is max torque produced in the wound-rotor induction motor?
when the resistance of the rotor cct is approximately equal to the rotor reactance.
The starting torque of a wound-rotor motor is improved by
inserting resistance across the secondary terminals M1, M2, and M3
To improve the torque output and speed regulation of the wound-rotor motor
the resistance in the rotor cct is decreased as the speed of the motor increases.
why is the efficiency of a wound-rotor motor low when resistance is left connected in the secondary cct?
a great deal of energy is being converted into heat at the secondary resistors.
The wound-rotor motor is intended to be a
variable torque motor
List the advantages of a wound-rotor motor
high starting torque with low current smoother acceleration under heavy load no abnormal heating during starting good running characteristics after the starting resistance is removed adjustable speed
List the disadvantages of the wound-rotor motor
physical size
higher initial and maintenance costs
more complicated control
poor speed regulation when the secondary resistance is left in the cct.
Applications of the wound-rotor motor
overhead cranes
ball mills
driving loaded conveyors
why is it that increasing the resistance of the rotor of the wound-rotor motor cct increases the torque output of the motor?
Increasing the resistance improves the power factor of the rotor thus bringing the rotor flux more in phase with the stator flux
with the secondary terminals M1, M2, M3, short-circuited, what will the starting torque of the wound-rotor motor be? Low or High?
low
to decrease the speed of a wound-rotor motor, resistance
added to the rotor circuit
why is it not practical to control the speed of a wound-rotor motor below 50% slip
speed becomes unstable
Describe the rotor of a synchronous motor
essentially a DC electromagnet with the same number of field poles as in the stator
poles remain constant and do not change polarity
Describe the poles in a synchronous motor rotor
the poles are of the salient type i.e. they project out from the rotor
the synchronous motor direct-current field pole windings are connected to two
slip-rings mounted on the rotor shaft
the synchronous motor direct-current field pole windings are marked
F1 and F2
List the three ways of supplying a synchronous motor with DC field current
external converters
built-in exciter
brushless exciter
Describe an external converter for supplying the DC current to the field in a synchronous motor
usually a simple rectifier
Describe a built-in exciter for supplying the DC current to the field in a synchronous motor
a small DC generator that is mounted on the same shaft as the synchronous motor
field excitation is done by placing a rheostat in series with the shunt field of dc exciter motor
Describe a brushless exciter for supplying the DC current to the field in a synchronous motor
a three-phase alternator and rectifier mounted on the rotor of the machine.
the alternator, rectifier and field of the synchronous motor all rotate together so no need for brushes.
Why are synchronous motor not self-starting?
when trying to start it produces torque in one direction for half the time and torque back in the other direction the rest of the time. Net 0 torque
amortisseur windings is used to
accelerate the synchronous motor close to synchronous speed or in other words used to start the synchronous motor and dampen any speed fluctuations
what happens to the amoritsseur windings if they carry the load too long?
can overheat
Define Pull-in torque for a synchronous motor
max constant under which the motor will pull it connected inertia load into synchronism when its field excitation is applied.
Define Pull-out torque for a synchronous motor
the max sustained torque that the motor can develop at synchronous speed. If it is exceed it will pull the motor out of synchronism
In a synchronous motor the amount of cemf produced is the stator
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is the phasor sum of the voltage applied to the stator and the emf induced into the stator by the rotating field
In a synchronous motor the two voltage vectors, Es and Ec, add together to yield the resultant voltage: Er. This voltage divided by the impedance of the stator windings (Zs), determines the stator current: Is. therfore
anytime Er increases, Is also increases
A synchronous motor with an under-excited rotor field will run with a
lagging power factor and the stator must supply the magnetizing current that the rotor is not supplying
A synchronous motor with an over-excited rotor field will run with a
leading power factor and the stator must supply a demagnetizing current to compensate for the over-magnetization caused by the rotor
synchronous motors with out an output shaft are called
synchronous capacitors and are used solely for power factor correction.
For a synchronous motor, at what point in the starting process is the DC excitation applied to the field?
When the rotor is moving at 95 to 97% of the synchronous speed.
Why is no current induced in the amortisseur winding when the rotor is rotating at synchronous speed
the rotor is locked with the rotating field and not cutting any lines of flux
