Mid Term Flashcards
Connection to the armature of an alternator is normally done using what
Slip rings
The higher power output alternators use a
Revolving field
Non rotating part of an alternator
Stator
Rotating part of an alternator
Rotor
An alternator that has a separate mall alternator mounted on the shaft of its rotor is equipped with a
Brushless exciter
Output frequency of an alternator is controlled by
Number of stator poles
Rotor speed
Alternators should be connected in
Parallel
What is a field discharge resistor used for?
A field discharge resistor is used to limit voltage and prevent damage to an alternator when rotor dc excitation is removed.
What can be used instead of a field discharge resistor?
Diode
What does a synchroscope do?
Test phase angle of an armature
Advantages of a brushless exciter?
Less parts to wear. (No brushes)
Max kva of a rotating armature alternator?
25kva
The amount of emf produced in an alternator is proportional to what?
Number of conductors in the armature
Magnetic field strength
Speed of rotation
Describe a salient pole rotor
Has many poles on the outside of the rotor. Turned at low speeds.
Describe a cylindrical pole rotor
Have 2-4 poles
Used at high speeds
Cylindrical shape with poles inside
Purpose of “f” leads on an alternator
Provide excitation or field current
Two types of stators in a rotating armature alternator?
Permenant magnet
Electro magnet
Purpose of a rotor in a rotating field alternator?
The rotor is rotated by the prime mover to produce a current
What is a prime mover?
The force used to rotate the armature in an alternator. (Harry)
What are 4 conditions that must be met when paralleling alternators?
Phase rotation must be the same
Phase sequence must be the same
Frequency
Voltage output
Output frequency of an alternator is dependent on?
Speed
Number of poles
3 factors that determine output voltage of an alternator
Number of conductors in armature
Magnetic field strength
Speed of rotation
What is electromagnetic induction?
When a conductor cuts magnetic lines of force, a voltage is induced into the conductor
Describe lenzs law
When a closed loop of wire cuts through magnetic lines of flux, current is induced into the loop, creating a magnetic field. This magnetic field opposes the original field.
Is a synchronous motor an induction motor?
No
What is synchronous speed?
Speed of rotating magnetic field in a motor
How is a squirrel cage rotor constructed?
Bars between 2 end rings
Motor with high starting current?
Squirrel cage
Typical percent slip of a squirrel cage motor?
2-5%
Describe single phasing
Lose a single phase of a 3 phase of a three phase motor.
Motor with low starting current and high starting torque?
Wound rotor motor
What are amortisseur windings?
A set of squirrel cage bars on the rotor of a synchronous motor used to start the motor.
What is a synchronous condenser?
A synchronous motor with no load used to correct the power factor of another motor or set of motors.
How is the rotor in a seldom motor constructed?
3 windings connected in wye
What are wound rotors motors used for?
High starting torque, low inrush current
Variable speed control
How are 3 phase motors alike?
All operate on the principle of a rotating magnetic field
All have the same stator design
How are 3 phase motors different?
Have different rotor designs
What is the purpose of the m leads?
Connect your rotor circuit to the resistors in a wound rotor motor.
Describe the starting sequence of a wound rotor motor
Resistance is connected to the rotor circuit to limit inrush current
Energize stator, creating rotating magnetic field
Voltage induced in the rotor is shunted out through the resistors
Resistance is removed making the rotor start to turn
At full rpm all resistance is removed
Starting sequence of the synchronous motor
Energize stator, creating rotating magnetic field
Field cuts amortisseur winding
Current through the amortisseur winding produces a magnetic field that is attracted to the stator field causing the winding to turn
When close to synchronous speed dc is applied to the rotor, causing the rotor windings to become electromagnets
Rotor and stator fields lock in step and rotate at synchronous speed
Describe the wiring configuration of the stator in a wound rotor motor
Windings 120 degrees apart setting up a rotating magnetic field
Stator is the same in all 3 phase motors
Describe the construction of a squirrel cage rotor
Bars connected to 2 end rings
Describe the construction of a wound rotor motor
Has 3 separate windings 120 degrees apart
Same poles per phase as the stator
Coils connected to slip rings on the rotor shaft
Describe the construction of a synchronous motor
Dc excited wound coil with a squirrel cage added to it
What are the brushes in a synchronous motor used for?
Dc excitation to the rotor
Describe how a rotating magnetic field is set up by the stator
The pairs of poles in the stator are wound so that when current flows flow through the winding they develop opposite polarities.
When the stator is energized a rotating field is set up which we call phase rotation.
It rotates at synchronous speed
Windings are 120 degrees apart
How are shynchronous and wound rotor motors the same?
Same stator
Both use wound coils. Wound rotor has 3, synchronous has 1
How are synchronous and squirrel cage motors the same?
Same stator
Synchronous motor used a set of squirrel cage bars in the rotor that are used to start the motor. (Amortisseur winding)
Why are motors important
Cause fuck you that’s why
