Reading...
Play button
Play button
Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
41 Cards in this Set
- Front
- Back
|
Describe the operation of a DC counter E motor starter
|
As the motor starts, the rising potential difference energizes relays which act to cut out the armature resistance as the motor cemf builds up
|
|
|
Describe the operation of a DC current limit motor starter
|
measures the armature current flow and reduces the resistance in the circuit as the starting current decreases
|
|
|
Describe the operation of a DC time limit motor starter
|
Cuts out armature resistance at definite time intervals using time-closinng contacts energized in sequence
|
|
|
Define dynamic braking
|
the motor armature is dosconnected from the power supply and connected to a suitable resistor while the shunt field is maintained at full strength. the motor behaves as a generator and feeds current to the resistor, disspating heat at a rate equal to I^2R
|
|
|
Dynamic braking is also called
|
resistive braking
|
|
|
Define regenerative braking
|
an overhauling load will drive a DC motor faster than it usually operates with a given applied voltage, causing its counter emf to become greater than the applied voltage and result in a generator action
|
|
|
if dynaic braking is used, what must be taken into consideration?
|
allowing a cool down period to allow the heat to disspate from the resistors
|
|
|
What is an advantage of regenerative braking?
|
slows a motor faster from its base speed to stop condition
|
|
|
Define self excitation
|
a small DC generator with permanent magnets is installed on the main rotor shaft. residual magnetism allows some voltage and current to be generated as the generator comes up to speed. output of the DC generator is supplied to the main rotor winding of the AC generator
|
|
|
Define separate excitation
|
separate DC supply is used to provide excitation
|
|
|
Describe brushless excitation
|
a small AC generator is installed on the main shaft. AC voltage is rectified by a 3 phase rotating rectifier also on the shaft. DC voltage is applied to the rotating main field which induces voltage in main stator field. AVR senses the voltage in the supply conductors, converts AC alternator output to DC voltage to supply the stationary exciter field. amount of current supplied is directly proportional to the difference between the actual output voltage and desired output voltage
|
|
|
What are the purposes of a voltage regulator
|
to control system voltage; to regulate the division of reactive power shared between machines running in parallel; control the field circuit closely to keep machine in synchronism with the system when operating at near unity or leading power factor; boost the excitation under system fault conditions to keep machine in synchronism with system
|
|
|
Describe the operation of a direct acting rheostat regulator
|
rolling contacts are arranged so that clockwise movement of pivots cause each sector to roll over a bank of contacts to increase the resistance in exciter field current, counterclockwise to decrease resistance
|
|
|
What controls the movement of the pivots in a direct acting rheostatic regulator
|
controlled by a drum which has a retaining torque exerted by springs and an operating torque derived from the machine terminal voltage
|
|
|
Describe the operation of a silverstat voltage regulator
|
uses a series of silver buttoned leaf springs which close or open contacts in succession. high voltage causes all the cilver contacts to be open resulting in maximum rheostat resistance. low voltage closes all contacts, minimizing rheostat resistance
|
|
|
Describe the operation of an indirect acting rheostatic regulator
|
contacts are continually oscilated by a motor driven cam. under steady voltage, no contact is made. contacts will close circuits between wither voltage raising relays or voltage lowering relays.
|
|
|
describe the principle of an electronic regulator
|
if generator voltage is above normal, the lamp resistance in the bridge circuit becomes greater than that of the lonear bridge arms. output of the bridge is amplified and is employed to reduce the main exciter field current
|
|
|
What is an amplidyne
|
a special type of motor generator which acts as a power amplifier.
|
|
|
how does an ampliydyne work
|
is has a separately excited field arranged so that the excitation power may be varied by a small amount (input or control signal) thus causing a large variation in output power
|
|
|
Describe a magnetic amplifier regulator
|
two windings are wound on a core material with ideal B-H characteristics (flux density-ampere turns). one coil is a series impedance in an AC circuit, the other carrying DC. impedance of the AC circuit can be varied by altering the current flow in the DC coil. small variations in control current can produce large changes of current in the AC circuit. control winding is supplied by a voltage transformer connected to the generator terminals, taken as 3 phase supply from a shaft alternator, connected in series with the main exciter field winding, so changes in output voltage directly control the main field current
|
|
|
Describe what an AVR does
|
it causes the excitation of the generator to be automatically increased or decreased so terminal voltage is maintained by modulating the DC voltage suppplied to the exciter field of the generator
|
|
|
What is the formula for voltage regulation
|
voltage regulation%= (no load volts - full load volts)/(full load volts)x100
|
|
|
what is the difference between a thyristor and a diode
|
thyristors are designed with a gate that allows the amount of rectified DC voltage to be varied, diodes transmit half of the AC sinusoid
|
|
|
What is slip speed?
|
the difference between synchronous speed and actual speed expressed as a percentage
|
|
|
What is a split phase motor?
|
the single phase AC current is split by employing two windings on the stator core (start and run winding). the two windings have different characteristics which causes them to differ in phase
|
|
|
What is a capacitor start motor?
|
a single phase AC current is split into two windings on the stator core (start and run winding). a capacitor is used to make a phase angle between the two windings
|
|
|
what is a repulsion start motor?
|
a single phase AC motor with a rotor cimilar to a DC motor armature. during start up and run, brushes riding the commutator are short circuited. resultant circulation currents in armature produce magnetic poles which are offset from the stator poles. repulsion occurs which rotates the armature
|
|
|
What is a reluctance start motor?
|
contains a start and run wonding. starts as an induction motor. the start winding is cut out ~75%, then the salient poles of the rotor are attracted to the salient poles of the stator. the motor pulls into synchronism (path of least reluctance)
|
|
|
What is a synchronous motor?
|
a revolving voltage is created when AC voltage is applied to the stator, revolving at synchronous speed. separate DC source is supplied to rotor poles thru slip rings to set up permanent magnetic poles which lock into synchronous speed with the rotating stator field
|
|
|
What is required to start a synchronous motor?
|
a drive motor starter or damper windings
|
|
|
What is a drive motor starter?
|
an external exciter motor (AC or DC) brings synchronous motor rotor up to 90% of its synchronous speed
|
|
|
What are damper windings?
|
A special squirrel cage wonding fitted into slots on the rotor pole faces. allows the machine to be started as an induction motor directly across the line
|
|
|
What will happen if a synchronous motor is started without proper starting methods?
|
produces heavy stator currents which trip the machine offline
|
|
|
Which transformer construction is more suited to high voltage applications?
|
core type
|
|
|
Which transformer construction os more suited for channelling flux?
|
shell type
|
|
|
What are four factors that affect tranformer performance?
|
kVA rating
impedance temperature rise harmonics |
|
|
What does lower transformer impedance mean?
|
fewer losses but increases the amount of short circuit current available in the power distribution system
|
|
|
What is an example of a core loss in a transformer?
|
energy required to sustain the magnetic field in the steel core, results from constant magnetizing and de-magnetizing of the core (hysteresis). circulating currents induced by magnetic material generate heat (eddy currents)
|
|
|
What type of connections would be used in an unbalanced transformer?
|
star windings
|
|
|
If the impedance of two paralleled transformers is different, what will happen?
|
current may circulate between the two transformers
|
|
|
List different types of transformer safety controls
|
fast and slow gas detection
oil temperature alarms low oil level protection winding temperature alarms overcurrent and undervoltage protection synchronization checks overezcitation ground fault protection phase sequennce relays dissolved gas monitoring differential protection |
