J2 Flashcards
List the two ways to start polyphase, squirrel-cage motors
full-voltage (across the line)
reduced-voltage starting
What are the advantages of full-voltage starting
simplest control
lowest component cost
max start torque
min acceleration time
When is reduced-voltage starting used?
where full-voltage starting may cause problems
What is the main function of reduced-voltage starters?
lower the inrush current drawn by the induction motor.
Reduced-voltage starters are usually categorized as
open transition or closed transition
Describe an open transition reduced-voltage starter
the motor is very briefly disconnected from the line as the control transfers from the start to the run position
Describe a closed transition reduced-voltage starter
the voltage to the motor is maintained as the control transfers from the start position to the run position.
Since reducing the starting voltage reduces both stator flux and rotor current, the starting torque is
directly proportional to the square of the starting voltage
A motor that will not start a load under full voltage will
definitely not start a load under reduced voltage
where a motor is started and stopped frequently
a larger-than-normal contactor must be used if the normal life expectancy of the contacts is to be obtained.
the most significant factor in determining whether a motor should be started across the line, or reduced voltage is
whether the power source can handle it.ie
State the advantages that reduced-voltage starting has over across-the-line starting for AC motors
reduced inrush current
reduced torque
less disturbance (line drop) to the power system
Describe primary-resistance starters
a power resistor is connected in series with each of the three-phase lines
when the motor reaches a certain speed the resistance is removed giving the motor full voltage
what is the major disadvantage of the primary-resistance motor starter?
the large power loss in the resistors during starting.
Describe an auto-transformer starter
steps down the starting voltage using taps on an auto-transformer
The auto-transformer starter provides more
starting torque per line ampere of current than any other type of reduce-voltage starter using transformer action
Which three-phase auto-transformer connection can sometimes result in unbalanced starting phase voltages? Wye or Delta
Delta
See page J2 - 19
Why are auto-transformers used for motor starting generally intended for intermittent duty?
if they are left in the cct too long they may overheat and become damaged
the auto-transformer starter with a closed-circuit transition is sometimes called the
Korndorfer-connection
What standard taps are available on motor-starting auto-transformer starters?
80%, 65%, 50%
Auto-transformer starters are sometimes referred to as
compensator starters
Describe the wye-delta motor starter
also called the star-delta requires access to each phase winding only six-lead or 12-lead motors can be used has only one starting step connects in wye for starting connect in delta for running
Since each phase lead is brought out from the motor the rating of the overload relay is based on the
phase value
List the two common methods of automatic acceleration for wound-rotor motors
definite-time acceleration
speed-sensing acceleration
a speed-sensing acceleration starter uses a frequency-sensing relays to sense the speed of the rotor and
time the removal of the secondary resistance according to the load
A regeneration starter used on a wound-rotor-motor replaces the resistors with an electronic cct that
recaptures the energy and transfers it back into the primary cct.
what happens to the magnitude and frequency of the voltage induced in the rotor as the motor accelerates
decrease
most electrical problems fall into one of following categories
opens
shorts
grounds
high resistances
List the parts of a synchronous motor starter
out-of-step relay
field discharge resistor
field contactor
polarized field frequency relay (PFR)
Describe the function of the PFR
monitors the rotor speed to time the application of the DC field current at about 92% and 97% or sync speed
time application so the DC field will be aligned with opposite poles in the synchronous field
disconnects the DC field excitation should an overload or voltage dip cause the motor speed to drop and reapplies it should the speed return to within sync speed before the out-of-step relay opens its contact
List the types of motor brakes
friction or electromechanical brakes plugging dynamic braking regenerative braking eddy-current braking
Most friction motor brakes are
spring-applied and electromagnetically released
Advantages of friction-type brakes
hold motor shaft from rotating after motor has stopped
few control cct components
automatically applied when a power failure occurs
brake cannot reverse the motor
Disadvantages of friction-type brakes
friction parts require maintenance and replacement
tend to be bulky
initial cost of the brake is high
Advantages of plugging
fastest deceleration method
no friction parts
suitable for large motors and severe duty
Disadvantages of plugging
requires reversing controller
requires a zero-speed switch or consistent time-delay switch
will not operate if a power failure occurs
draws large peak currents
may require special bracing to handle the high stress
motor is free to rotate after stopping
if a time-delay relay is applied too long motor may start in reverse
Dynamic Braking or DC injection braking
stopping the motor by making it act as a generator
Advantages of dynamic braking
can not reverse the motor is applied too long
gives smooth positive retardation
braking torque is adjustable by varying DC applied to the stator
no friction parts
Disadvantages of dynamic braking
DC excitation is required during braking
braking is not available if source voltage fails
motor is free to rotate after stopping
frequent stopping may cause motor overheating
Regenerative Braking
makes the motor act like a generator and feed energy back into the system
Disadvantage of regenerative braking
only effective above synchronous speed
Advantages of eddy-current brake
cannot reverse the motor if applied too long
adjusted by varying the voltage across the brake coils
no friction parts
Disadvantages of eddy-current braking
more expensive
not available if source voltage fails
motor is free to rotate after stopping
braking action drops off as the speed of the motor decreases
