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
