Motor Control Flashcards
Why is a 3-phase induction motor more commonly controlled by an across-the-line starter, rather than a reduced voltage starter?
The across-the-line starter has a high starting torque, low cost and simple construction
Which breaking method will apply a constant waveform to the stator of a motor to bring it to a stop
DC injection
Why are NEMA magnetic motor starters rated in horse power
To be able to safely interrupt the locked-rotor-line current of the motor
During an overload condition of a SCIM, the line current to the motor is interrupted by what?
Main contactor power contacts
Plugging on a 3-phase induction motor is accomplished by which method
Momentarily reversing the phase sequence to the stator
Which reduced voltage starter is an open transition
Wye/Delta without power circuit resistors
A paper winder is controlled by a VFD, what is the most appropriate torque setting for this application
Constant horsepower
Which “quick stopping” method will generally decelerate a 3 phase induction motor in the shortest possible time
Plugging
If automatic restarting of a motor after a power failure represents a safety hazard for a motor drive, then what should the control circuit provide
Low voltage protection
When starting a WRIM with starting resistors, the highest starting torque achieved when the secondary circuit is what
At maximum resistance
If an accidental ground fault develops just before the motor coil while the motor is running, what will be the most likely result
The motor will stop due to the fuse opening
Which starter provided the lowest starting line current for a given motor
Wye/Delta (1/3 starting current)
VFD’s can cause electrical problems? What is not one of the problems that can be caused by a VFD
High inrush current
When initially energized, what percent of line voltage across each individual coil or a 6 lead 3 phase SCIM motor when using a Wye/delta starter
57.7%
Which of the following is not one of the main components of a VFD
Contactor
A simple stop start 3-wire motor control circuit will run the motor if the start button is held on but stops if the button is released, what is the likely cause of this problem
Defective holding contact
Which method of starting a SCIM would have the highest starting torque
Autotransformer set to 80% tap
Which starting method would lend itself to the smoothest acceleration?
Three-stage wound rotor
What is one disadvantage of using a primary resistance starter
Power losses
With regard to a workers electrical safety, what does the term isolate mean?
To disconnect all energy sources
If automatic starting after a power failure does not represent a safety hazard for a sump pump, then the control circuit should be wired to provide what?
Low voltage release
Yours asked to troubleshoot a conveyor that has stopped working, It is controlled by a VFD, what is the first thing you should do?
Check the VFD for faults
With primary impedance starters, reactors are used instead of resistors for motors with higher currents because the resistors could overheat (T or F)
True
Dynamic breaking on a 3 phase SCIM is achieved by what method
Disconnecting the AC supply and applying a DC voltage to the stator leads until the motor comes to a stop
What does the term combination starter generally refer to?
A starter that has a disconnect, a contactor, with over current and overload protection
How many SCR’s would be in each line of a typical 3-phase AC soft starter full wave controller
2
What will a normally closed time contactor of an ‘ON’ delay timer do?
Be delayed on opening after the relay has energized
In an across the line starter, the minimum number of conductors in the power circuit required to connect to a 12 lead non reversible 3-phase motor to the supply line is 6 (T or F)
False
A pulse width modulation variable frequency drive controls the torque characteristics of a motor by varying the following electrical properties
Voltage and frequency
A time-closing or timed-to-close contact for an off-delay timer is what type of contact
Normally- closed contact
A pilot light indicating that a motor is off due to overload would normally be wired in series with what
A normally open overload relay contact
A 3 phase SCIM branch circuit would require how many power line conductors between wye/delta reduced voltage starter and the motor
6
What is the difference between a 3 phase contactor and a 3 phase motor starter
The starter has an overload relay and the contactor does not have an overload relay
What will a normally closed time contactor of an ‘ON’ delay timer do
Be delayed on opening after the relay has energized
What is the purpose of the discharge resistor
To prevent a high voltage build-up across the field
