Motor Control Flashcards
A coil is designed to energize at what percent of its rated voltage?
85%
A zero speed switch used for plugging is NO or NC?
NO
A zero speed switch used for anti plugging is NO or NC?
NC
What are the four types of reduced voltage starters?
Auto TX, primary impedance/resistance, solid state, part winding
What is the reduced current starter?
Wye Delta
Which starting method provides max torque with minimum acceleration time?
Across the line / full voltage
What is the disadvantage of an open transition start?
Because the motor is disconnected from the line, a torque dip can occur causing line disturbances.
What is the main consideration for whether a motor can be started across the line?
Permissible voltage fluctuations to the power distribution system (usually determined by Hydro)
What value of starting resistance is used in a primary resistance starter?
Between 65-80% of full load voltage
What is the disadvantage of a primary resistance starter?
Large power losses in the resistor during start up
How many resistors are used in a three point primary resistor starter?
Two in each phase
What is the usual voltage limitation for primary resistance starter?
600V
What is the disadvantage of the primary reactance starter?
Lowers the power factor even more than a primary resistance start. Poor voltage regulation because of poor power factor
Why does the auto transformer provide more starting torque per ampere than any other starter?
The motor starting current is greater than the line current because of the transformation in the auto tx
What are the disadvantages of the open delta auto tx
Unbalanced phase voltages for larger induction motors
The moment that the circuit is open during transition can cause current line surges
What is the usual auto tx duty?
Intermittent
To protect an auto tx from overheating, what type of switch is wired into the control circuit?
A manually-reset temp switch
What is the simplest and most economic control of the reduced voltage starters?
The open delta auto transformer
What are some components used to shunt the resistors from the resistance starter?
Permanent magnets with a Hall effect sensor, current transformers, speed sensor, timing relays
Auto TX’s are used for which type of motor?
SCIM. Auto TX starters are inductive type loads and affect the power factor during start up.
Open transition auto tx starters are generally used on what size motor?
Size 5 and smaller
Closed transition auto tx starters are generally used on what size motor?
Size 6 or larger
The overload heaters of a wye delta starter are rated by what current value?
The phase current.
If two current values are listed on a nameplate, the lower current value is the phase current
How should a good SCR read on an ohmmeter?
Near infinity resistance.
Zero resistance means bad SCR
What are the two main requirements that must be met for a wye delta starter to be used?
The motor must be designed for the stator windings to be connected in delta for the run period
All stator winding leads must be accessible.
What type of starting is generally used for synchronous motors?
Across the line, wye delta, reactor, and sometimes Auto TX
What are the components necessary for a synchronous motor start?
Out of step relay, field discharge resistor, field contactor, PFR, and field rheostat
What is the function of the out of step relay?
A timer that contains a current operated coil. Disconnects the motor from power if the rotor takes too long to excite. Prevents excessive current from burning up the motor.
What is the function of the field contactor?
Contains a DC coil that is energized by the excitation current of the rotor. Controls the application and disconnection of the DC excitation field and the discharge resistor. Provides a positive overlap to ensure the field circuit is never open.
What is the function of the PFR?
Sensing rotor speed and controls the operation of the field contactor.
How do you adjust the speed at which the PFR drops out and the sync motor becomes synchronized?
Add or remove shims or spacers to the face of the relay core.
Adding shims: lowers speed
Removing shims: increases speed
What are the advantages of a wound rotor motor?
Higher starting torque, smoother acceleration
What are the two types of automatic starting for a wound rotor motor?
Timed acceleration, and speed sensing acceleration
What is the disadvantage of the time delay wound rotor motor start?
Timers are not load sensitive and can’t always match the requirements of each load.
Which wound rotor motor starter is considered to be the best option?
Frequency control
What are the four necessary components of a VFD?
Rectifier, Inverter, DC Bus, CPU
In the control section of the VFD, what is the purpose of the inverter?
Varying the frequency and magnitude of the output wave
What are the three basic inverter schemes?
VVI: variable voltage inversion
CSI: current source inversion
PWM: pulse width modulation.
Which motor ratings determine which size VFD is required?
HP
How does an inverter function?
Changes DC to AC using a mechanical switch inverter, or electronic switching with an SCR, transistor, or an IGBT
What are the components used in a PWM VFD?
Diode bridge rectifier, filter section, and IGBTs
What are the two most common types of braking?
Dynamic and regenerative
What is an IGBT?
Insulated gate bipolar transistor
What is an advantage of IGBT drives?
Reduces problems caused by harmonics
What is a disadvantage of IGBT drives?
Can cause voltage spikes which will destroy the motor
What type of motor should be used with an IGBT drive?
Inverter rated motor.
What is ramping?
The ability (parameter) to control speed increase and decrease time
What is switching frequency?
The rate that the IGBTs switch per second
What is the benefit of Sensorless Vector Control (SVC)?
Allows AC motors to be controlled with the same precision as DC motors
What is phase control in relation to an AC soft start?
Delaying each SCR gate signal to the appropriate point of each half cycle
What are the electrical characteristics used to control the speed and direction of a 3 phase induction motor?
Speed: frequency
Direction: phase sequence
What is constant torque operation?
The torque required for the load is the same during the entire speed range, constant torque is the motors ability to maintain constant flux.
Ex) conveyor
What is constant HP operation?
Load requires a constant force as radius changes.
Ex) winder
What is reflected wave voltage?
Occurs in PWM VFD’s. When voltage pulses reach the motor they encounter a different impedance than the cable provided, pulses reflect back into the VFD doubling the voltage level. The longer the cable run the higher the voltage.
