Motor Control

Question 1

A coil is designed to energize at what percent of its rated voltage?

Answer 1

85%

Question 2

A zero speed switch used for plugging is NO or NC?

Answer 2

NO

Question 3

A zero speed switch used for anti plugging is NO or NC?

Answer 3

NC

Question 4

What are the four types of reduced voltage starters?

Answer 4

Auto TX, primary impedance/resistance, solid state, part winding

Question 5

What is the reduced current starter?

Answer 5

Wye Delta

Question 6

Which starting method provides max torque with minimum acceleration time?

Answer 6

Across the line / full voltage

Question 7

What is the disadvantage of an open transition start?

Answer 7

Because the motor is disconnected from the line, a torque dip can occur causing line disturbances.

Question 8

What is the main consideration for whether a motor can be started across the line?

Answer 8

Permissible voltage fluctuations to the power distribution system (usually determined by Hydro)

Question 9

What value of starting resistance is used in a primary resistance starter?

Answer 9

Between 65-80% of full load voltage

Question 10

What is the disadvantage of a primary resistance starter?

Answer 10

Large power losses in the resistor during start up

Question 11

How many resistors are used in a three point primary resistor starter?

Answer 11

Two in each phase

Question 12

What is the usual voltage limitation for primary resistance starter?

Answer 12

600V

Question 13

What is the disadvantage of the primary reactance starter?

Answer 13

Lowers the power factor even more than a primary resistance start. Poor voltage regulation because of poor power factor

Question 14

Why does the auto transformer provide more starting torque per ampere than any other starter?

Answer 14

The motor starting current is greater than the line current because of the transformation in the auto tx

Question 15

What are the disadvantages of the open delta auto tx

Answer 15

Unbalanced phase voltages for larger induction motors

The moment that the circuit is open during transition can cause current line surges

Question 16

What is the usual auto tx duty?

Answer 16

Intermittent

Question 17

To protect an auto tx from overheating, what type of switch is wired into the control circuit?

Answer 17

A manually-reset temp switch

Question 18

What is the simplest and most economic control of the reduced voltage starters?

Answer 18

The open delta auto transformer

Question 19

What are some components used to shunt the resistors from the resistance starter?

Answer 19

Permanent magnets with a Hall effect sensor, current transformers, speed sensor, timing relays

Question 20

Auto TX’s are used for which type of motor?

Answer 20

SCIM. Auto TX starters are inductive type loads and affect the power factor during start up.

Question 21

Open transition auto tx starters are generally used on what size motor?

Answer 21

Size 5 and smaller

Question 22

Closed transition auto tx starters are generally used on what size motor?

Answer 22

Size 6 or larger

Question 23

The overload heaters of a wye delta starter are rated by what current value?

Answer 23

The phase current.
If two current values are listed on a nameplate, the lower current value is the phase current

Question 24

How should a good SCR read on an ohmmeter?

Answer 24

Near infinity resistance.

Zero resistance means bad SCR

Question 25

What are the two main requirements that must be met for a wye delta starter to be used?

Answer 25

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.

Question 26

What type of starting is generally used for synchronous motors?

Answer 26

Across the line, wye delta, reactor, and sometimes Auto TX

Question 27

What are the components necessary for a synchronous motor start?

Answer 27

Out of step relay, field discharge resistor, field contactor, PFR, and field rheostat

Question 28

What is the function of the out of step relay?

Answer 28

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.

Question 29

What is the function of the field contactor?

Answer 29

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.

Question 30

What is the function of the PFR?

Answer 30

Sensing rotor speed and controls the operation of the field contactor.

Question 31

How do you adjust the speed at which the PFR drops out and the sync motor becomes synchronized?

Answer 31

Add or remove shims or spacers to the face of the relay core. Adding shims: lowers speed Removing shims: increases speed

Question 32

What are the advantages of a wound rotor motor?

Answer 32

Higher starting torque, smoother acceleration

Question 33

What are the two types of automatic starting for a wound rotor motor?

Answer 33

Timed acceleration, and speed sensing acceleration

Question 34

What is the disadvantage of the time delay wound rotor motor start?

Answer 34

Timers are not load sensitive and can’t always match the requirements of each load.

Question 35

Which wound rotor motor starter is considered to be the best option?

Answer 35

Frequency control

Question 36

What are the four necessary components of a VFD?

Answer 36

Rectifier, Inverter, DC Bus, CPU

Question 37

In the control section of the VFD, what is the purpose of the inverter?

Answer 37

Varying the frequency and magnitude of the output wave

Question 38

What are the three basic inverter schemes?

Answer 38

VVI: variable voltage inversion CSI: current source inversion PWM: pulse width modulation.

Question 39

Which motor ratings determine which size VFD is required?

Answer 39

HP

Question 40

How does an inverter function?

Answer 40

Changes DC to AC using a mechanical switch inverter, or electronic switching with an SCR, transistor, or an IGBT

Question 41

What are the components used in a PWM VFD?

Answer 41

Diode bridge rectifier, filter section, and IGBTs

Question 42

What are the two most common types of braking?

Answer 42

Dynamic and regenerative

Question 43

What is an IGBT?

Answer 43

Insulated gate bipolar transistor

Question 44

What is an advantage of IGBT drives?

Answer 44

Reduces problems caused by harmonics

Question 45

What is a disadvantage of IGBT drives?

Answer 45

Can cause voltage spikes which will destroy the motor

Question 46

What type of motor should be used with an IGBT drive?

Answer 46

Inverter rated motor.

Question 47

What is ramping?

Answer 47

The ability (parameter) to control speed increase and decrease time

Question 48

What is switching frequency?

Answer 48

The rate that the IGBTs switch per second

Question 49

What is the benefit of Sensorless Vector Control (SVC)?

Answer 49

Allows AC motors to be controlled with the same precision as DC motors

Question 50

What is phase control in relation to an AC soft start?

Answer 50

Delaying each SCR gate signal to the appropriate point of each half cycle

Question 51

What are the electrical characteristics used to control the speed and direction of a 3 phase induction motor?

Answer 51

Speed: frequency Direction: phase sequence

Question 52

What is constant torque operation?

Answer 52

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

Question 53

What is constant HP operation?

Answer 53

Load requires a constant force as radius changes. Ex) winder

Question 54

What is reflected wave voltage?

Answer 54

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.