Deck 5: DC Motors, Control Circuits, AC Motors and Variable Speed Drives

Question 1

The poles of a magnetic are determined by the electronic makeup of the material.

Answer 1

True

Question 2

Flux lines exit from the north pole and re -enter through the south pole.

Answer 2

False

Question 3

The magnetic strength of an electromagnetic cannot be increased beyond the point of saturation.

Answer 3

True

Question 4

The amount of torque that a motor produces is measured by dividing the force a motor exerts by the distance between the center of the shaft and the point at which the force is applied.

Question 5

One pound -foot is greater than one newton meter.

Question 6

If counter torque is greater than torque of the motor, then the motor doesn't turn but torque is still produced.

Question 7

Starting torque is equal to stall torque.

Question 8

Motor power is work multiplied by time.

Question 9

Eddy currents cause the temperature to rise in the core of the rotor.

Question 10

A laminated motor core can effectively eliminate flux losses.

Question 11

The shaft of the motor has the laminated core and commutator pressed into it.

Question 12

The narrower the air gap the stronger the magnetic field.

Question 13

If brushes wear out, they can damage the commutator.

Question 14

Shunt motors start with a very large load attached.

Question 15

The series motor controls rpm while at high speeds.

Question 16

The commutator segments continually change polarity from positive to negative, which causes the magnetic field in the armature to change polarity form north pole to south pole.

Question 17

In a series motor, stall torque occurs at the locked rotor condition.

Question 18

The input power supply to the series motor is equal to the voltage drop across the field winding plus the voltage drop across the armature resistance plus cemf.

Question 19

In a series motor, the polarity of the field winding is typically reversed to control the direction of rotation.

Question 20

Universal motors can operate on either DC or single phase AC power.

Question 21

In a shunt motor, the current flows from the positive power supply terminal through the shunt winding to the negative power supply terminal, with s similar current path through the armature winding.

Question 22

The cemf produced in a shunt motor has no effect on the magnetic field of the shunt windings.

Question 23

The speed of a shunt motor can be controlled beyond its rated value by changing the amount of voltage across either the field or the armature winding.

Question 24

The short shunt cumulative compound motor is used most often because the shunt field has a relatively constant current.

Question 25

The differential compound motor and the cumulative compound motor are the same except the connection to the shunt field terminals.

Question 26

If the polarity of the interpole winding is not matched to the polarity of the shunt field winding, the motor will overheat.

Question 27

The speed of the compound motors can be controlled with an adjustment to the shunt winding current.

Question 28

Solid state devices like the H -bridge eliminate shoot -through.

Question 29

In a thyristor drive, the SCR controls the amount of current that flows through the armature of a DC motor.

Question 30

The magnetic field strength of an electromagnet cannot be increased by

A) the amount of current.
B) the type of core.
C) the type of material.
D) the number of coils in the wire.

Question 31

Which of the following is the same in the DC series motor and the universal motor?

A) Field strength
B) Iron core composition
C) No -load speeds
D) Input power

Question 32

The speed of a shunt motor can be controlled by

A) varying the armature current.
B) varying the field current.
C) varying the voltage across the field winding.
D) A and C
E) all of the above

Question 33

Which of the following is not a type of compound motor?

A) Interpole
B) Integrator
C) Differential
D) Cumulative

Question 34

Which of the following is not a component of a DC motor starter?

A) Arc quencher
B) Contactor
C) Overload relay
D) Heater
E) Braking relay

Question 35

Match the overload block with its operation.

-Eutectic

A) Uses dissimilar materials
B) Uses motor voltage
C) Uses motor current
D) Uses melting poin

Question 36

Match the overload block with its operation.

-Bimetallic

A) Uses dissimilar materials
B) Uses motor voltage
C) Uses motor current
D) Uses melting poin

Question 37

Match the overload block with its operation.

-Phase loss

A) Uses dissimilar materials
B) Uses motor voltage
C) Uses motor current
D) Uses melting poin

Question 38

Match the technique t o the component i t uses for braking,

-Dynamic

A) Resistor
B) Inductor
C) Power supply

Question 39

Match the technique t o the component i t uses for braking,

-Regenerative

A) Resistor
B) Inductor
C) Power supply

Question 40

Match the technique t o the component i t uses for braking,

-Plugging

A) Resistor
B) Inductor
C) Power supply

Question 41

AC motors are available as single phase motors, typically less than 3 hp.

Question 42

AC power to the motor creates the magnetic field in the rotating assembly by current flow rather than induction.

Question 43

The AC motor is better suited than the DC motor for those applications that require variable speeds.

Question 44

In the AC motor as in the DC motor, the coil of wire wrapped around the poles form an electromagnet.

Question 45

The alternating field causes the rotor to rotate on the shaft, making one revolution for each half -cycle of the input AC power.

Question 46

Slip is the difference between the speed of the rotating field and the speed of the rotor.

Question 47

The stator core and windings are coated in an epoxy resin to seal them from moisture invasion.

Question 48

AC motors can be designed to be operational in an explosive and dust -ignition environments.

Question 49

The wound rotor construction is not a common AC motor in industry.

Question 50

The centrifugal start switch is usually mounted on the shaft of the AC motor.

Question 51

The current required to run the AC motor flows through the thermal overload heater and bimetallic switch contacts.

Question 52

Resistance -start motor has a start winding, which is a coil made of many turns of heavy gauge wire.

Question 53

In the resistance -start, induction -run motor, the current in the start winding leads the input voltage.

Question 54

In a resistance -start, induction -run motor, the phase difference between the current in the start winding and the run winding is 90 degrees.

Question 55

In the capacitor -start, induction -run motor, the capacitor generates a current in the start winding that lags the phase of the voltage input.

Question 56

The performance of the resistance -start, induction -run motor and the capacitor -start, induction -run motor is basically identical.

Question 57

In a capacitor -start, capacitor -run motor, the centrifugal switch disconnects a start capacitor and connects a run capacitor.

Question 58

The shaded pole induction motor is a single phase motor, which has no start and run windings.

Question 59

In a three -phase motor, the voltages are 180 degrees out of phase with each other.

Question 60

The line -to -line voltage is greater for a delta -connected three -phase motor than for awye -connected three -phase motor with both motors having the same phase voltage inputs.

Question 61

In a squirrel cage three -phase induction motor, the rotor resistance is equal to the rotor inductive reactance at the pull -out point.

Question 62

In a wound rotor induction motor, slip rings connect the rotor electrically to an external resistor network.

Question 63

The synchronous motor efficiently converts electrical power to mechanical power.

Question 64

The damper windings in a synchronous motor are similar to those in an induction motor.

Question 65

In a synchronous motor, the salient poles project outward from the shaft of the motor.

Question 66

A resistor in a DC exciter discharges the current generated in the field of a synchronous motor after synchronous speed has been achieved.

Question 67

In a system with a leading power factor, the phase of voltage leads the phase of the current.

Question 68

In a system with a lagging power factor, the phase of current lags the phase of the voltage.

Question 69

One method for correcting plant power factor is connecting banks of capacitors across the input lines.

Question 70

Delta configuration windings are connected in parallel as are wye configuration windings, thus providing for low -voltage input.

Question 71

The number of poles set the speed of the motor; the more poles the higher the speed.

Question 72

When selecting a motor starter, the current rating of the starter must be less than the actual current that the starter will switch.

Question 73

On a motor data plate, the type specifies the motor's enclosure.

Question 74

An advantage of the variable voltage drive is good speed control.

Question 75

Which of the following is not a component of the stator?

A) End plates
B) Terminal box
C) Windings
D) Core
E) Enclosure

Question 76

Which of the following terminal connections would be made to run this single phase AC motor in the low voltage mode?

A) T1 to T3 to T5, T2 to T4 to T8, T6 to T7, and input across T1 and T2
B) T1 to T5, T2 to T3, T6 to T7, T3 to T8 and input across T1 and T4
C) T2 to T3 to T5, T4 to T8, T6 to T7, and input across T1 and T4
D) T1 to T5, T2 to T3 to T8, T6 to T7, and input across T1 and T4
E) T1 to T3 to T5 to T7, T2 to T4 to T6 to T8, and input across T1 and T2

Question 77

Which of the following is not an advantage of a three -phase motor over a single -phase motor?

A) Torque output is constant
B) Manufacturing costs are less
C) Size is small relative to its horsepower
D) Rotor is laminated iron

Question 78

When approximating the characteristics of a three -phase induction motor, which is not a rule of thumb?

A) The full load current is equal to a constant times the ratio of input power to stator voltage.
B) The starting current is roughly 6 times the full load current.
C) The no load current is roughly 0.3 times the full load current.
D) The rotor inductive reactance is equal to the rotor resistance in the stalled condition.

Question 79

Which is not an advantage of a wound rotor induction motor over a squirrel cage induction motor?

A) Good running characteristics
B) Higher starting torque
C) Lower cost to produce
D) Smoother acceleration under heavy loads

Question 80

For a motor with service factor of 1.2, a 5 horsepower motor can actually supply

A) 6 hp.
B) 5.5 hp.
C) 5.2 hp.
D) 12 hp.
E) none of the above