Three-Phase Motors and Starters

Question 1

What terminal numbers are expected at a dual voltage motor’s junction box?

Answer 1

Terminals 1 through 9.

Question 2

What would be the designation of a 3 lead motor?

Answer 2

A single voltage, wye OR delta motor.

Question 3

What would be the designation of a six lead motor?

Answer 3

A single voltage wye/delta motor, (3 groups of 2 leads); or

A split winding motor, (2 groups of 3 leads).

Question 4

What is the best way to change a motor’s direction?

Answer 4

Interchange any two line conductors to the motor.

Question 5

What is the most common voltage to run a dual voltage motor?

Answer 5

240/480 V.

Question 6

How many windings does a dual voltage motor have?

Answer 6

Six.

Question 7

Is high or low Voltage connection the same for wye and delta dual voltage motors?

Answer 7

The high voltage (series connection);

T4 - T7

T5 - T8

T6 - T9.

Question 8

What are the two main types of starters?

Answer 8

Across the line (full voltage);

Reduced voltage.

Question 9

Why might using a reduced voltage to start be an issue?

Answer 9

If the available torque is not enough to rotate the load.

Question 10

Is a standard motor controller considered a disconnect by the CEC?

Answer 10

No, it uses magnetic (not manual) means to control the circuit.

Question 11

What type of starter is a part winding?

Answer 11

Full voltage, reduced current/torque.

Question 12

What is the typical inrush current of a part winding starter?

Answer 12

60 to 65 percent of full V starting current.

Question 13

What would happen if the second winding contactor refused to engage, in a part winding starter?

Answer 13

The first winding would overheat under rated load.

Question 14

What symbol signifies an on delay contact?

Answer 14

A single pole switch with an arrow tail (feather end).

Question 15

What symbol signifies an off delay contact?

Answer 15

A single pole switch with an arrow head ( sharp end).

Question 16

What ratio is the tap setting of an autotransformer starter?

Answer 16

Reduced voltage to full voltage.

Question 17

Why is line current a product of tap² in an autotransformer starter?

Answer 17

To satisfy VA_IN = VA_OUT.

Question 18

Why is torque a product of tap²?

Answer 18

Because torque is based on the interaction of stator and rotor flux; and

Flux is directly proportional to voltage;

Transformer action says if we reduce V_Stator by a factor of X, V rotor also gets reduced by a factor of X and the combined field strength is reduced by X².

Question 19

What kind of autotransformer transition offers better performance?

Answer 19

Closed transition.

Question 20

Does a wye delta starter require a six lead motor?

Answer 20

Yes.

Question 21

What is the inrush deration factor when using a wye/delta starter?

Answer 21

One third for both current and torque.

Question 22

What current do wye/delta overloads monitor?

Answer 22

Delta phase current.

Question 23

Why is a wye delta starter always open transition?

Answer 23

Otherwise we would produce a line to line short.

Question 24

Which two acceleration modes could be provided by an electronic soft starter?

Answer 24

Ramp starting;

Current limit starting.

Question 25

What is a kick start?

Answer 25

Soft starters providing a slight inrush current surge for extra starting torque.

Question 26

What kind of load would prove most challenging to start with reduced voltage?

How might we make this easier?

Answer 26

Constant torque loads;

A mechanical alteration which reduces starting torque (like a decompression valve).

Question 27

What is the starter which produces high torque and low current at start?

Answer 27

WRIM and controller.

Question 28

What is the purpose of a phase converter?

Answer 28

To change single phase voltage into roughly 3 phase.

Question 29

Which is the less expensive type of phase converter and what are it’s main drawbacks?

Answer 29

Static;

It can only supply one motor, non ideal output.

Question 30

In a rotary type phase converter, what is the main component?

Answer 30

An induction motor generator.

Question 31

What is the high capacitance electrolytic capacitor used for?

Answer 31

Starting a motor or the motor generator.

Question 32

Which two power ratings would be on a rotary phase converter?

Answer 32

Running hP;

Starting hP (biggest motor).

Question 33

Why does a static phase converter rely on the motor to generate the third line?

Answer 33

Because, unlike a rotary converter, there is no generated induced voltages, the load simply draws current at 90 degrees lead by the cap.

Question 34

When is a static phase converter most balanced?

Answer 34

At full load.