Operation of Three-Phase Induction Motors

Question 1

How is synchronous speed of a motor calculated?

Answer 1

Π_SYNC = (120 • f_{Hz}) / N_POLES

Question 2

What is slip speed?

Answer 2

It is the speed difference between: Π_SYNC and Π_ROTOR;

Π_S = Π_SYNC - Π_ROTOR.

Question 3

What is slip?

Answer 3

The ratio of slip speed to synchronous speed;

s = Π_S / Π_SYNC.

Question 4

When comparing two motors of the same horsepower ratings and synchronous speeds, which one will be more efficient?

Answer 4

The one with the higher full-load speed;

It will have less %Π regulation.

Question 5

What is the slip of most induction motors?

Answer 5

2% - 5% of Π_SYNC.

Question 6

What are the synchronous speeds for the following motors at 60 Hz.

• 2-Pole SCIM.
• 4-Pole SCIM.
• 6-Pole SCIM.
• 8-Pole SCIM.
• 10-Pole SCIM.
• 12-Pole SCIM.

Answer 6

• 3600 rpm;
• 1800 rpm;
• 1200 rpm;
• 900 rpm;
• 720 rpm;
• 600 rpm.

Question 7

Which quantities in the rotor will change as a motor accelerates?

Answer 7

• V_Rotor;
• _{<span>f</span>Rotor}​;
• _{<span>X</span>Rotor}​;
• _{<span>Z</span>Rotor}​;
• _{<span>pf</span>Rotor}​;
• _{<span>I</span>Rotor}​.

Question 8

At running speeds, which quantity is directly proportional to torque?

Answer 8

Active current.

Question 9

Which rotor parameter is considered constant?

Answer 9

Resistance.

Question 10

Which parameters are directly proportional to slip?

Answer 10

Rotor Voltage;

Rotor frequency;

Rotor Reactance.

Question 11

Rotor impedance decreases ___________ as the rotor accelerates.

Answer 11

Non - linearly.

Question 12

Does reactive rotor current produce torque?

Answer 12

No. It is simply necessary to establish the magnetic field which the rotor needs to operate.

Question 13

How is a motor’s stator current affected during acceleration?

Answer 13

The rotor current will be reflected using winding ratios, which will change the stator current [Magnitude and power factor].

Question 14

When are rotor Voltage, frequency and Reactance at their maximum?

Answer 14

At the instant of start.

Question 15

When does breakdown torque occur?

Answer 15

When Active current equals reactive current;

When X_Rotor = R_Rotor.

Question 16

Could an overloaded motor just have a tiny change in slip?

Answer 16

Yes. If a rotor has a low full load slip then a small increase in slip will result in a large increase in load.

Question 17

When do most NEMA A or B rotors experience breakdown torque?

Answer 17

At 75 - 80% of Π_SYNC.

Question 18

When will a motor stall?

Answer 18

If load counter torque exceeds breakdown torque.

Question 19

Is the current drawn by the stator a good indicator of motor output power?

Answer 19

No.

Question 20

Why does the rotor never reach Π_SYNC?

Answer 20

Because the rotor must lag the magnetic field slightly to induce current which causes torque.

Question 21

As the load is increased beyond rated values, how is slip affected?

Answer 21

Slip will increase at a greater rate than during normal operations.

Question 22

What is pull out torque?

Answer 22

Breakdown torque.

Question 23

Why would a motor’s starting torque need to be higher than it’s full-load torque?

Answer 23

So that it can start a higher inertia load.

Question 24

How is inductance increased in a SCIM?

Answer 24

By embedding the bars deeper in the core;

This increases the flux produced by the currents in the bars, which increases reactance.

Question 25

How is starting torque improved?

Answer 25

By increasing rotor resistance.

Question 26

What is the most efficient rotor design during operation?

Answer 26

A.

Question 27

What is the most common rotor design?

Answer 27

B.

Question 28

What is the best rotor design for starting high inertia loads?

Answer 28

D.

Question 29

What rotor design uses two sets of bars? What is it’s main disadvantage?

Answer 29

C.

The rotor does not dissipate heat well.

Question 30

Why is resistance switched into a WRIM using slip rings?

Answer 30

To improve the starting torque and reduce the inrush current.

Question 31

Why are WRIMs the best all around rotor?

What are the disadvantages?

Answer 31

They can start like a D and run like an A;

Higher cost, bigger frame, required auxilliary equipment, and extra maintenance.

Question 32

What are the drawbacks of resistor connected speed control?

Answer 32

They cause an increase in %Π regulation;

Which decreases efficiency.

Only practical from half load to full load speed.

Question 33

What is the best way to achieve smooth torque and speed control with a WRIM?

Answer 33

Connecting a VFD.