Operation of Three-Phase Induction Motors Flashcards
How is synchronous speed of a motor calculated?
ΠSYNC = (120 • f{Hz}) / NPOLES
What is slip speed?
It is the speed difference between: ΠSYNC and ΠROTOR;
ΠS = ΠSYNC - ΠROTOR.
What is slip?
The ratio of slip speed to synchronous speed;
s = ΠS / ΠSYNC.
When comparing two motors of the same horsepower ratings and synchronous speeds, which one will be more efficient?
The one with the higher full-load speed;
It will have less %Π regulation.
What is the slip of most induction motors?
2% - 5% of ΠSYNC.
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.
- 3600 rpm;
- 1800 rpm;
- 1200 rpm;
- 900 rpm;
- 720 rpm;
- 600 rpm.
Which quantities in the rotor will change as a motor accelerates?
- VRotor;
- <span>f</span>Rotor;
- <span>X</span>Rotor;
- <span>Z</span>Rotor;
- <span>pf</span>Rotor;
- <span>I</span>Rotor.
At running speeds, which quantity is directly proportional to torque?
Active current.
Which rotor parameter is considered constant?
Resistance.
Which parameters are directly proportional to slip?
Rotor Voltage;
Rotor frequency;
Rotor Reactance.
Rotor impedance decreases ___________ as the rotor accelerates.
Non - linearly.
Does reactive rotor current produce torque?
No. It is simply necessary to establish the magnetic field which the rotor needs to operate.
How is a motor’s stator current affected during acceleration?
The rotor current will be reflected using winding ratios, which will change the stator current [Magnitude and power factor].
When are rotor Voltage, frequency and Reactance at their maximum?
At the instant of start.
When does breakdown torque occur?
When Active current equals reactive current;
When XRotor = RRotor.
Could an overloaded motor just have a tiny change in slip?
Yes. If a rotor has a low full load slip then a small increase in slip will result in a large increase in load.
When do most NEMA A or B rotors experience breakdown torque?
At 75 - 80% of ΠSYNC.
When will a motor stall?
If load counter torque exceeds breakdown torque.
Is the current drawn by the stator a good indicator of motor output power?
No.
Why does the rotor never reach ΠSYNC?
Because the rotor must lag the magnetic field slightly to induce current which causes torque.
As the load is increased beyond rated values, how is slip affected?
Slip will increase at a greater rate than during normal operations.
What is pull out torque?
Breakdown torque.
Why would a motor’s starting torque need to be higher than it’s full-load torque?
So that it can start a higher inertia load.
How is inductance increased in a SCIM?
By embedding the bars deeper in the core;
This increases the flux produced by the currents in the bars, which increases reactance.
How is starting torque improved?
By increasing rotor resistance.
What is the most efficient rotor design during operation?
A.
What is the most common rotor design?
B.
What is the best rotor design for starting high inertia loads?
D.
What rotor design uses two sets of bars? What is it’s main disadvantage?
C.
The rotor does not dissipate heat well.
Why is resistance switched into a WRIM using slip rings?
To improve the starting torque and reduce the inrush current.
Why are WRIMs the best all around rotor?
What are the disadvantages?
They can start like a D and run like an A;
Higher cost, bigger frame, required auxilliary equipment, and extra maintenance.
What are the drawbacks of resistor connected speed control?
They cause an increase in %Π regulation;
Which decreases efficiency.
Only practical from half load to full load speed.
What is the best way to achieve smooth torque and speed control with a WRIM?
Connecting a VFD.
