• Author / Uploaded
• Alvaro De La Cruz

Citation preview

Homework 7 Induction Motor 1 1. What are those two type rotors of asynchronous machine? Please try to explain their features. (1 Remarks) -

Squirrel cage motor These consist of a series of conductive bars arranged inside grooves carved on the face of the rotor and short-circuited at one of its ends by means of large short-circuit rings. This design is called a treadmill rotor because the drivers, if examined on their own, look like one of the exercise wheels where squirrels or hamsters run.

-

Winding type Has a complete set of triphasic windings which are mirror images of the stator windings. Generally, the three phases of the rotor windings are Y-connected, and the ends of the three rotor wires are attached to the slip rings on the rotor shaft. The rotor windings are shorted by means of brushes mounted on the slip rings. Therefore, the wound rotors of induction motors have rotor currents accessible in the stator brushes, where extra resistance can be examined and inserted into the rotor circuit.

2. Please explain how does induction motor work (2 Remarks) Asynchronous or induction motors are practically three-phase motors are based on the actuation of a metallic mass by the action of a rotating field are formed by two armatures with coaxial fields one is fixed and the other mobile is also called respectively stator and rotor, The rotor winding that conducts the alternating current that is produced by induction of the directly connected stator winding consists of conductor copper or aluminum cast into a rotor of steel mills. Short-circuit end rings are installed at both ends of the squirrel cage or at the ends in the case of the wound rotor

3. Please express the relationship of nominal power P N and nominal voltageU N , nominal current

I N for an induction motor? (1 Remarks)

P N = √3∗U N ∗I N ∗η∗cos ⁡(φ) P N =Rated Power U N =Nominal Voltage I N =Nominal Current η=Efficiency

4. A three-phase asynchronous machine has the following parameters related, its number of pair pole of p, the stator frequency f 1, rotor frequency f 2, the slip s, synchronous speed n1, rotating speedn , the rotor MMF F 2 and its relative rerating speed n2 with respect to rotor. Please try to complete the table below. (The negative value means that the rotating direction is opposite to the air gap MMF rotating direction.) (12 Remarks)

p

f 1(Hz)

f 2(Hz)

s

n1 (rpm)

n

n2 (rpm)

1 2 3 4 5 2

50 50 50 50 50 60

1.5 5 50 10 91.67 3

0.03 0.1 1 -0.2 1.83 0.05

3000 1500 1000 750 600 1800

2910 1350 0 900 -500 1710

90 150 1000 -150 1100 90

5. There is a three-phase induction motor with its stator winding applied by rated voltage of 50Hz. (4 Remarks) a. If the number of effective turns of the stator winding for each phase decreases, then the magnetic fluxes for each phase Φ m would________ (increase or decrease). b. If the air gap length is enlarged, then the motor no load current would __________ (increase or decrease). c. If the value of voltage applied to stator winding is fixed, but its frequency is changed to 60Hz, then the exciting reactance would __________ (increase or decrease).current would __________ (increase or decrease).