V. Match the words and word combinations (a-f) to the sentences (1-6)
a) induces
b) slip
c) friction
d) accelerate
e) air-gap
f) interacts
1. The magnetic field in a normal induction motor is induced across the rotor … .
2. This magnetic field … with the rotating stator flux to produce the rotational force.
3. When the rotor starts to … in the direction of the rotating field, the rate at which the magnetic field cuts the rotor windings reduces and the induced rotor voltage decreases proportionately.
4. As the magnetic field rotates, the lines of flux cut across the rotor conductors and this … a voltage in the rotor windings.
5. The amount of … is determined by the load torque required to turn the rotor shaft.
6. The rotor settles at a speed slightly less than the rotating flux, which provides enough torque to overcome bearing … and windage.
VI. Answer the questions to the text
1. What currents set up a changing magnetic field which rotates around the inside of the stator?
2. What is a synchronous speed?
3. What is the rotating field produced by in the simplest type of 3-phase induction motor?
4. How many rotations for every cycle of the supply voltage does the flux perform when the three stator windings are connected to the 3-phase power supply?
5. What is a 2 pole motor?
6. Where is a magnetic field induced in a normal induction motor?
7. What must be present across the rotor bars to establish a current flow in the rotor?
8. What speed must the rotor rotate at to produce torque?
9. What is a slip?
10. What is a slip speed?
11. What is the amount of slip determined by?
12. What does the direction of the rotating stator flux depend on?
13. What is a phase sequence?
VII. Define the functions of Participle I and Participle II in the following sentences
1. When the induction motor is supplied from a power source of constant voltage and frequency, the current drawn by the motor depends primarily on the slip.
2. At no-load, the motor will draw only no-load magnetizing current.
3. When matching motors to mechanical loads, the two most important considerations are torque and speed.
4. Induced voltage reappears in the rotor, but in the opposite direction.
5. When set up a changing magnetic field rotates around the inside of the stator.
VIII. Say, which of the sentences are in the Active and which are in the Passive Voice
1. In contrast with a DC motor, the AC induction motor doesn’t have separate field windings.
2. The performance of the 3-phase AC induction motor has been described for the speed range from zero up to its rated speed at 50 Hz.
3. Typically, the slip varies between about 1% of synchronous speed at no-load to about 6% of synchronous speed at full-load.
4. The induction motor will always run at a speed lower than synchronous speed because, even at no-load, a small slip is required to ensure that there is sufficient torque to overcome friction and windage losses.
5. Inertia can be calculated using the formula.
IX. Translate the sentences paying attention to the Sequence of Tenses
1. The motor accelerates if the motor torque always exceeds the load torque.
2. At starting, the motor will not pull away unless the starting torque exceeds the load breakaway torque.
3. If the load torque increases, the motor speed drops slightly, slip increases, stator current increases and the motor torque increases to match the load requirements.
4. If the rotor speed was increased to the point that there was no slip, the induced voltage and current in the rotor fell to zero and torque output.
5. If the rotor speed is increased about the mentioned one, the rotor will run faster than the rotating stator field and the rotor conductors again start to cut the lines of magnetic flux.
6. When the motor torque is less than the load torque, the motor will stall.