Focused Practice

I. Answer the following questions:

1. Why is the turbine casing divided into structurally independent forward and rear assemblies?

2. How is each of the two bladed discs mounted?

3. Where are the first stage diaphragm and rotor housed?

4. What does the rear assembly carry?

5. Where does the abutment between the two assemblies lie?

6. What have special moulding techniques been developed for?

7. Why are cold setting plastics used in the construction of model diaphragms?

II. Analyse the grammar structures underlined in the above text.

III. Speak on: Forward and rear assemblies.

Unit 19 Grammar: Non-finite forms of the Verb. The Complex Sentences Word List:

1. motor losses	потери в двигателе
2. stray losses	потери на рассеяние
3. core losses	потери в сердечнике
4. zero slip	нулевое скольжение (пробуксовка)
5. slip losses	потери при пробуксовке
6. tooth edges	края зубца
7. flux density	плотность потока
8. in-plant	рабочий режим (на предприятии)
9. back iron	станина
10. stator impedance	полное входное сопротивление статора
11. idle to run idle	холостой ход работать на холостом ходу
12. idle readings	значения параметров на холостом ходу
13. core teeth	зубцы сердечника
14. motor imput watts	мощность мотора на входе
15. windage	сопротивление воздуха

Evaluating Individual Losses

Three of the motor losses appear, at first glance to be easy to evaluate. Stator I²R or “copper loss” is simply the product of winding resistance and current squared, summed up for all three phases. Both those items can thread quite accurately. With the motor running idle – that is, uncoupled from any load - only three losses are present rather than all five. Stray loss is nonexistent without load, and at essentially zero slip the rotor I²R, or “slip loss”, will not be present either. Total motor input watts will include only core loss, friction and windage, and a small stator I²R calculable from the no-load amperes. Taking idle readings of current and power as the voltage is varied permits separation of core loss and friction and windage from the total input. Unfortunately, the in-plant situation rarely permits adequate voltage variation, although in the repair shop it may be easy.

The core loss from such an idle test will not be quite the same at full load. Although the difference is slight, it can become important when small changes in loss are assigned high dollar value. Because core loss is associated with the magnetizing branch of this circuit, the value will depend upon the voltage across that branch. Under increasing load, the current through the stator increases while terminal voltage remains unchanged. Thus an increasing voltage drop occurs in the stator impedance, lowering the voltage across the magnetizing branch and decreasing the core loss.

Separate tests for core loss have become popular in recent years, primarily to evaluate possible damage incurred during motor repair. Those tests require the motor to be disassembled. It has been claimed that one testing device will verify actual motor core loss within 10 percent. Be skeptical of that. In the first place, what we call “core loss” in an assembled running motor is not the same as the power loss within a stator core alone, magnetized by a “core loss tester”. Even if the magnetic flux density in the core iron below the slots were exactly the design value, a motor’s total core loss includes major components other than that in the so-called back iron.

One is the loss in the core teeth themselves, especially along the tooth edges near the air gap. It is not present during the usual “'core loss test” because the teeth are not magnetized. Another component is the “surface pulsation loss” caused by magnetic flux crossing the air gap to link the rotor. It, too, is obviously not present during a core loss test. Hence, using the tester to estimate possible core damage prior to rewind is one thing; trying to accurately predict overall motor efficiency is quite another.