Fig. 2,019.—Diagram showing a method of operating a three phase motor on a two phase circuit, using a transformer having a tap made in the middle of the secondary winding, so as to get the necessary additional phase. While this does not give a true balanced three phase secondary, it is close enough for motor work. In the above arrangement, the main transformer supplies 54 per cent. of the current and the other with the split winding 46 per cent.
It is well to allow one kilowatt per horse power of the motor in selecting the size for the transformers, excepting in the small sizes when a little larger kilowatt rating is found to be the most desirable.
| Delivered voltage of circuit | Single phase transformer voltages | |||
|---|---|---|---|---|
| 110 volt motor | 220 volt motor | |||
| Primary | Secondary | Primary | Secondary | |
| 1,100 | 1,100 | 122 | 1,100 | 244 |
| 2,200 | 2,200 | 122 | 2,200 | 244 |
Very small transformers should not be used, even when the motor is large compared to the work it has to do, as the heavy starting current may burn them out.
The following tables give the proper sizes of transformer for three types of induction motor and the approximate current taken by three phase induction motors at 220 volts.
| Size of motor horse power | Kilowatts per transformer | ||
|---|---|---|---|
| Two single phase transformers | Three single phase transformers | One three phase transformers | |
| 1 | 0.6 | 0.6 | |
| 2 | 1.5 | 1.0 | 2.0 |
| 3 | 2.0 | 1.5 | 3.0 |
| 5 | 3.0 | 2.0 | 5.0 |
| 7 | 4.0 | 3.0 | 7.5 |
| 10 | 5.0 | 4.0 | 10.0 |
| 15 | 7.5 | 5.0 | 15.0 |
| 20 | 10.0 | 7.5 | 20.0 |
| 30 | 15.0 | 10.0 | 30.0 |
| 50 | 25.0 | 15.0 | 50.0 |
| 75 | 40.0 | 25.0 | 75.0 |
| 100 | 50.0 | 30.0 | 100.0 |
| Horse power of motor | Approximate full load current | Horse power of motor | Approximate full load current |
|---|---|---|---|
| 1 | 3.2 | 20 | 50. |
| 2 | 6.0 | 30 | 75. |
| 3 | 9.0 | 50 | 125. |
| 5 | 14.0 | 75 | 185. |
| 10 | 27.0 | 100 | 250. |
| 15 | 40.0 | 150 | 370. |
Transformer Connections for Motors.—Fig. 2,020 shows the connection of a three phase so called delta connected transformer with the three primaries connected to the lines leading from the alternator and the three secondaries leading to the motor.