Transformer Operation with Grounded Secondary.—The operation of a transformer with a grounded secondary has been approved by the American Institute of Electrical Engineers, and by the National Board of Fire Underwriters.
This method of operation effectually prevents a high voltage occurring upon the low tension wires in case of a breakdown or other electrical connections occurring between the primary and secondary windings.
Fig. 2,017.—Moloney pressure transformer adapted for switchboard work in connection with voltmeters, wattmeters the, etc., in sizes from 25 to 500 watts.
In case of a breakdown without the secondary grounded, any one touching a part of the low tension system, such as a lamp socket, might receive the full high pressure voltage. With the low tension grounded, the fuse in the high tension circuit will blow and the fault be discovered upon replacing it.
Transformer Capacity for Motors.—The voltage regulation of a well designed transformer is within 3 per cent. of its rated voltage on a non-inductive load such as incandescent lamps, but when motors are connected to the circuit their self-induction causes a loss of 5 per cent. or more, and if the load be fluctuating, it is better to use independent transformers for the motor, which will prevent considerable fluctuations in the incandescent lamps. Arc lamps do not show slight voltage changes as much as incandescent lamps. The proper rating of transformers for two phase and three phase induction motors is given in table on the next page.
Fig. 2,018.—Moloney current transformer switchboard or indoor type. It is used ordinarily for insulating an ammeter, a current relay, the current coil of a watt meter or watt hour meter from a high tension circuit, for reducing the line current to a value suitable for these instruments.
A three phase induction motor may be operated from three single phase transformers or one three phase transformer. While the one three phase transformer greatly reduces the space and simplifies the wiring, the use of three single phase transformers is more flexible and, in case one transformer burns out, the connection can be readily changed so that two transformers will operate the motor at reduced load until the burned out transformer is replaced or repaired.