Question

Another method of eliminating the ratio error is _________

(a) by reducing secondary turns

(b) by increasing the primary turns

(c) by increasing secondary turns

(d) by reducing the primary turns

The question was asked during an interview.

This intriguing question comes from Reduction of Errors in Potential Transformers in portion Extension of Instrument Ranges of Electrical Measurements

Answer 1

The correct answer is (a) by reducing secondary turns

Best explanation: In a P.T., at no load, we get

where, R is the ratio error

       n is the turns ratio

       Is is the secondary winding current

       Ie is the iron loss component

       Im is the magnetising component

From the above equation it is seen that to reduce the ratio error, actual ratio and nominal ratio must be made equal. This can be done by reducing the secondary turns.