20) The frequency of voltage generated by an alternator having 4 poles and rotating at 1800 R.P.M. is

21) A 50 Hz alternator will run at the greatest possible speed if it is wound for _____ poles

22) When speed of an alternator is changed from 3600 R.P.M. to 1800 R.P.M., the generated e.m.f. per phase will become

23) The power factor of an alternator is determined by its

24) For proper parallel operation, polyphase alternators must have the same