Please help with these questions Problem #3: A 120/12V transformer is to be conn

Question

Please help with these questions

Problem #3: A 120/12V transformer is to be connected as a step-up autotransformer. If VLv=115V, find the value of VHv. Be sure to show all steps of your work. Problem #4: A three phase motor is connected to a 60Hz electrical source. The motor spins at 360rpm. Find the number of poles in the machine. Problem #5: The output of the motor in problem 4 is connected to the input of a generator. The generator has 30 poles. Determine the electrical frequency of the generator output.

Explanation / Answer

Prob 3:

Given:

Designed rating:

Vin=120 V

Vout=12 V

turn ratio of T/F=N=120/12=10

Since T/F has to be used as Step up, So Vhv will become:

Testing input=Vlv=115 V (low voltage winding Voltage)

=>High voltage winding Voltage= Vhv=Vlv*N

=115*10

=1150 V

Prob 4:

Speed=360 rpm

frequency=60 Hz

For motor(both induction and synchronous)

Ns=120*f/P

Where Ns is speed of motor

=>P=120*f/Ns

=120*60/360

=>P=20 Poles

Prob 5:

For generators (both induction and synchronous)

Frequency=P*Ns/120

=360*30/120

=90 Hz

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