step up transformer

[Ibas]

Hi, I need to wire a transformer 120 volt primary to 240 volt secondary to power a 240 volt 1/8 hp motor. I have not seen the motor name plate yet. But I was wondering if some one could help me with the formulas to size the transformer. I am not sure how to size it.
1 hp= 746 watts
1/8 hp= 93.25 watts
NFPA 70 Table 450.3 B Primary and secondary protection current less than 9 amps = 167%
So do I size this at 93.25VA X 1.67%= 155.72VA
Or 240 Volts X Motor nameplate FLC= VA X 1.67%= VA
Or am I on the totally wrong path??

 

[jim dungar]

Hi, I need to wire a transformer 120 volt primary to 240 volt secondary to power a 240 volt 1/8 hp motor. I have not seen the motor name plate yet. But I was wondering if some one could help me with the formulas to size the transformer. I am not sure how to size it.
1 hp= 746 watts
1/8 hp= 93.25 watts
NFPA 70 Table 450.3 B Primary and secondary protection current less than 9 amps = 167%
So do I size this at 93.25VA X 1.67%= 155.72VA
Or 240 Volts X Motor nameplate FLC= VA X 1.67%= VA
Or am I on the totally wrong path??

HP is an output value. Using 746W/HP gives you the output watts of a motor. To convert output values into input values, you must take the motor effciency and power factor into account.

 

[Ibas]

Jim thank you for your quick response to my question.
I know VA= E X I
Power factor= Watts/ VA
I haven't seen the motor nameplate yet so I don't know the efficiency.
If I can calculate the VA of the motor do I size the transformer at 167% of the VA
Thankyou again for your patience and time

 

[LarryFine]

Looking at Table 430.148, and 2.2a for a 1/6hp motor, I'd say it would be okay to allow 2a for the current. Does 500 VA sound right, or should that be doubled?