Calculating Amperage

Are you trying to run a 3 phase motor on single phase power using a converter?

If you want to compare single phase a 3 phase motor loads look in the NEC table 430.249 and 430.250

The difference is the (square root of 3) 1.73... That is not factoring in any heat losses and such witch is usually minmal. If you are using a VFD for the conversion it is usally recommended to double the size of drive (5hp single phase - 10 hp VFD to the three phase 5hp motor)

Yes, the customer would like to run a 3ph motor with a 1ph service.

If the 3ph motor usually needs a 40amp dual-element fuse or a 60amp thermalmagnetic circuit breaker (under normal conditions): how do you know what he needs when using w/a 1ph service and transformer?

Thanks for your replies!

I have installed many VFD's for this application.... Just a word of advice Rotary phase converters are usally not the best solution, in fact very seldom. Just for the fact that the VFD's do a much better job, are more efficient, and almost zero maintenance once set up.

The constant in the conversion is power (kva). For single phase, KVA = V x A. For 3 phase, KVA = V x A x √3. Since the converter does not store power,

KVA_in = KVA_out

Thus, V_1ph x A_1ph = V_3ph x A_3ph x √3

Solving for current:

A_1ph = A_3ph x √3 x (V_3ph / V_1ph)

To input current for a given output load:

A_3ph = (A_1ph / √3) x (V_1ph / V_3ph)

Bear in mind that a Variable Frequency Drive will vary the voltage as the frequency changes to maintain a constant volts/hertz ratio. If your converter is a VFD, actual output voltage is not constant, and must be measured or calculated for the various frequencies in use in order for your calculations to be accurate.

To be really accurate, you need to consider the losses through the converter. However, for practical applications, the losses are usually negligible compared to the throughput.

Should we also considere the powerfactor in calculating the 3 ph curent...

This is not an easy question. I have furnish many control panels using the VFD to provide the three phase current. Here is the way I do it.

First, the three phase motor is does not distinguish where its driving current is produced. If the motor has a FLA of 30 amps, it will draw 30 amps regardless of where it gets the current. That is not the problem, however.

As has been posted earlier, the VFD 1) must be sized to twice the current rating of the motor FLA and 2) puts out a very clean, constant poewer to the motor. With this in mind, the VFD will protect the motor, so there is no need for a circuit breaker before the motor. There must be protection ahead of the VFD.

The solution, then is this:

1) - Select a VFD that is at least twice the current carrying capacity as the FLA of the motor and then...

2) - Select a circuit breaker to protect the VFD that is at least equal to the maximum current the VFD can accommodate.