BuickandDeere--power ??

In electrical machine controls, If I have a

25HP motor 3 phase 230 volts and the motor is running at full load what is my watts used.

I am asking this cause I would like to know how to calculate KW used for a 230volt motor and KW used for a 460 motor. I dont understand the formula I have in an old textbook from 1987.

It appears that the amps go down by half but the voltage goes up and the answer in the formula stays the same. in the electric bill are you paying for Kilowatts or amps? Please educate my thinking process. thanks.

I know that if the volts goes up from 230 to 460 the amps should drop and the electric bill should drop by 1/2 too.

A 25 hp motor running on 230 3phase will pull about 64 amps and on 460 will pull half that or 32 amps.
One horse power is equal to 746 watts, more or less. If you are running 25 hp with a motor you are using the same watts whether 230 or 460 volts. The amount of power used is the same, but the amps required are cut in half with 460 volts.
The electric meter measures watts used no matter the voltage. DH

I forgot to add that BuickandDeere can give you a whole lot more detailed explamation than I can.DH

Watts is Watts. 746 watts per horse regardless of voltage. They bill you on kilowatt/hours so no change in power usage by changing voltage. 1000watts consumption for one hour = 1KWH

Gun guru.....line voltage to a motor will not effect how much it costs you to operate the motor...(at least to a notable amount) But install costs may be less for larger motors...I mean big motors.....

746 watts per horsepower. Of course, the actual power may be greater than or less than the rated power; an electric motor will typically put out whatever power is required of it, up until it stalls or burns up.

Now, if you want to know the current draw of the different motors, that's an entirely different question. And typically a more important one.

You fellows were clear and covered all the bases. There isn't very much to add.

Assuming a dual-voltage motor, will the efficiency of the motor be better at the higher voltage?

CW:

The efficiency of the motor itself is unchanged when wired to run at a different voltage. The current through the windings is the same.

What changes is the voltage drop between the electrical service and the motor. Higher voltage allows the use of smaller conductors. Or, if the conductor size is the same, voltage drop is reduced, which increases the voltage at the motor. Paradoxically, more voltage at the motor can reduce current demand and further reduce voltage drop.

The only other reason NOT to switch from low voltage to high is your service....Do you have a 230 volt service now? To utilize 480V, you will need a transformer and sub panel, or flat out service change, leaving you w/ 277/480V, and you would still have to buck down to 120/240V for receptacle power and lights.......that is gonna get pricey....

Wiring size is dictated by two things: voltage drop and current capacity (aka "ampacity"). I'm going to assume that your 15 kW generator is really a 15 kVA, rated at 240 volts. That means the conductors need to be able to handle at least 63 amps. You will need at least AWG #6 copper to handle that much current. It wouldn't hurt to go to #4, but I imagine it will be tough enough connecting #6.

Voltage drop is going to be negligible over the short distance from your generator to the panel; there's no need to do any voltage drop calculations. But voltage drop will come into play for whatever cable you run from the panel to the load.

Thanks Mark. I checked the plate. It had a place for KW and KVA right next to it. It had the 15 under KW. It also said 240/120 volts and 62.5/125 amps.

The wires were large guage and had lugs on them for connections. It has 3 wires coming out of the motor and then one more wire grounded to the housing and motor for a total of 4 wires. I assume 2 hots, 1 neutral, and the 1 ground.

Guess the other question I've always had is what size main breaker to put in the panel. A 125 amp home type breaker or a 60 or 80 amp? With what I assume is 62.5 amps coming off each hot wire, I'm not sure. Thanks.

Something odd there with the 3rd insulated wire with a single phase motor.
A 15KW/20HP single phase motor is very unusual. We did you find it and what application?
Could you post all the motor data plate info please?
I'm wondering if it's a 208/240V three phase motor? A single phase motor of that size would draw approx 80amps on single phase 240V with power factor correction capacitors in use to obtain 0.9 lagging.

B&D, BC was talking about a generator, not a motor.

The kVA rating is more important that the kW rating. If no kVA was listed, then the kW rating can be assumed to be for a resistive load, meaning that volt-amps = watts = 15,000. That is backed up by the 62.5 amp output rating at 240 volts.

I'm speculating that the 125 amp rating at 120 volts is total current, in other words 62.5 amps per phase. (Even if a single phase is capable of 125 amps, you don't want to run it that way; you should split the load across both phases.)

You should use a 60 amp two-pole (i.e. 240 volt) circuit breaker. It will trip if either phase exceeds 60 amps continuous, but allows a brief overload.

If you install any 120v outlets in your panel, they should be divided across the two phases and each phase should have its own 20 amp circuit breaker.

Thanks guys. 60 amp double pole 240 volt breaker it is. Maybe that's part of why I didn't get the panel a long time ago. Finding a main breaker in that size in an exterior waterproof panel gets kinda pricey. As I recall they don't make one like that. I would have to mount a separate breaker in a main lug panel but I think I was looking at a 125 amp breaker that allowed the same size wire coming out as going in which was big and pricy.

I bought this generator years ago at an oilfield auction. It had been used to run a downhole water well submersible pump for a water well drilled to supply water for the oil well drilling. We used 3 phase on drilling rigs and split up the 120 lights onto separate phases.

Anyway right after I bought it at the sale, a guy from a electric shop said he needed a 3 phase for an oilfield purpose and offered to trade a same size single phase saying that if I was going to put it out at a farm, single phase is really best. I knew enough to know that 3 phase works but it is better if you can balance the 120 loads as much as possible. Nice guy that I was, I swapped with him even up. Don't know if it was a mistake or not or if the 3 phase was worth more.

The biggest reason for going to a higher voltage is to reduce the IsquaredR losses (amps X resistance) which is lost as heat in a motor and is the reason the electric company sends out 5000 volts over the power grid. Lower line losses as amperage is cut by 98% while resistance values remain the same for conductors and other delivery hardware.