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Motors - Calculating Electricity Consumption

01/27/2009 11:47 PM

if a 25hp.motor runs for an hour,how much electricity or units does it consumes?

is there some formula to calculate the consumption?

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#1

Re: how much electricity or units does it consumes,if a 25hp.motor runs for an h

01/28/2009 12:28 AM

One horsepower = 746 Watts. So 25 hp = 18,650 Watts.

If you run that motor for one hour, you will expend:

18,650 Watts * 1 hr = 18.65 kW-hours.

That's assuming 100% efficiency. Real efficiency is less. To get actual electrical input energy, divide 18.65 kW-hours by the efficiency. For instance, if the motor is 90% efficient, the input energy is:

18.65 kW-hr / 0.9 = 20.72 kW-hr

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#2
In reply to #1

Re: how much electricity or units does it consumes,if a 25hp.motor runs for an h

01/28/2009 12:41 AM

in US, what unit of power you guys use? Btu or KW-h?

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#3
In reply to #2

Re: how much electricity or units does it consumes,if a 25hp.motor runs for an h

01/28/2009 2:03 AM

For a motor, it is horsepower or kW. I haven't seen BTU used much recently, but I think of that more for HVAC work. Maybe others can reply better - I'm more of an electrical type.

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#4
In reply to #2

Re: how much electricity or units does it consumes,if a 25hp.motor runs for an h

01/28/2009 11:42 PM

Strictly speaking, the BTU (British Thermal Unit) is a unit of energy, not power. Power is energy/time, so logical units of power would include BTU/second and BTU/hour. I get the impression that in some fields and in some regions, apparently including the US, the BTU is used to mean BTU/Hr. I strongly object to this practice. All measurements should include the correct units to avoid any ambiguity!

Likewise, the kWh is definitely a unit of energy. The kW is a unit of power.

Generally, in the US, the kW is used as the unit of power for large electrical devices other than motors/engines, and the hp is used for motors and engines, although it is also common practice to use the kw as a unit of power input to an electric motor and the hp as a measure of the output power of the motor. By doing this, manufacturers easily obscure the efficiency of their motors, making it easier to sell inefficient motors!

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#11
In reply to #1

Re: how much electricity or units does it consumes,if a 25hp.motor runs for an h

01/29/2009 10:27 AM

So close and yet so far. What you've correctly calculated is the KVA rating not the kilowatts. KVA does not care how much power is real and how much is reactive.

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#5

Re: Motors - Calculating Electricity Consumption

01/29/2009 4:16 AM

Surely the load on the motor plays a heavy role? If a pump is actually pumping or just running free with no load.......that must make a difference....

I would say that the load current in amps would need to be measured while running and the mean average taken for any calculations.....

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#9
In reply to #5

Re: Motors - Calculating Electricity Consumption

01/29/2009 9:29 AM

Though post 1 has got GA, the answer is wrong.

Andy I agree with you. The consumption is variable depending on the load. With no load, it consumes some power which is minimum energy required to run the motor. Then afterwards, the energy consumed goes on increasing with load, though not linearly proportional.

If I am not wrong energy consumed is V.I.Cos (phi)

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#13
In reply to #5

Re: Motors - Calculating Electricity Consumption

01/29/2009 1:03 PM

An excellent response, and completely true. It is essential to know the load on the motor.

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#6

Re: Motors - Calculating Electricity Consumption

01/29/2009 4:42 AM

"if a 25hp.motor runs for an hour,how much electricity or units does it consumes?

is there some formula to calculate the consumption?"

Not enough information to determine anything useful without assumptions.

Just because a motor is rated 25HP does not mean it is consuming 25HP. Motors only consume what they need to move the load plus whatever they waste in doing the job. If you hook up a 25HP motor to a load that only needs 10HP to do what you want it to do, then the motor will only consume 10HP (plus waste).

So the only true way to know what the consumption is will be to measure it. That is what a kWH meter does.

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#7

Re: Motors - Calculating Electricity Consumption

01/29/2009 5:17 AM

Electricity consumption = power x time

Power consumption (active) = (Supply voltage x sq root 3 (for 3 phase) x Amps drawn x cos phi at load point) / Efficiency

e.g. (25hp) full load 400v 33Aflc 90%eff 0.82 cos phi (100% load)

If you know the cos phi, you can calculate the VA (Watts) and VAr (apparent power) as needed, this is the part many people do not measure.

If you are accurate re motor voltage and no load current, you can interpolate to approximate load and power.

e,g, Supply 400v, motor rating 400v 33A etc, no load current = 13A. Half load approx 13A + ((33-12)/2)) = 23A

50% load cos phi and eff from motor table, therefore approx power, say eff is 87% but cos phi now 0.69.

The main danger is taht for lighter loads you may not have a reasonable idea of cos phi so you have to measure.

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#8

Re: Motors - Calculating Electricity Consumption

01/29/2009 7:54 AM

Running a 25 HP motor for one hour is still lacking a whole lot of information.

Is the motor doing any work? Pumping water or operating a cooling fan maybe?

The mechanical load the motor is developing will determine the electrical energy the motor is consuming .

Also, a motor is an inductive load so the powerfactor is less than unity .

In an industrial environment you may be paying for KWHrs ( the energy consumed) plus maximum demand influenced by the motor( KVA)

So the short answer is the amout of energy ( in KWHrs) is determined by the amount of work the motor is doing.

The 25 HP is a maximum limit the motor can do safely.

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#15
In reply to #8

Re: Motors - Calculating Electricity Consumption

01/29/2009 7:30 PM

Check the data plate on the motor for full load amps and service factor.The service factor is a multiplier for the horsepower rating of the motor.A 1.25 service factor motor can perform satisfactorily at 125% of the name plate rating,etc. What is the voltage supply to this motor? 3 phase DELTA, WYE? If your utility charges for power factor (KVAR) this will also affect your total cost to operate, and if the utility charges for peak demand, a soft start could be beneficial. Many variables to consider.Not enough info to address them all. HTRN

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#10

Re: Motors - Calculating Electricity Consumption

01/29/2009 10:22 AM

Why didn't I think about having you guys do my homework while I was in school? Oh right, I value being able to do the work myself. How silly of me.

Well this group approves wrong answers all the time. So I guess he'll get his reward in the end.

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#12
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Re: Motors - Calculating Electricity Consumption

01/29/2009 10:49 AM

This reply is not only to redfred but the others who said post #1 was wrong because the load wasn't specified.

Disagree with redfred in his other post talking about kVA vs. kW. Original question asked how much electricity would be used by 25 hp motor. I answered that question accurately. The original question did not ask, "How much current will be drawn from mains power, mains power being..." The OP did not ask what the meter would read. It was a simple question about energy unit conversion. Now maybe the OP meant to ask these more involved questions, and if so, then my answer was only part of it. But I don't read minds well and couldn't say, off hand, if this was a dc motor (what potential), a single phase ac motor running off 115 Vac, or 220 Vac, or 440 Vac, or if it was operating across two phases, or three...

As to the effect of the load on the current draw, I had to make the assumption, based on the OP, that the load was the motor rating, else you can't answer the question. If the load runs directly off the motor, then the electrical power in will scale with the load. If there is substantial gearing between motor and load, there will be a certain amount of power lost in the transmission that will load the motor regardless of any other load.

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#14
In reply to #12

Re: Motors - Calculating Electricity Consumption

01/29/2009 2:42 PM

emc c,

Ok, I will admit to a minor faux pas on my part. I did assume that a motor as large as 25 hp would be AC powered. My point that I was trying to make is that the power rating of an electric motor is the largest real sustainable power that can be produced by that motor. But as others here have agreed with me, the actual power produced by the motor depends also on the mechanical load presented to the motor. When the motor is an AC driven motor, then nearly regardless of the mechanical load, most induction motors maintain the same current magnitude as the mechanical load changes. The phase angle between current and voltage changes with the mechanical load. The instantaneous power produced is actually a nearly useless quantity solved by the time dependent equation P(t)=V(t)*I(t). Real (average) power though is calculated by the product of the two magnitudes times the cosine of their phase or P=VIcosΘ. This does accurately imply that one could have a 25 hp motor that is producing no real power. This happens when a prime mover spins the motor shaft at just the right speed to balance all of the other losses.

This does not mean that knowing the rated horsepower of a motor produces, no usable electrical values can be ascertained. When one ignores the angle of these two vector components, one can directly produce the VA or kVA value that you calculated so well. The mechanical load will not matter for this calculation for load changes will modify the angle between vectors, not the magnitude.

Now back to the initial mea culpa in my reply, if this is a DC motor then there is no phase angle to change with load. Then the current does drop off as the mechanical load decreases. This comes from the fact that the motor produces a backward emf voltage that tends to cancel the voltage applied externally. Similarly if a prime mover exactly compensates for all mechanical losses, no current and therefore no power is produced by the motor.

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#16
In reply to #12

Re: Motors - Calculating Electricity Consumption

01/29/2009 9:34 PM

I was hoping, you will accept the incompleteness of your 1st Post, but seems, you are not mood to do so.

Though the originator of the thread didn't ask / provide many more things, you could have given really correct answer.

You are raising many more questions about the type of motor etc.

But, irrespective of type of motor, the electricity used will predominently depend upon the load. Thus, your answer, though ok to some (small) extent, it is incomplete.

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Users who posted comments:

Andy Germany (1); Anonymous Poster (2); Blink (1); djacob (1); dkwarner (1); emc_c (3); gsuhas (2); JRaef (1); redfred (3); VUC (1)

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