How to tell horsepower on electric motor?

[Black Forest]

I have a relatively large electric motor that has no faceplate. It is three phase.

Is there any way to tell how many KW/HP this motor might be? I want to use it to drive a hydraulic pump so I need to know how many HP so I know what pump to buy.

I thought maybe based on the number of ampere it draws it might be possible to calculate. But as usual I have no clue!

[The Artful Bodger]

Unless I am mistaken power is volts x amps x 3 for a 3 phase motor.

That will give you an answer in watts. 746 watts = 1 horsepower.

Of course this is the power consumption of the motor and the actual horsepower out will always be something less depending upon efficiency, however 3 phase is quite efficient and big motors are more efficient than small ones so assuming output power is the same as input power you cannot go wrong. IMHO of course.

If it does not have a name plate how can you determine full load current?

[Weston Bye]

The current rating for a motor is defined in terms of FLA or full load amps. Just measuring the current of an idling motor will not give a true picture. You would have to subject it to a known load.

Even then, you wouldn't get the whole picture, as an overloaded motor may deliver the overloaded output torque without slowing down too much until the windings overheated. Temperature rise is also a factor in motor performance.

[The Artful Bodger]

Weston, is there any rule of thumb relating to full load current and momentary stall current?

[Weston Bye]

The term "locked rotor" current comes to mind. I don't recall the specifics. However, any motor starting from zero rpm briefly experiences locked rotor current, the highest current the motor could be expected to encounter, until the rotor begins to turn.

An exception would be if the motor were connected to a "soft start" motor starter. These are uncommon except in very large motors or large motors with high inertia loads.

[Black Forest]

So basically I am screwed! I wonder if there is any way to do it if I would connect to a hydraulic pump of known output at a certain rpm's and then measuring the pressure created or something along those lines? Knowing that it takes a certain amount of HP to generate a given GPM'S at a certain PSI?

[JCHannum]

If it is a standard motor and frame, a motor rewind shop can tell you. There will be a charge as they will have to partially dismantle it as connections as well as windings and wire size determine the rated horsepower as well as motor speed.

If you have an idea of the application, you could try to duplicate it, monitor motor amps and temperature rise over a period of time of an hour or so to see if it can handle the load.

[MuellerNick]

The size of a motor gives a good clue of its power. If it is very old, the power is just "better horses" than now.


Nick

[Black_Moons]

Kick it hard enough to make it roll over on its frame. Count how many toes you have broken. Thats how many HP it is.

[J Tiers]

You can tell approximately.

There are tables of current draw vs HP. The problem is that you need to know that you have the full load, and there is the problem that motors these days often don't draw as much current as motor used to for the same power. So it depends on the age of the motor and the table you have.

We had this discussion in part a little while ago, in the infamous laser exhaust thread.... As Weston Bye mentioned, an electric motor, induction or other, HAS NO defined rating...... by itself.... You have to add something else, like heat rise, into the picture to get a rating.

Generally a motor will simply pull harder until it either stalls or burns up. In the most usual case, the motor (or anything electrical) is heat limited.

You can make a guess.... run the motor on its nominal voltage, and measure unloaded amps.

Assume (yes, I know) that unloaded amps are between 30% and 50% of FLA, say 40% for our purposes. Then figure the FLA.

Go to the motor tables and see what motor corresponds to that. It could be correct, or might be the one higher or lower, depending.

A 3 phase motor that draws 3A at 230V no load, then.... (I made up the number, so....) assume that is 40% or FLA, which gives 7.5A FLA at 230V 3 phase. The motor table I have handy here suggests 6.8A for 2 HP, and 9.6A for 3 HP.

So it could be either one, given the inherent inaccuracy of the method. But it is almost surely bigger than 1.5 HP (5.2A), and smaller than 5 HP (15A).

[wierdscience]

Google "Prony Brake Test",if you can determine the torque and the RPM you can determine the HP.

[J Tiers]

Google "Prony Brake Test",if you can determine the torque and the RPM you can determine the HP.

But there is always the heating issue...... you ALSO need the rated insulation rise... you can assume say 40 deg, but you don't KNOW.......

And it takes more equipment. A brake, scale, and lever, at least, plus the usual ammeter and a tach

[JMS6449]

Frame size will give you a rough idea or starting point for hp/kw-yes there are exceptions.

[wierdscience]

But there is always the heating issue...... you ALSO need the rated insulation rise... you can assume say 40 deg, but you don't KNOW.......

And it takes more equipment. A brake, scale, and lever, at least, plus the usual ammeter and a tach

Yabbut my thinking is he probably has that stuff laying around the house.

If the nameplate RPM most commonly found (in our case 1750 or 3450 @60cycles) is achieved in the brake test and the torque measured at that point then we should have an educated guess as to the HP.

Given the fairly wide increments motors are mfg in it should work.

[gwilson]

I am a terrible electrician. The rule I was told was 3 amps per H.P. @ 220 V.. That may well be proven wrong,I'm sure.

[Black Forest]

THis motor runs on 360 volts at 16 amps and is probably 1425 rpm. I don't know if it draws 16amps per phase but you normally must have it available. I look to see if I have a motor anywhere close to the same size. This motor weighs close to 100lbs. I go and take a picture.

[Black Forest]

The motor is 350mm long and 220 mm in diameter. I have it next to a rattle can of paint for comparison.

http://i853.photobucket.com/albums/ab91/burnandreturn/electric_motor.jpg

[MuellerNick]

About 2 kW or more. Just compare the sizes from this site (http://www.elektromotorenmarkt.de/elektromotoren/BrowseCatalog__Standardmotoren.shtml?catsku=91) and you are done. No current measuring and guessing.


Nick

[The Artful Bodger]

I would fix the motor down and arrange some sort of brake, which may be nothing more that a length of belting fixed at one end which can be tensioned over the pulley.

I would put an ampmeter in one lead then start the motor and slowly apply the brake while watching the amp meter, as the brake is applied the current will rise but at some point the motor will start to slow and the current will rise quickly.

I would assume the motors current carrying capacity would be something less than that point. I could then readily calculate the horse power as per post #2.

[J Tiers]

360V 3phase and 16A is somewhere around 7.5 HP anyway.... About 10 kVA input, which if at a PF of about 0.6 comes to 8 HP.... close to 7.5.

Since it is not a US motor, HP ratings are not likely to be standard...... so ignore those numbers...... 8 HP would be around 6 kW.

[whitis]

THis motor runs on 360 volts at 16 amps and is probably 1425 rpm. I don't know if it draws 16amps per phase but you normally must have it available. I look to see if I have a motor anywhere close to the same size. This motor weighs close to 100lbs. I go and take a picture.
How did you determine this with no nameplate? Isn't 3-phase power there 400V?


Upper bound:
360volts*16amps*sqrt(3) = 13.378858 horsepower
This assumes 360V leg to leg, rather than 360V leg to neutral. Power factor and efficiency corrections will lower horsepower. Power factor perhaps 0.86 at full load. Efficiency about the same? Which would work out to 10 horsepower.

Either the voltage or the current in the phases will be sqrt(3) lower than
the voltage/current in each line, depending on whether wye or delta configuration; either way, power is a factor of sqrt(3) lower. Mulitplying by three (3 legs) and dividing by sqrt(3) is the same as multiplying by sqrt(3).
http://www.djtelectraining.co.uk/downloads/ADL%203Ph%20SuppliesSystems.pdf

Prony brake video:
http://www.youtube.com/watch?v=9Wi51Kf8bzA

[macona]

I have a 10HP three phase motor that is about the same size as that. The 15HP I had for a rotary converter is virtually the same size.

[MuellerNick]

Upper bound:
360volts*16amps*sqrt(3) = 13.378858 horsepower

As we have 230 V, 3 phase voltage is 400 V (230 * sqrt(3)).
I do not know where the upper limit of 16 A comes from, nor do I know where the factor of sqrt(3) in your calculation comes from.
Efficiency of such a motor is more about 80% than 50%.

Anyhow, all the suggestions to measure the power input or output are funny. The motors are standardized by size and power. So simply looking for one of the same size is enough.
It is true that you can find motors that do have power of the next bigger one, but that error would not hurt.


Nick

[mike os]

physical size will give an indication of power.... but thats all

if is is a standard mount then the spindle height (above the feet) will give the frame size

frame size data (http://www.tecmotors.co.uk/pdfs/products/TECC%20data-89.pdf)

frame power data (http://www.tecmotors.co.uk/pdfs/products/TECC%20data-90.pdf)

only for 1 manufacturer but to the best of my knowledge these are standards

[Greg Q]

Nick is correct:a visit to one of ABB's websites will reveal plenty of product data for their (many) motors. The local website has these listed under "low voltage" motors because their catalogue includes massive industrial and traction motors.

You can narrow it down to one or two h.p ratings by comparing the physical dimensions with those listed online.

FWIW I have an ABB motor a bit smaller than that rated for 3 kW. (4 h.p)

Greg

[Ian B]

I think this is the relevant ABB brochure:

http://www05.abb.com/global/scot/scot259.nsf/veritydisplay/8b5006a94d4fc479832576da0024f77b/$File/General%20performance%20motors%20EN%2002_2010.pdf

At 220mm diameter (216mm in their listing), it would be a 132 size motor. These range from 3 to 5.5hp. Is it a 28mm or a 38mm shaft? Maybe there are some other measurements that you can take and match to the catalogue, to find which one it is.

Ian

[lakeside53]

As Whitis mentioned, you do have to factor in both powerfactor and efficiency. If both are 0.8 / 80%, then you're down to 0.64 of 1.0/100%.

Here a simple calculator.

http://www.engineeringtoolbox.com/electrical-motor-calculator-d_832.html

[mike os]

motors have been standard sizes for decades....

frame size, body & shaft dims and no of poles and you have it ....

[lakeside53]

deleted.........

[whitis]

As we have 230 V, 3 phase voltage is 400 V (230 * sqrt(3)).
I do not know where the upper limit of 16 A comes from, nor do I know where the factor of sqrt(3) in your calculation comes from.
Efficiency of such a motor is more about 80% than 50%.

I found numbers close to 86% for both efficiency and power factor, for three phase induction motors, at full load which is where the HP rating is taken. At lower loads, these numbers are lower. But these are typical. However, the numbers came out very close to 10HP.

The sqrt(3) was explained to some extent in a paragraph and a link. Three phase motor wiring with wye and delta is more complicated than a single phase winding. Either the voltage or the current, depending on configuration, is split between multiple windings so what you see at the input leads is not the same voltage/current that is across the windings and thus multiplying voltage*current*3 doesn't give the right number, you have to do voltage*current*3/sqrt(3) which simplifies to voltage*current*sqrt(3). Sqrt(3) is related to the 120 degree phase difference between the phases. It is the same number which gets you from 230V to 400V and it is applied to voltage or current but not both in the power calculation. Then you multiply by power factor. In wye configuration, the currents you measure are correct but the voltage is reduced to the single phase voltage since each winding is between a power leg and a virtual neutral. In the delta configuration, the voltages you measure between legs reflect the voltage on the corresponding winding but the current you see going through the leg is divided between that winding and the other two. Either way, you need the magic fudge factor to get the same results you would get if you pulled the terminal plate off the motor and measured at the six leads actually going to the windings instead of measuring at the three leads going to the power line.

Yes, motor power vs frame size is standardized for motors of the same type (can't mix single phase and 3-phase, for example) which actually adhered to the standard and were manufactured in the appropriate time frame (the standards have changed over time) and manufactured in or for the appropriate country where the standard applies.

[Black Forest]

Is it a 28mm or a 38mm shaft?


It is a 28mm shaft.

[mike os]

so probably a 100l or 112m frame

is the body 160 or 190 diameter?

and what is the floor to center of spindle height?

[J Tiers]

Yes, motor power vs frame size is standardized for motors of the same type (can't mix single phase and 3-phase, for example) which actually adhered to the standard and were manufactured in the appropriate time frame (the standards have changed over time) and manufactured in or for the appropriate country where the standard applies.


And there is much of the issue.....

There are innumerable "special" configurations made for one purpose or another, which don't adhere to the standard.... Shaft size, which was recently mentioned, is PARTICULARLY variable, since it is easy to make special.

But frame size may be kept even though the motor is reduced in power, simply because the original machine is too expensive to change in the production quantities made....

And usually a "special" will be fitted into a standard frame, and use standard laminations, even though they may not be the ones corresponding to the actual rating as a standard. This is because the frame and lamination profile is expensive to change, while windings, lamination stack, shaft, etc, can be customized at low cost.

And, of course, temp rise, service factor, "duty", "air over" vs self cooled, etc, all change the 'rating".... A "10 HP motor" might be 14 HP for a 30 minute rating..... A motor for raising a door might be 1 HP 10% duty, and really be a 1/3 HP motor....

You simply don't "know", and unless you perform the tests and re-rate the motor formally, *you* *are* *guessing*.

Frame size is a decent starting place, but per the above, may be bogus.

Electrical tests show electrical performance, but NOT necessarily heating issues, etc, which directly affect the actual rating (remember motors are heat limited) .

If the simple electrical tests PLUS the frame size both point to the same result, then you can be fairly sure.

[gary350]

There are several standard motor frame sizes they can be wound with different size wire to make that frame be any hp you like with in a certain range. A frame can be wired to be 2 hp, 2.5 hp, 3 hp, 3.5 hp etc. Just looking at the frame size only tells you it could be in certain range.

Locked rotor current is usually more than 3 times full load current.

You can run a motor harder that it should be run and it will run until it over heats. It is possible to get 6 hp from a 5 hp motor but the motor will over heat if you run in continious. Short runs are ok as long as the motor has time to cool down between each run.

Motors have built in safety factor they will run at about 10% more than the name plate rating.

There are a few ways to guess pretty close to the correct hp rating. Connect the motor to a hydraulic pump and turn it on. Turn up the relief valve pressure until the motor starts to get warm it may take more than an hour run time for the motor to warm up. Check the temperature of the motor body and keep turning up the relief valve pressure until your thermometer reads 150 degrees F after running for several hours. Once you have the motor running pretty stable at 150 deg F all day check the wires one at a time with an amp clamp they should all read pretty close to the same. Use a motor chart to look up the current it will tell you the motor size. This is still a guess it depends on the frame size and a few other things how correct the guess will be.

Another thing take the motor apart and mic the wire diameter. A motor rewind shop can give you a very very accurate guess at the hp rating by knowing the wire size.

[lakeside53]

Here's the Nema "standard" - shows both the current (1964) and past revisions (1952), and orginal frame size for both 3 phase ODP and TEFC, verses speed, frame and poles.

None of this covers the IEC motors, and as pointed out, there are many variations...

http://www.motorsanddrives.com/cowern/motorterms2_tbl1&2.html



On the same site.. there is some very good info on Motors :

http://www.motorsanddrives.com - left side under - "more info"; click on "cowen papers".