Motor Amps

Just going on past experience I'd say no. I've had a motor test bed at two companies I've worked for. Off load current is dependant on motor speed (the lower the speed the higher the current). PF goes out the window with low speed motors.

Contact the motor manufacturers is the only advice I can give.

General saying is that, a medium size motor takes 25~35% of its full load current and it is magnetising part of I.

i just want to know, Is there any standard that a motor should take this much current?? or it is defined in any like MGI 1993 or somewhere else that it should reside in this much limit.

No, there isn't.

Measure it.

Well depending on what you mean by "unloaded" this can be easily estimated. Any electric motor that is not connected as an electric load to a voltage source will have zero amperes of current.

Ok, that was a cheap shot. It's not likely that you meant electrically unloaded, but you really weren't precise in your question. Now to be able to reasonably predict the current flowing in a motor requires a firm knowledge of a variety of basic sciences and mathematics. Allow me to start with the most abstract to the tangible knowledge required to attempt an estimation of the current. Since the goal of any motor is to move something you must know the mathematics of motion, Calculus. This then permits a firm understanding of the kinematics of Newton's laws of motion. Then one adds the physics of electric circuit analysis to be able to calculate how to provide electric power. Now things get complicated with the field of electromagnetic theory and the three dimensional Calculus required to apply and understand Maxwell's equations.

Now that one has this lengthy foundation of knowledge one can attempt to calculate the current in a motor that has only the mechanical loads of windage, rotor moment of inertia and bearing friction, along with the electrical loads of eddy currents and IR losses. This is how it is possible to calculate the amperes of an unloaded motor.

Ah man!

Lets have a walk to the moon.

i like the way you prompted.

Put a current probe on it, and measure it.

Run the motor on NO-LOAD,use clamp-on-ammeter to measure the ampere.Use motor data information to calculate for full load.find the no load percentage,whether its b/w 25%-35% u mentioned.

How to calculte is this:Use motor data information like active power P,VOLTAGE,POWER FACTOR,and calculate for full load ampere value,find 25% or 35% of the full load as the case may be and the value you get is the motor unloaded ampere.P=√3VICOSφ

P=√3VICOSφ, will be fine, unless the motor is wired star and you are measuring line to neutral voltage.... just saying...

It would be better if manufacturer provides it in the name plate.

Salam Arham.

Calculation of current in a motor, whether it is loaded or not loaded (no load condtion), is the same. It is determined through engineering design. It can be readily calculated or computed with formulas as long as data of the machine are available. Not the "possible to calculate" as you mentioned. We can even estimate the No Load or Full Load amperes of the motor without computing by direct measurement using ammeter or clamp meter.

Since you used the word calculate, we need the ff info:

Is the machine a DC or AC motor?

If it is DC what type is it? Shunt, series, compound etc.

If it is AC what phase? configuration?

Resistance/Impedance of windings, No. of poles, flux per pole, RPM etc.

So we need all these things and the use of basic formulas to calculate the current.

A rough calculation can be done by measuring the back emf, brush voltage drop, and subtract these from the supply voltage divided by the measured resistances of all the windings and apply the formula (DC motor):

I = [V-(Eb+Vb)]/(Total resistance of the windings depending on the connection).

For AC it is quite complicated... Hope this may help you in determining the current.

It is not possible to do a calculation without a more accurate definition of "unloaded". A bare shaft motor has variables imposed on it by the type of bearings fitted, state of the bearings, shaft alignment, winding imbalance, motor orientation, ambient temperature, and lots more I have not even thought of. Most are specific to the individual motor. If you can assign a value to all the variables and relate that value to the load imposed on the motor a calculation is possible, otherwise you are stuck with an estimate. 25-35% seems good to me.

If you run the motor up to speed, switch off the power and measure how long it takes to stop, you might be able to refine the estimate. If it takes a long time, the internal load is low so the current will be nearer 25%, if it takes a short time the internal load and current will be higher. With a thousand motors, a stop watch and an ammeter you could draw a correlation graph and devise a method of estimating the no load current more accurately just based on the time it takes to stop. This would be such a pointless exercise that one day I expect to see it as somebodies Ph D. research project.

If you had a totally unloaded motor, i.e. frictionless, the answer would be zero. Run it up to speed, switch off the power and it will continue to run fore ever. (Newton's first law)

As a final point. Why would you want to calculate this totally useless fact as nobody runs unloaded motors anyway?

Seen a few generalisations so here are some examples of manufacturer's data based on 400V 3 ph 50Hz

Ir = Rated Full Load Current I0 = No load current

0.12kW 4p Ir 0.45A I0 0.42A

4kW 4p Ir 8.4A I0 4.1A

225kW 4p Ir 364A I0 72A

so, you can see that the ideas of about 1/2 etc of FLC are not very accurate.

Of course, if the supply is lower, I0 will be reduced, if higher I0 will be raised.

hi to calculate amps of an unloaded motor is.... AC three phase motor is typicley 30% less than F L C but this is not spot on because depending on size and kw the rotor will never run 100% co efficient to EMF of coils unloaded. DC series motor impossible to say because unless loaded the shunts lagg the arm causing exsessive current draw untill put under load then emf is balanced and runs all day well under f l c DC compound motor same as above but under no load will damage comm and spark unless de-brushed due to heat

just to add !! a clip on ammeter WILL NOT BE TRUE as coil emf and rotor efficiency will not be accincrinos unloaded.

A clip-on ammeter (Hall Effect Device) will indeed read the current flowing to a motor. A scope readout would be advised, as the waveform may not be sinusoidal. He can always question the accuracy (typically 5 %), but it will be about as good as he can do.

Having had a day to think about it I would like to apologise retract some of the things I said in post 12.

Studying the conditions that apply to "unloaded" motors is not a waste of time as I suggested. All the internal loading is still present and adding to the current of loaded motors. Studying them is the first step to reducing their effect and increasing overall motor efficiencies further. Go for it Ph D's

You can't fix the problem if you don't know what to fix

True as, Motor Total I= Actual working I + Magnetizing I.

1st i 'd like to thank all, who shared their thoughts.

2nd: well i think, this magnetizing I play a vital role in determining the Power factor of any motor and it (Mag I) has its own significance too.

If a motor is taking 35% of its FLC, then its p.f 'll be low as compared to that taking 25%.

Actually my query began, What are the reasons behind that same capacity motors (30KW induction motor) take different amount of Io (25%~35%).

Where it should stand?

Is there any proper calculation involving? if not

is there any of our standardization like ANSI or NEMA etc have fixed the standard like in case of energy efficient motors there is a publication named as MGI1993?

How if a motor is taking suppose 35% of its FLC can be reduced to 25%?

what 'll be the results if we take a New motor, 7~8 years old motor and a same capacity rewinded motor?