No Load Motor Draw

There is no rule regarding no-load current that can be universally applied to any motor. Every motor is different.

Plus, just because they are pulling 5A each doesn't mean it they are presenting resistive power to the load - the sort of power that makes a kWh meter dizzy. Some of it is inductive because, each being a load of whirling coiled wire, they have self-inductance. So the phase angle between the voltage and the current will not be zero and one should not believe everything one is told.

1. Differentiate between two things i. Motor in Decoupled condition ii. Motor with No Load Condition

2. Basic concept lies to understand the relation between P.F and current in relation to the part load of Induction motors. Read the Link

https://www1.eere.energy.gov/manufacturing/tech_deployment/pdfs/10097517.pdf

3. Change in P.F with reduced load is different in case of different capacity motors. There is no exact relation as mentioned in the question i-e 1/2 of Full Load Current.

4. Sometime it is surprising that small capacity motors consumes less current when they drive the load in comparison to the decoupled condition.

5. Please elaborate your question. Mention full name plates of motors, type and detail of load being driven.

The information should be in the motor data sheets obtainable online as the current will vary from motor to motor.

There is no useful rule of thumb that I am aware of, I just go by the data sheet information.

I've never seen that info in a data sheet, but things may be different around the world. I don't see very much need for the information, either.

It depends on the motor supplier, sometimes it is there (or in an application note) sometimes not.

If not I usually just find a physically and electrically similar motor from another manufacturer who does publish the information (super accurate no load current has never been important for any applications I have been involved in).

The only time I have found "no load" amps to be even remotely useful is in using an Under Current Relay to detect a broken shaft or belt. In those instances, I just measure it at setup. I see no other value in having that information, yet people are constantly asking us in this forum how to determine it (or "calculate" it, as I have recently come to realize is probably a mis-translation of determine). For some reason they seem convinced that it is necessary to know it.

But it is not...

But it is not...

Unless you are trying to trace a winding fault in a motor resulting in a higher than expected running current (for example).

Perhaps these are just plain old homework questions that have little basis in real world motor selection and protection.

Correct.

Usefulness is Usually in detecting when pumps are running dry. But then, the phase angle (power factor) is used, since the P.F. will drop drastically on very low load.

I agree.... In the application of underload, which is what we are talking about really, in my line of work, submersible pumps, we need to set an underload limit, which is 80% of running current. do not confuse this figure (running current) with the name plate value of Full Load Current.

Running current for submersible pumps is a figure constantly monitored by the electronic motor controller, informing the operator as to the health and correct operation of the pump/motor. A drop in running current and the following shutdown of the motor will save the motor, cable and pump in the case of no fluid to lift, broken shaft or something else. In the case of no fluid to lift past the motor, the motor will overheat, as that fluid passing over the motor is the ONLY thing that keeps the motor cool. So low running current means no fluid, no fluid means burnt motor!

Yes we have table for the idle amps of any motor, but that is just the motor, add to it protectors, gas separators and pumps, then the idle amps increases..... by monitoring the running current and setting the underload trip correctly we protect the equipment.

To the OP....As for the reason why you need to know the idle amps of any motor.. one can only guess.. I see no real need for it, as most surface motors do not have underload trips, and if running without load, what damage will be done.... none to the motor, maybe to a gearbox, but if the motor is idling then the gearbox damage has already occurred. If you have a motor monitor and wish to set the underload trip then use the 80% of running current.

Failing all that... get your motor in question, bolt it down (so it doesn't jump all over the workshop) connect it to power, switch on and measure the current.

What are these motors doing while energized?

An electrical motor has 2 currents, magnetising current and load current which is 90 deg out of phase from each other.The current you measure is the vector sum of both these currents.If there is no load theoretically, you will measure the magnetising current.I(mag)=Vp/(2*pie*f*L).If you can work out L, you can calculate your no load current.

Dear Mr.pewpew,

The Induction Motor NO LOAD CURRENT will be in the Range of 35% to 40% of Full-Load Current.

Perhaps, 1/2 Full load appears to be on the high side, if so the Power Factor will be very Low. Mainly it depends upon the POWER FACTOR for the Induction Motor, and at NO-LOAD, the POWER FACTOR will be in the range of 0.3 to 0.4. and as the Load increases, the Power Factor will improve. Lower the No-Load Speed, Lower will be the Power Factor, hence slightly more current.

The Induction Motor Current will follow the CIRCLE DIAGRAM, Pl. refer Standard Text Book.

DHAYANANDHAN.S

Re comments saying it is not necessary to know the No-load current are a little off the mark.

Compressor motors consume current when off-load (not quite the same as motor no-load) and this affects the running cost. Attempts to fit compressors with motors to reduce current appears to save money but in doing so could affect the power factor when off-load enough to incur penalties imposed by the electrical supply company.

It probably doesn't matter much for small compressors, but on larger ones a reciprocating compressor when off-load could consume about 10-15% of full-load current, whereas an oil-flooded screw compressor of the same air output when off-load might consume about 70-80% full-load current.

But I am not aware of a general rule of thumb that links on-load to off-load current in any useful way.

As for knowing a lot about 3-phase motors, I know enough to get by in my small world, but whether it is a lot, I can't say. How much is there to know?

• "Re comments saying it is not necessary to know the No-load current are a little off the mark.
• Compressor motors consume current when off-load (not quite the same as motor no-load)"

You said it, it is not the same. Then you go on about how the "not the ame" values have merit. You addressed a valid point, but one that is non-sequitur to the question asked. So saying that the comments that DID address the question as asked are off the mark is a little off the mark itself...

I imagine motor no-load to mean the current consumed when the motor is running but not connected to anything (eg. maybe via a clutch or gearbox). Whereas off-load is the current consumed by the motor when the connected device is not doing any work (eg. compressor running but not pumping).

So even if there was a strict rule that linked motor no-load to off-load current, the numerous mechanical devices driven by motors adds to the likelihood that in practice there is no general rule-of-thumb for motors that links no-load to full-load.

I was making a comment about those who have asked why, or have said it is not necessary to know the no-load current - when in fact it often is - and at the same answering the implied question about the current -and hopefully the explicit question asking about the existence of of anybody out there who knows a lot about 3-phase induction motors. I suppose there is - but it is not me. I know just enough for my comfort zone.

Thanks for the replies everyone. I ended up reaching the manufacturer and they did have a data sheet with that info.

• "Thanks for the replies everyone. I ended up reaching the manufacturer and they did have a data sheet with that info."

OK, but please indulge us (me). Why was this information necessary? This question gets asked a lot in this forum for some reason and I don't understand why. I learned one more potential reason from Jack of All Trades' response (thanks for that by the way). Is there something else I have missed? Enquiring minds want to know...

Dear,

JRaef,

Kindly may note that, no-load currernt data of 3-phase induction motor is required, when one wants to install capacitor bank across the motor for improvement of power factor; since kVAR capacity of the capacitor should be restricted to 90% of no- load kVA of the motor.

Regards,

Manindra.