AC Motor 415V

The reason the motor needed rewinding is that inadequate motor overload protection was provided in the supply circuitry when it was approaching burn-out point, Mildred.

We shouldn't be surprised if the motor is running >90% of full load current if it is doing some work, and <33% with a phase shift between the voltage and current if it isn't. That is normal. But if you are asking it to do more work than it is rated for, then don't be surprised if you have to rewind or replace it again.

The electrical power absorbed by any pump is the volumetric flowrate, multiplied by the rise in pressure across it, divided by the efficiency, as has been stated in this forum on a dozen or more occasions.

Dear Anonymous Poster # 1

You asked two questions and the practical answers are

Q 1

-Is it related with the pump, process flow or the new winding?

Yes it is related with all the above condition. But first compare it with the pervious or original condition of the motor loading pattern.

If loading on the motor has not changed after rewinding it, please

· Enhance your motor protection system if you are planning to run in the same condition.

· Otherwise think of using the pump in optimum level.

· Good engineering practice to load the prime mover/motor to 80% of it FLC.

Q-2

If the winding problem how we want to check the winding? Hopefully anyone can help me.

• For winding problems, it is always a good practice to take guidance from the manufacturer, who should have all the design data of the motor. He will be your best guide.

Load current of the motor is definitely depends on driven equipment( here pump ). Please check previous records of loading pattern of the motor for comparison i.e whether there is any defect in the pump or process flow. In general 75 to 80 percent (of FLA) loading is done to get maximum efficiency at rated voltage. However any motor can safely run at rated current(FLA). Protection circuit must be checked thoroughly for proper protection of motor against overload or shortcircuit condition. No-load current as indicated seems normal. You have not mentioned condition of the rotor, bearings, fans etc. which have direct impact for healthy running of the motor.

After rewinding work of the motor, following test should be done at vendor's workshop :

Insulation value check by megger(before and after H.V test)

P.I. test.

H.V(A.C) withstand test.

No-load running of motor at rated voltage to check no-load currents in three phases(should be balanced), r.p.m of the motor,watt consumed,and p.f.

Blocked rotor test and measurement of power consumed(allowing to flow FLA) ,p.f. etc.

In addition to the above percentage impedance and resistance can be measured and compared to check healthiness of windings; normally the difference of values should be within 5%. Also if possible full flux loop test may be done before rewinding work to check condition of the stator core.

Manindra

Nowadays rewinding is not advised, instead they recommend new replacement motors. Also as no-load current is not mentioned in motor name plates(why?) after rewinding how are you going to check, you can check only the full load current.

WHAT?

In which technical paper did you read this?

Please tell me the difference between a re-wind and newly built motor?

Is there any difference between the way the coils are wound and placed into the laminations, the skill, the training the attention to detail? No!

Is there a difference between the way a shaft bearing is put onto the shaft of a refurbished motor and that of a new motor? No!

Is a new motor built by hand? Yes

Is a re-wound motor built by hand? Yes!

Would you now like to retract that statement? I hope so!

I can't remember the source now but I can recollect that they mentioned about laminations losing its properties due to heat produced by burning of winding and insulation. Due to the increased losses the efficiency became less and using that became uneconomical in this "age of energy saving".

So what your saying is this.. and please feel free to correct me if I'm wrong...

Steel laminations are damaged when copper burns/melts @ 1357.77 K, 1084.62 °C, 1984.32 °F!!

Steel melts @ approximately 1,375 °C (2,507 °F). The damage damage to the laminations caused by a winding shorting to the lamination or to itself will be extremely fast and there will be insufficient time to heat up the lamination to the point where the crystalline structure of the steel will be affected as in heat treatment, that is, annealing and tempering (http://en.wikipedia.org/wiki/Steel).

To cause a change in the properties of the laminates the temperature first has to be within the "austenitie" range, from approx 900C to 1,500C and then rapidly cooled or quenched. To reach that temperature ALL of the laminates must be within the temp range. I doubt that will happen as the copper will melt way before the "mass" of the motor has increased that much.

As for a flash over short to earth.. the heat caused would never be substantial enough to cause anything more than a blob of copper on the laminate, which can be simply removed.

So I fail to see how it would affect the efficiency of the motor

Now if a motor is in a fire that is all consuming and has "cooked" for many hours, and the motor is totally, then yes, throw it away, claim on the insurance and buy new.

But we are NOT talking about that.. the OP is asking about rewinds.. so lets stay on point shall we!

One last thought.. was this earth changing item of literature produced by a motor manufacturer?.... Methinks it was!!

What our friend is saying that the core of the motor gets damaged during the burn-off process of the winding.

When a motor is rewound, the resin in the winding need to be softened, in order to extrude the copper from the core slots. Normally this gets done in a burn-off oven (smaller repair shops use open gas flames - no control) at a maximum temperature of 350°C. Should the temperature on the core of the motor exceeds this, the lamination material used in the core will breakdown resulting in higher iron losses in the core of the motor and obviously lower efficiency of the motor. The burn-off oven normally gets packed before the end of the day and switch on over night as this process takes 8 to 12 hours. In the morning when the shift starts the resin is soft and the copper wires are extruded from the slots.

The problem with the burn-off oven, because it operates on a gas flame the temperature is not constant in the entire oven and it gets overloaded with motor cores (less gas for higher output). The temperature on all the cores does not reach an acceptable level to soften the resin so they battle to get the wires out of the core. So to prevent this they up the temperature on the oven causing a temperature higher than the allowable 350°C on some of the cores in the oven, which results in higher iron core losses.

could you explain "iron core losses" please?

the losses in the core of the motor, made up of hysteresis losses and eddy current losses. should the lamination material of the iron core get damaged the eddy current losses will increase resulting in the motor operating at higher temperatures and a decrease in efficiency.

Damaged in what way?

The only thing that will damage the laminations to the point where they become useless is extreme heat.

I refer you to my earlier post regarding heat treatment and the temperature range of steel.

It is clear you did not read my earlier posting, it is not the steel that are damaged but the material used to form the laminations. This material is normally silicon or similar and have a far lower breakdown temperature than the steel?

So you are saying that the laminations are made from silicon?

the entire motor is made of silicon, winding, beari......

I think you are making this up.. please send me info on your silicon motor, with silicon windings, bearings, shaft, rotor, housing, laminations etc.

Or a link to the manufacturer, as I've looked and can't find one...

Thank you to all of you for giving lots of opinion and advice... i will try to solve the problem...thanks a lot friends!

The no load current will always be less then the load current. Its even normal with every then doing work. The only thing u need to check is,u will not be drawing more current than the full load current which is stated by the manufacture on the name plate. If we are within, we need not to worry

Sounds like the motor is running normally when coupled to the pump. All of our motors run 80-90% when in normal operation when decoupled around the 30% mark. As for rewinds anything under 40Kw we scrap. All larger motors get 2 rewinds before replacement as the there is about a 1-2% loss in effiency.

Have you tried testing a new motor?

Take look at the nameplate full load current of your NEW motor, connect it to the correct voltage, and once you bolted it down, switch it on with nothing for it to turn, that is, not connected to anything. Use a clamp meter or something similar to measure the current drawn by the motor running at what would be called "idle amps" and compare it to the FLC of the motor. See what % of the FLC the idle amps is

As for the load on the motor, (you said you are connecting this motor to a pump), will be dependent on several factors, the SG of the fluid you are pumping, the TDH of the tubing, the rate of flow of the pump... these all add to the HP required to do this work and over come certain losses in the system of pipe work etc, etc.....

So if your motor even after a rewind is running and driving a pump with a current of 90% FLC then you have no problem, if that's what it was doing before. The no-load current of 32% is OK..

So I would say you don't have a problem and you have nothing to worry about, your motor is OK and working fine

If you are STILL not sure, then you could try a clamp flow meter on the pipework to obtain a fluid flow reading to ensure that the fluid rate is as per spec of your process.

Brich,read comment bu Woobster 1.

your point is?

He has mentioned as for rewinds "anything under 40kW,we scrap". You can clarify from him why scrap and who recommended the 40kW limit.

Why would I want to clarify why he or his company scrap motors?

It is now somewhat obvious to me that you do not deal with or have to required experience to comment on motor rewinds.

I can hazard a guess why small motors are scrapped, but to someone who is in the industry that would be very obvious and as you can't see it.. I'm not going to educate you.

Maybe someone else would like to shine a little light into your world.. why not ask Woobster1?

Mr.Brich is absolutely correct. Rewinding of motor is depending on various factors like, age of motor, condition of stator cores, rotor condition(for example, die cast aluminum rotor cracks/damage repair is not feasible).Otherwise, rewinding of motors is standard practice in industries, except, small rating motors, which are not economically viable.Also one should keep in mind while repairing, original efficiency should be maintained failing wihch, repairing may not be economically viable in long run.

Manindra

We scrap motors under 40 Kw as our discount on new motors is less than rewind cost. Also a 40Kw motor is a small one. We have 1200 motors on site from 160Kw upwards. These are one we have rewound