We have a couple of induction machines tested under no load condition, two of them when ran 10% above rated voltage experience higher losses than the others, what could be the cause of this ??

Hi Lesedi. Are they all identical machines?

The machines are different, 3 hp (208 V) and 50 hp (575 V)

Hi Lesedi.
Are you comparing the 3 hp with the other 3 hp's, and the 50 hp with the other 50 hp's?
If so - and they are identical machines - then can we assume that their no-load losses are equal up to rated voltage - and that only when run at 10 % above rated voltage, do their losses increase? I would find this highly irregular because identical machines will be designed to operate at the same magnetic flux density point, so I would expect them all to behave very similarly above rated voltage. The only loss parameter that can increase here is iron-losses (unless your no-load current increases so drastically above rated voltage that the associated I2R loss plays a significant part - and I would highly doubt that, because the iron-losses would also increase drastically anyway in such a case).
So the "two" machines you refer to: Is one a 3 hp and the other a 50 hp? Can you describe which losses increased? This is easy to do with a varied voltage no-load loss test, with extrapolation of the "zero-voltage" point to give mechanical / Friction and Windage losses; then you know what your iron losses are. I would bet it's iron losses that are different, but then the motors are not identical and would have been rewound at some stage, or just have been manufactured differently. In any event - if you complete the test above (no-load test with voltage varied from say 30 % to 110 %, and recording current and dissipated power) - you will be able to disseminate the losses. Not a difficult task - good luck :-).
Take care.
Robert.

Hi Robert,

I am working with Lesedi on this issue and I will provide you with the results we got by testing 3 different machines under no-load condition.
The 50 HP and 3 HP are the ones which behave abnormally when we apply 10% overvoltage. The third machine (7.5 HP) is a machine that reacts normally under the same condition.
What we mean by abnormal behavior is the input power of the machine that will increase dramatically under only 10% overvoltage which is not the case with most of the induction machines. This can be seen by the numbers given below.



50 HP, 575V

Under 10% overvoltage:
Friction & Windage Losses increase 0.2%
Core loss increases 102%
Stator Copper Loss increases 107%


3 HP, 208V

Under 10% overvoltage:
Friction & Windage Losses increase 8%
Core loss increases 34%
Stator Copper Loss increases 63%



7.5 HP, 460V

Under 10% overvoltage:
Friction & Windage Losses decrease 1%
Core loss increases 22%
Stator Copper Loss increases 31%

Till now, we couldn't diagnose the exact reason that pushes those two machines to behave in such way.

Strange that for the 7.5HP machine the friction & windage losses actually decrease?

How do you measure the losses, and how do you determine how the losses are divided into the three categories?

If you had measured differences for identical machines (power rating and manufacturer) I could have suggested that the degree of "slightly-out-of-center" for the rotors could cause some differences, and if you were measuring on-load or overload characteristics, small rotor differences could play in, such as the depth and distribution of the small balancing holes in the cast rotor windings.

Hi, Maher,
I agree in completely with your opinion in aspect of warming of your electrical motors.
Difference in active power losses in ferromagnetic cores of your electrical motors with rated powers 50HP and 3HP which is a consequence of increasing of voltage for 10% is too high and could be the reason because ferromagnetic cores of your electrical motors work in the area of saturation what has a consequence, as you know, unallowed warming of ferromagnetic cores of your electrical motors and could have a consequence permanent damaging of ferromagnetic cores of your electrical motors.
Of course, in case when you have too high value of no-load current which flows through stator's winding, you also have too high active power losses (Joule's losses) in the stator's winding because they depend of square of value of no-load current which flow through stator's winding.

Hi Ole,
The losses are measured according to the IEEE Standard 112-2004 Test Procedure for Polyphase Induction Motors and Generators by running the machine at no-load with different level of input voltage and then obtaining the necessary values to calculate the losses. Linear regression will be involved.
The "slightly-out-of-center" theory might be good idea.
We replaced the bearings of the 3 HP aiming to solve the problem, but with no use.
I don't know about the reason of the decease in the friction & windage loss of the 7.5 HP machine when applying the 10% overvoltage, but that what I get!



Hi Bojan,

It is a common knowledge that losses will increase by increasing the voltage which in turn will rise up the current. This increase in losses in most of the induction machines is within normal ranges. The question is, why the increase in losses is much higher than normal in those 3 HP and 50 HP. You can see that although the 7.5 HP is bigger than the 3 HP, but the increase in losses by applying 10% overvoltage in the 3 HP is double the 7.5 HP!