IS Tolerance for Motor Efficiency

Vide Clause 17.1 of IS 12315:

"For compliance with the requirements of this standard, the values of efficiency subject to IS 325 tolerance, listed under appropriate efficiency class (either improved efficiency - eff 2 or high efficiency - eff 1) in Tables 1 to 4, shall be met as specified below".

And vide Table-2 of IS 12315, a 11kW, 4 Pole Motor shall have a "nominal" efficiency of 91% - as has been rightly mentioned by you.

But, vide IS 325, Table-1, there is a tolerance of -15% (minus 15 percent) in the efficiency of motors upto 50kW.

And, vide Clause 12.1 of IS 325, a manufacturer is permitted to declare superior values of performance charectersitics - subjected to verification. In your case, the manufacturer has offered a superior efficiency than siad in the Standard. It is okay as per IS.

thanks for your valuable replied.

i need one more clarification with respect to IS standard. IS tolerance is -15% for upto 50KW.then every manufacture shows that >95% with conditions of IS 325.my question is that shall i ask motor manufactures to supply without negative tolerance.kindly advice me.

Note:- for cables also as per IS drum cable length is +- 5%.but as per client request they will ready to manufacure without IS tolerance.this rule is applicable in motor efficieny or not.

regards,

N.Ganesan.

Motor manufacturers are capable to provide you motor with above normal efficiency.

Mention any of the desired guaranteed values you are after and clearly specify in the Order; performance testing of motor at the factory as a pre-condition to acceptance.

dear Sir,

i would like to know why all the manufactures are showing 91,92,93%.. efficiency.and put some notes above values are as per IS standard.what is the need of tolerance. in this case how can i ensure that which manufatures give maximum efficiency. advise me.

regards,

N.Ganesan

Hi Ganeshakce-electrical, I would like think that the manufactures efficiency is correct if the load connected is constant and at the full load capacity, if not the efficiency will alter dependant on load and would drop off as load the load is decreased.

Cheers

Joe

thanks for replied joe.

than why standard relaxation like effciency of the motor up 50 KW is -15% tolerance. This tolerance shall take advantage of the manufacture.they simply put note as per standard.

kindly advise me

regards,

N.Ganesan

Let us see the clause from the manufacturers point of view. The material which is used in the motor has certain tolerance in its electrical properties like sheet steel material (Watts/Kg), magnetizing characteristics of this material, conductivity of the copper, etc. Then there would be tolerances on the physical dimensions of the motor (length, diameter etc.), tolerance on the insulation thickness on the core stampings like that many more.

Then next point that should be considered about the proving the efficiency to the customer at manufacturers work. The measuring instruments, accuracy of the meters, class of accuracy of the meters to be used for testing, method of testing. All these factors call for a tolerance on the guaranteed quoted efficiency. Therefore, a manufacturer quotes efficiency based on the data available with him for number of motors of various designs manufactured and tested.

As far customer, they should go by assuming that the minimum efficiency that could be available for his motor, will be quoted efficiency with allowable tolerance. Any higher value of efficiency, obtained shell a bonus.

thanks for your valuable reply.

then how shall i compare the manufacture efficiency with their competitor.

Always compare the minimum possible (taking negative tolerance) efficiency of all the manufacturers. If you want then to compare the cost, then compare the losses and bring all the motors at one base by adding capitalized cost of extra loss difference.