Voltage Drop Of The Motor During Normal And Starting Conditions

IIRC, NEMA (National Electrical Manufacturers' Association, U.S.A.) stipulates that motors shall be able to start successfully at 85% of nominal voltage. That number may vary with other local/regional codes.

Please can you provide the exact reference in these codes,

Also, what about the formulas?

Thanks

I doubt that there are any universal formulas, because it depends on too many particulars of whatever system is supplying the motor(s).

Ohm's Law applies to the conductors feeding a motor, though.

Standard practice is not to allow more than 10%.

The motor torque is directly proportional to the square of voltage; and at starting, the starting torque shall be at least 70% more than the load torque in order to ramp up the motor. If the voltage drop is allowed to drop more than 10%, the starting torque will be less than the required 1.7 times the load torque; and because of this the motor may stall during start up time.

unfortunately, I am unable to recall the formula and calculation.

I'd get copies of the references given above, go to sources such as :Engineering ToolBox and otherwise secure the knowledge you desire some place where you can cite your own references, other than saying that you got your information from an anonymous forum.

Granted there are members here who are very well informed.
Maybe start by learning How to Search the Internet - Hannon Library

Your instructor/interviewer/boss/supervisor or customer might want more proof that you are knowledgeable in the subject at hand and you should have the correct reference material at your disposal to prove your case to them.

I have already checked the internet and didnt find what i have requested in my original query, which are:

· The formulas to calculate the voltage drop of the motors during normal and starting conditions (with code/standard references)

· Is there any requirement in any standard/Code concerning the maximum allowable voltage dip for motors

Yes. It's all in British Standard 7671.

Please check below link for the formula

http://www.openelectrical.org/wiki/index.php?title=Cable_Sizing_Calculation

For transformer voltage drop See IEC 60076-8 clause 7.4 The voltage drop equations.

For cable voltage drop see standard IEC 60364-5-52 or IEEE Standard 141 ch. VOLTAGE CONSIDERATION

I have seen two formulas in Nema ICS-14 (Application Guide for Electric Fire Pump Controllers) section 2.3 which are :

Percent voltage drop during motor running : (ZxMFLAx1.15xVLxSqrt(3))/(Transformer size in KVA x1000) Percent voltage drop during motor starting:(ZxMFLAx6xVLxSqrt(3))/(Transformer size in KVA x1000)

I AM NOT ABLE TO TRACK THE CODE REFERENCES OF THE ABOVE FORMULAS, AND IF THEY APPLY TO OTHER MOTORS (OTHER THAN FIRE PUMP)

Fire pumps are a special situation in that they "must start" when called upon to operate; therefore, the formulas are conservative and provide a wider tolerance band for undervoltage operation.

As you have discovered, most codes simply give you the limits, they are "what to do" guidelines, the "how to do it" is left to the designers and engineers. There are plenty of handbooks out there with different formulas for different situations. As long as your calculations yield results within the standards you should be covered, but since you have your doubts you should hire a Professional Engineer to assist you and stamp your drawings thereby shifting your doubts to someone who doesn't have any.

I do not know which country you hail from. But, here in India, the Indian Standard Specification for 3 Phase Induction Motors (IS 325) specifies that the motor shall continue to perform satisfactorily at +/- 6% of the rated voltage, which means that the permissible steady state voltage drop is 6% at the motor terminals. Another Indians Standard, the IS 12615, the Indian Standard Specification for Energy Efficient Motors, mentions this limit as +/- 10%, which means the permissible voltage drop during steady state is 10% at the motor terminals.

As regards the acceleration state voltage drop no such authentic references are available, though. The general practice is to limit the voltage drop to about 15% at the motor terminals.

And, I do not understand what do you mean by formulas for calculating the voltage drop. The Indian Standard Code of Practice for Selection, Installation & Maintenance of Cable, IS 1255, mentions the maximum permissible voltage drop in various types & sizes of cables, as V/km/A. This can be used to calculate the actual voltage drop in the cable supplying power to the motor, taking the steady state Amperes as well as Starting Amperes appropriately, to calculate the actual steady state voltage drop & the acceleration state voltage drop and comparing the same with the above mentioned limits.

This is how I had been sizing the cables all thro my career, with respect to voltage drop based sizing.

Hope this is of help to you.