Factors Effecting Motor rating for LV/MV system

[JatTiw]

Hi,
Normally at each plant we had both medium voltage & low voltage motors. The low voltage or medium voltage distinction is based upon kW rating of motors. I am interested to know the factors which identifies the maximum size of motor (kW) is suitable for a particular system voltage.
Thanks in advance

 

[Shortstub]

Essentially the availability of electrical components and devices to effect proper installation at each voltage level. However, in most cases it boils down to copper reduction!

As an example I once studied the economic impact of 830 V distribution for a grass roots refinery! The savings was formidable. But it was rejected... the real problem being the ego's of those who object to "change"!

 

[jbartos]

Suggestion: Reference:
Robert W. Smeaton "Switchgear and Control Handbook," 2nd Edition, McGraw-Hill Book Co., 1987, page 9-9, Table 3b
provides the HPs versus motor rated voltages.

 

[busbar]

Reference IEEE Std 141-1993 Recommended Practice for Electric Power Distribution for Industrial Plants [Red Book] Table 3-11 Standard Voltages and Preferred Horsepower Limits for Polyphase Induction Motors

 

[JatTiw]

HI,
I want to confirm, for making the LV/MV distiction based upon kW rating, is power distribution system of the plant play any role?

 

[gsimson]

The following factors are considered for selecting the rating of LT/ HT motors.
- The total connected load
- Availability of power supply, LT/HT
- The cost of LT/HT switchgear
- Permissible voltage drop
- Cost of HT/LT motor
- Efficiency of motors
- Repair facility available in & around
- Cost of Energy

Considering all the factors , for a very large system of app. 250 MW & 22000 motors, it was found that the optimum rating is 415 V ac for <200 kW,6.6kV for > 200 kW & 3MW for 11 kV.
However specific calculation is to be made considering the factors.

 

[jbartos]

Suggestion: The plant power distribution system has a significant impact on the motor kW selection for particular voltage level. Major components to be considered are transformers (their impedances), bus ratings, switchgear ratings (there will be motor short circuit current contributions), plant area (e.g. acres, square miles, etc., which impacts lengths of motor feeders), etc.