Motor Rating

Not so; there are motors with higher voltage ratings than that.

And lower, of course.

You must be asking about the 10% voltage tolerance rating? All electrical utilization equipment, or most of it, has a built in tolerance for up to a 10% from optimal voltage rating. Meaning, the motor will work as advertised at up to + or - 10% of name plate voltage.

Anything outside that 10% range, either high or low, the motor is not guaranteed to perform as per design.

Because + or - 10% tolerance is NEMA (National Electrical Manufacturers Association) standard for motors.

Here is the quote from NEMA MG 1 - 2006

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10.30 VOLTAGES

a. Universal motors-115 and 230 volts

b. Single-phase motors

1. 60 hertz-115, 200, and 230 volts

2. 50 hertz-110 and 220 volts

c. Polyphase motors

1. 60 hertz-115*, 200, 230, 460, 575, 2300, 4000, 4600, and 6600 volts

2. Three phase, 50 hertz - 220 and 380 volts

NOTE-It is not practical to build motors of all horsepower ratings for all of the standard voltages.

*Applies only to motors rated 15 horsepower and smaller.

12.44 VARIATION FROM RATED VOLTAGE AND RATED FREQUENCY

12.44.1 Running

Alternating-current motors shall operate successfully under running conditions at rated load with a variation in the voltage or the frequency up to the following:

a. Plus or minus 10 percent of rated voltage, with rated frequency for induction motors.

b. Plus or minus 6 percent of rated voltage, with rated frequency for universal motors.

c. Plus or minus 5 percent of rated frequency, with rated voltage.

d. A combined variation in voltage and frequency of 10 percent (sum of absolute values) of the rated values, provided the frequency variation does not exceed plus or minus 5 percent of rated frequency, and the voltage variation of universal motors (except fan motors) does not exceed plus or minus 6 percent of rated voltage.

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- MS

It depends upon the supply voltage.In India Common LV voltage is 415Volts for 3 phase and hence all 3 phase motors are rated for 415Volts.HT motors are rated for 3.3kv,6.6 and 11 KVin India.However outside India it is 380V In Thailand and Indonesia.Similarly in case of HT it is mostly ^KV, 3 KV and 10KV.In Japan 3phase 220V motors are used as well as 200V 3 phase in some parts.As regards the variation In India we need +-10% as Voltage fluctuation is high.Outside it is mostly +_6%.It is not uncommon to have other voltage varitions specified like 415V+-12.5% or +6% and -10% etc which become special execution.Similarly Frequency variation and combined voltage and frequencies are also there.The performance figures like efficiency and power factor are however declared and hold good at rated voltage.What is implied is the motor can deliver the required torque and within permitted temperature rise when running at extreme voltage or frequency conditios.