12 Electric Motors 1.2.2 Power and Torque The rated output of electric motors is defi ned within a standard range. This standardisation allows users to choose between diff erent motor manufacturers for specifi c applications. The “standard” output range and its increments diff er from country to country and region to region. It is recommended to fi nd out what manufacturers defi ne as standard in their catalogues. On average, motors with frame size up to 315 (ca. 200 kW) can be regarded as standard motors with standard dimensions. Horsepower [hp] is the imperial unit used for motor power. If this unit is specifi ed, it can be converted as follows: 1 hp = 0.736 kW or 1 kW = 1.341 hp.
Table 1.1 shows the typical industrial standard rated output power in [kW] and [hp]. Table 1.1 Rated motor output power Besides power, torque is an important characteristic of the motor. Torque indicates the strength of rotation of the motor shaft. Power has a direct relationship to torque and can be calculated when torque and speed are known. P = T x n 9.550 P = Power [kW] T = Torque [Nm] n = Speed [RPM] The factor 9.550 used in the formula results from the conversion of units:
• Power from the base units W (watt) to nameplate units kW (kilowatt) • Speed from the base unit s-1 (revolutions per second) to nameplate min-1 (revolutions per minute) kW 0.18 0.25 0.37 0.55 0.75 1.10 1.50 2.20 3.00 4.00 5.50 7.50 11.0 hp 1.00 2.00 3.00 5.00 7.00 10.00 15.0 kW 15.0 18.5 22.0 30.0 37.0 45.0 55.0 75.0 90.0 110.0 132.0 160.0 200.0 hp 20.0 30.0 40.0 50.0 60.0 75.0 100