*Locked rotor kVA is equal to the product of the line voltage times motor current divided by 1,000 when the motor is not allowed to rotate; this corresponds to the first power surge required to start the motor. Locked-rotor kVA per horsepower range includes the lower figure up to but not including the higher figure.


The design letter is an indication of the shape of the torque speed curve. Figure 11.2 shows the typical shape of the most commonly used design letters. They are A, B, C, and D. Design B is the standard industrial duty motor which has reasonable starting torque with moderate starting current and good overall performance for most industrial applications. Design C is used for hard to start loads and is specifically designed to have high starting torque. Design D is the so-called high slip motor which tends to have very high starting torque but has high slip RPM at full load torque. In some respects, this motor can be said to have a 'spongy' characteristic when loads are changing. Design D motors particularly suited for low speed punch press, hoist and elevator applications. Generally, the efficiency of Design D motors at full load is rather poor and thus they are normally used on those applications where the torque characteristics are of primary importance. Design A motors are not commonly specified but specialized motors used on injection molding applications have characteristics similar to Design B. The most important characteristic of Design A is the high pull out torque.

Figure 11.2

Figure 11.2


Efficiency is the percentage of the input power that is actually converted to work output from the motor shaft. Efficiency is now being stamped on the nameplate of most domestically produced electric motors. See the section 11.14.

Frame Size

Motors, like suits of clothes, shoes and hats, come in various sizes to match the requirements of the applications. In general, the frame size gets larger with increasing horsepower or with decreasing speeds. In order to promote standardization in the motor industry, NEMA (National Electrical Manufacturers Association) prescribes standard frame sizes for certain horsepower, speed, and enclosure combinations. Frame size pins down the mounting and shaft dimension of standard motors. For example, a motor with a frame size of 56, will always have a shaft height above the base of 3- 1/2 inches.


This is the frequency for which the motor is designed. The most commonly occurring frequency in this country is 60 cycles but, internationally, other frequencies such as 25, 40, and 50 cycles can be found.

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