Book at a Glance

PART I - Electric Motors, Drives and Energy Saving

Chapter 2. Motor Torque, Load Torque and Selection of Motors

Motor speed-torque curve
NEMA rotor designs
Special designs of rotors
Effect of starting current on torque
Load torque or opposing torque
Selection of motors
Time of start-up and its effect on motor performance
Thermal withstand time
Inching or jogging
Number of starts and stops

Motor speed–torque curve
Pull-out, breakdown or maximum torque, obtainable over the entire speed range. In a good design this should occur as close to the rated slip as possible to ensure that the motor runs safely, even during momentary overloads, load fluctuations exceeding the load torque, or abrupt voltage fluctuations, without harmful slip losses (Equation (1.9)). In some specially designed rotors, however, to achieve a high starting torque sometimes the pull-out torque Tpo may not be available on the speed–torque curve. It is possible that in such cases the Tst may be the highest torque developed by the motor in the entire speed range (Figure 2.2).

NEMA rotor designs
As a further step towards standardization and to achieve more harmony in motor sizes and designs, for better interchangeability in the motors produced by different manufacturers, in the same country or by other countries,

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