Machinery's Handbook, 31st Edition
2646 ELECTRIC MOTOR STANDARDS Design B: Locked-rotor torque as shown in Table 2, breakdown torque as shown in Table 3, locked-rotor current not exceeding that in Table 1, and a slip at rated load of less than 5 percent. Motors with 10 or more poles may have a slightly greater slip. Design C: Locked-rotor torque for special high-torque applications up to values shown in Table 2, breakdown torque up to values shown in Table 3, locked-rotor current not ex- ceeding values shown in Table 1 and a slip at rated load of less than 5 percent. Design D: Locked-rotor torque as indicated in Table 2, locked-rotor current not greater than that shown in Table 1 and a slip at rated load of 5 percent or more. Torque and Current Definitions.— The definitions which follow have been adopted as standard by the National Electrical Manufacturers Association. Locked-Rotor or Static Torque: The locked-rotor torque of a motor is the minimum torque which it will develop at rest for all angular positions of the rotor, with rated voltage applied at rated frequency. Breakdown Torque: The breakdown torque of a motor is the maximum torque which the motor will develop, with rated voltage applied at rated frequency, without an abrupt drop in speed (see Table 4). Full-Load Torque: The full-load torque of a motor is the torque necessary to produce its rated horsepower at full load speed. In pounds at 1-foot radius, it is equal to the horsepower times 5252 divided by the full-load speed. Pull-Out Torque: The pull-out torque of a synchronous motor is the maximum sustained torque which the motor will develop at synchronous speed with rated voltage applied at rated frequency and with normal excitation. Pull-In Torque: The pull-in torque of a synchronous motor is the maximum constant torque under which the motor will pull its connected inertia load into synchronism at rated voltage and frequency, when its field excitation is applied. Pull-Up Torque: The pull-up torque of an alternating current motor is the minimum torque developed by the motor during the period of acceleration from rest to the speed at which breakdown torque occurs. For motors which do not have a definite breakdown torque, the pull-up torque is the minimum torque developed up to rated speed. Locked Rotor Current: The locked rotor current of a motor is the steady-state current taken from the line with the rotor locked and with rated voltage (and rated frequency in the case of alternating-current motors) applied to the motor. Table 4. NEMA Standard Breakdown Torque of Polyphase Wound-Rotor Motors with Continuous Ratings — 60- and 50-Hertz
Speed, rpm
Speed, rpm
1800
1200
900
1800
1200
900
Horsepower
Horsepower
Percent of Full-load Torque … … 250 … … 250
Percent of Full-load Torque
275 275 250 225
250 250 225 225
225 225 225 225
1
7 1 ∕
2
10 15
1 1 ∕
2
2 3 5
275 275 275
275 275 275
250 250 250
20-200, incl.
… … … …
These values represent the upper limit of the range of application for these motors. Standard Direction of Motor Rotation.— The standard direction of rotation for all non- reversing direct-current motors, all alternating-current single-phase motors, all synchro nous motors, and all universal motors, is counterclockwise when facing that end of the motor opposite the drive. This rule does not apply to two- and three-phase induction motors, as in most applica tions the phase sequence of the power lines is rarely known.
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