Table 2. Characteristics and Applications of Polyphase AC Motors
Polyphase Type
Ratings hp
Speed Regulation
Speed Control
Starting Torque
Breakdown Torque
Applications
General-purpose squirrel cage, normal stg current, normal stg torque. Design B Full-voltage starting, high stg torque, normal stg current, squirrel-cage, Design C Full-voltage starting, high stgtorque, high-slip squirrel cage, Design D
None, except multispeed types, designed for two to four fixed speeds None except multispeed types, designed for two to four fixed speeds None, except multispeed types, designed for two to four fixed speeds Speed can be reduced to 50% of normal by rotor resistance. Speed varies inversely as the load
Constant-speed service where starting torque is not excessive. Fans, blowers, rotary compressors, centrifugal pumps, woodworking machines, machine tools, line shafts Constant-speed service where fairly high starting torque is required at infrequent intervals with starting current of about 500% full-load. Reciprocating pumps and compressors, conveyors, crushers, pulverizers, agitators, etc. Constant-speed service and high-starting torque if starting not too frequent, and for taking high-peak loads with or without flywheels. Punch presses, die stamping, shears, bulldozers, bailers, hoists, cranes, elevators, etc. Where high-starting torque with low-starting current or where limited speed control is required. Fans, centrifugal and plunger pumps, compressors, conveyors, hoists, cranes, ball mills, gate hoists, etc.
100 to 250% of full-load
200 to 300% of full-load
0.5 to 200 Less than 5%
200 to 250% of full-load
190 to 225% of full-load
3 to 150 Less than 5%
275% of full-load Will usually not stall until loaded to its maximum torque, which occurs at standstill
Drops about 7 to 12% from no load to full load With rotor rings short-circuited drops about 3% for large to 5% for small sizes
275% of full-load depending on speed and rotor resistance
0.5 to 150
Up to 300% depending on external resistance in rotor circuit and how distributed 40% for slow speed to 160% for medium speed 80% p-f designs. Special high-torque designs
200% when rotor slip rings are short circulated
Wound-rotor, external-resistance starting
0.5 to several thousand
Pull-out torque of unity-p-f motors 170%; 80%-p-f motors 225%. Special designs up to 300%
25 to several thousand
None, except special motors designed for two fixed speeds
For constant-speed service, direct connection to slow-speed machines and where power-factor correction is required.
Synchronous
Constant
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