Motor Designs A, B, C, D ECE 441

Cross-Sections of NEMA Motors ECE 441

Induction Motor Applications Design B Broadest field of applications Centrifugal pumps, fans, blowers, machine tools Design A Same characteristics as Design B, but with higher breakdown torque Higher starting current limits applications ECE 441

Induction Motor Applications continued Design C Higher locked-rotor torque but lower breakdown torque than Design B Use to drive plunger pumps, vibrating screens, and compressors Design D Very high locked-rotor torque and high slip Used for high-inertia loads such as elevators and hoists ECE 441

Induction Motor Applications continued Design E High-efficiency Drive loads similar to Design B, but with lower locked-rotor, breakdown, and pull-up torque ECE 441

NEMA Tables Tables 5.1 – 5.7 give values of locked-rotor torque, breakdown torque, and pull-up torque for specific horsepower, frequency, and synchronous speed ratings. ECE 441

Example 5.1 Determine the values of locked-rotor torque, breakdown torque, and pull-up torque that can be expected from a 3-phase, 10-hp, 460-V, six-pole, NEMA design C motor whose rated speed is 1150 r/m. ECE 441

Solution ECE 441

Locked-Rotor Torque From Table 5.1, page 171 of the text, the minimum locked-rotor torque of a 10-hp design C motor with a synchronous speed of 1200 r/min should be 225% of full-load torque. ECE 441

Breakdown Torque From Table 5-3 ECE 441

Pull-up Torque From Table 5.6, ECE 441

Motor Performance as a function of Machine Parameters, Slip, and Stator Voltage Use the “complete” equivalent circuit model, including both the rotor and stator. ECE 441

Equivalent Circuit for an Induction Motor with the rotor and stator as separate circuits ECE 441

Equivalent Circuit for an Induction Motor with all parameters referenced to the stator ECE 441

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Power, Torque, Speed, Losses, and Efficiency ECE 441

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Approximate Equivalent Circuit for an Induction Motor Move the magnetizing branch to the left of resistor R1. ECE 441

Shaping the Torque-speed curve ECE 441

Slip at Which Maximum Torque Occurs Take the derivative of the expression for the developed torque Solve for the value of slip that makes the derivative equal to zero. Slip is directly proportional to the rotor resistance. ECE 441

Slip at Which Maximum Torque Occurs Applications which require a very high starting torque are designed with enough resistance so that the maximum torque occurs at blocked rotor (s = 1). ECE 441

Slip at Which Maximum Torque Occurs ECE 441

Independent of Rotor Resistance! Maximum Torque at maximum torque Independent of Rotor Resistance! ECE 441