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Motor Designs A, B, C, D ECE 441

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**Cross-Sections of NEMA Motors**

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**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

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**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

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**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

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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

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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

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Solution ECE 441

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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

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Breakdown Torque From Table 5-3 ECE 441

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Pull-up Torque From Table 5.6, ECE 441

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**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

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

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

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**Power, Torque, Speed, Losses, and Efficiency**

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**Approximate Equivalent Circuit for an Induction Motor**

Move the magnetizing branch to the left of resistor R1. ECE 441

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**Shaping the Torque-speed curve**

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**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

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**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

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**Slip at Which Maximum Torque Occurs**

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**Independent of Rotor Resistance!**

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

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