1. Motor Designs A, B, C, D
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2. Cross-Sections of NEMA Motors
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3. 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
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4. 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
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5. Induction Motor Applications continued
Design E
High-efficiency
Drive loads similar to Design B, but with lower locked-rotor, breakdown, and pull-up torque
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6. 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.
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7. 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.
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8. Solution
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9. 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.
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10. Breakdown Torque
From Table 5-3
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11. Pull-up Torque
From Table 5.6,
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12. Motor Performance as a function of Machine Parameters, Slip, and Stator Voltage
Use the “complete” equivalent circuit model, including both the rotor and stator.
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13. Equivalent Circuit for an Induction Motor with the rotor and stator as separate circuits
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14. Equivalent Circuit for an Induction Motor with all parameters referenced to the stator
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17. Power, Torque, Speed, Losses, and Efficiency
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20. Approximate Equivalent Circuit for an Induction Motor
Move the magnetizing branch to the left of resistor R1.
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21. Shaping the Torque-speed curve
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22. 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.
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23. 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).
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24. Slip at Which Maximum Torque Occurs
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25. Independent of Rotor Resistance!
Maximum Torque
at maximum torque
Independent of Rotor Resistance!
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