ECE 4411 Motor Designs A, B, C, D. ECE 4412 Cross-Sections of NEMA Motors.

ECE 441 1

Motor Designs A, B, C, D

ECE 441 2

Cross-Sections of NEMA Motors

ECE 441 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

ECE 441 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

ECE 441 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

ECE 441 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.

ECE 441 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.

ECE 441 8

Solution

ftlbT

T

Tnhp

rP

fn

rated

s

67.455252

)1150(10

5252

min/12006

)60)(120(120

ECE 441 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.

ftlbT rotorlocked 8.102)67.45)(25.2(

ECE 441 10

Breakdown Torque

• From Table 5-3

ftlbTbreakdown 8.86)67.45)(90.1(

ECE 441 11

Pull-up Torque

• From Table 5.6,

ftlbT uppull 4.75)67.45)(65.1(

ECE 441 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.

ECE 441 13

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

ECE 441 14

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

ECE 441 15

phasegmagnetizinX

phasecorelossR

N

Na

aEEEa

II

XaX

RaR

M

fe

r

s

BRs

r

BR

r

/

/

2

2

22

22

ECE 441 16

0

tan

tan

r

fe

M

BR

I exciting current per phase

R actual rotor resis ce per phase

I core loss component of exciting current

I magetizing component of exciting current

X actual blocked rotor reac ce per p

��

��

��

1

r

hase

I actual rotor current per phase

V actual voltage per phase applied to the stator

I actual stator current per phase

��

��

��

ECE 441 17

Power, Torque, Speed, Losses, and Efficiency

22 2

0

2 0

2 0

1

fe M

fe M

P

in P

RZ jX

sR jX

ZR jX

Z ZZ

Z Z

Z Z Z

��

��

��

��

��

2

1

1

2

in

P

VI

Z

E I Z

EI

Z

��

��

��

��

ECE 441 18

21 1

,

3

1

746

scl

gap rcl

mech f w strayshaft

P I R

P Ps

P P PP hp

22 23

1rcl

mech rcl

P I R

sP P

s

22 23

1rcl

mech rcl

P I R

sP P

s

ECE 441 19

2

22 2

21.12

21.12

r rD

s

Ds

I RT

sn

I RT

sn

223

corefe

EP

R

223

corefe

EP

R

ECE 441 20

Approximate Equivalent Circuit for an Induction Motor

Move the magnetizing branch to the left of resistor R1.

ECE 441 21

2

2

2

1 1 2

2

2 22

1 1 2

2 2

2 22

1 1 2

2

21.12

( ) ( )

21.12

( ) ( )

r r

D

s

D

s

D

I RT lb ft

sn

VI R

R jX jXsV

IR

R X Xs

RV

sTR

R X X ns

T V

��

��

Shaping the Torque-speed curve

ECE 441 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.

2

max 2 2

1 1 2( )DT

Rs

R X X

ECE 441 23


• Applications which require a very high starting torque are designed with enough resistance so that the maximum torque occurs at blocked rotor (s = 1).

2

max 2 2

1 1 2

1( )DT

Rs

R X X

ECE 441 24


2

max 2 2

1 1 2

2 2

2, 1 1 1 2

1( )

( )

DT

s

Rs

R X X

R R X X

ECE 441 25

Maximum Torque

2 22

1 1 2( )

RR X X

s

2 2

2 22

1 1 2

21.12

( ) ( )D

s

RV

sTR

R X X ns

at maximum torque

2

,max 2 2

1 1 2 1

21.12

2 ( )D

s

VT

n R X X R

Independent of Rotor Resistance!

ECE 4411 Motor Designs A, B, C, D. ECE 4412 Cross-Sections of NEMA Motors.

Documents

Transcript of ECE 4411 Motor Designs A, B, C, D. ECE 4412 Cross-Sections of NEMA Motors.