26 Induction Motor Design

8/13/2019 26 Induction Motor Design

1/21

Design of Induction Motor

NEMA Class B,

Squirrel-cage Type


2/21

UCF NEMA Design ClassNational Electrical Manufactures Association (NEMA)

breakdown torque: pullout torque

locked-rotor torque: starting torque


3/21

UCF NEMA Rotor Shape

Class B, A Class C Class D

double cage


4/21

Rotor Peripheral Speed

The maximum allowable peripheral speed of the rotor is a central consideration in

machine design. With present-day steel alloys, rotor peripheral speeds of 50,000

ft/min (or about 250 m/s) represent the design limit. 1 ft/min = 0.0051 m/s

This is typically not a problem for low speed motor.


5/21

UCF Maximum FluxExperienced formula is:

in Wattpoweroutput:polesofnumberthe:

08.0)

003.000145.0(

out

outm

PP

f

P

P

Dl

PB

P

DlB mm

mm

2

2

Also


6/21

UCF Number of Turns per Coil on Stator

meffmeff fNfNE 44.421

rD 21.1

wceff

kPqNN sdpw kkkk

where

The denominator 1.1 in Neff is to considering leakage flux.

mw

rated

cfPqk

VN

2

07.1

small)areand(because97.0 11,1 XRVE rated


7/21

UCF Stator Volume and Size

The unit for D and l is inch.

cooled)(watermoreor10hpforlb)ft/(in6~53

0 V

Typically

cooled)(airlessor10hpforlb)ft/(in10~9 30 V

If we design D=l, we have the stator bore (inner)

diameter estimated

3

1

0 )(TVDestimated

coolingondepends1

,(constant) 00

2

sJVV

T

lD


8/21

UCF

Estimated Stator Core diameter (outer diameter):

inch647.0)03.1

175.1(,0 estimatedestimated DPD

Estimated frame outside diameter:

festimatedestimatedf tDD 2,0,

tf is wall thickness, typically 0.5 inch

frameNEMAfromuppickedisfD

Frame Diameter Estimation


9/21

UCF NEMA Standard Frame Size (inch)


10/21

UCF

Stator Core diameter (outer diameter):

03.1175.1

inch647.00

P

DD

Frame outside diameter:

ff tDD 20

tf is wall thickness, typically 0.5 inch

page.lastinframeNEMAfromuppickedisfD

Stator Bore diameter (inner diameter):

Stator Bore Diameter and Length

Stator active region length:

20

D

TVl


11/21

UCF Air GapExperienced formula

gkkg crcseff

where the Carters coefficients

2)5()5(

sss

sscs

bbgbgk

2

22

2

)75.04.4(

)75.04.4(

rrs

rscr

bbg

bgk

NEMA Class B

inch0072.0001.00016.0 lDg

Effective airgap

gDD r 2


12/21

UCF Stator Slot Design

S

Ds

sss bdb 42 1

in5.1in7.0 s

sss b 6.04.0

sss bt Use 0.015-in coil separator and slot liner for 230V motor

Use 0.030-in coil separator and slot liner for 460V motor

NEMA 284T or smaller frames: use 0.06 in slot wedgeLarger than NEMA 284T frames: use 0.125 in slot wedge

6.00.4, ss

s

s rb

r


13/21

UCF Stator Conductor SizeStator current density

a

rated

sa A

I

J,1

where Aa is stator (armature) conductor cross section

area and can be determined from the above formula

together with:

22 A/cm800400A/cm:cooled-Air saJ

pfV

PI

rated

outrated

,

,13

/

sa

rated

a

J

IA

,1

Typical Efficiency :75% for Pout = 1-5 hp

85% for Pout = 5-40 hp

90% for Pout = 50-200 hp


14/21

UCF Number of Slots (Bars) on Rotor

...)3,2,1(

3

)12(

k

kPSS

PkSS

r

r

Certain number of slots (bars) must be avoided because they

can produce detrimental cusps in the speed-torque curve.


15/21

UCF Rotor Effective Winding FactorRotor Coil pitch in electrical angle:

PS

PSfix

r

rr

/

)/(

Rotor Slot pitch in electrical angle:r

rS

P

srdrprwr kkkk

)

2

sin(

)2

sin(

rr

rr

d

q

q

k

)2

(sinc rsrk

PS

q rr3

2sin rprk

Skewed angle in electrical radian:r

rskew

r

S

PS ,

1typicallyskewed,slotsofnumbertheis,rskewS


16/21

UCF

Effective Number of Turns per

Phase for RotorEffective number of rotor slots per phase (may not be integer)

3

rS

Unlike the stator, the single cage rotor has only one conductor per slot

(or equivalent to single layer winding).

6

rwreffr

SkN


17/21

UCF Rotor Bar SizeRated current in rotor bar

ip)relationsher(transform,1, ratedeffr

eff

ratedb IN

N

I

Rotor bar current density ,b ratedsb

b

IJ

S

where Sb is rotor bar cross section area and can be

determined from the above formula together with:

22 A/cm775700A/cm:cooled-Air sbJ

Rotor bars can operate at higher temperatures than stator.

sb

ratedb

b

J

IS

,


18/21


19/21

UCF Rotor End Ring


20/21

UCF Rotor End Ring SizeRated current in rotor end ring

ratedb

r

ratedring

IP

SI

,,

Rotor end ring current density

ring

ratedring

ringsA

IJ

,

,

where Aring is rotor end ring cross section area and can

be determined from the above formula together with:

2

,

2 A/cm930775A/cm:cooled-Air ringsJ

Rotor end rings can operate at higher temperatures than rotor bar.

rings

ratedring

ring

J

IA

,

,


21/21

UCF ExampleDesign a 50 hp, 460 V, 60 Hz, 6 pole, Y-connected, 1175 rpm,

NEMA Class B squirrel cage induction motor, pf 0.85 lagging.

Pick up: 54 stator slots, 8/9 pitch, no skew on stator

51 rotor slots (bars), 1 slot skewed

Following the notes discussed, please study the MatLab codeimDesign.m by yourself.

Documents

26 Induction Motor Design