Motor Designs A, B, C, D
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Transcript Motor Designs A, B, C, D
Motor Designs A, B, C, D
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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
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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
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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
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NEMA Tables
• Tables 5.1 – 5.7 give values of lockedrotor torque, breakdown torque, and pullup torque for specific horsepower,
frequency, and synchronous speed
ratings.
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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 3phase, 10-hp, 460-V, six-pole, NEMA
design C motor whose rated speed is
1150 r/m.
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Solution
120 f (120)(60)
ns
1200r / min
P
6
Tn
hp
5252
T (1150)
10
5252
Trated 45.67lb ft
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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.
Tlocked rotor (2.25)(45.67) 102.8lb ft
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Breakdown Torque
• From Table 5-3
Tbreakdown (1.90)( 45.67) 86.8lb ft
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Pull-up Torque
• From Table 5.6,
Tpullup (1.65)( 45.67) 75.4lb ft
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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.
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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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R2 a 2 Rr
X 2 a 2 X BR
Ir
a
E2 Es aEBR
I2
a
Ns
Nr
R fe coreloss / phase
X M magnetizing / phase
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I 0 exciting current per phase
Rr actual rotor resis tan ce per phase
I fe core loss component of exciting current
I M magetizing component of exciting current
X BR actual blocked rotor reac tan ce per phase
I r actual rotor current per phase
V actual voltage per phase applied to the stator
I1 actual stator current per phase
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Power, Torque, Speed, Losses, and
Efficiency
Z2
Z0
I1
R fe jX M
Z2 Z0
ZP
Z2 Z0
V
Z in
Z in Z1 Z P
E I1 Z P
E
I2
Z2
R2
jX 2
s
R fe jX M
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Pscl 3 I12 R1
Prcl 3 I 22 R2
1
1 s
Pgap Prcl
Pmech Prcl
s
s
Pmech Pf , w Pstray
Pshaft
hp
746
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21.12 I r2 Rr
TD
sns
Pcore
21.12 I 22 R2
TD
sns
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3E22
R fe
19
Approximate Equivalent Circuit for an
Induction Motor
Move the
magnetizing branch
to the left of resistor
R1.
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Shaping the Torque-speed curve
21.12I R
T
lb ft
sn
2
r
r
D
s
I
2
V
R
R jX jX
s
V
2
1
I
1
(R R ) (X X )
s
2
2
2
2
1
T
D
1
21.12V
1
T V
D
R
s
2
2
2
(R R ) (X X ) n
s
2
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2
2
2
1
2
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s
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.
s
R
R (X X )
2
T D max
2
1
1
2
2
Slip is directly proportional to the rotor resistance.
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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).
s
R
1
R (X X )
2
T D max
2
1
2
1
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Slip at Which Maximum Torque Occurs
s
R
1
R (X X )
2
T D max
2
1
R
2
1
2
R (X X )
2
2,s 1
1
1
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2
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Maximum Torque
R
21.12V
s
T
(R R ) (X X ) n
s
2
2
D
2
2
1
1
R
R (X X )
s
2
2
1
1
2
2
s
at maximum torque
2
2
21.12V
2n R (X X ) R
2
T
D ,max
2
s
1
2
1
2
1
Independent ofECERotor
Resistance!
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