INDUCTION MOTOR :
kVA input = Q = (11 kω . Bav x 80-3) D2 Lns = Co D2 Lns.
Main Dimensions of Induction Motor :
| Design criteria | Length to Pole pitch ratio, τ |
| Good power factor | 1 to 1.25 |
| Best power factor | τ = √18 L |
| Minimum cost | 1.5 to 2 |
| Good efficiency | 1.5 |
| Good overall design | 1.0 |
Peripheral speed of Induction Motor:
For normal design of motors a limiting value of peripheral speed as 30 m/s is used. However, standard constructions have speeds up to 60 m/s and in very special cases up to 75 m/s.
Ventilating ducts in Induction Motor:
The radial ventilating ducts of 8 to 10 mm are provided after each stack of 10 cm core length.
Stator windings in Induction Motor:
Turns per phase,
Ts = Es / (4.44 f φm k ωm)
current density of 3 to 5 A/mm2 is generally used.
For diameters up to 3 mm round SWG conductors are used with proper insulation. For larger machines bar or strip conductors are used.
Stator slots in Induction Motor:
For smaller machines up to 20 kW, 600 V and diameter less than 40 cm, semi-closed slots are used. For higher ratings, open type slots with insulation wedge are used. Use of semi-closed slots gives low air gap contraction factor, low value of magnetizing current low tooth pulsation losses and quieter operation as compared to open slots.
For Ss = number of stator slots
Slot pitch, yss = Gap surface / Total number o stator slots = π D / Ss
Total number of stator conductors = 3 x 2 x Ts = 6 Ts
Hence conductors per stator slot = Zss= 6 Ts / S s
Rotor design of Induction Motor:
Air gap length depends on:
(i) Power factor,
(ii) Overload capacity,
(iii) Unbalanced magnetic pull,
(iv) Pulsation loss,
(v) Noise.
For small motors, any one of the following relation is used for air gap length :'
lg = ( 0.2 + 2 √ ( DL))mm
lg= (0.125 + 0.35D + L + 0.015 Va)mm
lg = (0.2 + D) mm
where,
D = inner diameter of stator in meters
L = length of stator in meters
Va = peripheral speed, m/s.
Air gap for 4 Pole
Induction Motors:
|
D
(cm)
|
15
|
20
|
25
|
30
|
45
|
55
|
65
|
80
|
|
Lg(mm)
|
0.35
|
0.50
|
0.60
|
0.70
|
1.31
|
1.8
|
2.5
|
4.0
|
Squirrel Cage Rotor Design:
Rules for number of rotor slots:
The number of rotor slots Sr in comparison with number of stator slots Sa, is given by
(i) Sr = (1.15 to 1.30) Ss
(ii) To avoid Synchronous Cusps
Ss - Sr != ± p ± 2p or ± 5p;
(iii) To avoid magnetic locking in 3 phase motor,
Ss - Sr != ± 3p;
(iv) To avoid noise and vibrations,
Ss - Sr != 1,2, (p ±1 ) or (p ±2).
Rotor bar current for 3 phase machine is given by
Ib = 0.85 (( 6 Ib Ts) / Sr )
where Is, is stator current.
Skewing in Induction Motor:
In order to eliminate the effect of any harmonic, the rotor bars are. skewed in such a way that the bars lie under alternate poles of the same polarity.
To eliminate nth harmonic, the angle of skew will be
Q = 720 / (n x p) degrees mechanical.
In practice the rotor is skewed through one stator slot pitch.
Area of end rings in Induction Motor:
The area of end ring,
ar = ( Sr Ib ) / (π p δ c ) mm2
Slip in Induction Motor:
The value of full load slip s is determined by rotor copper loss. Some typical values are given below:
|
Output
(kW)
|
0.75
|
3.75
|
7.5
|
18.750
|
37.50
|
75.00
|
150.00
|
|
Slip
|
5.00
|
4.2
|
4.0
|
3.70
|
3.50
|
3.20
|
3.00
|
Design of wound rotor:
No. of turns per phase on rotor,
Tr = k ωs / k ωr . Er / Es. Ts
where,
k ωs = winding factor for stator ,
k ωr = winding factor for rotor,
Es = voltage per phase applied to stator,
Er = voltage per phase induced in rotor at stand still,
Ts = number of turns per phase on stator.
Minimum tooth width = Flux per pole / (Max. allowable flux density x Slots per pole x Net iron length)
Rotor core depth = φm/ 2 - Bcr x L2
No comments:
Post a Comment