Direct Current Machine (DC Machine) :
The output equation is
EIa = n (pφ) ( (IaZ) / a ) = n (pφ)(IzZ)
where,Iz = Ia / a = current per conductor φ is flux per pole, p is number of poles, n is speed in rps, Z is total number of conductor and a is the number of parallel paths.
Also, D2L = (1/Co) P/n
where Co is output coefficient, D is diameter and L is the length of machine.
Specific Loading of DC Machine:
| Machine rating,kW | 10 | 50 | 100 | 500 | 1000 | 5000 |
Maximum gap flux density Bg ,
( Wb/m2)
| 0.65 | 0.80 | 0.85 | 0.95 | 1.00 | 1.05 |
Specific Electric Loading of DC Machine:
| Machine rating, kW | 10 | 50 | 100 | 500 | 1000 | 50000 |
| Ampere conductors per meter ac | 17500 | 25000 | 28000 | 36500 | 41000 | 50000 |
Relation between Length and Diameter of Armature of DC Machine:
L = 0.7 (Dπ /p)
Pole pitch is limited as per the following table:
| No. of poles | 2 | 4 | 6 | more than 6 |
| t cm | 25 to 50 | 30 to 40 | 40 to 45 | 45 to 55 |
Slot design of DC Machine:
Knowing the value of current through the conductor Ia, its cross-sectional area ac can be calculated as
ac = I c/ δ
where δ is current density in the armature winding.
The insulation is provided as per the following practice.
(a) Round conductors of DC Machine:
| Diameter up to (mm) | Total insulation thickness (mm) |
| 1.5 | 0.20 |
| 2.0 | 0.30 |
| 3.0 | 0.35 |
| 4.0 | 0.40 |
(b) Rectangular Conductors of DC Machine:
| Width up to (mm) | Total thickness (mm) |
| 1 | 0.3 |
| 4 | 0.4 |
| 4 and above | 0.6 |
Slot insulation of DC Machine:
The slot is also provided with suitable insulation. A typical slot insulation is given below :
|
|
Width wise insulation
|
mm
|
|
(i)
|
Conductors wrapped with mica paper
|
2.0
|
|
(ii)
|
Coil wrapped with glass tape
|
0.2
|
|
(iii)
|
Coil varnish
|
0.2
|
|
(iv)
|
Slot linear
|
0.4
|
|
(v)
|
Clearance
|
0.2
|
|
|
Width wise insulation
|
mm
|
|
(i)
|
Conductors wrapped with mica paper
|
2 x 2 = 4.0
|
|
(ii)
|
Coil wrapped with glass tope
|
2 x 0.2 = 0.4
|
|
(iii)
|
Coil varnish
|
2 x 0.2 = 0.4
|
|
(iv)
|
Slot linear
|
3 x 0.2 = 0.6
|
|
(v)
|
Layer separator
|
1 x 2.0 = 2.0
|
|
(vi)
|
Wedge
|
1 x 5.0 = 5.0
|
|
(vii)
|
Clearance
|
1.6
|
Main Pole of DC Machine:
Axial length of DC Machine:
The axial length is 10 to 15 mm shorter than the core length of the armature. Width. The width can be calculated from the relation,
Width = Cross-sectional area of pole Width / Axial length of the pole.
Height:
The height is calculated from the relation height
= [(1.1 to 1.25) ((Ia Z) / (2p) ) ] / (1000 (√ ( dc kω) ))
Commutator of DC Machine:
The diameter of the commutator is taken as (0.65 to 0.70) D for medium and large machines and (0.75 to 0.80) D for small machines.
Standard size of commutator brushes is given below :
| Machine | Width (mm) | Height (mm) | Thickness (mm) |
| Small size machine | 15 | 30 | 6,8,15 |
| Medium size machine | 22 | 37 | 9,12.5,15,18 |
| Large size machine | 30 | 43 | 12.5,15,18,22,25 |
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