fundamentals of machine design

Fundamentals of designing a machine

Electrical machine types fundamentals of machine design
Electrical machine types

The fundamentals of machine design is done before desgn a machine and it is important part. Also It need to be consider all considerations about all factors.

Symbols – Fundamentals of machine design

n = speed in revolution per second(rps)

ns = synchronous speed(rps)

p = number of poles

P = number of pole pairs

a = number of parallel paths in the windings

z = total number of conductors in the winding

Tph = total number of series turns per phase

Kw = winding factor

Ƭ = pole pitcg(m)

Ia = armature current(A)

and Iz = current in each conductor(A)

Iph = current per phase(A)

E = induced emf(V)

Eph = induced emf per phase(V)

W = Power developed by armature

Ø = average flux per pole (Wb)

Principle Dimensions

Two magnetically active members stator(fixed) and rotor(armature), capable of rotating and they are separated by a narrow annular air gap.

Principle Dimensions

  • The stator bore diameter(D)
  • Stator core length(L)

Active materials in an electric machine are iron and copper

Total magnetic loading – fundamentals of machine design

Total flux that leaves or enter the airgap around its entire periphery.

Btot = P Ø

p = number of poles of the machine

Ø = average flux per pole

Specific magnetic loading

Average flux density over the air gap of an electric machine. Indicates extent to which the magnetic material iron is utilized in the machine (there is magnetic loss)

Bsp = Total flux/Total area = P Ø/2π(D/2) L

Bsp = P Ø/πDL

Design Criteria

Use highest possible Bsp within around the permissible range of iron loss

Total electric loading

Total no of ampere conductors around the entire periphery of stator (or the armature) winding.

Atotal = IzZ

Specific electric loading

Number of armature conductors per meter length of the armature (or stator) periphery.

Specific electric loading (Asp) = Total armature ampere conductors/Armature periphery at the airgap

Asp = IzZ/ πD

Directly related to the mmf produced by the armature winding choice of its value also depends on the permissible I2R (i.e. Temperature rise) and the amount of cooling provided

Design Criteria – Fundamentals of machine design

Use highest possible Aspwithin permissible range of copper loss

Temperature rises

Breakdown of insulation

Output if DC machines

Power developed by the armature of the dc machines (Internal power)

Wa = generated emf X Armature current

Wa = E. Ia

But E = Ø.Z.(P/a). n. Ia                            Ia/a = Iz (current in each conductor)

Wa = (P. Ø). (Iz.Z.n)

Wa = Total magnetic loading X Total electric loading X speed(rps)

Output of the DC machine

Wa = (P. Ø). (Iz.Z).n

Wa = (πD.L.Bsp). (πD. Asp). n

Wa = π2. Bsp..Asp. D2.L.n = C0. D2L.n

Where the output coefficient C0 = π2. Bsp.Asp

Note – Fundamentals of machine design

D2L is proportional to the machine

Therefore, output of the machine is directly proportional to its volume for given values of electric and magnetic loadings.

Also from equation of Wa , high speed motor is smaller than a low speed motor for same power rating.

We should not confuse the power Wa developed by the armature with the rated output W of the machine. The relation between Wa and W depends on whether the machine is running as a motor or a generator.

Example, consider a DC shunt machine running in the generator mode

Power developed by armature in a generator in a generator: Wa

Wa = W + total copper loss

Also, Wa = input power – friction and windage loss

Wa = W/n – (friction + windage losses)

n is the generator efficiency

If the same machine running in the motor mode

Wa = output power +friction and windage losses

and

Wa = W + (friction + windage losses) Wa = W/n –(copper loss)

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