losses dc machines

LOSSES IN A DC MACHINES and efficiency

Following are the losses in the DC machines•Electrical or copper loss ( I²R Losses)•Core losses or Iron losses•Brush Losses•Mechanical losses•Stray load losses

07/28/14Professor MD Dutt HOD EX Department

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ELECTRICAL LOSSES

• Windings having resistance consumes certain losses, these are termed as copper losses because mostly windings are made of copper.

• Armature copper loss Ia²Ra ( Ia is armature current)

• Shunt field copper loss Ish²Rsh• Copper loss in the series field Ise²Rse


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ELECTRICAL LOSSES

• Copper loss in the interpole winding which are in series with armature Ia²Ri

• Compound machines both series field and shunt field copper losses are also there

• Copper losses are there in the compensating winding


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CORE LOSSES

• The core losses are the hysteresis losses and Eddy current losses. Since the machine usually operates at constant flux density and speed, these losses are almost constant. These losses are about 20% of Full load losses.


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BRUSH LOSSES

• There is a power loss at the brush contact with commutator and the carbon brushes. This loss can me measured by the voltage drop at the brush contact and armature current.

• Pbd = Vbd Ia• The voltage drop is more or less remains

constant over a wide range of Ia and it is assumed 2V ( approx)


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MECHANICAL LOSSES

• The losses associated with mechanical effect are called mechanical losses. These consists of friction losses at bearing and windage losses ( fan losses) . the fans are used to take away the heat produced due to I²R losses and iron losses inside the machine.


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STRAY LOAD LOSSES

• These are miscellaneous losses which are due to the following reasons:-

• Distortion of flux due to armature reaction• Short circuit currents in the coils due to

commutation.• These losses are difficult to find out, However

they are taken as 1 % of full load power output.


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EFFICIENCY


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Input from

prime-mover

Elec-

magnetic

Power =EaIa

Arm. terminal

power = Vta Ia

Output power

= Vt IL

No-load rotational loss (friction+windage+core)+stray load loss

Arm. copper loss Ia

2Ra+brush contact loss

Series field loss IL2Rs

+shunt field loss If2Rf

Input power from

mains =Vt IL

Elec-

magnetic

Power =EaIa

Arm. terminal

power = Vta Ia

Output available

at the shaft

No-load rotational loss (friction+windage+core)+stray load loss

Arm. copper loss Ia

2Ra+brush contact loss

Series field loss IL2Rs

+shunt field loss If2Rf

DC Motor

DC Generator

EFFICIENCY

Let us assume that the • R = Total resistance• I = Output current• Ish = Current through the shunt field• Ia armature current I +Ish• V is the terminal voltage• Power loss in the shunt field = V Ish


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EFFICIENCY

• Mechanical Losses = Friction losses at bearings+ friction losses at commutator + windage losses

• Stray losses Core losses mechanical losses and shunt field copper losses are considered as combined fixed losses.

• ή = Output / Input• = VI / ( VI + Ia²Rat +Pk +VbdIa)


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EFFICIENCY

• Ia = I + Ish• Since Ish compare to I is very small we can

consider• Ia I≡• ή = VI/ ( VI+ I²Rat +Vbd I +Pk)


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EFFICIENCY

• LOAD for Maximum efficiency• Ifl = Full load current at maximum efficiency• Im = current at maximum efficiency• Im ² Rat = Pk• Im ² = Pk/ Rat• Current at Maximum ή =

F.L Current X( Pk/F.L Copper loss) ²


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losses dc machines

Engineering