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Losses dc machines
1) There are several types of losses that reduce the efficiency of DC machines, including electrical or copper losses, core losses, brush losses, mechanical losses, and stray load losses. 2) Electrical losses include losses from the armature winding resistance, shunt field winding resistance, series field winding resistance, and interpole winding resistance. 3) Core losses are hysteresis and eddy current losses and account for around 20% of full load losses. 4) Brush losses are due to the voltage drop and current at the brush contact with the commutator.
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Losses dc machines
- 1. LOSSES IN ADC 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 07/28/14 Professor MD Dutt HOD EX Department SRCT Bhopal 1
- 2. ELECTRICAL LOSSES • Windingshaving 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 07/28/14 Professor MD Dutt HOD EX Department SRCT Bhopal 2
- 3. ELECTRICAL LOSSES • Copperloss 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 07/28/14 Professor MD Dutt HOD EX Department SRCT Bhopal 3
- 4. CORE LOSSES • Thecore 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. 07/28/14 Professor MD Dutt HOD EX Department SRCT Bhopal 4
- 5. BRUSH LOSSES • Thereis 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) 07/28/14 Professor MD Dutt HOD EX Department SRCT Bhopal 5
- 6. MECHANICAL LOSSES • Thelosses 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. 07/28/14 Professor MD Dutt HOD EX Department SRCT Bhopal 6
- 7. 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. 07/28/14 Professor MD Dutt HOD EX Department SRCT Bhopal 7
- 8. EFFICIENCY 07/28/14 Professor MD DuttHOD EX Department SRCT Bhopal 8 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 2 Ra+brush contact loss Series field loss IL 2 Rs +shunt field loss If 2 Rf 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 2 Ra+brush contact loss Series field loss IL 2 Rs +shunt field loss If 2 Rf DC Motor DC Generator
- 9. EFFICIENCY Let usassume 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 07/28/14 Professor MD Dutt HOD EX Department SRCT Bhopal 9
- 10. 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) 07/28/14 Professor MD Dutt HOD EX Department SRCT Bhopal 10
- 11. EFFICIENCY • Ia =I + Ish • Since Ish compare to I is very small we can consider • Ia I≡ • ή = VI/ ( VI+ I²Rat +Vbd I +Pk) 07/28/14 Professor MD Dutt HOD EX Department SRCT Bhopal 11
- 12. EFFICIENCY • LOAD forMaximum 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 07/28/14 Professor MD Dutt HOD EX Department SRCT Bhopal 12