1.2.3 losses & efficiency Topic 2: DC Motor Power stage : various power stages in the case of a dc generator are shown below: Mech. Power input = output of driving engine elect. Power developed in armature = EI watts elect. Power output = VI watts Iron and frication losses Cu losses A B C
1.2.3 losses & efficiency Topic 2: DC Motor Following are the three efficiencies: 1. Mechanical efficiencies: η = = ℎ = I 2. electrical efficiencies: η = = = I I 3. Overall or commercial efficiencies : η = = ℎ Ir is obvious that overall efficiencies η = η + η good generator value efficiencies may be high (95%)
the efficiency of DC generator DC generator have the following efficiencies due to the different power stages. 1. Mechanical Efficiency ɳm = 2. Electrical Efficiency ɳe = 1.2.3 characteristics og DC generator DC MACHINE
3. Commercial or overall efficiency ɳ c = electrical power output Mechanical power input Therefore ɳ c = ɳ m x ɳ e commercial efficiency is always to be taken if it is not specified. Since commercial efficiency is the produce of mechanical and electrical efficiency, ɳ c is always less than ɳ m and ɳ e 1.2.3 characteristics og DC generator DC MACHINE
THE FORMULAE OF TORQUE torque T, we know that the mechanical power P is given in equation watts 60 2 NT P where: P : Mechanical power Nr : Speed rotor Ta : armature Torque Tsh : output torque 2 n PNZIa 2 n EbIa Ta 60 N n 2 n Poutput Tsh 2 n ironloss& friction Ta Tsh 1.2.3 characteristics og DC generator DC MACHINE
Example 1.The armature of a DC machine has a resistance of 0.1Ω and is connected to a 230V supply. Calculate the back EMF when it is running as a motor taking 60A. 2. A supply 8.5kW generator, 220Volt. Calculate emf if armature resistance is 0.5Ω and shunt resistance is 70Ω. 3. A supply 6.5kW short shunt compound, 240V. Shunt resistant , serial resistant and armature resistant is 100Ω, 0.4Ω and 0.3Ω. This generator has 8 poles and rotated at the speed 600psm. Total number of conductors 788 pieces and is made by winding waves. Calculate the emf can be issued, load resistanced can be supplied and the desired strength of the magnetic flux. 1.2.3 characteristics og DC generator DC MACHINE