Compred to genertors DC MOTORS Prepred by Engr. JP Timol Reference: Electricl nd Electronic Principles nd Technology The construction of d.c. motor is the sme s d.c. genertor. the generted e.m.f. is less thn the terminl voltge. often used in power sttions to drive emergency stnd-by pump systems Bck e.m.f. When d.c. motor rottes, n e.m.f. is induced in the rmture conductors. By Lenz s lw this induced d e.m.f. E opposes the supply voltge V nd is clled bck e.m.f., Bck e.m.f. nd the supply voltge, V is given by: V E I R or E V I R Exmple A d.c. motor opertes from 240V supply. The rmture resistnce is 0.2 Ω Determine the bck e.m.f. when the rmture current is 50A. Exmple The rmture of d.c. mchine hs resistnce of 0.25 Ω nd is connected to 300V supply. Clculte the e.m.f. generted when it is running: () s genertor giving 100A, nd (b) s motor tking 80A.
Torque of d.c. motor EI T newton meter 2 n p ZI T newton meter c An 8-pole d.c. motor hs wvewound rmture with 900 conductors. The useful flux per pole is 25mWb. Determine the torque exerted when current of 30A flows in ech rmture conductor. Determine the torque developed by 350V d.c. motor hving n rmture resistnce of 0.5 nd running t 15 rev/s. The rmture current is 60A. A six-pole lp-wound motor is connected to 250V d.c. supply. The rmture hs 500 conductors nd resistnce of 1. The flux per pole is 20mWb. Clculte () the speed nd (b) the torque developed when the rmture current is 40A. The shft torque of diesel motor driving 100V d.c. shunt-wound genertor is 25 Nm. The rmture current of the genertor is 16A t this vlue of torque. If the shunt field regultor is djusted so tht the flux is reduced by 15 per cent, the torque increses to 35 Nm. Determine the rmture current t this new vlue of torque. A 100V d.c. genertor supplies current of 15A when running t 1500 rev/min. If the torque on the shft driving the genertor is 12 Nm, determine () the efficiency of the genertor nd (b) the power loss in the genertor.
Types of d.c. motor nd their chrcteristics () Shunt wound motor Supply Voltge Bck e.m.f. Supply current Equtions V E I R E V I R f I I I Smple Problem A 240V shunt motor tkes totl current of 30A. If the field winding resistnce R f =150 nd the rmture resistnce R =0.4, determine () the current in the rmture, nd (b) the bck e.m.f. Chrcteristics of Shunt Torque is proportionl to rmture current Chrcteristics of Shunt speed of rottion, E n V IR n Note In prctice, the speed flls by bout 10 percent between no-lod nd fulllod on mny d.c. shunt-wound motors. Due to this reltively smll drop in speed, the d.c. shunt-wound motor is tken s bsiclly being constntspeed mchine nd my be used for driving lthes, lines of shfts, fns, conveyor belts, pumps, compressors, drilling mchines nd so on.
Chrcteristics of Shunt Smple Problem A 200V, d.c. shunt-wound motor hs n rmture resistnce of 0.4Ω nd t certin lod hs n rmture current of 30A nd runs t 1350 rev/min. If the lod on the shft of the motor is incresed so tht the rmture current increses to 45A, determine the speed of the motor, ssuming the flux remins constnt. (b) Series wound motor Equtions Supply Voltge Bck e.m.f. V E I( R R ) E V I( R R ) f f Chrcteristics of Series
Chrcteristics of Series Smple Problem A series motor hs n rmture resistnce of 0.2Ω nd series field resistnce of 0.3 Ω. It is connected to 240V supply nd t prticulr lod runs t 24 rev/s when drwing 15A from the supply. () Determine the generted e.m.f. t this lod (b) Clculte the speed of the motor when the lod is chnged such tht the current is incresed to 30A. Assume tht this cuses doubling of the flux. (c) Compound wound motor Long nd Short Shunt Motor There re two types of compound wound motor: (i) Cumultive compound, series winding ssists the shunt winding. (ii) Differentil compound series winding opposes the shunt winding. Note A compound-wound motor hs both series nd shunt field winding, nd is usully wound to hve chrcteristic similr in shpe to series wound motor. A limited mount of shunt winding is present to restrict the no-lod speed to sfe vlue. Note However, by vrying the number of turns on the series nd shunt windings nd the directions of the mgnetic fields produced by these windings (ssisting or opposing), fmilies of chrcteristics my be obtined to suit lmost ll pplictions. Generlly, compound-wound motors re used for hevy duties, prticulrly in pplictions where sudden hevy lod my occur such s for driving plunger pumps, presses, gered lifts, conveyors, hoists nd so on.
Chrcteristics of Compound Motor Chrcteristics of Compound Motor The efficiency of d.c. motor output power efficiency, 100 input power VI losses 100 VI 2 VI I R I fv C 100 VI Smple Problem A 320V shunt motor tkes totl current of 80A nd runs t 1000 rev/min. If the iron, friction nd windge losses mount to 1.5kW, the shunt field resistnce is 40 Ωnd the rmture resistnce is 0.2 Ω, determine the overll efficiency of the motor. Smple Problem A 250V series motor drws current of 40A. The rmture resistnce is 0.15 Ω nd the field resistnce is 0.05 Ω. Determine the mximum efficiency of the motor. Smple Problem A 200V d.c. motor develops shft torque of 15Nm t 1200 rev/min. If the efficiency is 80 per cent, determine the current supplied to the motor.
D.C. motor strter D.C. motor strter Additionl resistnce must be dded to the rmture circuit t the instnt of closing the switch to strt the motor fuses protecting the motor will burn out if d.c. motor whose rmture is sttionry is switched directly to its supply voltge rmture resistnce is smll, frequently being less thn one ohm. Speed control of d.c. motors Methods of speed control for shunt motors Shunt-wound motors The speed of shunt-wound d.c. motor, n, is proportionl to V I R shunt field regultor vrible resistor in series with the field winding nd such resistor Smple Problem A 500V shunt motor runs t its norml speed of 10 rev/s when the rmture current is 120A. The rmture resistnce is 0.2Ω. () Determine the speed when the current is 60A nd resistnce of 0.5Ω is connected in series with the rmture, the shunt field remining constnt (b) Determine the speed when the current is 60A nd the shunt field is reduced to 80 per cent of its norml vlue by incresing resistnce in the field circuit. Speed control of d.c. motors Series-wound motors The speed of series-wound d.c. motor, n, is proportionl to V IR k diverter vrible resistor connected in prllel with the series-wound field to control speed
Methods of speed control for series motors Smple Problem On full-lod 300V series motor tkes 90A nd runs t 15 rev/s. The rmture resistnce is 0.1 nd the series winding resistnce is 50m Ω. Determine the speed when developing full lod torque but with 0.2Ω diverter in prllel with the field winding. (Assume tht the flux is proportionl to the field current). Smple Problem A series motor runs t 800 rev/min when the voltge is 400V nd the current is 25A. The rmture resistnce is 0.44 Ωnd the series field resistnce is 0.2 Ω. Determine the resistnce to be connected in series to reduce the speed to 600 rev/min with the sme current. Motor Cooling Motors re often clssified ccording to the type of enclosure used, the type depending on the conditions under which the motor is used nd the degree of ventiltion required. Motor Cooling The most common type of protection is the Screen-protected type ventiltion is chieved by fitting fn internlly, with the openings t the end of the motor fitted with wire mesh. Drip-proof type similr to the screen-protected type but hs cover over the screen to prevent drips of wter entering the mchine. Motor Cooling Flme-proof type cooled by the conduction of het through the motor csing. Pipe-ventilted type ir is piped into the motor from dust-free re, nd n internlly fitted fn ensures the circultion of this cool ir.