Alternator Secrets.pdf - Cd3wd.com

Electric Motorslower starting currents(three times runningcurrents) than other motorsof same horse power. SeeFigure 7.The National ElectricalManufacturer’s Assn hasagreed on four standardspeeds for shunt motors:1140, 1725, 2500, andFigure 73450 rpm. The speed is normally controlled by varying thearmature supply voltage. Speed varies linearly witharmature supply voltage and torque is unaffected.Shunt motors are typically used for loads which requiregood speed regulation and fair starting torque. If very heavyloads are to be started, a starting circuit may be required.Starting circuits connect progressively smaller resistancesin series with the armature. Runaway can occur in shuntmotors if the field current is interrupted when the motor isturning but not loaded. Dynamic braking and reversibilityare both options with shunt motors.Series MotorsIn series motors, the field coil is connectedin series with the armature loop. The fieldcoil has a large current (the full armaturecurrent). Heavier copper is used for thefield coil but not many turns are needed.Series motors are usually less expensiveand smaller in size than other motors ofthe same horsepower because less copperis used.SERIES FIELDSERIES MOTORDue to the small number of turns and the resulting lowinductance, series motors can operate on both ac and DCpower. For this reason, series motors are often calleduniversal motors. Power to both the field and armatureloops reverses at the same time when operated on acpower and so the resulting magnetic force remains thesame. Series motors may perform differently on ac than DCbecause of the difference in impedance of the windings.One shouldn’t assume all series motors are universal.Some may be optimized for a particular power supply andperform poorly or fail prematurely if not operated on thecorrect supply.As the motor’s speed is decreased by heavy loads, themotor supplies high torque to drive the load. This helpsprevent stalling and provides high starting torque. Startingcurrents are also high but are not usually a problembecause series motors are normally small motors. SeeFigure 8. The speed of series motors can be adjusted byvarying the supply voltage with a rheostat, variabletransformer orelectronic controls.Series motors arenot normally used ifconstant speed overa range of loads isrequired.Series motors areFigure 8very commonmotors in household appliances and power tools. They areused in blenders, juicers, food processors, and hand powertools such as drills. They are very versatile and have thehighest horsepower per pound and per dollar of any motorthat operates on standard single phase ac power. Theydeliver high motor speed, high starting torque and widespeed capability. Series motors are usually operated atspeeds over 7000 rpm or more. In routers, small grindersand sanders, speeds of 25,000 rpm are not uncommon.Series motors are often connected to a built-in gear train toreduce shaft speed and/or provide more torque. Gear trainsalso provide loading which prevents runaway.Series motors have comparatively high maintenance.Brushes and bearings need to be regularly replaced. Theyare the only motors that are usually given an intermittentduty rating. Other disadvantages of series motors are thatthey are not usually designed for dynamic braking andreversibility. They should not be run without a load asrunaway can occur.Series motors have a moderately low power factors —normally between 0.5 and 0.7. Resistors have a powerfactor of one. The more reactive a component, the lower itspower factor. Low power factors can be a problem formodified sine wave inverters. Appliances with low powerfactors may run three quarter speed. Sine wave invertersdo not have trouble with power factors less than one.Series motors are typically small motors and so their highstarting currents are not usually a problem for inverters.Compound MotorsA compound motor provides a mixtureof the characteristics of both shunt andseries motors. Its field coil is split into aseries field which is connected in seriesCOMPOUND MOTORwith the armature and a shunt fieldwhich is connected in parallel with thearmature. The magnetic fields can either aid (cumulativecompound) or oppose each other (differential compound).Cumulative and differential compound motors have differentspeed/torque characteristics. Cumulative compound motorsprovide more torque than shunt wound motors and betterspeed regulation than series wound motors. Differential50 Home Power #34 • April / May 1993