GE SX Series Installation And Operation Manual

GE SX Series Installation And Operation Manual Manual pdf 4 page image

BASIC OPERATION AND FEATURESSX TRANSISTOR CONTROL Page 4RevisedMay 2003Section 1. INTRODUCTIONSection 1.1 Motor CharacteristicsThe level of sophistication in the controllability of tractionmotors has changed greatly over the past several years.Vehicle manufacturers and users are continuing to expectmore value and flexibility in electric vehicle motor andcontrol systems as they are applied today. In order torespond to these market demands, traction systemdesigners have been forced to develop new approaches toreduce cost and improve functions and features of theoverall system. Development is being done in a multi-generational format that allows the market to takeadvantage of today’s technology, while looking forward tonew advances on the horizon. GE has introduced a secondgeneration system using separately excited DC shuntwound motors. The separately excited DC motor systemoffers many of the features that are generally found on theadvanced AC systems. Historically, most electric vehicleshave relied have on series motor designs because of theirability to produce very high levels of torque at low speeds.But, as the demand for high efficiency systems increases,i.e., systems that are more closely applied to customers’specific torque requirements, shunt motors are now oftenbeing considered over series motors. In most applications,by independently controlling the field and armaturecurrents in the separately excited motor, the best attributesof both the series and the shunt wound motors can becombined.NO LOAD CURRENTFULLLOAD CURRENTSTARTINGCURRENTARMATURE CURRENTFigure 1SPEEDTORQUEAs shown in from the typical performance curves of Figure1, the high torque at low speed characteristic of the seriesmotor is evident.In a shunt motor, the field is connected directly across thevoltage source and is therefore independent of variations inload and armature current. If field strength is heldconstant, the torque developed will vary directly with thearmature current. If the mechanical load on the motorincreases, the motor slows down, reducing the back EMF(which depends on the speed, as well as the constant fieldstrength). The reduced back EMF allows the armaturecurrent to increase, providing the greater torque needed todrive the increased mechanical load. If the mechanicalload is decreased, the process reverses. The motor speedand the back EMF increase, while the armature current andthe torque developed decrease. Thus, whenever the loadchanges, the speed changes also, until the motor is againin electrical balance.In a shunt motor, the variation of speed from no load tonormal full load on level ground is less than 10%. For thisreason, shunt motors are considered to be constant speedmotors (Figure 2).NO LOAD CURRENTFULLLOAD CURRENTSTARTINGCURRENTARMATURE CURRENTFigure 2SPEEDTORQUEIn the separately excited motor, the motor is operated as afixed field shunt motor in the normal running range.However, when additional torque is required, for example,to climb non-level terrain, such as ramps and the like, thefield current is increased to provide the higher level oftorque. In most cases, the armature to field ampere turnratio can be very similar to that of a comparable size seriesmotor (Figure 3.)NO LOAD CURRENTFULLLOAD CURRENTSTARTINGCURRENTARMATURE CURRENTFigure 3SPEEDTORQUEAside from the constant horsepower characteristicsdescribed above, there are many other features thatprovide increased performance and lower cost. The