GE SX Series Installation And Operation Manual

GE SX Series Installation And Operation Manual Manual pdf 5 page image

BASIC OPERATION AND FEATURESSX TRANSISTOR CONTROL Page 5RevisedMay 2003following description provides a brief introduction toexamples of some of these features.Section 1. 2 Solid-State ReversingThe direction of armature rotation on a shunt motor isdetermined by the direction in which current flows throughthe field windings. Because of the of the shunt motor fieldonly typically requires about 10% of the armature current atfull torque, it is normally cost effective to replace thedouble-pole, double-throw reversing contactor with a lowpower transistor H-Bridge circuit (Figure 4).By energizing the transistors in pairs, current can be madeto flow in either direction in the field. The armature controlcircuit typically operates at 12KHZ to 15KHZ, a frequencyrange normally above human hearing. This high frequencycoupled with the elimination of directional contactors,provides very quiet vehicle operation. The field controlcircuits typically operate at 2 KHZ.The line contactor is normally the only contactor requiredfor the shunt motor traction circuit. This contactor is usedfor both pre-charge of the line capacitors and foremergency shut down of the motor circuit, in case ofproblems that would cause a full motor torque condition.The line can be energized and de-energized by the variouslogic combinations of the vehicle, i.e. activate on key, seator start switch closure, and de-energize on time out of idlevehicle. Again, these options add to the quiet operation ofthe vehicle.Section 1. 3 Flexible System ApplicationBecause the shunt motor controller has the ability tocontrol both the armature and field circuits independently,the system can normally be adjusted for maximum systemefficiencies at certain operating parameters. Generallyspeaking, with the ability of independent field andarmature, the motor performance curve can be maximizedthrough proper control application.Section 1. 4 More Features with Fewer ComponentsField weakening with a series wound motor isaccomplished by placing a resistor in parallel with the fieldwinding of the motor. Bypassing some of the currentflowing in the field into the resistor causes the field currentto be less, or weakened. With the field weakened, the motorspeed will increase, giving the effect of “overdrive”. Tochange the “overdrive speed”, it is necessary to changethe resistor value. In a separately excited motor,independent control of the field current provides forinfinite adjustments of “overdrive” levels, betweenmotor base speed and maximum weak field. Thedesirability of this feature is enhanced by theelimination of the contactor and resistor required forfield weakening with a series motor.With a separately excited motor, overhauling speedlimit, or downhill speed, will also be more constant. Byits nature, the shunt motor will try to maintain aconstant speed downhill. This characteristic can beenhanced by increasing the field strength with thecontrol. Overhauling load control works in just theopposite way of field weakening, armature rotationslows with the increase of current in the field.Regenerative braking (braking energy returned to thebattery) may be accomplished completely with solid-statetechnology. The main advantage of regenerative braking isincreased motor life. Motor current is reduced by 50% ormore during braking while maintaining the same brakingtorque as electrical braking with a diode clamp around thearmature. The lower current translates into longer brushlife and reduced motor heating. Solid state regenerativebraking also eliminates a power diode, current sensor andcontactor from the circuit.For GE, the future is now as we make available a newgeneration of electric traction motor systems for electricvehicles having separately excited DC shunt motors andcontrols. Features that were once thought to be onlyavailable on future AC or brushless DC technology vehiclessystems are now achievable and affordable.FUSELINECAPARM F2F1Q3Q4Q5Q6Q1POSNEGFigure 4A1 +A2 -Q2