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| Home > Articles > Chopper for Battery-operated Vehicles & Applications |
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Chopper for Battery-operated Vehicles & Applications |
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The armature current builds up, and energy stored in the reactor connected in series with the armature. When the chopper is off, armature current is forced in to the supply. So, energy stored in the reactor from the armature is thus released to the supply.
If a series motor is used, an additional reactor in series with the armature may not be necessary. The series field winding stores energy during the chopper on interval and releases energy to the source during the chopper Off interval. However, the series field should remain unidirectional during motoring and regenerative braking operation.
Where does the energy go that is fed back during the regenerative braking? Normally, there are other accelerating motors or equipments served by the same supply bus. These loads use the energy returned during regenerative braking. However, if the line is non-receptive during regenerative braking, the line voltage increases and only a 20% increase in line voltage can be tolerated. So, during regenerative braking, the line voltage should be sensed continuously. If it exceeds a certain preset value, the regenerative braking should removed, and either friction braking or dynamic braking is to be applied.
Closed-loop Control of Chopper Drives
DC motors are widely used in many speed control drives. Open-loop operation of DC motors may not be satisfactory in many applications. However, if the drive requires constant speed operation, the PWM has to change to maintain a constant speed. This can be achieved in a closed-loop control system through attaching DC tachogenerator to the motor shaft. The actual speed signal will be fed back continuously and compares with the applied speed refer, the error-En of that will be used to control the armature voltage.
If the motor speed decreases due to application of additional load torque, or due to input voltage reduction or any other reason, the speed error-En increases, which increases the control signal EC. "This in turn changes the on signal of the chopper switch, and thus increases the motor armature voltage. An increase in the motor voltage develops more speed to restore the set speed of the drive system. The system thus passes through a variation until the developed speed matches the applied speed.
A closed-loop system is preferable and effective. It has the advantages of greater accuracy, improved dynamic response, and reduced effects of disturbances such as loading. When the drive requirements include rapid acceleration and deceleration, closed-loop control is necessary. In a closed-loop system, even the drive characteristics can also be modified.
Thus, the system can be made to operate at constant torque or constant horsepower over a certain speed range, which requires in traction systems. The chopper circuit protection like short circuit, overcurrent, overspeed, undervoltage, overvoltage, active current limit, earth fault and many more can be provided effectively in a closed-loop control system. In fact, most industrial drive systems operate as closed-loop feedback systems.
Current Control
The study of number of industrial applications that it would be beneficial to limit the current to some maximum allowable value. This objective can be achieved by using complex control circuit, where the maximum current drawn by DC motor can be adjusted to some present value. This can be done through in-built electronic current sensor, which continuously monitors the current drawn by motor and feedback to electronics comparator to keep the voltage to motor in such a manner that motor current does not exceed the present value.
So, actually both the speed controller and current controllers works together. Both P controllers and PI controllers are used for the speed and current control loops. For regenerative braking, current control loop is necessary.
Application
The chopper drives concept now finds a lot of applications other than in battery operated vehicles like glass industries, fertilizer industries and tyre manufacturing industries. Continuous process plants, where the motor should not stop even for a fraction of a second, use chopper DC drives with DC motor as a load drive system along with DC power source comprising of battery charger and battery on float conditions. This becomes an efficient and cost-effective solution for process needs. This becomes a reliable, simple and cost-effective alternative against the alternative of UPS system with AC drives and AC motor package. The same concept is being used in glass industries for a) Lehar drives for pulling the semi liquid glass sheet, b) for rolling machine (with 4 drives under synchronization with master-slave controls), which draws glass sheet from furnace, and which becomes a replacement for complex servo gearboxes.
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| Posted : 8/12/2005 |
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