In the following discussions, we will see through different closed loop configurations which are used in electrical drives irrespective of the type of supply they are fed, i.
Control of Electrical Drives 3rd Ed
During the starting, we know if precautionary measures are not taken there is a chance of huge current flow through the motor circuit. To limit the current and sense the current fed to the motor, current limit controller is installed. The feedback loop does not effect the normal operation of the drive but if the current exceeds the predetermined safe limit, the feedback loop activates and the current is brought down below the safe limit.
Once the current is brought down below the safe limit the feedback loop again deactivates and in this way the control of current takes place. This type of torque controller is seen mainly in battery operated vehicles like cars, trains etc. Speed control loops are perhaps the most widely used feedback loops for drives.
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If we first see the block diagram of this loop then it will be a lot easier for us to understand. We can see from the diagram that there are two control loops, which can be said as an inner loop and outer loop.
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The inner current control loop limits the converter and motor current or motor torque below the safe limit. Now we can understand the function of the control loop and drive by practical examples. Now the inner loop increases the current keeping it under maximum allowable current.
This signal closes the speed-control loop. The feedback is compared with reference setpoint and fed to the differential error amplifier, ad infinitum. Motor speed will remain constant until the speed setpoint changes. In general, the speed of a D. Since the field flux is kept constant below base speed, the motor speed can be varied by increasing or decreasing the armature voltage.
This is done by adjusting the variable voltage produced at the output of the SCR bridge drive phase controlled rectifier or the average variable voltage produced by that SCR bridge that controls the duty cycle of the D. When the maximum output voltage of the converter is reached, additional speed can be achieved by reducing the field flux. This is called field weakening. In field weakening, the speed range is usually limited to about to ensure stability and good motor commutation.
The motor has full torque over the normal speed range and even at standstill. In field weakening, torque falls in proportion to speed but the output power remains constant. Selecting or sizing an electronic D.
Capital costs vary with rating of the drive. Larger drives with more performance features will cost more. By way of comparison, A. Efficiency is the ratio of output power to input power.
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But, controlled SCR rectifiers or D. However, for low impedance power supplies, input chokes or isolating transformers will need to be added on the D. Information needed for fast response. Simple and easy speed control methods.
Electric braking can be applied easily. Pollution free. Wide range of speed, power, torque ratings. Efficiency is higher. Short time overload capacity. Function a variety of work environments, such as explosive, radioactive and submerged. Self-starting—no need of external starting equipment. Compared to hydraulic and diesel prime movers, its operation is cleaner, less noisy and less maintenance. Types of D.
Modeling, Simulation and Control of Electrical Drives
Motor Control There are three general types of D. Functions of D. Motor Control Whether a D. Stopping There are three forms of stopping: coasting, braking or a deceleration ramp. It is accomplished through a control switch or via an electronic drive controller and entails reversing the polarity of the armature connections, which can be done with a push button control of mechanically interlocked forward and reverse buttons or programming the electronic drive controller for reverse operation 13 Speed Control A D.
Drives An electronic D. Capable of full torque at standstill without a clutch. Very large speed range without needing gearboxes. Clean operation. Safe operation in hazardous environments. Immediate use no warm up time Low no-load losses. Low acoustic noise.
Excellent control ability. Four-quadrant operation: forward motoring, forward braking, reverse motoring and reverse braking. The advantages 27 of a closed-loop system are: More accurate and reliable Responds to environmental changes Effects due to non-linearity and distortion are reduced Preferred when disturbances and variations are unpredictable A closed-loop, speed-control system 28 consists of reference circuit the speed setpoint , a differential error amplifier, a firing pulse generator, an SCR bridge and a speed feedback signal from a tachometer or an encoder.
C Drive Selection Selecting or sizing an electronic D. Recent Posts What is the average AC induction motor life expectancy? What is the difference between an AC motor and a DC motor? What are thermistors and how do they limit inrush current? What is electrical torque and mechanical torque? Motor Sales Inquiry Form.
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