Electrical drives have become one of the most essential components in electric motors and other rotating machines today. We know that electrical drives mainly fulfill three types of functions,
- Starting.
- Speed control.
- Braking.
It can be said that electric drives enable us to control the motor from every aspect. But the control of electrical drives is also important because all the functions performed by the drives are basically transient operation i.e. the variation in terminal voltage, current etc. is very high which can damage the motor temporarily or permanently.
Hence the need to control the drives increases and there are various methods and devices to control the various parameters of the drives which are discussed later.
Closed Loop Control of Drives:
In control system, there are two types of systems, one is open loop and the other is closed loop control system. We know In open loop control system, the output has no effect on the input, i.e. the controlling phenomenon is independent of the output, on the other hand, closed loop control system is more advanced and scientific, here the output is connected to the input terminal. is fed. Determines the amount of input to the system, for example if the output is greater than a predetermined value then the input is reduced and vice versa. Feedback loops or closed-loop control in electrical drives fulfill the following requirements.
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Protection.
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Enhancement of speed of response.
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To improve steady-state accuracy.
In the following discussion, we will go through the various closed-loop configurations that are used in electrical drives, regardless of the type of supply they are supplied with, i.e. DC or AC.
Current Limit Control:
During starting, we know that there is a possibility of excessive current flowing through the motor circuit if precautions are not taken. A current limit controller is installed to limit the current and sense the feed current to the motor. The feedback loop does not affect the normal operation of the drive but if the current exceeds a predetermined safe limit, the feedback loop is activated and the current is brought down below the safe limit. Once the current is brought below the safe limit, the feedback loop is deactivated again and thus the current is controlled.
Closed Loop Speed Control:
Speed control loops are probably the most commonly used feedback loops for drives. If we first see the block diagram of this loop, it will be very easy for us to understand it.
We can see from the diagram that there are two control loops, which can be called inner loop and outer loop. An internal current control loop limits the converter and motor current or motor torque below a safe limit. Now we can understand the function of control loop and drive with practical examples. Suppose that the reference speed Wm* increases and there is a positive error ΔWm, indicating that the speed needs to be increased.
Now the inner loop increases the current keeping it below the maximum allowable current. And then the driver accelerates, when the speed reaches the desired speed the motor torque is equal to the load torque and the reference speed decreases to Wm indicating that no further acceleration is required but slow down. must occur, and braking is performed by the speed controller at the maximum allowable current. So, we can say that while controlling the speed the function is transferred from motoring to braking and from braking to motoring continuously for smooth operation and running of the motor.
