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Post: #1


.ppt  ELECTRIC DRIVES.ppt (Size: 1.25 MB / Downloads: 197)

Electrical Drives

About 50% of electrical energy used for drives
Can be either used for fixed speed or variable speed
MEP 1523 will be covering variable speed drives

Components in electric drives

Control unit

Complexity depends on performance requirement
analog- noisy, inflexible, ideally has infinite bandwidth.
digital – immune to noise, configurable, bandwidth is smaller than the analog controller’s
DSP/microprocessor – flexible, lower bandwidth - DSPs perform faster operation than microprocessors (multiplication in single cycle), can perform complex estimations
Electrical isolation between control circuit and power circuit is needed:
Malfuction in power circuit may damage control circuit
Safety for the operator
Avoid conduction of harmonic to control circuit


Sensors (voltage, current, speed or torque) is normally required for closed-loop operation or protection
Electrical isolation between sensors and control circuit is needed for the reasons previously explained
The term ‘sensorless drives’ is normally referred to the drive system where the speed is estimated rather than measured.

Overview of AC and DC drives

AC motors used in high performance applications – elevators, tractions, servos
AC motors favorable than DC motors – however control is complex hence expensive
Cost of microprocessor/semiconductors decreasing –predicted 30 years ago AC motors would take over DC motors

Elementary principles of mechanics

As the motor speed increases, the kinetic energy also increases. During deceleration, the dynamic torque changes its sign and thus helps motor to maintain the speed. This energy is extracted from the stored kinetic energy:

Thermal considerations

Electrical machines can be overloaded as long their temperature does not exceed the temperature limit
Accurate prediction of temperature distribution in machines is complex – hetrogeneous materials, complex geometrical shapes
Post: #2
Post: #3
Electrical drives have become the most essential equipment now days in the electrical motors and other rotating machines. We know that electrical drives mainly accomplishes three kinds of work,


Speed control


It can be said that the electrical drives enable us to control the motor in every aspect. But control of electrical drives is also necessary because all the functions accomplished by the drives are mainly transient operations i.e the change in terminal voltage, current, etc are huge which may damage the motor temporarily or permanently. That’s why the need of controlling the drives rises and there are various methods and equipment's to control different parameters of the drives which are discussed later.

Closed Loop Control of Drives

In a control system, there are two types of systems, one is open loop and the other is closed loop control system. 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 much more advanced and scientific, here the output is fed back to the input terminal which determines the amount of input to the system, for example if the output is more than predetermined value the input is reduced and vice-versa. In electrical drives feedback loops or closed loop control satisfy the following requirements.


Enhancement of speed of response

To improve steady –state accuracy

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.e DC or AC.
Current Limit Control

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.

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