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Control of drive systems

DC motors are widely used in many speed-control drives. Open-loop operation of DC motors may be satisfactory in many applications. When the load increases the speed of the motor drops and the new operating point of speed is obtained after the transient.

Lecture notes in electrical drives and traction systems for students (7th semester B.Tech)
Lecture notes in electrical drives and traction systems for students (7th semester B.Tech) – on photo: comsiderbrescia.it; credit: comsider srl

For getting constant speed i.e. the initial operating point the open loop does not work. So, closed-loop control system is required. The basic block diagram of closed-loop control system is shown.

If the motor speed decreases due to application of additional load torque, the speed error εN increases, which increases the control signal Ec . This in turn changes the firing angle of the converter, and thus increases the motor torque to restore the speed of the drive system.

The system passes through a transient period until the developed torque matches the applied torque. A closed-loop system improves the dynamic response specially during acceleration, deceleration and disturbances such as loading in drive system.

The response of a closed –loop system can be studied by using transfer function techniques.

Basic block diagram of a closed-loop speed control system
Figure 1 – Basic block diagram of a closed-loop speed control system

Separately Excited DC motor Drives

Armature voltage control is inherently a closed loop control system in DC motor drives. However, the output speed signal can not be measured and the speed error is not found properly.

This closed loop is further extended by using a feed back tachogenerator with speed controller and converter for modern control drives.


Solid State Control

DC motor speed control by solid state can be done by two methods: DC-DC Chopper control and Phasr rectifier control method.


Chopper control of Dc motor drive: (separately excited)

This is one of the simplest power-electronic/machine circuits. With a battery, it is currently the most common electric road vehicle controller. The ‘chopper‘ is also used for some d.c. rail traction applications.

The principal difference between the thyristor-controlled rectifier and the chopper is that in the former the motor current always flows through the supply, whereas in the latter, the motor current only flows from the supply terminals for part of each cycle.

Chopper control of Dc motor drive
Chopper control of Dc motor drive

The chopper may use transistor, thyristor, MOSFET or IGBT as switches.

A single-switch chopper using a thyristor can supply positive voltage and current to a DC motor, and is therefore restricted to quadrant 1 motoring operation. When regenerative and/or rapid speed reversal is called for, more complex circuitry is required, involving two or more power switches.

When the motor voltage is less than the battery, the step down chopper is used and when the motor voltage is greater than the battery voltage, a‘step-up’ chopper using an additional inductance as an intermediate energy store is used.

Function:

Vdc = V,CH1 on
Vdc = 0,CH1 off
D1 on

Title:Lecture notes in electrical drives and traction systems – A course in 7th semester of Bachelor of technology programme in electrical engineering by Department of Electrical Engineering Veer Surendra Sai University of Technology, Burla
Format:PDF
Size:2.00 MB
Pages:65
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Lecture notes in electrical drives and traction systems
Lecture notes in electrical drives and traction systems

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