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Speed Control Of A Separately Excited Dc Motor Engineering Essay

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Published: Mon, 5 Dec 2016

There are countless industrial applications that are currently using DC motor. The continuous development of dc motor is very essential for the industries. Speed control has been the most important factor in dc motor system. DC motor has a long line of history for providing excellent control of speed for acceleration and deceleration. DC motor, because of their simplicity, better reliability, ease of application, high efficiency and favourable maintenance cost are more preferred in industries compare to AC motor. Besides that, DC motors are capable of providing starting and accelerating torques much higher than the rated. There are two types of DC motor, Separately Excited and Self Excited. In this project speed control drive will be developed to control the speed of the separately excited dc motor. Traditionally rheostatic

 control method was widely used in the industries for the speed control power dc motors. However due to rheostatic

 control method just eligible to use in some low power dc motor, proportional integral-derivative (PID) controllers have been used as the replacement because of the desired torque-speed characteristic could be achieved. In recent years neural network controllers (NNC) were effectively introduced to improve the performance of nonlinear systems as the PID controllers require exact mathematical modeling, if there are any parameter variation, the performance of the system will be affected[1]. The application of NNC is very promising in system identification and control due to learning ability, massive parallelism, fast adaptation, inherent approximation capability, and high degree of tolerance. The limitation of NNC was the controllers not well suited for system with unstable inverses[2]. Most of the speed control of dc motor product in the market was using ac as power source. Due to all the limitations of the above mention speed control methods, this encouraged me to take up the challenge to develop a speed controller well suited to use in separately excited dc motor. In this project, Pulse Width Modulation (PWM) method will be use to control the speed of dc motor. Power electronic converter will also be used to convert ac power source to dc power (Controlled Rectifiers) and dc power source to dc power (DC Choppers).

1.2) Background

DC motor speed control has been widely used in many industrial applications because of the simplicity, ease of applications, reliability and favorable cost. Besides that, DC motor is considered a single input and single output system having torque/speed characteristics compatible with most mechanical loads.

1.3) Objectives

To design and fabricate a half-wave controlled rectifier and its firing circuit for the control of a separately-excited DC motor.

1.4) Aims

The aim of this project is to use power electronic converters to convert AC power source to DC power (Controlled Rectifiers), DC power source to DC power (DC Choppers) and control the speed for the single phase speed control of separately-excited DC motor.

Chapter 2 Literature Survey

2.1) Introduction

There is no exact date that can be traced back of modern day motor invention. However, motors find the most practical use in our everyday life in form of modern gadgets, machines, devices and appliances. It has been a gradual process with many prominent names and contributions from the scientific world. Separately excited dc motors (SEDCMs) are extensively used in industrial variable-speed drive applications. A SEDCM is basically represented by linear equations in the armature control region. Therefore, it is easy to apply the linear control techniques to the motor control system. In an operating condition where the full armature voltage has been applied and the operation demands for further increase in the speed, the field weakening is required to obtain speeds above the base speed [3].

2.2) Model of Separately Excited DC motor

When a separately excited motor is excited by a field current of If and an armature current of Ia flows in the circuit, the motor develops a back EMF and a torque to balance the load torque at a particular speed. The If is independent of the Ia. Each winding are supplied separately. Any change in the armature current has no effect on the field current. The If is normally much less than the Ia[4].

2.3) Single Phase Half-Wave Rectifier

VF = RFIF

Ea = KIF (t)

T = KIFIa

Combining the equation will get :

or

As seen from these equations, speed control can be done by varying Ra, Va, and IF. Varying Ra can only increase the speed. Also, by varying Ra, we increase the losses I2R. IF can only be decreased. This is accomplished by putting resistance in the field. Again, this increases

the I2R losses. Control of Vf is limited because of saturation. Besides that, Half-Wave Rectifier can act as an AC to DC converter. The DC output voltage can be controlled by varying the firing angle of the thyristors.

2.4) Unijunction Transistor (UJT) Relaxation Oscillator

The main function of UJT Relaxation Oscillator circuit is for triggering purposes. This circuit is ideally suited for triggering an SCR since UJT is capable of generating sharp, high powered pulses of short duration whose peak and average power don’t exceed the power capabilities of the SCR gate for which they are intended. When power is applied to the given circuit, capacitor C starts charging exponentially through R to the applied voltage VCC. The voltage across C is the voltage-Ve applied to the emitter of UJT. When C is charged to Vp, then UJT turns ON. This greatly reduces the effective resistance between emitter and base1 of UJT. A sharp pulse of current flow from base1 to emitter, discharging C through Rb1. When the capacitor voltage drops below Vp, UJT is brought back to the previous state and the capacitor again begins to charge towards Vbb. This produces a sawtooth wave[5].

2.5) DC Choppers

A dc chopper is a dc-to-dc voltage converter. It is a static switching electrical appliance that converts an input fixed dc voltage to a variable dc output voltage without inductive or capacitive intermediate energy storage. Chopper connects source to load and disconnect the load from source at fast speed so a chopped load voltage is obtained from a constant dc supply of magnitude Vs. During the period Ton, chopper is on and load voltage is equal to source voltage Vs. During the period Toff, chopper is off, load voltage is zero. In this manner, a chopped dc voltage is produced at the load terminals[6].

2.6) Control Technique Comparison

Technique

Pulse-Width Modulation (PWM)

Method

Process of switching the supply and load on and off at a very fast pace. It controls the motor speed by driving the motor with short pulses. These pulses vary in duration to change the speed of the motor. The longer the pulses, the faster the motor turns, and vice versa

Advantages

Power loss in switching devices is low.

Good load efficiency

produce more torque in a motor by being able to overcome the internal motor resistances more easily.

Reduce harmonics

Disadvantages

Reduction of available voltage

Fast controlling of speed cannot be used

Technique

Rheostatic Control

Method

Insert of external series resistance in the armature circuit. By increasing the resistance armature the voltage applied across the armature terminals will decrease, hence speed is reduced.

Advantages

Easy and smooth speed control below rated

Disadvantages

Power loss in heat

Difficult to achieve uniform speed

Limited to low power motor only

Technique

Proportional Integral-Derivative (PID)

Method

Proportionalcontrol makes the actuation by multiplying the current error. Integral control, takes into account the error history. Derivative control acts based on the rate of error, as it is an indicator of how the error is going to evolve.

Advantages

Desired torque-speed characteristic could be achieved

Disadvantages

Susceptible parameter variation

Poor performance in non-linear system

Noise in derivative

Can only handle single input and single output system

Technique

Neural Network Controller

Method

Mathematical modelling which consist of three layers.

Advantages

High degree of tolerance

Fast adaptation and inherent approximation capability

Disadvantages

Susceptible to unstable inverses system

There are two speed control techniques in separately excited DC motor, by varying the armature voltage for below rated speed and by varying field flux should to achieve speed above the rated speed. Based on the table of content above, there are several methods use to control the speed of DC motor such as Rheostatic

 control method, proportional integral-derivative (PID) controllers and Neural Network Controllers. Rheostatic

 control method has been replaced by many other application long ago because it is just suitable to use in low power motor. Rheostat incurs several drawbacks such as power loss and reliability. When Rheostatic operating in long hours, the resistance will heat up and the consumption of the electric energy turned into heat, a harmful rise in temperature occurring in the rheostat and will probably damage the circuit as electronics components are vulnerable to heat. The difficulty of realizing a uniform speed control is also one of the disadvantages. As for proportional integral-derivative (PID) controllers, exact mathematical modeling is used to prevent any errors from occurring. Although desired torque-speed characteristic could be achieved but this is not suitable to use in speed control of dc motor because the performance of the controller will be affected when there are any parameter variations. There are many advantages of Neural Network Controllers such as high degree of tolerance, fast adaptation and inherent approximation capability but vulnerable to unstable inverses system. By comparing with Rheostatic

 control method, PID and NNC, pulse width modulation (PWM) method were the better choice due to efficiency. By using PWM, power supply to the load without wasting as the controller will act like switch that is turned on and off rapidly to mimic the variable power supply. PWM also a proven method as it has been using in the industries for quite some time.

Chapter 3 Project Activities

3.1) Project Costing

Below shows the costing of this project :

Component

Quantity

Unit Price ( RM )

Total Price ( RM )

DC motor

1 no.

50.00

50.00

Thyristor

2 nos.

3.00

6.00

Resistor

20 nos.

0.10

2.00

Capacitor

5 nos.

0.10

0.50

Relay

1 no.

5.00

5.00

MOSFET

1 no.

5.00

5.00

Hardware

1 set

30.00

30.00

Switch

2 nos.

0.50

1.00

555 IC

2 nos.

4.00

8.00

741 IC

1 no.

2.00

2.00

TOTAL

109.50

3.2) Gantt Chart


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