19222 14 dc machine control

8/3/2019 19222 14 Dc Machine Control

1/18

1

DC Machine ControlDr Andrew Cruden

Electronic & Electrical Engineering

University of Strathclyde

R: R3.08H

E: [email protected]

T: 0141-548-2991

19222 Electrical Machines & Control


2/18

219222 DC Machine Control

Overview of Presentation

Instill an understanding of the typicalmethods and equipment used to control(start, stop and regulate) DC motors,

including relays and thyristorsSummary

See accompanying notes entitled 19222DC Machine Control


3/18


Quick Review

There are 2 main types of DC motor: series and

shunt motors

Shunt motors have been shown to be virtually

constant speed motors w.r.t. the applied loadSeries motors have been shown to be variable

speed, high starting torque motors

Typical applications for both motors have beengiven


4/18419222 DC Machine Control

DC Machine Control

The DC motor can be accelerated and decelerated

quickly and easily, and the speed can also be accuratelycontrolled over a wide range by, amongst othermethods, by varying the applied voltage to the motor

However most industrial factories and premises aresupplied with AC power therefore conversion to DC isnecessary to utilise DC machines

Additionally DC motors are generally more expensivethan AC machines and these two factors have promotedincreased use of AC motors and variable speed drives

rather than use of a DC



Basic Motor Control -

Starting!

The first aspect of DC motor control is to successfully start them!

Remember from previous that at starting ( = 0rad/s) that theback emf, E, will equal zero volts (there can only be an emfwhen the motor armature is turning)

If E = 0, then the armature current will be very high (limitedonly by the armature resistance which is typically quite small)then precautions must be taken to prevent motor damage

Usually small motors, say up to 1.5kW, can be direct-on-line(DOL) started, whereas large motors will typically employ eithervariable resistance or reduced voltage starting



Variable Resistance

The armature resistance can be varied manually, howevergenerally this is not viable

More appropriate automatic regulation of armature resistance canbe achieved using various relays, operated either by the back emfitself or on a definite time basis

1CR11M1+

OL

Resistor

2CR1

1M2

Start Stop

2CR

ARM __field

series

1M

1CR

OL

normally closed

normally open

Relay

1M

1M2

1M1

Symbols



Relay Operation

When the relay drive coil (symbol

1M) experiences current flow, amagnetic field is establishedattracting the relay contact tomove.

This moving contact (either 1M1or 1M2) will either open or closedepending on its initial condition

The control circuit (supplying thedrive coil) may be completelyseparate from the contact circuit,

hence isolation can be achieved

normally closed

normally open

Relay

1M

1M2

1M1

Symbols

Drive Coil

Contact



Relay Operation - example

Start switch pressed

Current flows to 1M

1M1 closes current flows to

resistor and motor armature

1M2 closes start switch canbe released and is bypassed by1M2

As back emf, E, increases as

speed increases, 1CR operates

1CR1 closes and half of resistoris bypassed, therefore motorsupply voltage increases andmotor speeds up until 2CR

operates 2CR1 closes, resistor is

completely bypassed andmaximum supply voltagereaches motor. Maximum

motor speed achieved andmotor has started in controlledfashion

1CR1

shunt field

1M1+

OL

Resistor

2CR1

1M2

Start Stop

2CR

ARM

fieldseries

1M

1CR

OL



Voltage Controlled Relays

(1CR & 2CR)

Relays 1CR and 2CR are specially

controlled relays in the previous exampleThe contacts (1CR1 and 2CR1) only

operate when the voltage across theirdrive coils (1CR & 2CR) are above userdefinable threshold values

Contacts 1CR1 & 2CR1 behave likevoltage controlled switches

ar a e es s ance



ar a e es s ance -contd

The extra resistance added into the armature circuit at start up

(marked Resistor) is at a maximum at start up (i.e. zero speed) As the motor speeds up the two voltage sensitive relays (1CR &

2CR) progressively operate and switch out part, then all, of theextra resistance out of the armature circuit

These relays work as the back emf across the motor builds up,first 1CR operates and shorts out part of the resistor, then asthe speed and emf increase further, 2CR operates and shorts

out the complete resistor. Relay 1M operates as a latching relay and a thermal overload

(O/L) relay is included

Definite time relays are essentially timers - the relay changesstate after a definite period of time

Th l O l d R l



Thermal Overload Relay

(bimetallic switch)

A bimetallic strip commonly acts as a thermal overload protection on a motor.

Two strips of different metals are heated by current passing through them. The heat causes the

metals to expand and, due to the different metals, the expansion causes the fused strip to curve. This curvature draws the bimetallic strip away from a set of contacts and hence breaks the electrical

circuit.

Source: Wikipedia

C C


12/18


Reduced Voltage AC to DC

power electronics

The voltage to a DC motor may be altered by a variety ofmeans - a potentiometer or by power electronic means

A typical DC shunt connected motor, and the AC/DC conversionequipment used to power it from the mains is shown below.

The armature voltage may be adjusted via controlling thyristor,

T

MV D

T

Gate

E

RL I

field

P El t i


13/18


Power Electronics -

Thyristors A thyristor is a controllable diode - when a thyristor starts to conduct may becontrolled by means of a positive pulse to the gate relative to the cathode

Once the thyristor is turned on it will turn off only when the current through it

has reduced to zero (and remained at zero long enough for it to return to theblocking state) - similar to idea of a latching relay

By varying the firing angle, for an AC supply, the output DC voltage may bevaried

cathode

AC waveform

lower equivalent DC

signal due to thyristoaction at 90 degrees

Vpeak

2

Vpeak

anode

Gate

equivalent DC signal(the rms value)

0

degrees

180

Output for conduction

Vpeak

Vo

lta

g

0at 90

P El t i


14/18


Power Electronics -

Thyristors (contd) One problem with thyristors is that they can not be

turned off by a control pulse This has led to the development of a GTO - Gate TurnOff thyristor which can be both turned on and off by

separate applied pulses There are alternative power electronic devices available

to facilitate AC to DC conversion - IGBT (Insulated Gate

Bipolar Transistor) as well as power MOSFET (MetalOxide Semiconductor Field Effect Transistor) that arebecoming increasingly popular as replacements for low -

to medium power thyristors

Red ced Voltage DC to DC


15/18


Reduced Voltage - DC to DC

power electronics A chopper circuit is one where

the applied DC voltage isvaried by a simple powerelectronic switch

As the time the switch is on is

varied the resultant appliedvoltage is varied

Generally, for motor

applications, there is sufficientinductance in the circuit toenable current to continue toflow even during the off

periods

'Chopped' Voltage

Corresponding curre

'Chopped' Voltage

Corresponding curre

V

Power ElectronicSwitching Device

M


16/18


Motor Control

Motor speed control can also be implemented through

the various techniques discussed in this lectureMost commonly field weakening for a shunt connected

DC motor can be achieved using a power electronic

device (e.g. a chopper) to allow speed controlMotor direction can be changed (reversed) by changing

the polarity of either the field or armature

Typically the current in either the field or armature isreduced to zero, a changeover contactor is operated,and then the current is increased again to operate the

motor in the reverse direction

M t C t l B ki


17/18


Motor Control - Braking

Braking is a very important motor control!!

The motor may be left to coast to a stop, braked using mechanical

means or either employ dynamic braking or regenerative braking Dynamic braking dumps the motors energy via a resistor placed across

the armature as heat

Regenerative braking feeds the motors kinetic energy back into the

supply - it is generating

1M1

__

Stop

1M4

Start

Shunt Field

O/L

+

Dynamic Braking

Resistor

1M O/L

Variable Field resistance

1M3

M

1M2


18/18


Summary of Presentation

Studied variable resistance starting usingrelays

Provided overview of relays

Studied reduced voltage starting usingboth ac/dc and dc/dc power electronics

Studied braking of motors using relays

19222 14 dc machine control

Documents