Download TERMINAL CHARACTERISTIC of a SHUNT DC MOTOR

ENERGY CONVERSION
DC MOTORS AND GENERATORS
DC MOTORS AND GENERATORS
Summary
1. The Equivalent Circuit of a DC Motor
2. The Magnetization Curve of a DC Machine
3. Separately Excited and Shunt DC Motors
- The Terminal Characteristics of a Shunt DC Motor
- Nonlinear Analysis of a Shunt DC Motor
- Speed Control of Shunt DC Motors
- The Effect of an Open Field Circuit
4. The Permanent-Magnet DC Motor
5. The Series DC Motor
- Induced Torque in a Series DC Motor
- The Terminal Characteristic of a Series DC Motor
- Speed Control of Series DC Motors.
6. DC Motor Starters
- DC Motor Problems on Starting
- DC Motor Starting Circuits
7. Introduction to DC generators
8. Separately Excited Generator
- Terminal Characteristic of a separately Excited DC Generator
- Control of Terminal Voltage
- Nonlinear Analysis of a Separately Excited DC generator
DC MOTORS AND GENERATORS
INTRODUCTION
• The same physical dc machine can operate as either motor or a
generator & it depends on direction of power flow
• Introduction to DC motors:
• dc motors have a significant fraction of machinery purchased
each year through 1960s
• Reasons: existence of dc power system in cars, trucks and
aircraft
• Another application: when wide variations in speed are needed
• Before widespread use of power electronic rectifier-inverters, dc
motors were dominant means of speed control
• Even without a dc power source, solid-state rectifier & chopper
circuits used to create necessary dc power & dc motors used to
provide speed control
• Today induction motors with solid-state drive packages are
preferred choice over dc motors for most speed control
applications, while still in some applications dc motors preferred
DC MOTORS AND GENERATORS
INTRODUCTION
• DC motors are compared by their speed regulation:
SR= [ωnl-ωfl]/ωfl x 100%
• It is a rough measure of shape of motor’s torque-speed
characteristic
• A positive regulation means speed drops with increasing load &
a negative speed regulation means speed increases with
increasing load
• Magnitude of S.R. approximately show how steep is the slope
of torque-speed
• Dc motors driven from a dc power supply (unless specified) and
input voltage assumed constant)
• Five major types of dc motor:
1- separately excited dc motor 2-shunt dc motor
3-permnent-magnet dc motor 4- series dc motor
5-compounded dc motor
DC MOTOR
EQUIVALENT CIRCUIT
• Figure below shows a dc motor equivalent cct.
• Armature cct. represented by an ideal voltage source
EA & a resistor RA
• This is thevenin equivalent of entire rotor, including
coils, interpoles & compensating windings
• Brush voltage drop represented by a small battery
Vbrush opposing direction of current flow
DC MOTOR
EQUIVALENT CIRCUIT
•
•
A simplified equivalent circuit eliminating the brush
voltage drop and combining Radj with the field
resistance shown in (b)
Some of the few variations and simplifications:
1- brush drop voltage is often only a very tiny fraction
of generated voltage in the machine. where it is not
too critical, brush drop voltage may be left out or
included in the RA.
2- internal resistance of field coils is sometimes
lumped together with variable resistor and total is
called RF
3- Some generators have more than one field coil,
all of which appear on the equivalent circuit
DC MOTOR
EQUIVALENT CIRCUIT
• The internal generated voltage is given by:
EA = K φω
• and the torque induced is
ind = K φ IA
The Magnetization Curve of a DC Machine
- EA is directly proportional to flux and the speed
of rotation of the machine
- EA is therefore related to the field current
- field current in a dc machine produces a field
mmf given by mmf=NFIF
DC MOTOR
EQUIVALENT CIRCUIT
• mmf produces a flux in the machine in
accordance with its magnetization curve
DC MOTOR:EQUIVALENT CCT
• Since If is proportional to mmf & since EA is
proportional to flux, magnetization curve can
represented as a plot of EA vs field current for a
given speed ω0
SEPARATELY EXCITED AND SHUNT
DC MOTORS
• Equivalent cct. of separately excited dc motor
shown below
SEPARATELY EXCITED AND SHUNT
DC MOTORS
• separately excited dc motor is a motor whose
field cct. is supplied by another constantvoltage supply
• shunt dc motor is a motor whose field circuit
gets its power directly from armature terminals
of motor
• When supply voltage to a motor assumed
constant, there is no practical difference in
behavior between these two machines
• Kirchhoff’s voltage law KVL equation for
armature cct. of these motors is: VT=EA+IARA
TERMINAL CHARACTERISTIC of a
SHUNT DC MOTOR
• Terminal characteristic of a motor is a plot of
output torque versus speed
• If load on shaft of a shunt motor is increased,
then load torque Tload exceed induced torque
Tind & motor will start to slow down
• & Its internal generated voltage EA=Kφω
decrease
• Then IA= (VT-EA)/ RA increases
• consequently Tind=KφIA increases & finally Tind
will equal Tload at a lower mechanical speed
TERMINAL CHARACTERISTIC of a
SHUNT DC MOTOR
• O/P characteristic of shunt dc motor can be
derived using Tind, EA equations & KVl
• Combing these three equations:
VT=EA+IARA  VT=Kφω+IARA
& IA = Tind /(Kφ)  VT=Kφω+ Tind /(Kφ) RA
 ω = VT / (Kφ) - Tind/(Kφ)^2 RA
• This equation is a straight line with a negative
slope
TERMINAL CHARACTERISTIC of a
SHUNT DC MOTOR
• Torque – speed characteristic of a shunt or
separately excited dc motor
TERMINAL CHARACTERISTIC of a
SHUNT DC MOTOR
• Armature reaction affect the torque speed
characteristic
• As shown in last slide, as load increase, flux
weakening effect reduce the flux in shunt motor
• And according to speed equation, reduction in
flux will increase speed
• If a motor has compensating winding, then
there would be no flux weakening & flux remain
constant