3. DC Machines.
45
Chapter 2 DC Machines
description
lecturenote
Transcript of 3. DC Machines.
Chapter 2 DC Machines
DC Machines
DC Machines are motors that convert dc electric energy to mechanicalenergy and generators that convert mechanical energy to dc electrical energy
Construction
Stator of dc machines
Construction
Rotor of dc machines
Construction
dc machines with housing
Part of the dc machines Stator (Field): does not move, the outer frame of the machines and
its made from ferromagnetic materials.
Rotor (Armature): free to move, the inner part of the machine and its made from ferromagnetic materials.
Field winding: is wound on the stator poles to produce magnetic field (flux) in the air gap.
Armature winding: is composed of coils placed in the armature slots.
Commutator: is composed of copper bars, insulated from each other. The armature winding is connected to the commutator.
Brush: is placed against the commutator surface. Brush is used to connect the armature winding to external circuit through commutator.
Introduction to DC Motors Earliest power systems is dc systems. 1890s ac power systems were winning
out over dc power systems. Why dc motors so common, when dc
power systems were fairly rare? Reasons:-◦ Vehicle – has a dc power system, it make
sense to consider using dc motor.◦ Provide the desired speed control.◦ There are still some applications where dc
motor are preferred.
Speed Regulation (SR) DC Motor are often compared by their speed regulation.
, ,
,
, ,
,
SR= 100%
SR= 100%
m nl m fl
m fl
m nl m fl
m fl
n nn
ω ωω−
×
−×
A positive speed regulation means a motor’s speed drops with increasing load.
A negative speed regulation means a motor’s speed increase with increasing load.
Types of dc motors
The separately excited dc motor The shunt dc motor The series dc motor The compounded dc motor The permanent-magnet dc motor
The Equivalent Circuit of a DC Motor
A mE Kφω=
ind AK Iτ φ=
Internal generated voltage
Induced torque developed
The Magnetization Curve
mmf = F FN I
Separately Excited DC Motors
Shunt DC Motors
The Terminal Characteristic of a Shunt DC Motor
( )2T A
m indV RK K
ω τφ φ
= −
It is important to realize in order for the speed of the motor to vary linearly with torque, the terminal voltage supplied by the dc power source is assumed to be constant.
Motor’s Speed
A terminal characteristic of a machine is a plot of the machine’s output versus each other. For a motor, the output quantities are shaft torque and speed.
The Terminal Characteristic of a Shunt DC Motor If terminal voltage is not constant, then
the voltage variations will affect the shape of the torque-speed curve.
Another effect internal to the motor that can also affect the shape of the torque-speed curve is armature reaction. If a motor has armature reaction, then as its load increase, the flux-weakening effect reduce its flux.
The Terminal Characteristic of a Shunt DC Motor
If a motor has compensating winding,of course there will be no flux-weakeningproblems in the machine, and flux in the machines will be constant.
Nonlinear Analysis of a Shunt DC Motor
net ARF FN I= −F F
Magnetomotive force due to armature reaction (AR),
The equivalent field current of a shunt dc motor is given by,
* ARF F
F
I IN
= −F
Induced Voltage,
'A mE K nφ=
Internal generated voltage is related to speed by,
0 0
mA
A
nEE n
=
Speed Control of Shunt DC Motors
Common Method
1. Adjusting the field resistance FR
2. Adjusting the terminal voltage
Less Common Method
3. Inserting a resistor in series with the armature circuit
Speed Control of Shunt DC Motors1. Changing the field resistance
Speed Control of Shunt DC Motors2. Changing the armature voltage
Speed Control of Shunt DC Motors2. Changing the armature voltage
Speed Control of Shunt DC Motors3. Insert a resistor in series with the armature circuit
The Series DC Motor
ind AK Iτ φ=
The Terminal Characteristic of a Series DC Motor
Speed Control of DC Series Motors
Unlike with the shunt dc motor, there are only one efficient way to change the speed of a series dc motor. That method is to change the terminal voltage of the motor. If the terminal voltage is increase, the first term in Equation torque-speed relationship is increase, resulting in a higher speed for any given torque.
1 A STm
ind
R RVKcKc
ωτ
+= −
The Compounded DC Motor
( )T A A A SV E I R R= + +
A L FI I I= −
TF
F
VIR
=
The Torque-Speed Characteristic of a Cumulatively Compounded DC MotorThe torque-speed full-load rating
The Torque-Speed Characteristic of a Cumulatively Compounded DC MotorThe torque-speed no-load rating
The Torque-Speed Characteristic of a Differentially Compounded DC Motor
DC Motor Efficiency Calculations
Copper losses Brush drop losses Mechanical losses Core losses Stray losses
Introduction to DC Generators
Separately excited generator Shunt generator Series generator Cumulatively compounded generator Differentially compounded generator
* There is no real difference between a generator and a motor except forthe direction of power flow.
Power Flow Diagram for DC Machines
Equivalent Circuit DC Generator
Voltage Regulation
nl fl
fl
V VVR= 100%V−
×
The Separately Excited Generator
Terminal Characteristic Separately Excited Generator
With compensating windings
Without compensating windings
Control of Terminal Voltage1. Change the speed of rotation.
2. Change the field current.
, FF
F
A m
T A A A
VIR
E K
V E I R
φ
φ ω
= ↑↓
= ↑
= ↑ −
A m
T A A A
E K
V E I R
φω= ↑
= ↑ −
Effect of Decrease in Field Resistance on the Output Voltage
The Shunt DC Generator
Terminal Characteristic Shunt DC Generator
The Series DC Generator
Terminal Characteristic Series DC Generator
The Cumulatively Compounded DC Generator
Terminal Characteristic Cumulatively Compounded DC Generator
Thank [email protected]
