EEE373 Electric Motor Drive
Lecture 3
Asst. Prof. Dr. Mongkol Konghirun
EE, KMUTT
Separately Excited DC Motor
{
Equivalent circuit of separately excited
DC motor.
1
Separately Excited DC Motor
{
The independent control of magnitude flux and
electromagnetic torque.
φ ∝ if
Te ∝ ia
{
As a result, it is easy to generate varying
torque for a given’s speed and hence make
torque generation independent of the
operating speed.
In steady state,
{
Note: Ke = Kt, Unit: Ke (V/rad/sec), Kt (N.m/A)
{
e = K eφωm
Te = K tφ ia
Separately Excited DC Motor
{
For a given constant flux, φ then
e = K eωm
V = e + ia Ra ⇒
ωm =
⎛T
V −⎜ e
⎝ Kt
ωm =
Ke
Te = K t ia
e = V − ia Ra
e V − ia Ra
=
Ke
Ke
⎞
⎟ Ra
⎠ = V − ⎛ Ra ⎞ T
⎜
⎟ e
Ke ⎝ Kt K e ⎠
2
Separately Excited DC Motor
{
To increase speed, input voltage must
be increased.
{
As load torque increased for a given
input voltage, the speed is decreased.
ωm =
V ⎛ Ra ⎞
−⎜
⎟ Te
K e ⎝ Kt K e ⎠
Shunt Excited DC Motor
{
Equivalent circuit of shunt excited DC
motor.
3
Shunt Excited DC Motor
{
In state-state,
Te = K t i f ia
e = K e i f ωm
T
e
⇒ ia = K i
t f
V = e + ia Ra
{
For a constant input voltage V, ia is also
constant,
e = V − ia Ra = K ei f ωm
⎛ T
V −⎜ e
⎜ Kt i f
V − ia Ra
⎝
=
ωm =
K ei f
K ei f
⎞
⎟⎟ Ra
⎠
Shunt Excited DC Motor
⎛ T
V −⎜ e
⎜Ki
V − ia Ra
⎝ t f
=
ωm =
K ei f
K ei f
⎛ 1
⎝ Ke
ωm = ⎜
⎞⎛ V
⎟ ⎜⎜
⎠⎝ if
⎞
⎟⎟ Ra
⎠
⎞ ⎛ Ra ⎞ ⎛ Te
⎟⎟ − ⎜
⎟ ⎜⎜ 2
⎠ ⎝ Kt Ke ⎠ ⎝ i f
⎞
⎟⎟
⎠
Equivalently,
⎛ K t K ei 2f
Te = − ⎜
⎜ R
a
⎝
⎞
⎛ Kt i f
⎟⎟ ωm + ⎜
⎝ Ra
⎠
⎞
⎟V
⎠
4
Shunt Excited DC Motor
{
For a fixed DC input voltage, the
electromagnetic torque VS speed
characteristic of the shunt excited DC
motor.
Shunt Excited DC Motor
⎛ K K i2
Te = − ⎜ t e f
⎜ R
a
⎝
Te ↑, ia ↑,
{
⎞
⎛ Kt i f
⎟⎟ ωm + ⎜
⎝ Ra
⎠
⎞
⎟V
⎠
e ↓(small), ωm ↓(small)
The drop in speed from its no-load
speed is relatively small. Because of
this, this motor is considered a
constant-speed motor.
5
Shunt Excited DC Motor
{
In variable input voltage operation, an
independent control of field and
armature current is lost, leading to a
coupling of flux and torque production
in the motor.
{
Such a feature makes it unsuitable for
variable speed applications.
Series Excited DC Motor
{
Equivalent circuit of series excited DC
motor.
6
Series Excited DC Motor
{
In state-state,
e = K ei f ωm = K eiaωm
{
Te = Kt i f ia = Kt ia2
⇒
V = e + ia ( Ra + R f
)
ia =
Te
Kt
For a constant input voltage V,
e = V − ia ( Ra + R f ) = K e iaωm
Series Excited DC Motor
V −i (R + R )
ω =
a
m
a
f
K eia
⎛ V ⎞ ⎛ Ra + R f ⎞
=⎜
⎟−⎜
⎟
⎝ K eia ⎠ ⎝ K e ⎠
⎛ V ⎞ K t ⎛ Ra + R f ⎞
=⎜
−⎜
⎟
⎟
⎝ K e ⎠ Te ⎝ K e ⎠
⎛
⎜
⎝
ωm = ⎜ V
Kt
Ke
⎞ 1 ⎛ Ra + R f ⎞
−
⎟
⎟ T ⎜⎝ K e ⎟⎠
⎠ e
7
Series Excited DC Motor
{
For a fixed DC input voltage, the
electromagnetic torque VS speed
characteristic of the series excited DC
motor.
Series Excited DC Motor
⎛
⎜
⎝
ωm = ⎜ V
{
K t ⎞ 1 ⎛ Ra + R f ⎞
−⎜
⎟
⎟
K e ⎟⎠ Te ⎝ K e ⎠
Te ↑, ωm ↓
For starting torque ( ωm = 0 ),
⎛
Kt ⎞ 1
⎛ R + Rf ⎞
=⎜ a
⎜V
⎟
⎜ K e ⎟ T ⎝ K e ⎟⎠
⎝
⎠ e
Te =
V 2 Kt
(R
a
+ Rf
)
2
8
Series Excited DC Motor
{
There is also no independence
between the control of the field and the
armature currents.
{
The electromagnetic torque is
proportional to square of the armature
current.
Permanent-Magnet DC Motor
Stator (permanent-magnet)
Rotor
N
S
Example of permanent-magnet: ceramic, alnico,
and rare-earth varieties such as
samarium-co bolt and boron-ironneodymium magnets.
9
Permanent-Magnet DC Motor
{
{
„
„
Advantages:
Compactness of the filed structure
Elimination of resistive losses in the filed
winding
These features contribute to a compact
and cool machine, desirable features
for a high-performance motor.
EEE373 Electric Motor Drive
Lecture 3
Asst. Prof. Dr. Mongkol Konghirun
EE, KMUTT
10