3 Phase-controlled DC motor drives
• 3.1 Introduction
• Two types of speed control: armature
control and field control
3.2 Principles of DC motors speed Control
• The induced voltage is dependent on the
field flux and speed
e = Kφfωm
• The flux is proportional to the field current
φf ∝ if
• The speed is expressed as
ωm ∝
e
e ( v − ia R a )
∝ ∝
φf if
if
• The rotor speed is dependent on the applied
voltage and field current.
• In field control, the applied armature
voltage v is maintained constant.
• Then the speed is represented as
ωm ∝
1
if
• In armature control, the field current is
maintained constant. Then the speed is
derived as
ωm ∝ ( v − ia R a )
• Hence, varying the applied voltage changes
speed. Reversing the applied voltage changes
the direction of rotation of the motor.
• The advantage: the field time constant is at
least 10 to 100 times greater than the armature
time constant.
• Armature control is idea for speeds lower
than rated speed; field control is suitable
above for speeds greater than the rated speed.
• Fig. 3.1
• By combining armature and field control, a
wide range of speed control is possible.
• The relationship between armature current
and torque
Te = Kφfia
• The rated torque
Ter = Kφfriar
• The normalized version
φf ia
Te
Kφf ia
=
= ( )( ) = φfn ian , p.u.( per unit )
Ten =
φfr iar
Ter Kφfr iar
• Similarly, the air gap power is
Pan = enian, p.u.
where en is the normalized induced emf.
• As ian is set to 1 p.u., the normalized air gap
power becomes
Pan = en, p.u.
• The steady-state power output is kept from
exceeding its rated design value, which is 1
p.u.
• The air gap power constrains the induced emf
and flux field as
Pan = 1 p.u. = enian = φfnωmnian
• If ian is equal to 1 p.u., then
φfnωmn = 1 φfn = 1/ωmn
• Hence, the normalized induced emf is
en = 1
• In the field weaken region, the power output
and induced emf are maintained at their rated
values by programming the field flux to be
inversely proportional to the rotor speed.
3.2.5 Four-quadrant operation
• During a steady speed of ωm, stoping a machine
need to slow down at zero speed first.
• Four-quadrant dc motor drive characteristic
function Quadrant Speed
Torque
Power
FM
I
+
+
+
FR
IV
+
RM
III
+
RR
II
+
-
• Fig. 3.4 shows four-quadrant torque-speed
characteristics
• Fig. 3.5 illustrates the speed and torque
variation.
• Converter requirements: The voltage and
current requires four quadrant operation.
• The relationship between armature voltage and
armature current
Operation Speed Torque Voltage Current Power Output
FM
FR
RM
RR
+
+
-
+
+
+
+
-
+
+
+
+
-
• Two basic methods by using static converter
The First method: using phase-controller
converter to converter the ac source voltage
directly into a variable dc voltage.
The second method: Ac source voltage → fixed
dc voltage → variable dc voltage.
• Thyristor devices:
SCR, Transistors, GTOs, MOSFETs, ect.
3.3 Phase-controlled converter
• Fig 3.6 is a single-phase controlled-bridge
converter (positive average value).
• The bridge conduction is delayed in latter
beyond positive zero crossing.
• The delay angle is measured from the zero
crossing of voltage waveform and is
generally termed α.
• Thus, this voltage is quantified as
1 α+ π
2 Vm
Vdc = ∫α Vm sin( ωs t )d ( ωs t ) =
cos α
π
π
• Fig. 3.7 is the controlled-converter operation
with negative average voltage (α > 90º).
• In the case that the load current is discontinuous, the
average output is
Vdc =
1 α+ γ
Vm
V
sin(
ω
t
)
d
(
ω
t
)
=
[cos( α) − cos( α + γ )]
∫α
m
s
s
π
π
where γ is the current conduction angle.
• For certain value of γ, the output voltage for
discontinuous conduction can be greater than that
for continuous conduction.
• For example, let α+γ=π, and α = 30°
Vm
V
[cos α − cos(α + γ )] = m 1.866
π
π
2V
2V 1.732 1.732Vm
=
Vdc (con ) = m cos α = m
2
π
π
π
Vdc (dis) =
• The source inductance can be introduced to
reduce the rate of rise of current in the
thyristors.
• If the source inductance is Lls, the voltage
lost due to it is
Vx =
1 α +µ
Vm
V
sin(
ω
t
)
d
(
ω
t
)
=
[cos α − cos( α + µ)]
∫α
m
s
s
π
π
where , the overlap conduction period is
µ = cos−1[cos α −
πωs Lls Idc
]− α
Vm
• Fig. 3.10 is a three-phase thyristor-controlled
converter.
• The thyristor requires small reactors in series
to limit the rate of current rise, and
snubbers, which are resistors in series with
capacitors across the devices, to limit the rate
of voltage rise.
• The transfer characteristic of the three-phase
controlled rectifier is derived as
1 2 π / 3+ α
3
Vdc =
∫π / 3+ α Vm sin( ωs t )d ( ωs t ) = Vm cos α
π/3
π
• The characteristic is nonlinear (as shown in
Fig. 3.13).
• A control technique to overcome this
nonlinear characteristic is that the control
input to determine the delay angle is
modified to be
α = cos−1(
vc
) = cos −1( v cn )
Vcm
where vc is the control input and Vcm is the
maximum of the absolute value.
• Then the dc output voltage is
⎡ 3 Vm ⎤
3
3
3
−1
Vdc = Vm cos α = Vm cos(cos v cn ) = [ Vm ]v cn = ⎢
⎥vc = Kr vc
π
π
π
π
V
cn ⎦
⎣
• Fig. 3.14 is a schematic of a generic
implementation.
• The maximum delay angle is usually set in
the range from 150 to 155 degree.
Control modeling
• The gain of the linearized controller-based
converter is
Kr =
1.35V
,
Vcm
• The converter is a sampled-data system.
The sampling interval gives an indication of
its time delay.
• The delay may be treated as one half of this
interval
Tr =
60 / 2
1 1
× ( time period of one cycle) = ×
360
12 fs
• The converter is then modeled with
G r (s) = K r e − Tr s
• The above equation can also be
approximated by as
G r (s) =
Kr
(1 + sTr )
• For the case that the transfer characteristic
is nonlinear, the gain of the converter is
obtained as a small-signal gain by
δVdc
δ
Kr =
=
{1.35V cos α} = −1.35 sin α
δα
δα
• Fig. 3.15 is current source converter.
• Fig. 3.16 current source operation.
• Fig. 3.17 is a half-controlled converter.
• Fig. 3.18 is the converter with freewheeling
diode.
• Fig. 3.20 shows the converter configuration
for a four-quadrant dc motor drive.
3.4 Steady-state analysis ...
• Average values: The steady-state performance
is developed by assuming that the average
values only are considered.
• The armature voltage equation
va = Raia + e
• Termed by average values
Va = RaIa + KΦfωmav
• Average electromagnetic torque is
V − KΦ f ωmav
1.35V cos α − KΦ f ωmav
Tav = KΦ f Ia = KΦ f { a
} = KΦ f {
}
Ra
Ra
• The normalized electromagnetic torque
T
Tav
KΦ f {1.35V cos α − KΦ f ωmav } [1.35V cos α − KΦ f ωmav ]
=
=
Ten = av =
Φ fn
Ter KΦ fr Iar
KΦ fr Iar R a
Iar R a
Ten =
[1.35Ven cos α − Φ fn ωmn ]
Φ fn , p.u.
R an
• Positive and motoring torque is produced
when
cos α >
Φ fn ωmn
1.35Vn
• Steady-state solution including harmonics
accurately predict its electromagnetic torque
• The speed of the machine and the field
current are assumed to be constant. Then
R a i a + La
dia
+ K bωm = v a
dt
where va = Vmsin(ωst +π/3+α), 0<ωst<π/3
• The induced emf is a constant under the
assumption of constant speed, hence its
solution is
V
E
ia ( t ) = ( m ){sin( ωs t + π / 3 + α − β) − sin( π / 3 + α − β)e − t / Ta } − ( )(1 − e − t / Ta ) + iaie − t / Ta
Za
Ra
where ωs = 2πfs, β = tan-1(ωsLa/Ra) = machine
impedance angle, Ta=La/Ra = armature time
constant, iai = initial value of current at time t =
0, Za = Ra+jωsLa= motor electrical impedance.
• Critical triggering angle αc: when the armature
current is barely continuous (iai = 0).
αc = β + cos−1{
( E / Vm ) 1
π
⋅
⋅ (1 − e −( π / 3 tan β ) )} − + θ1
c1
cos β
3
• When the current becomes discontinuous, the
voltage across the machine the is the induced
emf itself.
• The steady state is
R a i a + La
dia
+ E = Va , 0 < ωs t < ωs t x
dt
ia = 0, ωs t x < ωs t < π / 3
Va = Vm sin(ωs t + π / 3 + α), 0 < ωs t < ωs t x
= E, ωs t x < ωs t < π / 3
• Fig 3.24
3.5 Two-Quadrant three-phase convertercontrolled DC motor drive
• Fig. 3.26 is speed-controlled two-quadrant
dc motor drive
3.6 Transfer functions of the subsystems
• Fig. 3.27 is DC motor and current-control
loop.
• Fig. 3.28 is step-by-step derivation of a dc
machine transfer function.
• Converter (after linearization)
V (s )
Kr
Gr = a
=
Vc (s) 1 + sTr
• The current and speed controllers of
proportional-integral type are
K c (1 + sTc )
sTc
K (1 + sTs )
G s (s ) = s
sTs
G c (s ) =
• The gain of current feedback is Hc.
• The transfer function of the speed feedback
filter is G ω (s) = Kω
1 + sTω
3.7 Design of controllers
• Fig 3.29 is the block diagram of the motor drive
• Fig 3.30 is current-control loop.
• Speed controller: Fig. 3.32 is the representation
of the outer speed loop in the dc motor drive.
• The closed-loop transfer function of the
speed to its command is
ωm (s)
ω*r (s)
=
1 + 4T4s
1
[
]
2
2
3
3
H ω 1 + 4T4s + 8T4 s + 8T4 s
3.8 Two-quadrant DC motor
drive with field weakening
• Fig. 3.38 is its schematic.
3.9 Four-quadrant Dc motor drive
• Fig. 3.39 is its schematic.