power dc motor
TRANSCRIPT
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The connection diagram on the
ight shows the circuit of “DC
Motor with 3 Step Starter” taken
rom one of the demos
power_dcmotor) available in
MATLAB- SIMULINK’s
SimPowerSystems Library.
The following document is
ntended for description of
various components present in
his demo along with a basic
nsight into how to select various
parameters and simulate a dc
motor.
The diagram has been divided
nto various blocks as can be
een (Blocks 1, 2,3,4). We take
up these blocks one by one and
tart with the components
connected in that block and then
a brief about what parameters
are required in the component
and what’s the use of that
component.
A basic knowledge of various
curves of dc motor and the
ormulae for calculation are
mperative for this demo,
otherwise it’s better to sit withome reference text on electrical
machine and drives.
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BLOCK 1: MOTOR INPUT
TIMER:
This component is used to give a constant input signal according to the time
fed by user. The parameters are as shown below:
In this demo, we use this element to trigger the switch. The value at the start of
simulation i.e., t=0s is set at 0 and the value at t=0.5s is set as 1. We use the value as 1
because it is the required at the Ideal Switch to trigger it to ON state.
This element can be found in:
Library: SimPowerSystems>Extra Library>Control Blocks
Enter a matrix (1xn) with the various
transition times.
Enter the matrix (1xn) with the magnitude
of signal at nth transition instant
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IDEAL SWITCH:
This element behaves as a switch in parallel with a series RC snubber circuit, based
upon gate signal. 0 implies switch OFF and some signal (in our case 1) implies switch
ON. The parameters are shown below:
We use this element to start Switch ON our circuit.
This element can be found in:
Library: SimPowerSystems>Power Electronics
DC BATTERY:
This element can be found in:
Library: SimPowerSystems>Electrical Sources>DC Voltage Source
Enter the ON resistance of the switch. Usua
this should contain a very low value of resist
Enter the default configuration of switch
Enter the value of Snubber Resistance and
capacitance. To remove the snubber circuit,
Rs “inf” which stands for infinite and Cs as “0
Check this box if you want to measure the c
though the switch.
This element is used as voltage supply of our motor. It has just one input
parameter and a measurement option as:
Enter the amplitude or magnitude of
voltage.
Use this option if we want to use
multimeter in measurements.
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BLOCK 2: STARTER
This block uses a subsystem which is used to hide
the complexity of a bigger circuit and also speed up
the compilation.
Double clicking on this element takes us to the actual components connected in the
subsystem, which are as shown below:
A subsystem can be created as:
Library: Simulink>Ports & Subsystems.
Then double click on the subsystem and then for delete the inport and
outport . Replace them with Connection Ports available in
Library: SimPowerSystems>Elements>Connection Port.
Now we will discuss the elements in this subsystem.
STEP: This element is used to give a step pulse with input parameters as:
Enter time (t) to start the signal
Enter default value
Enter the step value after time‘t’
Enter the Sample time. Usually, it should be let 0
To avoid errors, check these options
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This can be found as
Library: Simulink>Sources>Step.
BREAKER: This is circuit breaker which is used as a switch according to the time input.
The input parameters are as:
This element can be found in:
Library: SimPowerSystems>Elements> Breaker.
This starter is used limit the starting current in the motor armature. The calculation of
values of resistances can be understood from any standard text on Electrical
Machines.
We use the breaker to cut the resistances in step by using the pre-defined step pulses
at an interval of 2 seconds given by step. This time is enough for current reach its
steady state value. As the motor approaches steady state, all the resistances are cut
off from the armature circuit as all the current passes through the minimum
resistance path provided by the breakers.
Enter the ON resistance of the switch. Usuall
this should contain a very low value of resista
Enter the default configuration of switch
Enter the value of Snubber Resistance and
capacitance. To remove the snubber circuit,
Rs “inf” which stands for infinite and Cs as “0
Check this box to use external timing signal.
unchecked, we need to specify the transition
times in a 1xn matrix
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BLOCK 3: SEPARATELY EXCITED DC MOTOR
The way we connect the field and armature terminal distinguishes between series,
shunt machine. For permanent magnet machine, we have to check in theconfiguration of dc machine as shown:
Once configured, select the parameters for your machine. This element has following
parameters:
TL/W : Load torque (N-m) or speed (rad/sec)
m : Measurement port
A+/A- : Armature terminals
F+/F- : Field winding terminals
The following element can be found in:
Library: SimPowerSystems>Machines>DC
Select a pre configured machine or leave this ‘n
you want to make your own model.
Select type of feedback input, i.e. torque or spe
Select permanent magnet if you want a PM ma
otherwise leave ‘wound’ if you want a series or
shunt machine
Enter the value of armature resistance and
inductance. Usually, these must be low.
Enter field resistance and inductance. Field resistan
must be large in order to limit the current and also
because of its construction
Enter mutual inductance between field and armatu
Enter rotor inertia.
Enter friction constants
Enter initial speed of machine, this should have a
small value so that machine starts and does not
changes direction.
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Since in the demo, we use separately excited motor, thus, we can see that the field
windings are supplied separately by another 240V battery.
The mutual inductance can be calculated from rated values of speed and field winding
current. Positive input in TL means the machine is in motoring mode otherwise in
generating mode. m is a multiplexed signal for measurement of parameters in the
following order:
1. Speed in rad/sec
2. Armature Current in Amperes
3. Field Current in Amperes
4. Developed torque in N-m
The signals are separated using a demultiplexer which can be found in:
Library: Simulink>Signal Routing>Demux.
We need to select the number of output ports as parameter.
We need four outputs from the measurement port of machine as indicated above. So
we set the number of outputs as 4.
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BLOCK 4: MOTOR PARAMETER MEASUREMENT
We have the signals with us after simulation, but in order to observe them, we use
scopes. These elements can be found in:
Library: Simulink>Sinks
The number of plots in a scope can be increased by changing the parameter by
following these steps:
Double click a scope>Click on Parameters button >in the window that opens,
select General tab> then change the number of axes from 1 to desired number.
The 3 scopes are used to show speed (w), armature current (Ia) and developed torque
(Te). The XY Graph is used to plot speed vs armature current characteristics.
SCOPEs for plotting the magnitude of
function against time
TERMINATOR
XY GRAPH for plotting
between two signals
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SIMULATION:
We now simulate the circuit for 10 seconds using ode23tb as solver with adaptive
algorithm and a tolerance of 1e-4. This is done merely to speed up the process of
simulation. The various plots are as follows:
Armature current in Amperes
Speed
in
rad/sec
The plot starts with 0 Ia and 1 w.
As the ideal switch is turned ON,
The armature current grows very
high rapidly due to low value of
Laa and Ra. This is limited by the
starter resistances. As the motorspeed increases, back emf grows
and thus armature current
automatically reduces.
The three highlighted
points are the points
where the breakers are
switched ON and the
starter resistances are
cut. The current spikes
are a result of sudden
increment in voltage
across the armature by
shorting of starter
resistance. The
sharpness of the spikes
can be varied by altering
the values of armature
winding inductance.
Time in seconds
Armature
Current
n Amperes
Time in seconds
Torque in
N-m
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As we can see in the plots, Torque developed is directly proportional to armature
current and the Rotor speed is directly proportional to the armature voltage. The
spikes in voltage are result of inductive nature of windings.
The initial lag in the plots is because we start the motor at t=0.5 sec using the ideal
switch.
otor speed
Rad/sec
Time in seconds
Time in seconds
Armature
Voltage in
Volts