power dc motor

8/13/2019 Power Dc Motor

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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.



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



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.


Library: SimPowerSystems>Power Electronics

DC BATTERY:


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.



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



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:


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



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.



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.



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



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



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

power dc motor

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