DC machine characteristics.

AIM- To perform dc machine characteristics.

OBJECTIVE-

1. Implement DC machine torque speed characteristics equations in MATLAB script file. It asks user to input data and plots speed (RPM) versus torque (Nm). Submit the .m file for evaluation and attach the screenshots of all the required plots.
2. How can you distinguish between power cables and signal cables and also prepare a table where these cables are used in the wiring of an electric vehicle?
3. What voltage levels are considered as high & low for a battery pack of electric vehicles?

EXPLANATION-

Study 1-> In this problem, we have to implement a DC machine torque speed equation in matlab to find the plot.

Torque speed behaviours of a DC motor

• When the load on the motor is constant, speed is proportional to supply voltage and when the supply voltage is constant, speed is inversely propotional to the load on the motor.
• The inverse relationship between speed and torque means that an increase in the load (torque) on the motor will cause a decrease in speed.
• This means torque speed curve is a descending line,with a negetive slope.
• The torque speed curve begins at the crossing of the Y axis, where torque is maximum and speed is zero. This is the holding torque (maximum torque when the motor is running at nominal voltage).
• The line falls downward untill it intersects the X-axis i.e zero torque and max speed and it is known as no-load speed(the speded when running at nominal voltage and zero load).

DC motor torque speed formula-> T= V(k_flux)/Ra-((K_flux)^2/Ra)*w

where, t= motor torque

V= supply voltage

w= rotational speed

K_flux= flux constant

Ra= armature resistance

T= torque(Nm)

Matlab code :

%inputs for voltage and armature resistane
vt=input('enter the value of terminal voltage=');
Ra=input('enter the value of armature resistance=');

%motor characteristics
eb=1;
w=70;

%flux constant and max speed
k=eb*60/(w*2*pi);
wmax=vt/k;

%maxtorque
Tmax=vt*k/Ra;

%range of torque values
t=linspace(0,Tmax,500);

%speed calculation
speed=(wmax-(Ra/(k)^2)*t);
%plot
figure(1)
plot(t,speed,'color','g')
xlabel('torque(Nm)')
ylabel('speed(rad/s)')
grid on

Inputs and calculations:

OUTPUT:

Study >2

POWER AND SIGNAL CABLES

The cables are distinguished by their area of cross-section.

• Power cables have higher cross-section as they carry much higher current in them, for example, supply of power from battery to motor or ac supply for recharging EV etc. Power cables does not nesessarily need to be of higher voltage. The wires coming off from the car battery are power wires, because they carry a substancial amout of electric power.

• Signal cables have lesse cross-section compared to power cables as they carry less current, for example, supply of current to infotainment system, or to commute different signals from different sensors installed in an automobile. Signal cable carries low levels of current for regions that requires less than one ampere of current, because not much current is needed for contrrol purposes ex energizing a coil relay etc.

The cables can be differentiated by :

• Size of the cable
• Observing the cable connections, ex: power cables provide electricity for the main componenets such as motor, to the air conditioners

signal cables will be connected to the control subsystems like ECM, or speed sensor etc.

STUDY->3:

VOLTAGE OF A BATTERY

It means the potential difference between the positive and negetive terminal of the battery. The potential difference is directly propotional to the current flow, it is because of the movements of electrons that produces the current, so more the potential difference in the battery, more will be the current flow.Higher voltages are adotped because they boost the efficiency. The traditional internal combustion engines powered cars uses batteries which are of 12V and it was sufficient too. In case of New generation Hybrid vehicle the battery voltage became more higher to 48V.

Whereas in an electric vehicle battery, it consists of many hundreds of small , indivisual cells arranged in a series or parallel configuration to achieve the desired voltage. A common pack is composed of blocks of 18 - 30 parallel cells in series to achieve a desired voltage. for example, a 400V pack will contain 96 series block(used in tesla model3). The EVs , all require high voltage to power the motor. voltage levels of 400-800V or more is needed to generate sufficient power, While running the battery voltage of electric car , sometimes can increase over 800V and it is considered as High voltage levels. And if the battery voltages fall below the required 400V then the voltage level is considered to be low and power generation will be affected.