DC machine characteristics using MATLAB.

Aim: To perform DC machine characteristics

Objective:

1. Implement DC machine torque-speed characteristics equations in a MATLAB script file. It asks the 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 in the wiring of an electric vehicle?
3. What voltage levels are considered as high & low for a battery pack of an electric vehicle?

Introduction:

Characteristics of DC motors:

• A DC machine is an electromechanical energy alteration device. The working principle of a DC machine is when electric current flows through a coil within a magnetic field, and then the magnetic force generates a torque that rotates the dc motor. 
• Generally, three characteristic curves are considered important for DC motors which are, (i) Torque vs. armature current, (ii) Speed vs. armature current, and (iii) Speed vs. torque. These characteristics are determined by keeping the following two relations in mind.
  T_a ∝ ɸ.I_a and N ∝ E_b/ɸ
• These above equations can be studied at - emf and torque of DC machine. For a DC motor, the magnitude of the back emf is given by the same emf equation of a dc generator i.e. E_b = PɸNZ / 60A. For a machine, P, Z and A are constant, therefore, N ∝ E_b/ɸ
• The construction of the DC machine can be done using some of the essential parts like Yoke, Pole core & pole shoes, Pole coil & field coil, Armature core, Armature winding otherwise conductor, commutator, brushes & bearings. Some of the parts of the DC machine are shown below.

Speed vs. torque (N-Ta):

• This characteristic is also called a mechanical characteristic. From the above two characteristics of DC series motor, it can be found that when speed is high, torque is low and vice versa.

• The relationship—between speed and load (or torque)—is typically shown on the motor’s torque-speed curve.

• 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 can be demonstrated by the DC motor torque equation:

Where:

• T = motor torque
• V = supply voltage
• ω = rotational speed
• k = motor constant
• R = resistance

Of course, the motor constant (k) doesn’t change, and resistance (R) in the motor windings is constant. Therefore, when the supply voltage (V) is constant, torque (T) is inversely proportional to speed (ω).

Rearranging for speed, we can see the same inverse relationship to torque:

• The inverse relationship means that the torque-speed curve is a descending line, with a negative slope. The torque-speed curve begins at the crossing of the Y-axis, where torque is maximum and speed is zero. This is the stall torque—the maximum torque when the motor is running at a nominal voltage. The curve slopes downward until it intersects the X-axis—that is, zero torque and maximum speed. This point is known as the no-load speed—the speed when running at nominal voltage and zero loads.
• Because the torque-speed curve is a straight line, it’s simple to find the torque that the motor can produce at a given speed, or conversely, to find the motor’s speed for a given load (torque) on the shaft. Recall the equation for a straight line:

Where:

• y = value of y-axis variable, to be determined
• m = slope of the line; change in y divided by change in x
• x = value of x-axis variable, given
• b = y-intercept; point at which the line crosses the y-axis
• Using this equation for the torque-speed curve, we can find the motor’s torque at a given speed. In this case, the variables in the line equation represent the following:
• y = torque to be determined
• m = change in torque divided by change in speed
• x = given speed
• b = stall torque (value where the line crosses the y-axis)

The line equation can also be rearranged to find the motor’s speed at a given torque:

1) MATLAB Program:

clear all;
close all ;
clc

% Motor Speed in Radian (Rad/sec)
Speed = linspace(0,250,75);

% Constant Motor Speed (RPM/V)
N = input ('Motor Speed Value - ');

% Voltage Supply (Volts)
V = input ('Supply Voltage - ');

% Resitance Value of Armature (Ohms)
Ra = input ('Armature resistance - ');

% Back EMF Inputs
EMF = 1

% To calculate Motor Constant 
K = ((EMF*60)/(N*2*3.14))

% To calculate torque by using DC machine equation
T =((V*K)/Ra) - (((K)^2/Ra)*Speed)

% To calculate Total Current 

TC = (V-EMF)/Ra

% To Calculate the torque 
T_max = K*TC

% Converting RPM to RAD/Sec
Speed = Speed * (60/6.28)

% Plotting DC Machine  characteristics wrt Speed Vs Torque
plot(Speed,T,'color','r','linewidth',2);
xlabel ('Speed (RPM)');
ylabel ('Torque (Nm)');
title ('DC Machine Charactersitics wrt Speed Vs Torque');
grid On
axis ([0 2700 0 270]);

Graph:

• Motor speed = 200
• Supply voltage = 48
• Armature resistance = 0.02

2) How can you distinguish between power cables and signal cables in the wiring of an electric vehicle?

• Power cables are basically electrical conductors that are held together in order to transmit electric power on a large scale. It has many industrial applications that make the wires a popular choice to be used in ships, airports, industrial areas, underground, hospitals, commercial centers, and many more. When they are installed, they can be either exposed or buried deep within the ground or a building. There are different sorts of power cables such as flexible cables which can also be used for mobile tools as well as heavy machinery.

• Signal cables are low power and currently rated wires whereas power cable is high current rated and power. Signal cables will be smaller gauge and consist of multiple individually shielded twisted pairs. Signal cables are not a heavy enough gauge to prevent too much voltage drop or actual cable overheating and damage when used in a control circuit. Control circuits could easily be 1 to 5 amps.

• The main difference is, signal cables are used in test and measurement applications, whereas power cables are used in powering the motors, industrial machines, large power transmitting cables. Signal cable will use for low & high-level analog signals such as filed instruments, RTDs T/c's. etc.. Power cable will use for powering the instrument or machinery. if you are using a 4 wire micro motion transmitter. the signal cable will carry 4 to 20 mA signal and power cable will carry 24DC/110/230vac. both cables cannot be run on the same route.

3) What voltage levels are considered as high & low for a battery pack of an electric vehicle?

• An electric vehicle battery is often composed of many hundreds of small, individual cells arranged in a series/parallel configuration to achieve the desired voltage and capacity in the final pack. A common pack is composed of blocks of 18-30 parallel cells in series to achieve the desired voltage. For example, a 400V nominal pack will often have around 96 series blocks (as in the Tesla Model 3).
• Common nominal pack voltages in current vehicles range from 100V-200V for hybrid/plug-in hybrid vehicles and 400V to 800V and higher for electric-only vehicles. The reason for this is higher voltages allow more power to be transferred with less loss over the same diameter (and mass) of copper cable.

• The drawbacks of higher voltages include the necessity for higher-voltage-rated components in the entire system. They also prevent the ability to use DC fast-charging stations of a lower voltage without incorporating some type of DC-DC boost converter in the on-board charger.

Common battery capacity ranges, on the other hand, are as follows:

• Hybrid vehicles: 0.5 to 2 kWh - 100V to 200V
• Plug-in hybrid vehicles: 4 to 20 kWh - 100V to 200V
• Electric vehicles: 30 to 100 kWh or more - 400V to 800V and higher.

Conclusion:

• Implemented DC machine torque-speed characteristics equations using a MATLAB program.
• Distinguished between power cables and signal cables of an electric vehicle.
• Found what voltage levels are considered as high & low for a battery pack of an electric vehicle

Thus, we have performed the given objectives successfully.

References:

1. https://www.allaboutcircuits.com/technical-articles/introduction-to-electric-vehicle-battery-systems/#:~:text=Common%20nominal%20pack%20voltages%20in,and%20mass)%20of%20copper%20cable.
2. https://www.windpowerengineering.com/single-cable-power-signals-now-servo-drives/
3. https://control.com/forums/threads/difference-power-cables-and-signal-cables.25016/#:~:text=Friends%2C,machines%2C%20large%20power%20transmitting%20cables.
4. https://www.slideshare.net/vguardindustries/the-main-differences-between-power-and-control-cables-a-primer-24376868
5. https://www.elprocus.com/dc-machine-types-and-their-applications/
6. https://www.motioncontroltips.com/torque-equation/#:~:text=DC%20motors%20are%20relatively%20simple,the%20load%20on%20the%20motor.&text=Therefore%2C%20when%20supply%20voltage%20(V,proportional%20to%20speed%20(%CF%89).
7. https://www.electricaleasy.com/2014/07/characteristics-of-dc-motors.html