DC Machine Characteristics

DC MACHINE CHARACTERSTICS

AIM

• To study the Speed & Torque Behaviour of DC Motor.
• Classification of Power Cables & Signal Cables.
• To study on Level of Voltage in a Battery Pack of EV/HEV.

INTRODUCTION

DC Machine

Direct Current Machine can be of 2 types DC Motor or DC Generator, which works on Flemings Left Hand Rule & Flemings Right Hand Rule respectively. The DC Machines draws current from the Storage Devices like Battery. Most of the DC machines are equivalent to AC machines because they include AC currents as well as AC voltages in them. Any rotating machine (Motor/Generator), at one time acts as a Generator i.e., they develop an AC Voltage while cutting the Magnetic flux, this can be corelated to Back EMF concept in DC Motors. These machines are also named as commutating machines, as they use Commutators to apply electric current to Windings by reversing the current direction in the rotating windings each half turn, a steady rotating force (torque) is produced.

These Conventional DC Machines requires regular maintenance and are relatively inefficient and hence their usage is very less in continuous Torque delivering Applications. Automotive Starter Motor is one of the applications where these DC Motors are used to provide in initial torque to crank the Engine. For Traction Applications, in recent developments use Brush Less DC Motors (BLDC) are used.

Principle

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 which rotates the dc motor.

The above setup shows a simplified view of DC Motor which mainly consists of Stator Core with a Housing, The Stator mainly consists of the Permanent Magnets which acts as Poles. The rotating element of the Machine is Rotor, here is the Armature which can be of Lap or Wave winding. The selection of winding reflects on the selection of Number of Carbon Brushes and Magnetic Poles. The Armature windings are then drawn into Commutator. The main function of the commutator in the DC machine is to collect the current from the armature conductor as well as supplies the current to the load using brushes. And also provides uni-directional torque for DC-motor. The commutator can be built with a huge number of segments in the edge form of hard drawn copper. The Segments in the commutator are protected from thin mica layer.

Equations for DC Machines

The DC machine e.m.f can be defined as when the armature in the dc machine rotates, the voltage can be generated within the coils. In a Generator, the e.m.f of rotation can be called the generated emf, and Er=Eg. In the Motor, the emf of rotation can be called as counter or back emf, and Er=Eb. In Motors Back EMF developed is an undesirable characteristic feature which opposes the Supply Voltage.

A motor has coils turning inside magnetic fields, and a coil turning inside a magnetic field induces an emf. This emf, known as the back emf, acts against the applied voltage that's causing the motor to spin in the first place, and reduces the current flowing through the coils of the motor. At the motor's operating speed, enough current flows to overcome any losses due to friction and other sources and to provide the necessary energy required for the motor to do work. This is generally much less current than is required to get the motor spinning in the first place.

When the motor is spinning and generating a back emf e, the current is reduced to:

Let Φ is the useful flux for every pole within Weber

P is the total number of poles

z is the total number of conductors within the armature

n is the rotation speed for an armature in the revolution for each second

A is the no. of parallel lane throughout the armature among the opposite polarity brushes.

Z/A is the no. of armature conductor within series for each parallel lane

As the flux for each pole is ‘Φ’, every conductor slashes a flux ‘PΦ’ within a single revolution.

The voltage produced for each conductor = flux slash for each revolution in WB / Time taken for a single revolution within seconds.

As ‘n’ revolutions are completed within a single second and 1 revolution will be completed within a 1/n second. Thus, the time for a single armature revolution is a 1/n sec.

The standard value of produced voltage for each conductor

p Φ/1/n = np Φ volts

The voltage produced (E) can be decided with the no. of armature conductors within series I any single lane among the brushes thus, the whole voltage produced

E = standard voltage for each conductor x no. of conductors within series for each lane

E = n.P.Φ x Z/A

For any Motors,

• Electromagnetic torque T =k I e φ a developed by the motor acts along the direction of rotation.
• The load torque TL acts in the opposite direction of rotation or in opposition to Te.
• If Te =TL , motor operates with constant speed.
• If at any time Te >TL , the motor will accelerate.
• If at any time Te < TL, the motor will decelerate.

TORQUE-SPEED Characteristics of DC Machine

The above image shows the Torque Speed Performance of the DC Motor at Constant applied Voltage.

No Load Speed is the maximum Speed of the Machine at which it rotates when there is no load acting on the it. When this Load increases ultimately Torque acting on the Machine will increase and so does the Speed decreases.

Maximum torque can be achieved when the motor is fully braked (Stalled Torque) and no torque is experienced when the motor is at its maximum rotational speed. DC motors perform the best when they are producing the most mechanical power. Power is the product of torque and rotational speed.

Power = Torque * Rotational Speed

The Torque developed by Rotating Element/Armature is given by,

Where

T : Torque (Nm)

K : Motor Constant

Ra : Armature Resistance (ohms)

ω : Angular Velocity (rad/s)

Ø : Field Flux

Also,

Kø = Eb/ ω

Eb : Back Emf

ω = 2Πn/60

The above Formulae can be used to script a MATLAB file to Plot Speed-Torque Behaviour.

MATLAB PROGRAM TO EVALUATE DC MACHINE CHARACTERSTICS

%Torque-Speed Characterstics of DC Machine
clear all
close all
clc
 
%To define an array of Speed in Omega(rad/s)
Motor_Speed=linspace(0,1200,50);
 
N=(2*3.14*Motor_Speed)/60;
 
%Battery Supply Voltage
V=input('Enter the Battery Supply Voltage : ');
 
%Velocity Constant = No Load Speed/Supply Voltage
%This factor helps in Motor Sizing and Selection of Winding Parameters.
Velocity_constant=input('Enter the Velocity Constant Value : ');
 
No_Load_Speed=Velocity_constant*V;
 
%Speed is proportional to supply Voltage
%Hence Motor develops Back EMF porportional to Speed, as per Lenz's Law
E_b=1;
 
%Armature Resistance
R_a=input('Enter the Armature Resistance Value (ohm) : ');
 
%Motor Constant
M_c=(E_b*60)/(2*3.14*Velocity_constant);
 
%Torque Equation
T=((V*M_c)/R_a)-(((M_c)^2)/R_a)*N;
 
%Power Equation
P=T.*N;
 
%Current Drawn
I=(V-E_b)/R_a;
 
tiledlayout(1,2);
 
% To Plot Power Curve
ay1=nexttile;
plot(N,P);
xlabel('Speed (RPM)');
ylabel('Power (W)');
 
% To Plot Torque Curve
ay2=nexttile;
plot(N,T);
xlabel('Speed (RPM)');
ylabel('Torque (Nm)');

Program Description

• Motor Speed range has been defined to certain limits both in rad/s and RPM.
• INPUTS
  • Battery Voltage: Primary Criteria
  • Velocity Constant: Required for Motor Sizing; It is the ratio b/w No Load Speed to Peak Voltage.
  • Velocity Constant can be replaced by No Load Speed if Known.
  • Armature Resistance / Winding Resistance: Since varies from Winding to Winding and Temperature dependent.
  • Since we don’t have other values such as Armature Slots and no of Poles, Torque Equation is solved by using Motor Constants and Armature Resistance and Speed.

OUTPUT

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Power Cables & Signal Cables in Electric Wiring of Vehicles

Power Cables are used to transmit Electrical Power, and are diametrically larger in size which can conduct maximum desired current through it. Also, they transfer both high energy and low energy from the source to the equipment. The external sheath of the Power cables with its immense strength and protection enables the cables to transfer high volume of energy without any tension or strain. Power cables often have a unique colour code system that distinguishes the high voltage carriers from their low voltage counterparts.

Signal Cables are used to transmit Electric Signal used for Control the data transmission between Power Elements and Control Units. They are Diametrically very smaller in Size when compared with Power Cables. They are capable of transferring very low currents required for trigger of electric signals or information. Usually used for regions that require less than one ampere, these cables can carry signals up to 40 volts. These cables usually bear a PVC insulation that protects them from impacts and harsh climatic conditions.

Identifying Cables in Vehicle

• Size
  • Bigger: Power Cable
  • Smaller: Signal Cable
• Transmission
  • Battery, Motor, Alternator, Starter: Power Cables
  • Inverter, Power Modules, ECU: Signal Cables

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Voltage Level for Battery Pack of Electric Vehicle

A battery pack is a set of any number of (preferably) identical batteries or individual battery cells. They may be configured in a series, parallel or a mixture of both to deliver the desired voltage, capacity, or power density. Components of battery packs include the individual batteries or cells, and the interconnects which provide electrical conductivity between them.

The Voltage of the Battery / Battery Pack is a requirement of the Vehicle to meet its Power Demand and more the requirement, more will be the Battery Voltage.

The ICE Vehicles have 12V Battery System and every component of this vehicle are designed for this Voltage System.

The EV/HEV are having 48V Battery System in order to meet the High Electric Power requirement for the comfortable drive by Motors.

Higher the Voltage of the Battery, more will be the current flow and hence requires a better and efficient Battery Management System and DC-DC Converters.

Consider the Conventional Vehicle with 12V Battery System. The Vehicle Accessories draws current from this Battery. A typical 12V Battery has a nominal Voltage of 12.6V and is packed of 6 cells of 2.1V each and again this depends on the type of Battery.

Battery Voltage also depends on the SOC, where when Vehicle is on Run and al, the accessories draws the Current and the SOC goes on depleting. Now at this time the Battery Voltage may fall up to 11V or below.

If a Cell in the Battery gets damaged, suddenly there will be drop in the Voltage level and this can be termed as Low Voltage Scenario where the Battery fails to Crank the Starter Motor of Vehicle which requires an Optimal Voltage of 11.6-12V.

Simultaneously the Alternator also keeps on charging the Battery which is enough to maintain a Stable SOC and Voltage level in the Battery, but if Alternator fails to deliver Current to Battery can also leads to Low Voltage Condition.

A High Voltage can appear when there is an adverse heating in the Chemistry of Battery Cells, this could be anything beyond 13V and can damage the Circuit of many accessories and the Starter Motor.

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