****> []# Futuretron EV Course_CHAPTER3

# CHAPTER 6: MOTORS

## 6.1 Electric Motor

An electric motor is an electrical machine that transforms mechanical energy into electrical energy.

### The History behind an Electric Motor:

The connection between electricity, magnetism, and movement was discovered in 1820 by French physicist Andre-Marie Ampere (1775–1867) and that is the basic science behind an electric motor.

The discoverers who converted this scientific discovery into a practical bit of technology to power electric motors were Englishmen Michael Faraday (1791–1867) and William Sturgeon (1783–1850) and American Joseph Henry (1797–1878).

The basic concept of an electric motor is simple, electricity is provided at one end and an axle (metal bar) rotates at the other end which gives the power to drive any type of machine.

The principle behind the working of an electric motor is when a current-carrying conductor is placed in a magnetic field it experiences a magnetic field and starts rotating according to the direction given by Fleming’s Left-Hand Rule for a motor.

There are many types of electric motor available in the market and the choice of these motors are very important. Selection of these motors is based on voltage, operation, and application.

## 6.2 Types of Motor

In one of the classifications motors are classified based on the power i.e. AC or DC and their method for generating rotation.

**DC MOTORS**

* Brushed DC motor
* Brushless DC motor
* Stepper (STP)

**AC MOTORS**

* Induction motor (IM)
* Synchronous Motor (PM)

Both types of motor have their own advantages and disadvantages:


**Why does TESLA exclusively use AC motors?**

While most of the commercially available EV's in the market employ a BLDC motor in their powertrain. Tesla motors remains as an exception from the lot. It is the only major auto manufacturer to use AC motors. Let us understand the reason behind this decision in detail.

**Reasons for opting AC motors over DC motors**

* **Cost**
The biggest concern is the cost of the motor itself. BLDC motor uses permanent magnets which are very expensive (~$50 per KG) in contrast Induction machines have no magnets thus bringing the overall cost much lower. Although the development of an Induction motor controller is expensive and complex, it pays off in the long run.

* **Efficiency**
For motors with lower capacity the efficiency of both kinds of motors are comparable but with increase in capacity, the losses in a DC machine increases with size whereas in an Induction machine it remains in the same range.

* **Regenerative braking**
Implementing the Regenerative braking system is a simpler affair with an AC motor when compared to that of a DC machine.

## 6.3 Important Parts in an Electric Motor

Figure: Important Parts of an Electric Motor

Armature: It is the power generating part in an electric motor. It can be a rotating part or the stationary part of the machine. The armature[ABCD] has the rectangular iron core wrapped by copper wire through which current is passed and is placed between the two poles of a magnet. The armature has an axle to which commutator is attached.

Commutator: They are the split rings which reverses the direction of the current. They connect the brushes and coil and are made up of copper. The purpose of the commutator is to make sure that the current direction in the coil reverses every half time so that one side of the coil is pushed downwards and another side of the coil is pushed upwards. The contacts of the commutator are linked to the axle of the armature so they rotate with the coil. In the above figure, commutator rings are denoted as C1 and C2.

Brushes: They are the two pieces of metal or carbon. One end of the brush is connected to the commutator and other ends of the brushes are connected to the positive and negative terminal of the battery respectively. In the above figure, Brushes are denoted as B1 and B2.

## 6.4 Working of an Electric Motor:

An electric motor uses the magnetic push to rotate the current-carrying conductor.

Magnets S and N will generate the magnetic field that will rotate the current-carrying conductor. Instead of inserting the current-carrying wire, a current-carrying loop [rectangular loop] is introduced as in the figure. Each side of the coil will experience the force and hence the coil starts rotating. To find the direction of the force in each side of the loop Flemings Left-Hand rule for motors(also called motor rule) is used.

Fleming’s Left-Hand Rule:

Figure: Fleming’s Left Hand Rule

Hold your thumb, fore and center finger of your left hand such that three fingers are at right angles to each other.

If you point the center finger in the direction of the current and the forefinger in the direction of the field, your thumb will show the direction of motion.

Fore-finger = Field
Center finger = Current
Thumb finger= Motion

Figure: Working of an Electric Motor

To begin with, the plane of the rectangular coil ABCD is made parallel to the magnetic field by placing the coil in the horizontal position. Electric current is passed through the rectangular coil ABCD, which enters at A and leaves at D as shown in the figure.
When the current is passed AB and CD side of the coil which is perpendicular to the direction of the magnetic field experience force according to Fleming’s left-hand rule. As the current passing through AB and CD of the coil are in the opposite direction, the forces acting on them will also be opposite. Hence forces push AB in downward direction and CD in the upward direction. Thus the coil starts rotating in the anticlockwise direction.

The commutator rings C1 and C2 change their contact from brushes B1 and B2 respectively when the coil completes its half rotation. Because of this, the direction of current in the rectangular coil ABCD is reversed, due to which the direction of forces in the coil is also reversed. Hence side AB is now pushed in upward and CD in the downward direction. Similarly, the whole process is repeated for the continuous rotation of the coil.

## 6.5 Advantages of an Electric Motor:

* For the same horse-power rating, the initial cost of an electric motor is less compared to fuel engines.
* An electric motor has comparatively few moving parts and hence longer lifespan.
* Minimum maintenance is required.
* Electric motors don’t require fuel, hence there is no need for engine oil maintenance, also no greenhouse gas emissions.
* Electric motors are highly-efficient with the ratings that range from 80% to 95%.

## 6.6 BLDC Motor

Also known as BrushLess DC motor or BL motor, it is the DC motor which does not have brushes. Motor is the heart of an Electric Vehicle. It is called Brushless because it doesn’t have brushes and commutator arrangements. Here the commutation is done electronically.

#### PRINCIPLE OF BLDC MOTOR:

The working principle of BLDC motor is similar to Brushed DC motor. The reversal of current in Brushed DC motor is done by Commutator and Brushes whereas in BLDC motor sensors are used, mostly hall-effect sensors. The hall-effect sensors generate a high or low signal whenever rotor magnetic poles cross the hall sensors, which can be used to detect the position of the shaft.

#### TYPES OF BLDC MOTORS:

They are classified based on their design but their working procedure remains the same.

* Outer rotor design
* Inner rotor design

Figure: BLDC Motor Types

**OUTER ROTOR DESIGN**

The rotor of the motor is situated outside and it surrounds the stator which is located in the center of the motor has multi-phase winding. The windings are fed with current and are controlled(commutated) to effect rotation of the rotor. There is no need of external gear system in this type of design and in few instances the motor itself comes with inbuilt gears. Therefore, without any gear system this design makes the device less bulky. The magnets present in the rotor acts as an insulator and will not allow the heat to dissipate from the motor. Outer rotor designed motor has low torque and operated at low current rates.

**INNER ROTOR DESIGN**

As in the image the rotor is situated in the centre of the motor and is surrounded by the stator winding. The rotor magnet does not shield the heat inside and the heat will be dissipated effortlessly, thus increasing the torque.

#### WORKING OF BLDC MOTOR

Figure:Working of BLDC Motor

The BLDC motor has 2 main parts rotor and stator. The rotor is the permanent magnet and is rotating part. The stator is the armature winding and is the stationary part. In BLDC motor coils do not rotate as in Brushed DC motor instead they are fixed onto the stator. There is no need of commutator and brushes since coils are static.

Mechanical rotation is produced when the magnetic field generated by the permanent magnets(rotor) comes in contact with the field induced by the current in the stator windings. The magnitude and direction of the current into the coils are adjusted to control the rotation.

Hall effect sensors are mounted on the stator or rotor. As the rotor rotates the hall effect sensors senses the magnetic field and produces a high signal for one pole or low signal for opposite pole. These sensors are connected to the Electronic control unit. Electronic control unit switches the supply voltage between the stator winding as the rotor rotates and energize the correct winding at correct time in such a way that it rotates the rotor around the stator. According to these combinations of signals electronic unit will decide the next commutation sequence or interval to activate.

Some of the advantages of Brushless DC motors are they are highly efficient, exceptional controllability, produce high torque, higher speed range compared to other motors, Operating life is long due to absence of electrical friction losses, Operation is noise-less, high dynamic response. It has power-saving advantages also. Due to its traction characteristics they are most preferred motors in electric vehicle applications.

### Numericals

**1. Calculate the nominal current consumption of a 1KW BLDC motor operating at 36V.**

Sol:
P = V*I

I = P/V
I = 1000/36 = 27.78 A

**2. Calculate the rotor speed of a 8 pole, 60 HZ BLDC motor.**

Sol:

N = 120f/p
= 120(60)/8
= 900 rpm

**3. Calculate the efficiency of a 2KW motor with the following parameters: 48V, 1200rpm, 14Nm.**

Sol:
E = Pout/Pin

Pout = τ * ω

ω = rpm * 2π / 60
= (1200 * 2*π)/60
= 125.66 rad/s

Pout = 14 * 125.66
= 1759.29 Watt

E = Pout/Pin
= 1759.29/2000
= 0.8796
= 87.96%

**4. Calculate the torque of a 500 watt BLDC motor with the following parameters: 36V, 500rpm, 85%η**

Sol:
τ = (I * V * E *60)/(rpm * 2π)

I = P/V
= 500/36 = 13.89A

τ = (13.89 * 36 * 0.85 *60)/(500 * 2π)
= 8.11 Nm

## 6.7 Regenerative Braking System

#### WHAT IS REGENERATIVE BRAKING SYSTEM?

The technique of recharging the batteries while braking is called Regenerative Braking System. Regenerative Braking System is used in fully electric vehicles and hybrid electric vehicles that use an electric motor.

#### HISTORY OF REGENERATIVE BRAKING SYSTEM

In 1908, a smart car with Regenerative Braking System was Patented by C.J. Paulson.
In 1967 American Motors Cooperation (AMC) in cooperation with Gulton Industries developed the “Energy Regeneration Brake” system.
This Energy Regeneration from braking was later commercialized by the Japanese and both Ford and Chevrolet licensed it from Toyota for use in their domestic built hybrid vehicles.
During the late 2000s an Electronic Control Unit [ECU] used by BMW that engages the alternator during braking.

#### WHAT IS THE NEED OF REGENERATIVE BRAKING SYSTEM?

Using RBS technique, the fuel economy of vehicles can be improved. It further reduces greenhouse gas emissions. The enrgy which would have been lost due to thermal dissipiation is thus harnessed and restored in the system.

Normal Driving conditions

In the current world where energy and resource management has become very much important and it’s necessary to employ energy in all the form. Vehicles which are in motion have a lot of kinetic energy and when the brakes are applied all the kinetic energy that was propelling the vehicle forward are lost to the environment as heat which decelerates the car. This useful kinetic energy which could have been used to do some work is wasted as heat.

So, a braking system called Regenerative Braking System [RBS] was introduced, it is an energy recovery mechanism where the kinetic energy is converted into electricity which is used to recharge batteries in Electric Vehicles.

#### REGENERATIVE BRAKING SYSTEM IN FULLY-ELECTRIC VEHICLE
Regenerative action in EV

During acceleration, the motor draws the energy from the battery to move the wheels. During braking, the power from the battery is cut off by the controller [ECU] which slows down the motion of the vehicle. Hence the motor performs as a generator and the energy flows back to the battery.

#### REGENERATIVE BRAKING SYSTEM IN HEV

Regenerative action in HEV

When the driver applies a brake on hybrid electric vehicles, the electric motor generates a negative torque, that is in the reverse direction, which makes the motor act like a generator and thus recovering the energy which is stored in the battery, producing electricity which flows into the batteries.

To gauge the regenerative braking system, its effectiveness and efficiency should be seen. Efficiency states how competently the braking system captures the energy lost when the brake is applied. Effectiveness states how size-able impact(difference) the regenerative braking system actually makes.

#### BENEFITS OF RBS:

For smaller and personal electric vehicles, the regenerative braking system is not as beneficial or effective as in electric cars, still, it has some multitude of returns.

Regeneration occurs only when the brakes are applied and the resultant energy depends on the speed of the vehicle. By using the Regenerative Braking System, the energy usage efficiency can be increased and also driving distance of Electric Vehicles can be improved. This kind of brakes improves the driving range of Vehicles. All-electric vehicles in motion can utilize this advantage of regeneration to re-collect energy that would have been lost.

Some of the other advantages are the reduction in engine wear out, smaller components, increase the performance, increase in the overall efficiency of vehicles, increases the life span the friction braking system.

Some of the disadvantages are friction brakes are needed to stop the vehicle at low speeds, means energy is still wasted. RBS can increase the overall weight, cost of components, manufacturing and installation are high, maintenance required based on the complexity of the design.