Electric Motors Performance compared to IC Engine for vehicle application

Electric Motors Performance compared to IC Engine for vehicle application:

It was Charles F. Kettering who was the pioneer in employing an electric motor for an automotive application. He is credited with first successful application of a motorised starting of IC engines in the first decade on last century.

It was a DC Motor. But it was clear from the application that electrical motor is equipped to generate a much higher value of short term torque compared to its rated continuous duty torque. That explains why it generates a much larger peak torque for a small duration of time. Using this torque, it may even be possible in some cases to propel the entire vehicle for a short duration. The torque is controlled by the motor current for a DC motor. Current in turn is impacted by the applied voltage also.

This fundamental concept is the foundation and implementation of electrical motors for the vehicle propulsion kind of application. Control the motor output by supply voltage & current using controller.

DC motors are used for electric vehicle traction application because of the substantial amount of advantages it delivers over an IC Engine, such as: being lighter in weight, high starting torque, high power density, easy to service (less parts are integrated compared to IC Engine), higher efficiency, variable gear ratio (help to achieve variable torque and speed).

It is easy to tailor DC motor speed/torque curve for a given application through corresponding control of motor voltage/current within its specified rating. Because a DC Motor rotates due to the interaction between two magnetic fields like other electrical machines, one of two must be an electromagnet and other may be electromagnet or permanent magnet. Some limitations with permanent magnet are size constrain as well as strength of magnetic field constrain which limited the speed of the motor.

Speed of the DC motor is dependent on supply voltage, current & back e.m.f. By the variation of these three parameters motor speed, torque and power are controlled.

Without controller (current limiter), if voltage has been increased to the motor leads, it will increase torque tends to infinite because current flow will increase tends to infinite inside armature coil & field winding due to principal of Ohm’s law.

But due to increase of voltage, as much current will be pumped into the armature coil, the amount of back emf induced into the coil will be increased, that will oppose the current flow into the armature coil which will become the cause of reduction in torque. But due to increase in voltage motor speed will be increased and reached to its maximum range.

But as per the traction application, torque and speed both are required. High Torque required for starting vehicle (as per the application of starter motor), similarly for sudden acceleration during overtaking, climbing flyover or any uphill. Higher Speed is required for cruising condition.

During cruising very minimum obstacle needs to overcome, which needs very less amount of torque. Due to this reason the rate of current inside the conductor will be reduced.

Therefore motoring speed/torque curve can be shaped by use of electronic controller which controls the flow of current at a required rate to meet application demand. This enables us to deliver a constant maximum possible torque (governed by its torque rating) from zero speed to the value of base speed (rated speed with rated torque consuming rated amount of power). This region under performance carve is called constant torque region. In this zone as the torque has been constant at motor torque rating using current limiter and speed increase from zero to base speed. Within this zone output power will increase from zero to rated power limit.

Output Power (P) = 2πNT

In constant torque zone T= Constant, Hence P∝ N (N varies from zero to base speed).

After base speed, torque getting reduced accordingly due to high voltage, speed will increase to retain the constant power (rated power) output. This zone is called constant power region.

Output Power (P) =2πNT= Constant (Max Power rating)

In case of power, motor generates steadily increasing power at a constant rate, as the vehicle starts and reaches a speed corresponding to the motor base speed (Rated Speed) because of the high voltage source attached which causes higher amounts of the current flow till motor current rating to achieve max torque.

Motor Speed Torque relations:

Speed (N) ∝ Armature Voltage/ magnetic flux

Torque (T) ∝ Armature Current × magnetic flux

Flux (Ø) ∝ Field Current

Hence electric vehicle may not need any transmission system because vehicle speed and torque are controlled by magnetic flux which is controlled by field current. Example-Nissan Leaf. 

From the above figure, it is clear that to shape, the speed/torque curve, different gear ratios are required in case of engine, but motor can easily shape the curve by varying magnetic flux, which is controlled by the field current only by using a single paddle application. 

There are two types of DC motors in earlier days.

Series DC Motor: The Field winding & Armature winding are connected in series with battery. Therefore as field current varies the armature current also varies. Due to this reason torque performance increases. This kind of arrangement provides high starting torque to start the vehicle instantly. Hence starter motor used in vehicle are series wound DC Motor.

Torque (T) ∝ Armature Current × magnetic flux

 Flux (Ø) ∝ Field Current

Shunt DC Motor: The Field winding & Armature winding are connected in parallel with battery. Therefore as field voltage and armature voltage remain constant for any fixed supply voltage. Therefore if supply voltage varies speed (RPM) also varies. This kind of arrangement is suitable for highway crushing operation.

This kind of dc motor is used for fan, blower, and conveyer etc. This type of motor is suitable for constant speed application.

Speed (N) ∝ Armature Voltage/ magnetic flux

Flux (Ø) ∝ Field Current

Separately excited DC Motor: In this kind of arrangement, both field winding and armature winding is separately connected with the battery. Therefore both current and voltage is controlled separately for both armature and field using electronic control system. Hence ease of variation in torque and speed.

This arrangement is eliminated the limitations of Series Wound and Shunt Wound DC Motor.

Currently DC Motors is replaced by 3 Phase AC Motors because there are some limitations as follows.

1) DC Machines are much heavier than AC machines for the same output power rating, which causes lighting of vehicle, increased torque to inertia ratio.

2) DC Machines draws heavy current using current carrying conductors, which causes using of thicker conductor for motor winding. Due to this high current flow inside these thicker conductor, getting more heated. This heat can’t easily dissipate to the environment, which may cause failure of insulation system of the motor.

There is an inverter has been integrated with the system to convert the DC battery supply to AC supply.

In case of the torque speed characteristic of IC Engine (Petrol/ Diesel) has lots of limitations because construction & working of the engine is limited by displacement volume, ignition timing, valve timing etc. To shape the torque speed curve, reduction gearbox has been integrated. Even though the performance has been achieved upto a certain limit by doing lots of tuning of Engine Components as well as electronics. Even lots of tuning required in transmission system also.

In terms of working of heat engine, used in automotive application operates in Otto Cycle (Petrol Engine), Diesel Cycle (Diesel Engine). In both cases for four stroke engine, there is only one power stroke over two complete revolution of crankshaft. Therefore flywheel is used as a kinetic energy capacitor, which stores energy during the power stroke majorly & release energy to maintain the inertia for the rest of the strokes.

Form the figures it is clear that in power stroke, torque is maximum.

As per speed torque characteristics of the engine, the torque will increase as much speed will be increased, up to a certain limit where the maximum torque will be achieved.

Then after the maximum torque limit, torque will be reduced as speed increases. Because, as the speed increases, the duration of power stroke will be reduced and the frequency of power stroke will be increased within a time span as compared to low speed. Accordingly duration of other strokes also will be reduced. Hence the specific fuel consumption will be increased also, after achieving maximum torque limit.

In case of output power, increases gradually upto a maximum limit, which is beyond the maximum torque limit because over the limit the rate of toque decrease is lesser as compared to the rate of increase of engine RPM. As speed increases beyond a maximum power limit the requirement of toque is very less to overcome the obstacles on the road, hence output power will be reduced upto maximum speed limit.