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sqgdi

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I had a doubt while choosing an dc motor for a college EV project. While calculating the torque on my motor I had considered two factors :

1. The weight of the car.
2. Aerodynamic drag force.

So, while looking at motor datasheets and considering the load on motor, why should I consider the aero drag force? I feel that the motor is working against the weight of the car only. How does aero drag affect the torque load on it?

I know the car works against the air by pushing it out of the way and all, but wont the motor just need an extra amount of power to reach the required speed and have no affect on how much torque it is experiencing? Basically it comes down to the point whether a dc motor has change in load torque on it when it goes to higher speeds?

kennybobby

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Why should you include aero forces (and rolling resistance)? Because if you ignore these you will underestimate the loads and undersize your motor.

How are you calculating torque based upon weight?--Chances are a force is involved and it is no different for aero and rr forces.

Herein find an online calculatus tool:
http://ecomodder.com/forum/tool-aero-rolling-resistance.php

major

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....but wont the motor just need an extra amount of power to reach the required speed and have no affect on how much torque it is experiencing? ...........
An extra amount of power That is strange wording.

All power from the motor is always equal to the shaft torque times the rotational speed. P = T * S. Or, in familiar units: HP = lb.ft. * RPM / 5252. You can't get power from the motor without torque.

sqgdi

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Umm...i am new to this but I will try to raise my concern more clearly this time around. Basically what i want to ask is that as the aero drag increases the load on motor, load will increase as we increase speed...how am i supposed to cross check it with the motor datasheets that give us values tested on a dynamometer or something. Because suppose i take a cruising speed of 50km/hr for my car...and it gives about 22 Nm torque...i check the efficiency of of motor through datasheet at that value and be content....but if I want to increase speed...my torque will increase and thus i look at another efficiency rating at that new torque on the motor datasheet...so how should i choose my motor seeing these datasheet values....

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major

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...so how should i choose my motor seeing these datasheet values....
The first column is torque. Torque is the independent variable and is the x-axis on motor performance characteristic curves. Torque is the load on the motor, or the force opposing shaft rotation and the motor behaves differently or reacts to this load.

The second column is voltage applied to the motors terminals. In this case it is obviously an attempt to hold 72 Volts constant for this test. It is like having a constant voltage power supply instead of the battery.

All the other columns or values depend on these first two. The load and the applied voltage. So if your vehicle has a ideal 72 Volt battery, then the speed column shows you the maximum speed you can go at each load (torque) point. You can't go faster than that for that torque. You can go slower by virtue of the motor controller. It will reduce the motor voltage so you can go slower.

Also notice that this motor has a peak output of about 4500 Watts (~6hp).

So to address your question: Size the motor for the required power and gear it for the required torque. Make sure that your operational load/speed/voltage conditions fall within the area outlined by the performance curve. And that you are observing the thermal limitations of the machine for your anticipated duty cycle.

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