Choice of motor

There is a jungle of electric motors out there DC, AC, Series DC, permanent magnet and so on. The choice of motor is critical so you should invest some time in it. The fiat 500 R has an 18hp 594 cm³ engine producing about 35 Nm at 3000 rpm, max hp at 4500 rpm.

The original I.C.E without the "shell"

Give the values above and the fact that I will be using the original gear box I started looking for a suitable motor.

A good place to start looking for a suitable motor is http://www.evalbum.com/. You can browse through cars and even find your car model already converted.  The reason for choosing Agni as the motor manufacturer is because of its low weight, high rpm and high efficiency. The only back draw would be that it’s not a brushless motor.

I’ve used motenergy’s (http://www.motenergy.com/) ME0201013001 in my boat and the ME0708 for my mower and they work fantastic. The reason for not using them on the fiat is the low rpm.

This are the specs of the Agni 95-R

· Hi Torque 8" Diameter motor

· Highest Power to Weight ratio

· Kevlar banded for higher RPM performance.

· Bi Directional rotation

· Adjustable Timing

· Easy brush maintenance

Model	rpm/V	max. V	A cont. @48V	cont. output power@48V	max rpm	cont. output power	max. power for 5sec
							kW
95-R	71	84	220	9.5 KW	6000	16 kW@78V	ca. 30 kW

Agni uses the following model to calculate the size of engine for a specific vehicle.

Take the weight of the vehicle, I use 600 Kg, and multiply it with the intended top speed, let’s go for 100 Km/h. This gives me a value of 60000. If the value is more than 30000 you will need a gearbox if it’s lower than 30000 you can go for direct drive.

I’ll use the gearbox on the fiat and the rule of thumb is:

When in top gear with a gearbox, the overall reduction ratio needed will be about 3.6:1 with typical size small car wheels (145/70x12, approx. 20 inches diameter including tires).  The fiat 126 gear box and the top gear ratio is 0,87:1 and the final drive ratio is 4,87:1.

Gear box with integrated final drive

Let's try some math!

The gear ratio on the 4th gear is 0,87:1. The Agni 95-R gives me 71 rpm per volt, I'm guessing this is during a no-load scenario. I will be using a 72 volt system which gives me 71*72=5112rpm 5112/0,87 equals 5875 rpm before the final drive ratio. Rear end gear ratio is 4,87:1 and tier diameter is about 20 inches. This boils down to a maximum theoretical speed of 72 Mph. 

The rpm/v will be lower when under load, approx 4800rpm, and the voltage will decrease during high ampere draw from the battery pack so I'm guessing I'll end up somewhere around 67 Mph.