DC Motor Theory and Practice

Question

My first post here, I have been playing with a DC motor and attempting to upgrade the original batteries from SLA to Lipo, the scenario is as below and i'm looking for some advice on what happened and some advice on a motor upgrade to avoid what happened to the original motor in the future, apologies for the long post...

So I had a DC motor (400 W) powered by a 12V SLA 33Ah battery, I have bought some 4s Lipo batteries (80Ah total) to replace the 12v SLA, the Lipos are lighter and have a much greater capacity but in turn deliver above the original 12V spec of the motor (16.8V full, 14.8V nominal).

I was hoping for a slight speed increase and an increase in duration, what I got was a massive speed increase and now a burnt out motor !

Originally the underwater dive scooter this setup was working in used to run for one hour at around 30 meters per minute covering approximately 1800 meters before the SLA battery.

When I took it for a dive with the new Lipo batteries it ran for 80 minutes covered 3600m at 45 meters per minute then the power dropped off rapidly. I assumed the batteries were low, took it home recharged and tried to dive again but the motor was severely under powered then stopped completely, on examination at home the coil has melted in places.

So my basic knowledge leads me to believe that the increased voltage of the Lipo batteries lead to an increase in the speed and an increase in current drawn which lead to more heat and the motor eventually burning out; firstly could someone confirm i'm thinking along the right lines here ?

Its an old Bosch motor which I don't have the specs for all I know was its 12V and 400 W.

Main question is this: I would like to replace the motor with a 24 V 750 W, one of these as they are used in other 24V dive scooters:

http://www.everything-ev.com/index.php?main_page=product_info&products_id=242

What are the likely consequences of supplying the 24V motor with the 4s 80Ah Lipo configuration and delivering between 13.2V (min.) to 16.8V (max.) to it, i'm expecting this will decrease its speed and power considerably but not sure what implications this might have on the current it draws if its designed for 24V supply.

Second option will be to rearrange the battery pack, so its 8s and delivering between 26.4V (min.) and 33.6V (max.) and put in a regulator limiting it to 24V to the motor.

Any thoughts on this much appreciated

Answer 1

The higher voltage caused the motor to run at a higher speed.

As your load was a propeller, whose torque and so current demand goes as speed squared, this resulted in a larger increase in current. As heating in the motor goes as current squared, this resulted in a much larger heating, which burnt out the motor. So if we take only the 14.8 nominal voltage, the current would rise by (14.8/12)^2 = 1.5, and the armature heating by (14.8/12)^4 = 2.3. At max voltage, it's much more.

Supplied with less than its rated voltage, the 24v motor will run slower than its rated speed. It's still possible to burn it out if the propeller load is high, and so it demands too high a current.

Given you're driving a prop load, it would be a good idea to have a speed control. It's probably more convenient to have the battery voltage above that of the motor and buck it down, as this gives you 100% speed control, and a bit of overrange if you need it. Using a battery voltage below the motor requires a buck-boost converter, a bit less efficient than a simple buck.