# rheastat type device



## lou-ace (Jul 21, 2009)

Ok I'm not an EE, but when I used to work in a electric motor repar shop ( skyline electric Utah) we used to test our motors with a huge coil round device that had various resistnce built in on a huge coil circular device that i think was called a rheastat. It was calabrated at different amperages and we could run up the motors that we had rebuilt to a specified amperage and run them for 10-20 minutes and certify them these were the motors used at the Kenicot copper mine in those huge trucks that hauled the ore out of the pit and had to run all day diesel/electric configuration. So I'm really curious why we can't use that same kind of configuration for our EVs........... what s with these $1111111 controllers.


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## Qer (May 7, 2008)

lou-ace said:


> So I'm really curious why we can't use that same kind of configuration for our EVs........... what s with these $1111111 controllers.


Sure you can. You just need a bigger pack. A MUCH bigger pack. Something like a $2222222 bigger pack.

Knock yourself out!


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## OBeer-WAN-Kenobi (May 28, 2010)

Qer said:


> Sure you can. You just need a bigger pack. A MUCH bigger pack. Something like a $2222222 bigger pack.
> 
> Knock yourself out!


Are you talking about the rheostat being inefficeint or are you talking about running one of those huge motors? I think the OP is asking about using a rheostat for a throttle. I'm interested in this as well at 36-48v.

Bad idea?


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## Qer (May 7, 2008)

OBeer-WAN-Kenobi said:


> Bad idea?


I made a graph over a fictive situation where the pack voltage is 100 Volt (without any sag what so ever, impressive pack!) and plotted a graph over efficiency with a rheostat versus a controller where the voltage drop over the transistors are 1 Volt.









If you look at for example 50 Volt motor voltage and you want to get 10 kW out of the motor, that means that the motor current will be 200 Ampere, but since the rheostat only works like a giant heater with the rest of the pack voltage over it (50 Volt again) and the same current it means the rheostat burns 10 kW as well, resulting in that you drain 20 kW from the pack!

In the controller case the motor still has 50 Volt over it, but since the controller works like a buck converter you don't waste 50 Volt over it. An ideal controller won't waste a single Watt, no controller is ideal but I'm lazy so I'll pretend it is (despite the graph above) so that means those 10 kW in the motor is drain from the pack, which means that on the pack end have 100 Volt and 100 Ampere (10 kW) that is converted to 50 Volt and 200 Ampere (still 10 kW) in the motor end.

In reality you will lose some in the controller too (typically a few percent) but it's really nothing compared to the losses in the rheostat. Besides, a rheostat that can handle serious amounts of kW aren't exactly cheap. If you can't find one cheap in the junk yard I wouldn't even be surprised if it's even MORE expensive than a controller, and then you need to cool it too. As if that wasn't enough you also need a bigger pack to get the same mileage...

So yeah, bad idea. Very bad idea.


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## OBeer-WAN-Kenobi (May 28, 2010)

Qer said:


> So yeah, bad idea. Very bad idea.


Thanks,
Your graphs and explanation make sense and it makes sense to me that on any actual car conversion it would be a bad idea. Hence, for the OP, bad idea

However; do you think a rheostat is ok for a 36-48 volt system like my lawnmower conversion where I would only really be using it to soft start and then run at full throttle 99% of the time?

I still may use a controller but this would be interesting if it's easy and not too expensive. For the tiny amount of time it would be running at lower than full throttle, the inefficeincy probably won't be a big deal.


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## EVfun (Mar 14, 2010)

OBeer-WAN-Kenobi said:


> However; do you think a rheostat is ok for a 36-48 volt system like my lawnmower conversion where I would only really be using it to soft start and then run at full throttle 99% of the time?


Either a rheostat of possibly just a starting resistor. Since the time spend at less than full motor voltage is minimal the efficiency hit is insignificant. Some type of soft start is a good idea


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## DavidDymaxion (Dec 1, 2008)

Lou-ace, are you still in Utah? There is a Salt Lake electric car group:

http://groups.google.com/group/utah-ev-interest

Onto the rheostat thing: One option is to use a starting resistor just briefly to get rolling (some golf cart controllers work this way). It wastes power, but just for a moment. The second option is to use a rheostat to control the field of a shunt or sepex motor. This allows you to control the speed of the motor over a range of about 3:1 in rpm. Since the field takes a small fraction of the power of the armature, it makes for small losses, whereas a rheostat on the armature wastes much power.



lou-ace said:


> Ok I'm not an EE, but when I used to work in a electric motor repair shop ( skyline electric Utah) we used to test our motors with a huge coil round device that had various resistnce built in on a huge coil circular device that i think was called a rheastat. It was calabrated at different amperages and we could run up the motors that we had rebuilt to a specified amperage and run them for 10-20 minutes and certify them these were the motors used at the Kenicot copper mine in those huge trucks that hauled the ore out of the pit and had to run all day diesel/electric configuration. So I'm really curious why we can't use that same kind of configuration for our EVs........... what s with these $1111111 controllers.


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