# An EV without motor controller



## leonheart (Mar 2, 2011)

Hi
I'm new in this forum but I'm trying to understand as much as possible 
from your conversions. I want to understand if is possible have a 
simple potentiometer, in a very base conversion, and no controller.
If isn't possible, why? What is the function of a controller that we
need in a EV?


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## WarpedOne (Jun 26, 2009)

> I want to understand if is possible have a simple potentiometer, in a very base conversion, and no controller. If isn't possible, why? What is the function of a controller that we need in a EV?


Theoretically it IS possible, but you wouldn't want to do it that way.

Potentiometer would act as a voltage divider between motor and resistor. 
Both would always get the same current from the battery. When fully opened, the whole voltage would get applied on the motor and none on the resistor, but when fully closed whole voltage would heat up the resistor. Your system would always work at full power - when motor wouldn't spin, the resistor would turn all the available energy into heat. 
Inefficient and hot as hell.

So, you need a way to control how much energy gets to the motor = how much current flows out of the battery. This is the sole and main function of EV motor controller. It acts just like a very fast On/Off switch. It switches on and then off many thousand times a second - the energy from battery flowes to the motor in packets or drops. More packets mean more power, less packets less power.


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## leonheart (Mar 2, 2011)

WarpedOne said:


> Theoretically it IS possible, but you wouldn't want to do it that way.
> 
> Potentiometer would act as a voltage divider between motor and resistor.
> Both would always get the same current from the battery. When fully opened, the whole voltage would get applied on the motor and none on the resistor, but when fully closed whole voltage would heat up the resistor. Your system would always work at full power - when motor wouldn't spin, the resistor would turn all the available energy into heat.
> ...


Thanks for the complete response


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## DavidDymaxion (Dec 1, 2008)

It is correct if you try to control 10's of kW of power directly with a potentiometer, you'll get 10's of kW of heat wasted in the potentiometer, which probably would have burned up well before then anyways.

There is another trick, though (one I have done myself). You get a shunt or sepex motor. The armature gets started with just a contactor switch, and the motor spins at a high idle speed. You then use a potentiometer on the field to weaken the field, to further speed the motor up.

Yes, this also wastes energy as heat in that field potentiometer... But the field current is much lower (around 10 Amps) than the armature current (100 to 1000 Amps). Now you are wasting only around 1% of your power in that little field potentiometer, about on par with a modern silicon controller.

You need a manual transmission car with the clutch, and the motor idles. This does not scale well to a high voltage car. It has been demonstrated, however, on 48V and 72V cars, and can be adequate for a city car. It is cheap, and it gives regen. It's crude, but better than no EV at all!



leonheart said:


> Hi
> I'm new in this forum but I'm trying to understand as much as possible
> from your conversions. I want to understand if is possible have a
> simple potentiometer, in a very base conversion, and no controller.
> ...


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## EVfun (Mar 14, 2010)

Hey David, what happens if you suddenly back sharply off the throttle at, say, 35 mph? 

I've find that sep-ex idea very interesting, but I think the driver would have to relearn how to use the throttle.


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## DavidDymaxion (Dec 1, 2008)

With just 48V and a 2000+ lb car, not a problem. If you threw it down 4th to 1st; or turned it off down a hill, sped up, and then clicked it on, you could get a brief skid out of the tires -- but you had to work at it. On the slippery Salt Flats I could get 2 to 4 meters of wheel spin taking off if I popped the clutch, and a brief spin in 2nd, but if the clutch was slipped even for a fraction of a second no wheel spin (I could get unlimited wheel skid doing tow regen, but getting towed by a 260 kW charger (a truck) is not usual practice!).

The gas pedal tends to more choose speed than torque, and if you pressed the gas pedal down too far at low rpm it would actually accelerate less quickly! So you are right it did not perfectly mimic a gas car, but if I threw the keys to an average person that could drive a manual tranny they would get around OK.


EVfun said:


> Hey David, what happens if you suddenly back sharply off the throttle at, say, 35 mph?
> 
> I've find that sep-ex idea very interesting, but I think the driver would have to relearn how to use the throttle.


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## Lee Hart (Oct 16, 2009)

Remember that we've had EVs for well over 100 years -- long before there were transistors, or even vacuum tubes to make controllers!

I've driven a 1902 Baker Electric. It had a reasonably good speed control, forward and reverse, and even regenerative braking, with no clutch or transmission. It was all done with switches and a compound wound motor (both series and shunt fields).

It worked by switching the batteries in various series or parallel configurations, and powering one or both field windings. The switch was a big drum, with copper fingers sliding on copper segments. There were also two "starting" resistors, used only to get moving from a dead stop or to absorb braking energy when going too slow to regen into the batteries.

The 1970's Sebring Vanguard CitiCar used an even simpler setup. It had a series DC motor, coupled directly to the rear axle (no clutch or transmission). The battery pack consisted of two 24v banks (four 6v golf cart batteries each). Its controller consisted of three contactors (on/off, series/parallel, and forward/reverse) plus a starting resistor. The accelerator pedal operated three switches in sequence to select:

1. off
2. 24v and starting resistor to motor
3. 24v direct to motor
4. 48v direct to motor

This is about the minimum setup one can use and still get satisfactory results. I drove such a vehicle to work every day for years. Though crude, it works!


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## major (Apr 4, 2008)

leonheart said:


> Hi
> I'm new in this forum but I'm trying to understand as much as possible
> from your conversions. I want to understand if is possible have a
> simple potentiometer, in a very base conversion, and no controller.
> ...


Hi leon,

Here's what a pot would look like  

http://www.diyelectriccar.com/forums/showpost.php?p=204460&postcount=3 

Regards,

major


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## WarpedOne (Jun 26, 2009)

> 1. off
> 2. 24v and starting resistor to motor
> 3. 24v direct to motor
> 4. 48v direct to motor


I love this setup. No controller that overheats, breakes, goes mad etc.

I would do it in more steps though. With 12 12V battery blocks:
12s1p = 144 V
6s2p = 64V
4s3p = 48 V
3s4p = 36 V
2s6p = 24 V
1s12p = 12 v

6 levels of power might actually work very well. I need to try it


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## Lee Hart (Oct 16, 2009)

WarpedOne said:


> I love this setup. No controller that overheats, breaks, goes mad etc.
> 
> I would do it in more steps though. With 12 12V battery blocks:
> 12s1p = 144 V
> ...


Three steps is just barely enough to get by. The EV I drove with a 3-step controller also had a 3-speed transmission, so I had a total of 9 speeds. This made it perfectly adequate.

Six steps gets to be a lot of switches or contactors, and is likely to be more steps than you need. Start by building with a smaller number of steps, and see how you like it. Add more only if you find it necessary.

The "rectactor" circuit is a good modular way to build such controllers. Here's my write-up on it: http://www.evdl.org/docs/rectactor.pdf


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## DIYguy (Sep 18, 2008)

WarpedOne said:


> I love this setup. No controller that overheats, breakes, goes mad etc.
> 
> I would do it in more steps though. With 12 12V battery blocks:
> 12s1p = 144 V
> ...


No... just batteries that are totally unbalanced and prone to failure and terribly inefficient at using available energy. Get with the times folks...


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## EVfun (Mar 14, 2010)

DIYguy said:


> No... just batteries that are totally unbalanced and prone to failure and terribly inefficient at using available energy. Get with the times folks...


This method doesn't imbalance the batteries. The pack isn't being tapped, it is being put into parallel groups. It takes a number of contactors and diodes.


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## DIYguy (Sep 18, 2008)

EVfun said:


> This method doesn't imbalance the batteries. The pack isn't being tapped, it is being put into parallel groups. It takes a number of contactors and diodes.


Oh.... sorry... guess I didn't read it... just thought it was an extension of the tapped pack discussion. hmmm, ok no imbalance.... but enough electrical connections wire and contactors to make anyone wish they had a real motor controller....  Hmm, this might even cost more. lol

Really folks. . . this stuff is great for school project, learning, development, fun. etc. But, if you want to make a real "DIYElectric Car", which is kinda what I think most ppl are after, then you really need to get what works best. A "complete system" to make others wish they had a "real electric car". Yes?


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## WarpedOne (Jun 26, 2009)

> but enough electrical connections wire and contactors to make anyone wish they had a real motor controller.


This gets compensated by absense of a BMS. Batteries are always perfectly balanced. By design.

The sole problem are those high-current contacts.
But if DC motor brushes can work without melting and fusing, so can such a contactor.

It's not just the price, it's mainly reliability and no overheating problems.


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## vpoppv (Jul 27, 2009)

leonheart said:


> Hi
> I'm new in this forum but I'm trying to understand as much as possible
> from your conversions. I want to understand if is possible have a
> simple potentiometer, in a very base conversion, and no controller.
> ...


 Ok, I am the cheapest of the cheap when it comes to...well everything But in all honesty, I bought a 72v Curtis controller for $75 on eBay. I doubt you could find all the parts you would need for so cheap to make it work. If you are patient, you can get a controller that will work for you. The headaches you will go through to mimic a controller is just not worth it IMHO.....


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## WarpedOne (Jun 26, 2009)

> I bought a 72v Curtis controller for $75 on eBay.


The real question is is such a controller really good enough and up to snuff.
We have a saying 'round here that goes something like "little money, little music".

Ofcourse "real" controllers are better on pure technical merits but on the metrics of every day performance / price things aren't that well cut. Even Tesla Roadster has that Overheating control light, cooling fans that stuck and fail etc.

Simple is beautiful.


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## DIYguy (Sep 18, 2008)

WarpedOne said:


> This gets compensated by absense of a BMS. Batteries are always perfectly balanced. By design.


Lead acid probably goes out of balance worse than most chemistries. With floodes, ya, it doesn't matter so much since u boil them to balance at the top. With VRLA (sealed gels or AGM) you need to keep them balanced cause u can overcharge a few and they will vent/die early. I have LiFePo and they probably stay balanced better than most chemistries. I run with no BMS. When I had AGM's, I used a BMS.



WarpedOne said:


> It's not just the price, it's mainly reliability and no overheating problems.


Comes down to what you buy. U have to get one with a good reputation. There are no guarantees though, ur right about that. Walking is pretty reliable....unless u twist ur ankle.


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