Running a 48v controller and motor --- on a 60v system. Doable?

[Rayhaque]

I have been bugging Gary over at goldenmotor.ca and asking him a lot of technical questions. He thought you all might have a better idea on this one.

I have a 60v vehicle (that's right, FIVE batteries). I want to upgrade the motor from a 2.8Kw to a 5Kw. And I would rather not have to mess with the batteries and charging system. I am already way over budget on this project. ;-) So I need to either step it up to 72v or step it down to 48v. From what I know about electrical circuits (NOT MUCH) I would think that I want to step *DOWN* to 48v.

The specs on the VEC controller say that it will take anywhere from 48v-96v DC. But will it take 60v and work okay with it? From what I read it will take the 60v and apply the same 60v to the motor. But I imagine this will take a bit of programming too? I planned on buying the USB kit too.

I am looking for a bump in speed *AND* torque. I was thinking the 5Kw motor with the 300a VEC controller.

Thoughts?

Thanks,
-Ray

[Rayhaque]

Mercy 23:38:21   48v controller can bear 60v in short time

According to Mercy at Goldenmotor, this should work. I should have made it clear in my message that I am NOT attempting to run a 48v motor at 60v! I want the controller to accept 60v, but send 48v to the motor. It appears that I should be okay with firing up the controller at 60v and immediately program it to accept this new voltage as normal.

But if anyone can confirm - that would be great!!  :-)

[Morgen 3Eman]

Hi Ray,

Hey, I'm an old geezer and I'm having trouble understanding what you want to do.  Are you looking to buy a new motor and a new controller to use with your existing battery pack? Or just a new controller to use with your current motor and battery pack?

However, if I was advised "48 V controller can bear 60V in short time" by a manufacturers rep, I would not ever consider using those components at that voltage.   

The BLDC control circuits I am familiar with simply switch the supply voltage on and off to power the motor.  I haven't seen any that regulate the voltage applied to the switching FETs.

TTFN,
Dennis

[Rayhaque]

Are you looking to buy a new motor and a new controller to use with your existing battery pack?

Yes! I have a five battery system, which gives me the odd-ball 60v. So my current controller and motor are also 60v. As my options are non-existent with 60v I need to step up or down in volts to get to something normal. I was hoping that the controller would do that for me.

But the key word here is REGULATOR (thank you for that). If it switches 60v to my 48v motor, I am in trouble. I need it to regulate the power on down to what the controller and motor want.

So I guess I am back to trying to figure out how to pull a battery out of the mix to bring it down to 48v. And that could be difficult for a nameless vehicle, with no documentation whatsoever.  :-(

Maybe I should start by pulling a battery and seeing if the charge circuit still works?

[Morgen 3Eman]

Hi
I'm still not sure I completely understand your needs, but here goes.  If you want to keep using the 60VDC battery system you have,  you should buy a controller and motor that are rated at a higher voltage- like 72 VDC.  There is no danger to those items if you run them on a lower voltage.  You need to ensure that they will accept a lower voltage for the microporcessor/control circuits, but that is usually not a problem.  But always check!
 
You should not expect to develop rated power and speed with lower supply voltage, simply because of the lower supply voltage. 

As a rule of thumb, never subject electrical devices to more than the rated supply voltage.  It tends to let the smoke out of them.

TTFN,
Dennis
 

 

[Rayhaque]

The controller seems to accept any voltage from 48-96v. So firing it up with 60v would be just fine.

But pushing 60v to a 72v motor would make it run slower wouldn't it? I know very little about how these circuits work. I am just thinking that if I run 6v to a little 12v motor, it will run very slowly. But then if I could somehow crank up the amperage, the motor would run faster wouldn't it? Is that what I would be trying to do here?

My main goal here is to gain extra speed on flat land, and also some torque if I can pull it off! I would hate to get all of this installed only to have it run slower than it had before. :-D

EDIT: Found this on an e-Bike site ... makes perfect sense!

In general if you want more speed you run more voltage. And if you want more torque you run more amps, but…in general you get more speed and more torque whenever you pump up the wattage to the motor. Calculating wattage to the motor is simple: voltage X amps = watts to motor.

Giving 60v to a 72v motor will give me fewer RPM's so I should expect a calculated speed loss.

[Morgen 3Eman]

 Hi,

Like I said,
"You should not expect to develop rated power and speed with lower supply voltage, simply because of the lower supply voltage. "

In the gear head community there is a saying "How fast you can go depends on how much money you can spend", or variations on that theme.  If you want to keep the 60VDC supply,  that is what limits the speed.  I was addressing the reliability aspect.  60 Volts won't hurt a 72 Volt controller, but would probably fry a 48 Volt controller.  60 Volts applied to a 72 Volt motor (through the appropriately rated controller) wont hurt it, (it will probably lengthen the service life) but it wont allow it to run at the full rated speed.  60 Volts applied to a 48 Volt motor probably wont kill it immediately, but you should expect a reduced service life, and the motor will run faster than the rated speed.

Your quote is right, and it makes perfect sense, but you are applying it incorrectly.  Your choice of 60 as a supply sets the max speed.  I don't remember the formula right off hand, but you can Google it to learn more.



TTFN,
Dennis
PS more torque is always a good thing:)

[Rayhaque]

I don't remember the formula right off hand, but you can Google it to learn more.

I have found a couple of variations on that formula. Some are just wrong. This is the one that I think makes the most sense ...

To calculate the top speed of an electric golf car, use the equation below:
RPM’s of motor X Tire Radius (in inches) / Gear Ratio X 168 = Miles per Hour
Example: A golf car with 3000 RPM motor with an 8" tire and a 12.44:1 gear ratio (a standard in golf cars):

I went off on a tangent last night and didn't get to bed until the Green Bay Packers were getting back on the bus. I was attempting to figure out what my RPM's would be with a 72v motor while only running 60v through it. I couldn't find a reliable way to gauge that, and I think it's because there are just too many variables. Even with a good data sheet on the motor (Golden Motors provides one) they are not going to show me what 60v does. I guess my fear is that I will top it out and I will end up going 25mph like I do now!

I also went off looking at stepdown DC/DC transformers last night. Because if I could reduce the 60v to 48v before it enters the controller - I would be set. But as you can imagine, it's pretty hard to find a transformer that brings in DC and outputs DC at 48v (and at 400amp?).

PS more torque is always a good thing:)

Torque is over-rated!     The trick is to always drive down-hill every where you go.  ;-)  But seriously, I have mapped a route from home to work to take advantage of the lowest hills in my favor. This site is really cool: http://cycleroute.org/  If I failed to improve my speed, but gained a little torque, that wouldn't be a complete loss.

[Morgen 3Eman]

Hi ,

Kind of wasted your time Googling that one, eh?  Where is voltage in the calc?   

It seems your mind is already made up on how you want to proceed.

Have fun with your project.
 
TTFN,
Dennis
 

[Morgen 3Eman]

Hi,

This is the first thing I found…..

Hi Duncan:
An ideal DC motor's RPM is directly proportional to applied voltage.

RPM=K1*V where K1 is a constant, and V is applied volts.

Real-life motors have armature resistance and losses, which causes variation
from this equation. Think of the motor as a generator which turns as fast
as needed to generate a back emf = applied emf. However, armature
resistance drops some of the applied voltage, so that:

RPM=K2*(V-I*R)=K2*V-K2*I*R where K2 is a constant not=K1 above.
I is armature current, and R is armature resistance.

If you can measure armature current, you can empirically determine the K2
times R and add this term for more accurately determining RPM. (This is
often done in practice for speed control).

Summary:
* The first equation is approximate, depending on motor quality,
(i.e.,armature resistance and losses).

* The second equation can be used for more accuracy if you are willing to do
some calculations.

[Supchrgamx]

around  here its called cubic dollars
as in motor cubic inches

[Morgen 3Eman]

HI Supchrgamx,

That is sooo true!  Thanks for the laugh

TTFN,
Dennis

[Bikemad]

Maybe I should start by pulling a battery and seeing if the charge circuit still works?

That would definitely not be a good idea unless you use a 48V charger, your existing 60V charger would overcharge the 48V pack.

Alan
 

[AdamB]

Ray

You might try posting in the e-car sub-forum... You'll get better luck with answers. This is the general discussions forum but the e-car, e-motorcycle, etc sub-forum all use the motor/controller type you are talking about; as opposed to most GM customers in general use e-bike kit.

I may be wrong, it just seems like you are getting some confusion over what you are getting to do... Or perhaps I am confused & misunderstanding you...

Adam

[Rayhaque]

RPM=K2*(V-I*R)=K2*V-K2*I*R where K2 is a constant not=K1 above.
I is armature current, and R is armature resistance.

CRIKEY! I have no hope in hell in getting a good figure out of that formula. :-) I think I will stick to this formula instead ...

around  here its called cubic dollars as in motor cubic inches

       

I think the general consensus here is that I would be out of my mind to run 60v to a 48v anything, and think that it would be okay. With that, I guess I will order the 72V motor and hope for the best. I'm not sure how the math will work out. Voltage-wise I will be putting about %85-ish V to the motor. This will certainly drive it. To what extent, I don't know. I am only realistically trying to hit the 35-ish MPH mark. Any more than that and I will risk certain death!

Perhaps I can reach my desired speed with controller adjustments. Or this will be a huge failure, and my motor and controller will be back on the shelf until next summer when I can add a battery, a 72V charger, 72V contactor, etc.