Speed on go kart using 5kw GM motor and controller

[Tom E]

I posted a similar question in the endless-sphere forums but haven't gotten a reply in a couple days so thought I would try it here...

I want to build a go kart (have frame) into an electric kart.  I was thinking I would get a 5 kw GM bldc motor and controller but wonder how fast the kart will be able to go with that?  Also, I want to be able to switch select a lower top speed to allow smaller kids to ride safely and wonder if this is possible with the GM controllers? The frame has 11" wheels and I may use a 60 tooth sprocket on the axle, not sure about the sprocket for the motor shaft.

Also, I may either buy or build a battery pack.  I was wondering what the options are for a bms if I build my own packs using lipo cells?

Thanks
Tom

[GoldenMotor]

We tried using 5KW motor to drive 600kg weight of trike, at speed of 60km/h. So you can work out based on these numbers.

You can have a potentionmeter to limit the speed.

[truly_bent]

Let's see...

The specs on the HPM-5000B work out to a KV of 83.3. That's the RPM/Volt factor.
83.3 x 24V = 2000rpm
83.3 x 48V = 4000rpm
83.3 x 72V = 6000rpm

Assuming 48V for the moment, and a 12 tooth sprocket on the motor shaft (for easy calculation). Your 60 tooth gear on the axle gives a 5:1 reduction, for 800rpm (4000 / 5) at the wheel. Note that that's likely to be the "no load" speed.

11" (wheel diameter) x pi = 34.557" wheel circumference
34.557 x 800rpm = 27646"/minute
27646 x 60 = 1658760"/hour
1658760" / 63360" =  26.2mph  (there being 63360 inches in a mile)

Presuming a "loaded" motor speed of 70% (although a 5KW motor might not even feel a go-cart), then your top speed would be around 18.3mph. I'm guessing you want more speed than that. So, lower your gear reduction or increase your battery voltage.

As for LiPo's; those are used mostly by the RC community these days and I don't know if that bunch of outlaws are even aware of BMS's. Probably see them as just extra weight and a hindrance to spectacular flame-outs. But, you'd probably have to use LiPo's to get anywhere near the necessary C rate to drive that mother.

As the Emperor suggests, you're probably going to have to work out a simple resistor network to give you speed ranges. A two pole toggle switch would probably do it for switching in different resistor values to limit the throttle voltage to the controller.

Happy go-carting.


P.S. Please post any videos of spectacular flame-outs. Always a crowd pleaser.

[Bikemad]

The battery pack current output will have a huge impact on the acceleration, and I reckon at least 300A will be required for quick acceleration if geared for a high top speed, or use much lower gearing to produce ultra quick acceleration and masses of wheel-spin, but with a lower top speed.

You can have a potentiometer to limit the speed.

As Yao suggested earlier, the potentiometer would be the easiest option as it can be adjusted from anything between snail and cheetah mode and then simply switched on or off as necessary:



To give you some idea of the performance, the Vectrix electric scooter weighs 210kg and uses a 128V, 7kW continuous motor (20.2kW peak) which produces 65Nm of torque and can reach a top speed of ~68mph. It can accelerate from 0-31mph in just 3.6 seconds and 0-50mph in 6.8.


A go kart fitted with a 5kW motor should be a fair bit lighter and have even less aerodynamic drag, so it should have a similar (or even better) performance.

Alan
 

[Tom E]

Let's see...

The specs on the HPM-5000B work out to a KV of 83.3. That's the RPM/Volt factor.
83.3 x 24V = 2000rpm
83.3 x 48V = 4000rpm
83.3 x 72V = 6000rpm

First, thanks for all the responses, all good info.

RE: the KV numbers above, how is this calculated, I don't see it in the specs?  Also regarding specs in general, in looking at various motors I see some that give power as, say, 10kW continuous, and some that only give voltage and amps and (sometimes) torque.  Is there a good link for formulas for calculating all the specs so comparing is easier?

Assuming 48V for the moment, and a 12 tooth sprocket on the motor shaft (for easy calculation). Your 60 tooth gear on the axle gives a 5:1 reduction, for 800rpm (4000 / 5) at the wheel. Note that that's likely to be the "no load" speed.

I was thinking that 48V would be good to use as there seem to be quite a few choices of batteries in that range, and stepping up to 72 or higher seems to limit choices a bit more and/or raise the cost considerably.  There is a 20 tooth drive sprocket (on the site I am looking at for differential, bearings and sprockets - www.northerntool.com) so that would make the gear ratio 3:1 and with your calculations below using 70% load, gives about 30mph which would work.  I could then always up the voltage later if that proved to be too wimpy.

My question is, does that seem like a reasonable route or should I think about getting a larger 10kw motor even though the cost is nearly double? I'd rather not but if using the 5kW is just going to result in an underpowered cart and no fun....

Thanks again and will post info as I progress.  Hopefully no flames required :-)
Tom

[Morgen 3Eman]

Here is another data point.  My Morgen and I weigh about 400 Lbs.  My MP3 with 48V GM battery pack goes 24 MPH.  Surely your kart is much lighter than this, and 5KW is much greater than the 1KW MP3.  With the low RPM torque available in BLDC motors, you can drop the gear ratio a whole bunch to get much more speed.  I think with a 5:1 ratio you will have the ability to smoke the tires.

TTFN,
Dennis

[Tom E]

Here is another data point.  My Morgen and I weigh about 400 Lbs.  My MP III with 48V GM battery pack goes 24 MPH.  Surely your kart is much lighter than this, and 5KW is much greater than the 1KW MP3.  With the low RPM torque available in BLDC motors, you can drop the gear ratio a whole bunch to get much more speed.  I think with a 5:1 ratio you will have the ability to smoke the tires.

TTFN,
Dennis

Thanks, I will likely go with the 5kW then.  On controllers, is the max amp rating refer to the max amps the controller will supply to the motor?  What are the limiting factors?  Can I go with the HPC300 or would I want the HPC500?

Tom

[truly_bent]

Okay, zo...

5KW (5000W) / 48V = 104.2 amps. This is the nominal current for this motor, at this voltage.

The HPC300H is already way overkill at 360A (@ 48V). No way you're gonna pump more than 360A through that motor before it becomes a puddle of slag. And stand back, cause the ending is spectacular.

What you're gonna get with the 10K motor is a ton more torque (well, double anyway). Top speed is likely to be about the same (unless the motor pole counts are unequal). All that torque is really your amps in action. How many amps you can pump through the motor sorta equals how much fun you'll be having.

The problem comes in when you have to feed the monster. Your battery costs are some multiple of the motor cost (i think mine is about 4). This curve tends to get expensive quickly.

Later...

[Tom E]

Okay, zo...

5KW (5000W) / 48V = 104.2 amps. This is the nominal current for this motor, at this voltage.

The HPC300H is already way overkill at 360A (@ 48V). No way you're gonna pump more than 360A through that motor before it becomes a puddle of slag. And stand back, cause the ending is spectacular.

What you're gonna get with the 10K motor is a ton more torque (well, double anyway). Top speed is likely to be about the same (unless the motor pole counts are unequal). All that torque is really your amps in action. How many amps you can pump through the motor sorta equals how much fun you'll be having.

The problem comes in when you have to feed the monster. Your battery costs are some multiple of the motor cost (i think mine is about 4). This curve tends to get expensive quickly.

Later...

Cool, thanks.  I was thinking of getting the GM 48v 30ah pack unless I decide to roll my own...any opinions on GM batteries vs other choices?

Tom

Tom

[truly_bent]

The "C" rating is what you want to be paying attention to when it comes to batteries. That's the discharge rate. My LiFeP04 20AH has a C rating of 2. This means that it will comfortably provide 40A before the possibility of degrading the cells. It's rated to provide 60A max (3C), but what damage may be happening to the cells above 2C is a grey area. Probably not good.

The MP draws 30A nominal. Note that my battery is able to provide more amps comfortably than the motor will draw. This ensures that the battery discharges below it's 2C parameter and should remain healthy for a long time.

Now, your 5K motor draws a nominal 104A. A 30AH 2C battery will comfortably provide 60A before exceeding its 2C rating - with 90A being the max. You see the problem here? Some controllers allow current limit settings via software, but I haven't checked that on the HPC300H you're looking at. Suffice to say that without limiting the current, the 5K will draw as many amps as are available - right up to 104A (@ 48V).

I think you'll have to pay a visit to the RC Gang clubhouse and go with high C rate Lipo batteries. Lipo's were kinda dropped by many in the e-bike community because placing incendiary devices so close to one's privates is a bit unnerving (speaking for myself of course), but as long as you don't over-charge or discharge the cells beyond their working parameters, you should get plenty of tire smoke without burning down anything expensive. It just takes a bit more vigilance.

As a comparison, the same 30AH batteries in 10C Lipo's (i think the C ratings can go higher than that on Lipo's) would provide 300A - albeit only for about 10 minutes. Now you're back to making spectacular flame-out videos.

For all this free advice, I want dibs on yer first flame-out vid.

[Tom E]

The "C" rating is what you want to be paying attention to when it comes to batteries.

Thanks for the great explanation. Soooo, for a mere $2000 I could get two 30aH packs (in parallel) and be able to pull 120A comfortably (assuming the C rating works that way as well)? And one one each side would be better balanced! 

For all this free advice, I want dibs on yer first flame-out vid.

You really are convinced I am going to go up in flames, aren't you?  Do you know me or somethin' ? 
Tom

[truly_bent]

You got it Grasshopper!

And when you're looking for more thrills, you'll spend another $2000 for two more lumps and hop up your cart to 96V (the motor is rated for this voltage as well). This will nearly double your speed, not accounting for wind drag and the occasional bridge abutment.

It has a secondary benefit of allowing you to change your power wiring down to a lighter gauge (up the AWG scale actually). All components handling the drive power can be scaled down in specs because they don't have to handle as much current.

5KW (5000W) / 96V = 52.1A. This is the same motor's new nominal amp rating (@ 96V)

Translated this means that you get the same power (watts) for half the juice (amps) and twice the pressure (volts). Something to do with Ohm's Law I believe. Anyway, less juice equals less heat.

Sayonara

[Tom E]

And when you're looking for more thrills, you'll spend another $2000 for two more lumps and hop up your cart to 96V (the motor is rated for this voltage as well).

I only see 72V listed on the web site.  I understand that many motors can be run at higher voltages up to the (unknown) point where the insulation on the coil wires breaks down.  Not sure if I will get to the point of wanting to become one with bridge abutments anyway... 

Sayonara

This sounds so... Permanent
Tom

[truly_bent]

I took my numbers from the motor picture on the GM website (see attached). See that little "96V" tagged onto the end of the "Voltage:" list?

If you want to know just how much electrical pressure your motor can take, you'll have to dig deeper into the insulating material separating the windings. You're on your own there Grasshopper . I doubt GM would reveal such proprietary information anyway.

By now you're probably ready for the Big Leagues. You'll find a heap more info on pushing the envelope over at http://endless-sphere.com/forums/.

Just keep an eye open for marauding RCers. I hear they bite if you try to feed 'em.

[Tom E]

I took my numbers from the motor picture on the GM website (see attached). See that little "96V" tagged onto the end of the "Voltage:" list?

Interesting, I have been looking at the GM Canada site and the photo there is different (and only goes to 72V).  Perhaps they are trying to be a bit more conservative or something...

In fact, when I look at Goldenmotor.com at the bldc motor page, the HPM5000B shows that it goes to 120V!

Tom