mac hub 7t In 20"

[1KW]

since it's a fast wind, I'm going to lace it to a bmx rim and then use 16" moped tire. anybody else have a 20" mac?

what size spokes, what kind of lace? if used at 18s and lower amps will it be quick and last?

 

[crossbreak]

i run a 6T in 26" rim at 48V/120A Kelly KBS48121X. Tops out at 60kph, peaks at 5kW. The pervormance is really good. 50kph can be reached in less than 4 seconds.

I use 13g spokes single crossed with 4.5mm nippels to fit the normal bike rim: Mavic EX325
Single crossed might not work for a 16" moped rim. Maybe you should think about an 17". 12g should work well in a 17" rim if you use some washers. Try to use aluminium washers in a aluminium rim to prevent corrosion, dont use stainless washers

Since it got hot quite fast, i filled it with 80 ml oil, put in two 17x26x7 radial shaft seals and sealed the side covers with engine sealant. That helped. Stays much cooler now, the oil didn't leak out anywhere yet.
Edit: Yesterday I had some drops of oil where the phase wires exit the shaft. The oil seems to always find a way out :?

You should ask for 10k thermistor and two shaft seals if you order your upgrade-MAC from cellman

Dont use this power level without thermistor+CA V3 for temperature limiting. You risc burning the MAC in minutes.

 

[Ch00paKabrA]

I run a 6T in a 19" M/C tire that has a diameter of 25.5". The bike will max out at 38 mph with the wind at my back on 14S. What size tire are you going to run? A 2.5" moped tire would give you a wheel diameter of around 21".

In an email, Paul told me that the 6T is real hard to get the top speed from so the 7T may be a bit easier but still a bear to max out.

I tried running mine at 72 volts and even with low amps it still almost melted the phase wire insulation. I think you are really pushing it at 18S. I can run it all day long on 14S 25a with no problems. Maybe you should go for a bigger wheel and lower the volts a bit. Mine liked a 24 x 3.00 tire real well (and it looked good too).

Good luck to you with whatever decision you make.

 

[crossbreak]

In general a smaller tire reduces torque needed to get a certain acceleration. This reduces phase current for a given wind

The motor does not care about what specific winding or phase current you have. It cares about the sum of the currents flowing around the stator teeth.

if you have a 6T and do 100A phase current, this leads to a total current of 600A flowing around the teeth
if you have a 12T and do 50A phase current, this leads to a total current of 600A flowing around the teeth

In my experience, currents sums greater than 720A do not lead to more torque, seems like this is where the upgrade-MAC starts to saturate. For your 7T MAC it would be 720A/7T = 103A phase current. Pushing more into it just leads to more heat generated but not to much increased torque. A 6T has ~60KV or ~0.167 Nm/A (i shall measure again) at the rotor. So at round about 100 Nm @wheel, it stops producing more torque by pushing more amps.

To cut a long story short, you get a better acceleration with a 20" tire than with a 26", no matter what winding or voltage you have. You shall always set phase amps in your controller setup a way that you dont exceed the 100Nm. This is

Max Phase amps as a rule of thumb: 720A /turn-count)
6T = 120A
7T = 103A
8T = 90A
10T = 72A
12T = 60A

 

[Ch00paKabrA]

all true crossbreak. My response was based on simple anecdotal evidence regarding the heat generated by a specific voltage. All things being equal, yes a smaller tire will always reduce torque needed for acceleration. I based my response on the assumption that someone who chooses a speed wind values speed over torque. In that case, the juxtaposition is also true; A larger wheel will generally have a higher top speed than a smaller wheel (all things being equal and up to a point).

If you are going to throw a bunch of amps at a MAC though, it would probably be a good idea to upgrade the phase wires though.

 

[crossbreak]

The phase wires can't be upgraded. The shaft is too thin to drill a bigger hole. The 4mm² cellman uses fill the hole completely already. No room for improvement here, but you can cut the near the axle and solder thicker wires there.

Sure the faster winds have a handycap here. Still, the 4mm² wires are enough to take the current of 120A of a 6T wind (short term). They are still thicker than the actual windings in the motor and better cooled, so they wont fail before the windings burn.

The resistance of the 4mm² (AWG 11) phase wires is 4mOhm for a 1m cable. So for all 3 phase wires it is a total of 12mOhm. Not so much compared to winding resistance.
The 120A cannot be used continous:

For the 6T@ 120A it is 172.8 Watts of loss in the Phase wires for the 6T, not possible to use this continuously

for the 7T@103AA it is 127.3 Watts, not possible to use this continuously

for the 12T@60A it is only 43.2 Watts, this is continuously acceptable, still the motor windings wont take it continuously

A 4T or 5T would indeed be problematic even with short bursts.

So for the 6T, 7T fans it would be an improvement to cut the phase wires near the shaft and upgrade them to gains some efficiency. I wont do it. The improvement does not justify the investment IMO

 

[crossbreak]

based on simple anecdotal evidence regarding the heat generated by a specific voltage

Heat generated is P = I² *R

Voltage has no influence. You just need more voltage to go faster

I tried running mine at 72 volts and even with low amps it still almost melted the phase wire insulation

Your phase amps sum must have been higher than 720A then. This has absolutely nothing to do with the voltage you use.

You shall use a controller which can really limit phase current if you want to push it to it's limit. Infineons cannot.

 

[Ch00paKabrA]

You shall use a controller which can really limit phase current if you want to push it to it's limit. Infineons cannot.

That explains it then. I am using an infineon. Thanks