AC 3 phase motor fee spinning

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T1 Terry
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AC 3 phase motor fee spinning

Post by T1 Terry » Mon, 16 Sep 2019, 13:43

I want to start converting my VW Kombi to electric, but a few issues have held me back. The primary one is I want to tow it behind our motorhome with an "A" frame rather than on a trailer. One of the problems is the Kombi gearbox can not be towed for long periods in neutral. Much like most manual gearboxes, it requires the input shaft to be spinning to spin the gears themselves so the oil is splash fed throughout the gearbox. Towing in neutral with the input shaft not spinning would mean the shafts were spinning inside the gear sets and spinning the roller bearings but no oil being splash fed into them.
I know just about all EV's recommend not flat towing them because they will attempt to regen and overcharge the battery or for what ever reason, they just say no flat towing.
What I need to know is, if I disconnect the cables between the 3 phase motor and the controller, could I tow the Kombi in 4th gear resulting in the motor spinning at the same speed as it would spin when being driven at what ever speed. Probably a maximum of 100km/h, around 3500rpm? The motor should be free wheeling, but would it try to develop some sort of power generation anyway?

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Richo
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Re: AC 3 phase motor fee spinning

Post by Richo » Mon, 16 Sep 2019, 14:50

Well induction motor wouldn't have any of those problems.
Permanent magnet would produce a back EMF (AC volts) but if its disconnected would be the same as a power point with nothing plugged into it.
The power is only generated once it is connected to something.
So the short answer is NO but the long answer is YES.
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Re: AC 3 phase motor fee spinning

Post by T1 Terry » Mon, 16 Sep 2019, 15:59

The motor I'm looking at using is a rewound to 48v 3 phase WEG132 motor. First asked about it here http://forums.aeva.asn.au/viewtopic.php?f=34&t=4267 then again here http://forums.aeva.asn.au/viewtopic.php ... mbi#p50960
This is an induction motor so all should be fine, that will make life a lot easier than converting the rear hubs to free wheeling hubs or dropping the drive shafts every time we wanted to tow it.
Now to find the thread where those in the know here helped me with calculating the battery and other requirements to this 48v wound motor to do what I want at the rpm that suits. At 130km/h it is an absolute handful on the road, so my old age has determined that the 110km/h will be plenty enough for me .... my licence would last a bit longer as well and the money I'd save in fines could go towards a better build :roll:

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Richo
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Re: AC 3 phase motor fee spinning

Post by Richo » Tue, 17 Sep 2019, 12:51

195/75R14 has c=2.035m
5.857:1 diff
4th is 0.82:1

Total ratio 7.143:1
110kph=30.55m/s
15.01RPS=900.9RPM
Motor RPM = 6,435
6,435/1500=4.29xFn

Normally you wouldn't set the nominal voltage for 110kph/205Vac/291VDC as the motor will still spin at a higher RPM with less power.

90kph=5265RPM=168Vac=238VDC
80kph=4680RPM=150Vac=212VDC

I'd think 4500RPM is still acceptable which is 144Vac=203VDC.
So the short answer is NO but the long answer is YES.
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Re: AC 3 phase motor fee spinning

Post by T1 Terry » Tue, 17 Sep 2019, 16:18

My '74 Kombi has a 2.0ltr gearbox, one of the 5 rib super strong case models, these have the highest diff ratio of all the air cooled engine models.
These are the actual road speed to rpm for each different model gearbox as they left the factory. '67 and before had reduction hubs between the gearbox output shaft and the drive wheel each side so their original rpm to road speed was pretty poor. The 68 onwards had a floating axle CV at each end and the wheels stayed square to the road unlike the earlier models that looked as though they would fold under if they went over a jump ....
1967-1970 1600 47bhp@4000rpm 104km/h
1970-1971 1600 50bhp@4000rpm 109km/h
1971-1973 1700 66bhp@4800rpm 128 km/h
1973- 1975 1800 68bhp@4200rpm 109km/h
1975-1979 2000 70bhp@4200 rpm 126km/h

So 4,500 rpm would see maybe 150km/h .... well not really, it would probably see the Kombi off in the bushes well before it reached that speed :lol: A caption on the back of a restored Kombi at the Valla festival a few yrs back "Handles like a dog on lino" I have had it out past the 130km/h on the speedo a few times, but a real white knuckle ride.
The 4500rpm would make it quite sporty through the hills in 3rd gear though with all that torque down low in the rpm range.

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Re: AC 3 phase motor fee spinning

Post by Richo » Wed, 18 Sep 2019, 12:35

Whoops - I see the mistake - sorry
divided by 0.82 not multiply :oops:

Total ratio should be more like 4.80:1
4200 RPM would be 874.5RPM at the wheel.
Which comes out to 106kph.

So even having the nominal at 80kph/3169RPM that's 100Vac/142Vdc

What controller was this going on?
Big Mouses?
So the short answer is NO but the long answer is YES.
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Re: AC 3 phase motor fee spinning

Post by francisco.shi » Thu, 19 Sep 2019, 04:43

If you have an induction motor you do not need to disconnect the motor. As long as there is no power on the stator windings it will freewheel no problems.
As for the max speed induction motors are good in that once you run out of volts you can keep increasing the frequency and the motor will keep speeding up you just get less torque. So you could go over 4500 rpm with the 200v battery.

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Re: AC 3 phase motor fee spinning

Post by Richo » Thu, 19 Sep 2019, 12:36

francisco.shi wrote:
Thu, 19 Sep 2019, 04:43
So you could go over 4500 rpm with the 200v battery.
Not 200V - that was my mistake.
The battery voltage should be more like 120-140Vdc for the nominal at ~80kph.
It will still do 4500RPM and get to 110kph.

The actual max speed of the Kombi with eV system isn't relevant due to the fear factor. :lol:
So the short answer is NO but the long answer is YES.
Help prevent road rage - get outta my way!

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Re: AC 3 phase motor fee spinning

Post by T1 Terry » Thu, 19 Sep 2019, 12:52

Richo wrote:
Wed, 18 Sep 2019, 12:35
Whoops - I see the mistake - sorry
divided by 0.82 not multiply :oops:

Total ratio should be more like 4.80:1
4200 RPM would be 874.5RPM at the wheel.
Which comes out to 106kph.

So even having the nominal at 80kph/3169RPM that's 100Vac/142Vdc

What controller was this going on?
Big Mouses?
I have a Tritium Wavesculptor VFD that was programmed for the motor I bought from Ben that was going into his Magna project but he decided he was never going to have the time to complete it. I would prefer to use that motor in the big bus project, because Chris says that was what it was originally designed for, and use the monster VFD I bought from Weber & Coulomb when they decided it was too big for the MX5 project.
I'm not sure if Tritium still do the Wavesculptor tuning, but if they do, I was planning to send it up to them to get it set up for this motor and a 245vdc nom. battery pack as I have one in the plug in Prius so I could use that for the first experimental runs and build a bigger capacity one later on.

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Re: AC 3 phase motor fee spinning

Post by T1 Terry » Thu, 19 Sep 2019, 12:57

Ahhh.... just read your last post Richard, 245vdc battery might be a bit over the top then :lol: I could drive it in mostly 2nd and 3rd gear and just use 4th for highway cruising with a possible 160km/h and still have good torque to get it/hold it there .... be cool for drag racing eh

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Re: AC 3 phase motor fee spinning

Post by Richo » Fri, 20 Sep 2019, 12:52

Even with 80kph as the nominal voltage 140Vdc that's 18kW nominal and ~55kW peak.
More voltage - more power.

You'll have to check what the upper and lower cut-off limits are in the Wavesculptor.
So the short answer is NO but the long answer is YES.
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