View Full Version : Argument over Inductive or Restive load

Modelhovercraft - n/a
2-Jan-08, 01:32 AM
is there anybody out there who has good electrical and electronics knowledge. I dont im a mechanical engineer.

Having a discussion in the pub, something that was discussed which ended up in a debate.

the debate was. is a normal 1000 watt AC Mains powered Vacuum cleaner etc an Inductive or Restive load?

Why are arguing in a pub about vacuum cleaner motors I will never know and would think we defiantly need to get a life.

but thats us nerds for you.

Please put us out of our misery.

Don83000 - n/a
2-Jan-08, 06:51 AM
Here is the answer your looking for print it out and take it to the pub http://hovercraft.org.uk/images/icons/smiley_icons/icon_lol.gif

http://www.cod.edu/people/faculty/wolfson/phy2112/Chapter%20 (http://www.cod.edu/people/faculty/wolfson/phy2112/Chapter%2031_new.pdf) 31_new.pdf

Roger Drew - n/a
2-Jan-08, 03:49 PM
If you didn't understand the last posting then the easy answer is BOTH. Generally, motors rely on the force generated by a magnetic field cutting a current flowing in a coil of wire. Therefore there will be an inductive load. However the wire which forms the magnet and/or the rotor will have a resitance and therefore you will also have resistive element to the load.

Sounds like you drink in a lively pub!

kach22i - n/a
2-Jan-08, 04:17 PM
I've noticed that some of the brushless electric motors for RC aircraft have gearboxes on them. Is this because the batteries cannot deliver in amps the torque required to drive the propeller or ducted fan?

In what I've read they only mention slowing down the RPM to match prop speed using the gearbox.

profqwerty - n/a
2-Jan-08, 04:47 PM

With an electric motor stalled (or running at low rpm) it draws higher current than when running at higher rpm.

So yes sort of...if the motor is running at low prop speed you need higher current to turn it...therefore need thicker wires, larger battery (to deliver current) higher psec speed controllers, etc. All adding weight - especially the motor.

kach22i - n/a
2-Jan-08, 06:46 PM

With an electric motor stalled (or running at low rpm) it draws higher current than when running at higher rpm.

So yes sort of...if the motor is running at low prop speed you need higher current to turn it...therefore need thicker wires, larger battery (to deliver current) higher psec speed controllers, etc. All adding weight - especially the motor.

I will be hooking up two RC type NICAD 9.6V battery packs in series because a single battery per electric motor is not spinning fast enough and has very short battery life.

Series hook-up:

http://www.hobbyprojects.com/dc_theory/series_parallel_batte (www.hobbyprojects.com/dc_theory/series_parallel_batteries.html) ries.html

Volts: 9.6 V + 9.6 V = 19.2 V

Amps: 2.5 + 2.5 = 5 (discharge rate/current)

And if this website below is correct almost 48 watts of power. EDIT: I forgot to double the amps, I now get 95 watts of power!

http://www.sengpielaudio.com/calculator-ohm.htm (www.sengpielaudio.com/calculator-ohm.htm)

I will be putting 60% more voltage (19.2V verses 12V) than recommended though some cheep 12V motors, so I expect them to burn out fairly quicky. Hopefuly this is enough time to decide if the power is enough before I upgrade the motors to something more serious and perhaps more efficient.

I found a cheap electric motor in an old model box (about 25 years old) which originally sold for about $1.00, the same motor is tens times that now at the local hobby shop. Where is all this cheap stuff from China I keep hearing about? The $30 to $60 motors might be what I need, but money is tight (need five of them).

I'm looking at short operation times because the mass/weight of the fans is larger/heavier than the motor would normally be expected to drive. I may not get to the rpm's I need to at first. I just want to experiment and try to get lucky and perhaps learn through some rough knocks before I start going into debt on this thing.

profqwerty - n/a
2-Jan-08, 07:16 PM
What exactly are you trying to do?

putting the bats in series increases the V across the motors, so the I would increase too (and hence power transfered).

As I don't know what you're doing I can't say for sure...but if this is an R/C model then you'd be better off with using rc motors - which are rated at 9.6V or whatever - and the controllers to go with them. the 12V motors around tend to be bigger and do use a lot of current.

95W - this is a LOT. considering you are running it thru cheap, under rated motors, i suspect a large portion will be dumped as heat into the motor - burning out quickly - and being wasted.

but if just for experimenting then fine i guess. You'd find a proper rated motor would perform much better when you do get it though http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif

Jeremy - n/a
2-Jan-08, 09:53 PM
I've been playing with brushless RC motors and am impressed. I bought a relatively cheap motor and controller from my local model shop and it is amazingly powerful. The motor draws over 16 amps at 12V, (around 200W or a bit over 1/4 hp). I think the whole lot cost about 35 (about $70).

I've also been playing with brushless motors on my bike. The first motor I bought was a Chinese made Crystalyte. This easily gives well over 1kw into the front wheel, but was plain silly to ride. I've just picked up another brushless hub motor from China, a TongXin. This is much more civilised (and a lot lighter), with only around 350W or so.

The TongXin hub motor retails from China for just under 30, with the brushless controller being about another 12, pretty cheap really.

Looking at the way these motors are going, and the way the prices keep coming down, it looks possible that before long a reasonably effective (if not efficient) electric lift system might be an option. A friend has been helping with an electric paramotor project and the guy has flown quite well using two big model aircraft brushless motors. As these motors are very light for the power (a 5hp motor typically weigh around 1.5kg), the added weight of a big alternator on the main engine, plus a "buffer" battery to allow near instant lift adjustment, might not be too much of a penalty.


Don83000 - n/a
2-Jan-08, 11:15 PM
If you want serious power from model motors look here as these will run a 30" prop if you so desire and produce 12.5 KW and if you use Lipoly batteries you have shed loads of power at a lot less weight than old time Nicads you will note on a lot of these motors they do not state the RPM but they do state the KV and if you x the KV by the volts you intend to use it will give you the rpm of the motor.



Check out also the Hacker range



Check out the following for batteries I have purchased many from them with no problems

http://stores.ebay.co.uk/RevolutionShop_Rechargeable-Battery (http://stores.ebay.co.uk/RevolutionShop_Rechargeable-Battery-Charger_W0QQcolZ4QQdirZ1QQfclZ4QQfsubZ3080454QQfti dZ2QQtZkm) -Charger_W0QQcolZ4QQdirZ1QQfclZ4QQfsubZ3080454QQfti dZ2QQtZkm

profqwerty - n/a
3-Jan-08, 08:44 AM
wow I didn't realise how powerful these were!

I visited the building of the electric boat speed record holder..they were using 'lynch' motors - permanent magnets, not stator coils - 4 motors, 144Volts, ~1000Amps. They had to swap out the batteries between the runs!

The motors were about 30cm dia, and 10cm deep.

kach22i - n/a
3-Jan-08, 02:24 PM
Out of curiosity I started looking up information on small motorcycle replacement batteries which are Lipoly technology based.

Between the battery and motor cost I think I'll stick with the path I'm on. Old tech is just cheaper, for instance NICAD's are on sale at Radio Shack 40% off and they come with a charger.

However donations will be accepted. http://hovercraft.org.uk/images/icons/smiley_icons/icon_lol.gif

Modelhovercraft - n/a
6-Jan-08, 03:20 AM
So far with tests on a bench my brushless motor turns a 16x8 three blade propeller at 6000rpm at 24 volts.

The meter reads 37 amps, 850 watts at this speed.

If i put a 20x10 two bladed prop the motor slows down to 4500rpm at 24 volt and draws 53 amps amd 1145 watts. because the motor is struggling with this prop it get hot to quick so it wouldn't last long.

I am going to probably run 2 of these motors and a bigger one for lift http://hovercraft.org.uk/images/icons/smiley_icons/icon_eek.gif totaling somewhere around 130 to 150 amps and about 2.5 to 3 kw at 24 volts.

The only way i can get this sort of power for long periods is only from large lead acid battery's. probably two 100AH battery's should give 30 minutes of run time.

profqwerty - n/a
6-Jan-08, 08:02 AM
Actually a friend is very much into electric motors - when I showrd him the hover he raised various points like 'why do you invest 500 in an engine when you could put it into research and get a similar powered electric fan?'

Have you see the hybrid electric mini: http://www.pmlflightlink.com/archive/news_mini.html

What's it using for batteries / motors?