More info about automotive alternator used as motor

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dabit
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Hi guys,

Soon I will move to a new house with electricity in the garage. This would make it possible to start thinking EV for commuting to work (25km/16mi single trip) since I can finally charge the vehicle overnight. Normally I use a motorcycle to commute.

Now, I have done some math, and making myself a electric motorcycle with 50km range and cruise speed of 80km/h (50mi/h) is at best expensive and bulky. Lead-acid would be way too heavy and bulky, Li-ion would be too expensive, NiMH would be too expensive too; it would be hard to operate such a bike at the same running cost as a regular motorcycle.

Another option would be a bicycle conversion. At 35km/h (22mi/h) the amount of Wh/km needed is much lower, and I could use dedicated bicycle roads (I live in The Netherlands..) which are normally closed for cars and motorcycles. Not having to dress up for the motorbike trip every time could compensate the lower top speed also.

So, currently I am redoing the math for the bicycle. Buying one is not an option; there is no fun in that, and Dutch law restricts motor power to 200W max. I am more interested in a motor with about 750W output. I once broke my leg in a traffic accident, and since then puttung a lot of force on the bicycle pedals is a problem.

When researching suitable motors for the electric motorcycle, I was inclined to use a 3-phase induction motor with 360V battery pack. Using an industrial three-phase motor for the bicycle is way overkill, but soom I realised that a car alternator would also make a nice 3-phase motor. With the rotor shorted it is basically an induction motor, with the rotor powered it behaves like a brushless DC motor, to put it very simple.

Alternators can be obtained for little money, and I am quite capable of designing and building the required inverter myself. MOSFET's are cheap, as are the Microchip dsPIC30F signal processors with motor control unit inside. Regenerative braking would also be very easy to do. Now, that sounds like a fun project!

Thus, I am looking for more information about using a +/- 50Amp car alternator as motor. What kind of efficiency can I expect, what are the useable RPM ranges, would a 50A alternator withstand the use as a motor with ~750W output, etc?

Can you guys help?

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andrew
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Re: More info about automotive alternator used as motor

That sounds like an interesting challenge. I suspect a 50 amp alternator could withstand 750 watts output ok. They are probably designed with a fair amount of tolerance to avoid frying in the most demanding conditions. Like driving in 110 degree weather with the lights/radio/ect on, and at the same time recharging the battery, while the alternator is externally heated from the engine.

This could offer some neat advantages, like being able to run off just one 12v battery, meaning not much spent on a battery and charger.

I thought of an interesting thing to try. If you power the rotor to produce a constant field, and switch the armature, than you can vary the rotor field strength to alter the speed/torque characteristics, kind of like a transmission. They can do this for separately excited motor, which is very similar to what this will be (just with the switched field as the armature, and switching done by a motor controller).

Just a suggestion: work out driving the motor before you fabricate any motor mounts for the bike, or setup the bike for it. Getting the motor to work will probably be the most difficult part of the project.

spinningmagnets
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Re: More info about automotive alternator used as motor

Since automotive alternators are well-known to be built to withstand a lot of heat, perhaps you could try running 24/36 volts through a 12-volt alt, or even 36 volts through a 24-volt alt.

If there is some arc-ing on the brushes from using a higher voltage than it was designed for, you could narrow the brushes, or widen the spaces between the brush contact plates? they are cheap and easy to change.

This might produce higher RPM's and allow a different output gear ratio, which could be helpful. I fear the short armature may provide low torque, but, these alternators are designed to survive years of use at over 6,000 RPM's.

There has been some good results with using a separate internally geared hub as a jackshaft and transmission in order to get more useable power from a smaller motor.

Is it possible to put a 250 watt label on a 500 Watt motor?

If a 1,000 Watt motor is mounted inside a stealth luggage trunk with panniers for the batteries, and you are pedalling at the same speed as everyone else,...how would anybody know?

What are the fines in the Netherlands for having a bicycle that is too powerful? will it be confiscated by the police?

Here is a link for making a car alternator a permanent magnet generator, very easy to make it a motor. With no Watt label, the police couldn't figure it out, since its obviously a conversion:

http://www.fieldlines.com/story/2007/7/3/191357/4809
using trash microwave magnets in a car alternator to make
14 VDC @ 1,000 RPM

http://www.fieldlines.com/story/2007/7/4/225522/5838
14 VDC @ 1900 RPM = 300 Watts generated

Fechter
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Re: More info about automotive alternator used as motor

I sort of tried that once. It seems that shorting the field winding on the alternator does not make a good induction rotor. It would be much better to find an induction rotor from an AC motor and use that.

I have seen setups where the alternator rotor was replaced with a permanent magnet rotor, and used as a brushless PM motor. This arrangement works well.

I'd be inclined to suggest a hub motor for your bicycle application. A rear motor and pannier bags for the batteries will be very stealthy and the 'officials' will not notice you even have a motor.

LinkOfHyrule
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Re: More info about automotive alternator used as motor

+1 on the hub. I don't like them as much as motors with gears, but they're the stealthiest way to do things.

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andrew
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Re: More info about automotive alternator used as motor

Yea, but a hub motor's so easy to stick on. Where's the fun in the project? :(

dabit
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Re: More info about automotive alternator used as motor

andrew wrote:

That sounds like an interesting challenge. I suspect a 50 amp alternator could withstand 750 watts output ok.

My thoughts exactly. But what about efficiency?

Quote:

This could offer some neat advantages, like being able to run off just one 12v battery, meaning not much spent on a battery and charger.

I think a higher voltage and a higher speed could improve efficiency; it lowers the I2R losses in the copper. I'll go for an 48 Volt supply anyway; since I have to construct the 3-phase supply anyway it doesn't really matter what the actual supply voltage is. Generating a 12Vpp sine from an 48V supply is just as easy as generating a 12Vpp sine from a 13.8V supply. The 48 volts gives more headroom to work with and lowers the required wire gauge between batteries and controller.

Quote:

I thought of an interesting thing to try. If you power the rotor to produce a constant field, and switch the armature, than you can vary the rotor field strength to alter the speed/torque characteristics, kind of like a transmission.

Powering the rotor seems the best way to drive the motor; it's behaviour resembles a brushless PMDC motor then. Whether of not it is beneficial to vary the rotor current with speed is easily determined by experimentation. Personally I think it's better to keep the rotor magnet field strong, that improves low speed torque.

Quote:

Getting the motor to work will probably be the most difficult part of the project.

Sure, it's also the most interesting part :)

spinningmagnets wrote:

If there is some arc-ing on the brushes from using a higher voltage than it was designed for

I do not intend to overdrive the rotor, and the armature is not connected with brushes. That's the beauty of using an alternator: everything keeps running within it's original specifications.

Quote:

these alternators are designed to survive years of use at over 6,000 RPM's.

And that's also why I'm interested in converting one of those. They are made to withstand the hard life on a vehicle. Bearings must be strong also; there is a lot of tension on the regular V-belt.

Quote:

Is it possible to put a 250 watt label on a 500 Watt motor?

I'm not terribly interested in what the law prescribes me, especially not on a bicycle. If I did a motorbike or moped conversion, it would be a different story. I would need to have it certified because I need insurance on it. I don't need insurance on a bicycle.

Quote:

What are the fines in the Netherlands for having a bicycle that is too powerful? will it be confiscated by the police?

I have no idea. They might confiscate it, but the risk is so low. An 'overpowered' bicycle is flesh nor fish, so they don't know what to do with it without taking it to court, which they won't. After all it's not faster than what you can do using human power only.

Quote:

Here is a link for making a car alternator a permanent magnet generator, very easy to make it a motor.

Hmm, interesting. May be a nice step 2.

Fechter wrote:

I sort of tried that once. It seems that shorting the field winding on the alternator does not make a good induction rotor.

I think you are right. DC resistance is in the 3-5Ohm range.
Powering the rotor seems a better plan.

Quote:

I have seen setups where the alternator rotor was replaced with a permanent magnet rotor, and used as a brushless PM motor. This arrangement works well.

Indeed a beautiful plan, although one would still need sinewave-shaped drive and V/f or vector control instead of the rectangular drive and 3 Hall-sensors with which most PMDC motors are quite happy. Replacing the powered rotor with a PM rotor would be a nice upgrade and would save the power supplied to the rotor. But it should work with a powered rotor (= electromagnet instead of PM).

Quote:

I'd be inclined to suggest a hub motor for your bicycle application. A rear motor and pannier bags for the batteries will be very stealthy and the 'officials' will not notice you even have a motor.

Hmm. Expensive, a lot of mechanical trouble to fit that on an existing bicycle, and probably less powerful. Then I'd better buy an existing electrically supported bicycle and replace the drive electronics.

But before even attempting to run an alternator as motor I need a rough figure on the efficiency which I can expect. When used as generator the efficiency is not that high, but the electronics used to rectify the 3-phase AC and control the rotor current are not very sophisticated and loss-free either.

shroomer
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Joined: 05/07/2008
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Re: More info about automotive alternator used as motor

Here is a link where someone talks of using one for up to a 100v generator... Might also take 100v as a motor

Oh.. and hi to everyone. Been lurking for a while. First post.

http://islandcastaway.com/stuff/windpower/Alternator%20Secrets.htm

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