Max current from dc motor

[Ironchef]

I am making a wind generator using a 500 watt 48 volt pm dc motor. I assume the normal operating current it can produce at it's designed speed is around 10 amps. However the speed of the motor driven by a 15 mph wind may be greater than this. Is there a rule of thumb to estimate this. I need to size the cables and want to keep the weight and cost down.

 

[alec_t]

Output volts are roughly proportional to RPM. However, IMO it would be unwise (because of heating of the motor windings) to try to extract more than the rated 500W from the motor. So as volts rise above 48V wouldn't you need to reduce the current draw to keep it proportionally below 10A ?

 

[Ironchef]

Output volts are roughly proportional to RPM. However, IMO it would be unwise (because of heating of the motor windings) to try to extract more than the rated 500W from the motor. So as volts rise above 48V wouldn't you need to reduce the current draw to keep it proportionally below 10A ?

Thanks Alec, that is my concern. If my design produces too much current I can safeguard this with the Arduino based controller. The method would be to move the blades off wind by adjusting the yaw ( tail). However because the wind can gust and change direction for a short time I can have a spike in current that I thought would be good to deal with. I really don't have instantaneous controll of the wind speed. God does but he's not included in the design.
I thought it would be wise to but a safety factor in the wires to the 48 v battery bank.

 

[Nigel Goodwin]

However because the wind can gust and change direction for a short time I can have a spike in current that I thought would be good to deal with.

You're over-thinking it - current spikes would be no problems whatsoever - batteries are very hardy devices, and cables are rated for continuous currents not 'spikes'. The current specification on a cable is based on it getting hot, and short term high currents (even a good few seconds) aren't any concern as it doesn't have enough time to get hot.

 

[Ironchef]

OK, thank you all. I have a tendency to over-think. In this case I wanted to not make a fundamental mistake or over design with over-cost. This project is becoming interesting to my neighbors, passers by and even the birds!
I will be back on the forum as events unfold. Right now I'm waiting for the OK from the home owners association.

 

[tcmtech]

Whatever the motors rated amp draw is also what it can put out continuously as a generator.

Wattage wise is a bit different. If spun at its rated RPM's the output voltage will be lower thus the wattage will be lower as well.

However if spun faster than it's stock motor speed rating the output voltage can go higher thusly letting you get more peak wattage out.

 

[Ironchef]

Now that I understand about the peak and the continuous issues I have another question. I'm looking for a rule-of-thumb to determine how much to go above the design limits continuously ( the safety factor ). Does such a factor exist?

 

[tcmtech]

Amp draw wise things stay about the same. Short moderate overloads are no more of a problem than what overloading was when it was a motor.

Speed wise that depends on the motor most will easily take 2x their rated speeds without issue and some can go far higher as well. Thats sort of where you have to take into account what was the motors stock no load speed. If it was only a few thousand RPM or less the odds are it can take a considerable overspeed without problems.

 

[Ironchef]

I'm trying to find the no load rpm from Johnson Electric. The motor was from an electric lawn mower so I'm guessing the nl rpm is 2000. Johnson Electric is a Chinese company to my surprise but they have several sales office in the USA.

 

[tcmtech]

From what I can find online it looks like the majority of the larger 48 volt lawn mowers run at the standard 3000 - 3500 RPM range that their gas engine equivalents run at.

Looking at the parts breakdowns of the motors I found if yours is the direct drive PM type I would expect you could run that up in the 5000 - 6000 RPM range without doing any damage whatsoever.

Also being designed for lawn cutting work I suspect that this type of motor has a considerable short term overload capacity as well.

 

[Ironchef]

From what I can find online it looks like the majority of the larger 48 volt lawn mowers run at the standard 3000 - 3500 RPM range that their gas engine equivalents run at.

Looking at the parts breakdowns of the motors I found if yours is the direct drive PM type I would expect you could run that up in the 5000 - 6000 RPM range without doing any damage whatsoever.

Also being designed for lawn cutting work I suspect that this type of motor has a considerable short term overload capacity as well.

Wow! You'r doing more homework than me. When I get a definitive answer from Johnson I'll let you know how close you were. I had assumed a lower nl rpm. But I never checked to see what the gas mower's are.

 

[tcmtech]

Standard 4 stroke gas engine mowers are around 3600 RPM.

Not much homework. Just good at using google.

That and Johnson makes a lot of the chain store branded machinery. I just looked for the specs on a number of 48 volt push mowers and found which ones shared common design layouts that looked like they are based on the 48 volt Johnson units.

 

[Ironchef]

Let's sum up what we know / deduce:
Know- Johnson motor 550 watts, 48v dc, 12 amp spec on motor
Motor generator outputs to 48 v battery bank.
Deduced- no load rpm around 3000 rpm ,max around 6000 rpm

Therefor the output dc current can be in the range of 10-20 amp.

My original post asked how much current I could expect so I could spec the wire from the generator to the battery bank ~25 ft. This indicates an AWG 9 wire size.

 

[tcmtech]

Sounds reasonable enough to me. 9 ga is overkill for a 25 foot run at 20 amps or less. 12 ga copper would be just fine for that peak current range.

The next question is how are you going to get the motor spun up to 3000+ RPM in normal wind conditions?

 

[4pyros]

You may want to over size the wire anyway to reduce the voltage drop.
In this case bigger may be better.
Every piece of wire has resistance.

 

[Ironchef]

Yes it was overkill and over cost. I still don't know the wind speed vs rpm yet.
I suppose we can put it on the back of a pickup as some ( crazy ) people have reported, but not for me. However I can calibrate a jury rigged anemometer outside the car window and use a ir sensor with the Arduino to measure rpm. Or maybe just measure volts and amps from the generator and plot against wind speed.

 

[4pyros]

Some wind mills are geared up, but you maynot have the best motor for the job.

 

[Ironchef]

I should have said at the outset that this is a DIY project not for school credit and definitely an area I have interest in but not expertise. Thus is a first iteration ( i.e. Learning experience). Thus I'm not trying to optimize anything. I am doing this on a shoestring with fallbacks in case of failure or the Architectural Review Committee does not approve the project. For instance most of the components can be used for an electric tricycle or solar power. The blades and hub can be sold on eBay or local to fun the other projects.
But I've got to tell you if the ARC turns it down I will be on their doorstep with an attitude and a welding torch to get their attention. Why a welding torch? I don't know it just seemed like a good image. Perhaps their doors get welded shut! Ha! No I am not that kind of guy. But being Jesuit raised (high school and college) I would pursue some legal issue.
As you may have guessed I have a good sense of humor, it's my strength!

 

[tcmtech]

Just for reference I used to hand craft my own wind generator blades out of good quality treated 2" x 6" pine lumber.

A solid one piece two blade unit six feet long made from a piece of wood with a smooth clean even grain from end to end will easily take 3500+ RPM all day long! Granted it will also put the tip speeds into supersonic levels which makes a wicked amount of noise too!

BTW the rule of thumb I always went with was 1 foot of rotor diameter for each 1/8" of main shaft diameter. I never had a shaft failure going with that ratio.

 

[Ironchef]

I have come across many rules of thumb in this wind generator project. Thanks for this one. The shaft diameter is 17mm and the blade diameter is around 60 inches. So it may be OK . 5 X 1/8 = (5/8 )x 24.5 = 15.3 mm.