Limiting alternator output with resistors in parallel

In summary, the conversation is about the use of a bicycle-driven alternator to power a stationary bike for a workout. The user has found that adding resistors in parallel on the alternator output line helps to limit the production of the alternator to levels that can be sustained during a workout. They have also tried adding resistors in series on the field line, but it did not affect the output level. The user is seeking guidance on how to calculate the optimal resistance to use in order to get the best workout while still capturing power from the alternator. They also mention using a feedback circuit to adjust the alternator power output.
  • #1
knightscape
3
0
Hello, this is my first post here! I am building a bicycle-driven generator using an alternator and battery (12v) pulled from a dead car. I can easily spin it up to the point where the alternator self energizes and produces output, but on a dead or low battery, the load on the alternator is so high that it is unsustainable for more than about 5 minutes. After reading various sites describing how to accomplish what I'm trying, I've found that by adding several resistors in parallel on the alternator output line, I am able to limit the production of the alternator to levels I can actually pedal for an hour, around 0.9a (the idea here is to capture power during my normal workout on my stationary bike). I had a few resistors in my bin, so what I've added in are two 10Ω 10w and one 8Ω 20w resistor in parallel. These total up to just over 3Ω. By decreasing the resistance I should be able to let the alternator produce a bit more power, but I honestly don't understand why this is so or how I can calculate the optimal resistance to aim for. I would appreciate any guidance or advice to help me understand why these resistors help keep the alternator pedalable and how to dial it in for optimal output.

Here's my setup, except I cleaned up the giant roll of wire on the output line today:

DSC01121-1.JPG
 
Engineering news on Phys.org
  • #2
This is not an unusual problem. Amateurs have used Bicycle driven alternators have been used to power transmitters at Field Days for years. In order to get a usable level of power for a sustainable amount of effort requires and drastic gear down ratio. Something like using the lowest possible gear ratio on a ten speed and then stepping that down. Adding a fair sized fly wheel to the alternator should help keep things going.
I would point out the the purpose of an alternator is to put out a constant voltage no matter the RPM applied. Stepping down the voltage, doesn't really gain you much.
Gearing down the bicycle and removing the voltage limiters should make it work for you.
I would also point out that 15 mins pedaling as about all most athletically capable men can do. Talk to your local Amateur club, or contact the ARRL (www.arrl.org). There is a lot of literature available.
 
  • #3
A world class cyclist can put out 400w for 20 minutes. You can probably put out 1 hp for 20 seconds.
If I find my link for the all-day-work-output of a person I'll post it.

Use resistors in series with, or a pulse width modulated voltage across, your alt's field winding to adjust the alt's output to a level comfortable for you.
You have a speed/torque curve, just like an engine or a motor. You want to maximize the overall person/machine efficiency.
 
  • #4
A fairly common number tossed around by cyclists is 150w for sustained output. I'm a fairly strong rider so I anticipate that I should be able to get that much out of my legs. I know there's drivetrain loss in the system and other limiting factors. I'm basically looking to see how much of that I can capture and still get a good workout. So a key for me is reigning in the demand from the alternator. One of my basic requirements is that I can ride it for an hour.

I tried putting these same resistors in series on the field line but it didn't seem to affect the level at all, which led me to the question, "How do I figure out what level of resistance will get it under control?" Placing them in parallel on the alternator's output line does bring it under control, just a little too much. I'd love to zero in on the best solution without throwing money at resistors and guessing. Whether that solution is placing them in series on the field line or in parallel on the output line doesn't so much matter to me, but I'm at a loss as to figure out how to come up with a theoretical level of resistance to try on either.

I can measure that the field charging line draws about 3A when powered up. The output line with no resistors inline will put out something >5A and < 15A with me grinding as hard as I can against it for a few minutes.
 
  • #5
knightscape said:
I tried putting these same resistors in series on the field line but it didn't seem to affect the level at all, which led me to the question, "How do I figure out what level of resistance will get it under control?" Placing them in parallel on the alternator's output line does bring it under control, just a little too much. I'd love to zero in on the best solution without throwing money at resistors and guessing. Whether that solution is placing them in series on the field line or in parallel on the output line doesn't so much matter to me, but I'm at a loss as to figure out how to come up with a theoretical level of resistance to try on either..

PLACING RESISTORS IN PARALLEL REDUCES THE RESISTANCE ON THE OUTPUT!

1/Rt = 1/R1 +1/R2 + 1/R3...
 
  • #6
My guess is that you would like a fixed alternator voltage (and power) output, independent of the bicycle RPM. This is easily done using a simple feedback circuit to the armature which will reduce the alternator excitation as the RPM increases. Then a single knob can adjust the alternator power output to say 50, 75, 100, 125, 150 watts, etc.
Bob S
 
  • #7
I understand that resistors in parallel decreases overall resistance and I am familiar with calculating the combined resistance of them in parallel. I was talking about paralellel or series in two different places:

1. The resistors in parallel I tried were on the alternator's output line, where lower resistance, seems to produce higher output. I've got that hooked up to a rheostat right now, and it's working fairly well, though I still need to fine tune it.

2. Others have recomended as an alternative, placing resistors in series on the alternator's field charging line, where theoretically, higher resistance would decrease alternator output by limiting the field within the alternator.

What I'm struggling with is figuring out how much resistance to try out in either of those arrangements. So far, empirically, I've found that very low resistance on the alternator output line achieves the desired effect (I have a cluster of resistors with a combined resistance of 0.2-2Ω (rheostat involved), though I wonder about how much power I'm wasting with those resistors. With this arrangement, I'm able to sustain output of 4.2A.

I have had no luck with limiting current to the field line with resistors in series though, and this seems like it would be the more sensible approach to my mind. I'm curious about this idea of a feedback loop, what would I loop back to the shell? Not the alt output itself right?
 

1. How do resistors in parallel limit alternator output?

When resistors are connected in parallel with an alternator, they create a separate path for current to flow. This decreases the overall resistance in the circuit, causing the alternator to produce less output.

2. Why would someone want to limit the output of an alternator?

Limiting the output of an alternator can be useful in situations where the electrical load on the alternator is too high. This can prevent damage to the alternator and other components in the circuit.

3. How do you choose the right resistors for limiting alternator output?

The resistance of the resistors chosen should be equal to the desired output of the alternator. This can be calculated using Ohm's Law (R = V/I), where R is the resistance, V is the voltage, and I is the current.

4. Can resistors in parallel completely stop the output of an alternator?

No, resistors in parallel can only limit the output of an alternator. They cannot completely stop the output as some current will still flow through the circuit.

5. Are there any risks or drawbacks to using resistors to limit alternator output?

One potential risk is the possibility of the resistors overheating and failing if the electrical load on the alternator is too high. It is important to choose resistors with a high enough power rating to handle the current passing through them.

Similar threads

  • Electrical Engineering
Replies
7
Views
1K
  • Electrical Engineering
Replies
16
Views
2K
Replies
8
Views
934
Replies
20
Views
2K
Replies
12
Views
2K
  • Electrical Engineering
Replies
10
Views
4K
  • Electrical Engineering
Replies
6
Views
1K
Replies
14
Views
79K
Replies
12
Views
1K
  • Electrical Engineering
Replies
2
Views
2K
Back
Top