Build a Pedal-Powered Generator for Middle School Workshop | Engineering Project

[nautikal]

If you want some background information on the project I'm working on, continue reading. If you just want to help then you can just skip to the bolded part.

I'm supposed to build a pedal powered generator for an engineering class with a group. We are using a treadmill motor and pressing the back wheel of a bicycle against the motor so that it spins in reverse and acts as a generator. (We would have preferred to use a pulley or sprocket and chain to connect the wheel and motor for better efficiency, but time constraints did not allow this).

The purpose of this pedal powered generator is to be part of a workshop with middle school aged students. We want to be able to measure the power (in watts) they produce so that we will be able to have them calculate how long they would have to pedal in order to offset how much TV they watch a day or how much they use a computer per day etc. We are able to measure voltage fine (and get up to around 150 volts ) but are having trouble measuring resistance or current. We connected a multimeter in series with the circuit and somehow the reading went over 10 amps with the motor barely spinning. I'm positive it was connected properly so maybe the multimeter is bad... I will have to try another one. This would make the most sense as I do remember getting a reading at around 2-3 amps at top speed (130-150v) when we first started this project.

So since the current wasn't reading properly, I tried measuring the resistance of the motor but got something like 2500 ohms, which would put the current draw of the motor at 115v at 0.05 amps which would make sense maybe for a mouse treadmill :). So I'm guessing the multimeter is just broken?

On the motor itself it says it draws something like 4 amps at 115 volts, giving a resistance of 28.75 ohms (I don't have it in front of me, so the values are just an idea). Is the resistance of a motor/generator constant, and if I run a motor in reverse will these values be affected? In other words, can we just calculate the power output (watts) using the internal resistance of the motor calculated from the current/voltage rating on the label? Or can/should we measure the resistance at rest and use this value (with a multimeter that gives a reading that makes sense :D)?

 

[brewnog]

What is the load you are powering with the generator?

 

[Pumblechook]

You can't just measure DC resistance and draw any conclusions. It is the dynamic or AC impedance which governs the maximum load which can be applied and power factor comes into it. A DC generator is essentially and AC alternator with mechanical rectification (commutator) so AC theory applies. In other words an individual coil rotating in a magnetic field always generates AC. To power 50 or 60 Hz devices from a DC generator or AC alternator which will be of variable speed you will need to re-generate electronically the correct frequency and voltage or maybe store energy in a battery and then generate 50 or 60 Hz. Maybe a a small alternator/rectifier with defined max output charging a battery would be a better bet.

There must be figures for what a person can produce when pedalling in Watts or Wh over a given period. .. That is a starting point.

 

[Ivan Seeking]

If you try rotating the armature a bit and stop, you should find that the resistance changes. The minimum resistance measured should be the series resistance. If you know this, can you can use this R to calculate I²R losses in the motor.

As mentioned, your active impedence will be greater when the motor is running due to the back EMF generated by the collapsing magnetic fields, but if you measure the current and know the DC resistance, you can still estimate the internal losses.

This assumes that you are using a series wound, brush-type motor.

The average adult can produce ~100 watts all day long. Children [<15 years old] will top-out at about 200-300 watts for a few minutes, and many or most adults can only sustain 300-400 watts for a minute or two. Obviously this will vary, but it is typical. An olympic athlete can produce about 1000 watts for a few seconds. I happen to know this because I did extensive testing myself for a similar public demonstration project, for a science museum.

 

[nautikal]

The load is an inverter which is powering several light bulbs in series.

We don't really need to know the internal losses since there are going to be more losses due to the inefficiency of just butting up the wheel against the motor. We're more interested in just knowing how much usable power is produced so we can tell the students something like "this is how long you'd need to pedal at your maximum speed in order to watch TV for one hour." So if we just measure the current and voltage out of the generator, will we get that? Or would it be better to measure the current and voltage out of the inverter (although this would take into account the inefficiency of the inverter)?

My highest education in electricity and magnetism is AP Physics C which I took last year. I got a 5 on both parts of the exam, but I'm a little rusty on the topics and we didn't go too in depth conceptually so this is a little difficult for me to grasp. Thanks for all the help though.

 

[Ivan Seeking]

You would need to know the losses in the mechanical system as well. What you might do is drive the mechanism with another motor, instead of a person pedalling, and measure the power demands based on that. You might then map these numbers for your application in some appropriate manner. At that point I guess you can map the power demand based on the rpm.

Also, your internal losses in the generator will still likely be signficant, but by using another motor to drive the entire apparatus in order to get a baseline, everything is accounted for. But in order to do this, you will need to either use a true-power meter, like those used on a house, or you will need to know the phase angle of the motor.

 

[Ivan Seeking]

You will likely find that with a lossy mechanical system, you won't even need an electrical load, to a point. For example, you might find that you have two-hundred watts of mechanical losses at some nominal rpm, which isn't a lot of power, but signficant for children.

Also, just use frequency independent loads, like incandescent lightbulbs, or if you have access, high-power resistors [nichrome wire can be used and sized for the application]. Then you don't have to worry about inverters, and frequency issues.

 

[nautikal]

But don't we only need to know all the losses if we want to find out how much total power they are producing? If I understand this properly there is power (and energy) lost mechanically when the motion of the wheel is transferred to the motion of the generator. Then there is power (and energy) lost due to the internal resistance of the generator. And whatever is left is the usable power (or energy). Some of this energy is lost as heat since the inverter isn't 100% efficient and then the rest is used by the light bulbs. So how can we just measure the power that is left over - that is the total power produced by the student minus any losses. Can we not just measure the current and voltage out of the generator or inverter?

 

[Pumblechook]

The problem with ordinary bulbs is the low off resistance... Maybe ten times lower than the fully on resistance. It would like trying to be pedal with the breaks on and they come off as you pedal faster. Maybe an idea to dump most of the power into some high power resistors and just a proportion into some bulbs.

 

[Ivan Seeking]

Trust me. The test model seen in the background ultimately serviced millions of users. It was my first project [many years ago] after college. It required a constant power demand at the pedals, independent of rpm, and preprogrammed pwm loads cycled through according to an audio and graphical presentation.

You can see the 2hp drill attached at the pedal shaft, for testing.

http://img367.imageshack.us/img367/4007/ebikefirstpicsresizexm1.jpg

It is still being sold.
http://www.omsi.org/store/exhibitsales/unit.cfm?Title=Energy%20Bike

 

[Ivan Seeking]

The problem with ordinary bulbs is the low off resistance... Maybe ten times lower than the fully on resistance. It would like trying to be pedal with the breaks on and they come off as you pedal faster. Maybe an idea to dump most of the power into some high power resistors and just a proportion into some bulbs.

Yes, lightbulbs do have problems, but if the power curve is mapped anyway...

Nichrome wire is probably best. Just oversize the load so the maximum temp is not signficant. That way the load curve is approximately linear.

 

[Ivan Seeking]

But don't we only need to know all the losses if we want to find out how much total power they are producing? If I understand this properly there is power (and energy) lost mechanically when the motion of the wheel is transferred to the motion of the generator. Then there is power (and energy) lost due to the internal resistance of the generator. And whatever is left is the usable power (or energy). Some of this energy is lost as heat since the inverter isn't 100% efficient and then the rest is used by the light bulbs. So how can we just measure the power that is left over - that is the total power produced by the student minus any losses. Can we not just measure the current and voltage out of the generator or inverter?

You can do it that way, but it won't be representitive of what's at the pedals. Your mechanical losses may be twice the electrical load. If you just want to know the end electrical load, then measure the voltage and current at the load, and use DC to eliminate phase angle concerns.