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# Measuring and regulating power output of motor

#### ChronoFish

Oct 14, 2010
3
Hello, I'm looking for some suggestions:

I am a professional software developer, novice electronics hacker and experienced triathlete.

For the heck of it, this winter I would like to transform my mechanical resistance rollers (think treadmill for a bike) into computer controlled resistance rollers.

I've done enough electronics hacking that I can use a PIC to drive 7-segment displays and communicate status through a parallel or serial port. But that is about my limit.

Here's my (non-unique) idea:

I would like to hook up one of the rollers to a motor. The motor would be capable of spinning the roller (backwards) with a power output of 0 up to 1 HP (745 watts). The step increase is TBD - probably between 0.5 and 1 HP. This is a safe upper bound that I am not, even for a short period time, going to exceed.

Without knowing much about a given motor, how can I measure the output (or input) of the motor?

Do I have to do it via calibration and use of a lookup table, or can I determine this on the fly? And if calibration is the way to go - how do I do that?

Is it better (easier) to control the power to the motor via mechanical means (like a servo turning a rotary resistor) or to use some sort of PWM scheme?

The motors I have available are things like a 2HP AC circular saw motor and a DC motor from a batter powered weed-whacker (unknown power rating).

Are the power outputs of motors different when the motor isn't actually spinning (i.e. braked) - or spinning backwards?

In other words as I pedal my bike the motor is actually acting as a break trying to spin the roller in the opposite direction.

Thanks for your suggestions.

-CF

#### Resqueline

Jul 31, 2009
2,848
If you can mount the motor in a way that you can measure its torque (by the use of a strain gauge) then mechanical power is just a calculated product of torque & speed.
AC motors needs a complicated and expensive frequency inverter to drive it like you want. A permanent magnet motor is easily driven with a simple PWM circuit.
You say spin backwards, but is this what you actually mean? I see it as being driven instead of driving. A driven motor will produce electric power above a certain RPM.

#### ChronoFish

Oct 14, 2010
3
You say spin backwards, but is this what you actually mean? I see it as being driven instead of driving. A driven motor will produce electric power above a certain RPM.

Thank you Resqueline,

To clarify what I mean: I want to be able to electronically control the resistance. So my wheels sit on a set of "rollers" (kind of looks like two rolling pins) and as I ride the rollers "roll". I want the motor to attempt to drive the rollers in the opposite direction.

So lets say my legs can drive the rollers at 250 Watts - and it registers at 30mph. When I engage the motor, it will attempt to drive the rollers in the opposite direction. So even though the output from my legs is the same (250 watts) my speed will drop.

If I put in 250 watts of power to the motor, the motor should be at a speed of 0, as will be my wheel speed - as my legs are working but not able to turn the cranks.

The crux here is that I want to "know" how hard the motor is working (in watts) so that I can then determine how hard I'm working.

Thanks!
CF

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,509
If the motors act as generators when driven by your bike, the easiest way is to place a load across the motor.

The heavier the load, the greater the dynamic breaking effect.

The advantage of this technique is that the braking effort is proportional to speed (actually proportional to some function of speed, I'm not sure it's linear). In that way it mimics an incline, the effort you have to make increasing with speed.

Constant power, on the other hand, would make it very hard to start, but get progressively easier as you increase in speed -- it would be as if you had an automatic transmission on your bike that kept you at a constant speed, no matter how fast (or slow) you pedalled.

A simple solution may be to simply measure the voltage across the resistor bank. The power is V^2/R, so with a resistance of (say) 0.33 ohms, you could make a tabl like this:

1V = 3W
2V = 12W
3V = 27W
4V = 48W
5V = 76W
6V = 109W
7V = 148W
8V = 194W
9V = 245W
10V = 303W

Once you find the right value of resistor you could rescale an analogue meter to read watts, or you could refer to a table.

One important point to remember is that if you are reading 200W, the 200W are being dissipated in the resistor bank. You will need resistors capable of dissipating this amount of heat, and possibly a fan to blow cool air on them. If you don't do this, you will soon smell the smoke...

Your explanation of driving the motor backwards does not reflect what will happen. If you are capable of riding at 30mph and do so at an equivalent power of 250W, then on a static bike, with 250W going into a motor, it would still be running backwards and your wheels would be doing the equivalent of 30mph.

As you slow your pedalling the pedals would be driven backwards, kind of like rolling backwards down a hill. I don't know about you, but that's well out of the envelope for my style of riding.

#### ChronoFish

Oct 14, 2010
3
If the motors act as generators when driven by your bike, the easiest way is to place a load across the motor.

The heavier the load, the greater the dynamic breaking effect.

...

A simple solution may be to simply measure the voltage across the resistor bank. The power is V^2/R, so with a resistance of (say) 0.33 ohms, you could make a tabl like this:

...

Thanks so much for your response Steve. Are you suggesting to let the rollers spin the motor rather than the other-way around? And by putting a variable resistor across the leads of the motor, the amount of physical power required to turn the motor will change based on the setting of the resistor? That would be pretty cool....

....
Your explanation of driving the motor backwards does not reflect what will happen. If you are capable of riding at 30mph and do so at an equivalent power of 250W, then on a static bike, with 250W going into a motor, it would still be running backwards and your wheels would be doing the equivalent of 30mph.

As you slow your pedalling the pedals would be driven backwards, kind of like rolling backwards down a hill. I don't know about you, but that's well out of the envelope for my style of riding.
...

Well - you're right. Except that I would programatically control to ensure that the wheel is never actually driven "backward". Clearly there are issues with this- and given your suggested solution - I think I would much rather go with your idea.

Thank you,
CF

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