Using motor as generator for rotor design testing and getting some str

[rob43]

Have been testing rotor designs on a small prototype wind rig in a small wind tunnel using a 24V DC PM motor loaded with a variable resistive load as a generator. Results we have been getting seem out of whack with load characteristics.

Load is set at 12V 10A which given our small rotor size should be max power we are able to produce. Have hooked the load in series and used datalogging multimeters and anemometers to record "wind" velocity and amp and volt output.

everything seems fine until we approach 15 m/s. Over that speed the voltage and amperage start rising above 12V 10A. At 20m/s output on meters reads 24V 20A.

Have approximated rpms at 20m/s using a bootleg genset at about 15,000 rpm's. Around 7,000 rpms on genset gave us the load 12V 10A output.

We must be doing something wrong. Can anyone take a shot at what might be going on?

 

[ScottyUK]

A DC PM machine produces an internal EMF proportional to speed. The terminal voltage when the machine is generating will be slightly less than the internal EMF, but it's close enough for now. If you have a resistive load which consumes 10A at 12V and you double the speed you will produce 24V which will drive 20A through the resistive load. The power dissipated in the load increases with the square of speed, e.g. double speed = 4x power, triple speed = 9x power, etc.

So.... everything looks pretty normal: what is the anomaly you think you are seeing?


----------------------------------

Sometimes I only open my mouth to swap feet...

 

[waross]

We have built tachometers for a customer with small PM motors.
We used a toy 98 cent motor for the prototype. It worked well, but the customer objected to the toy motor. We changed to a better quality toy motor. $14.50, it worked well looked more robust and the customer was happy. We used a fixed resistor in series with a variable resistor for calibration, in series with a milliameter. The linearity was acceptable and the usable speed range was very good.
Sounds like you are doing much the same with a couple of magnitudes more current.
If the motor is rated at 10 amps, 10 amps will be the maximum safe loading. Use a bigger motor to test greater wind velocity on the same blades.
respectfully

 

[rob43]

Thanks ScottyUK

"So.... everything looks pretty normal: what is the anomaly you think you are seeing?"

Basically misconception on my part. EE is definately not my speciality and I thought that the output was limited by the voltage and current of the load.

Thanks again

Thanks Waross

"10 amps will be the maximum safe loading. Use a bigger motor to test greater wind velocity on the same blades."

What happens when it is producing more than 10A? Will it eventually burn out?

 

[ScottyUK]

Your motor will (should) have a rating plate which will detail the nominal voltage and rated current, or possibly rated power. If it lists power instead of current, the rated current can be implied from the relationship P = V x I.

You must have studied Ohm's Law in high school physics - the resistor you're using obeys Ohm's Law which is the relation ship between resistance, voltage and current R = V/I. Your resistor also has a power rating similar to the motor, so P = V x I applies again. I think waross is assuming that 10A was the current rating of the motor - whatever the ratings are, don't exceed them (or not for long anyway) otherwise the magic smoke gets out. It's magic because it won't go back in once it escapes...

I guess you were thinking the load would take a constant current regardless of the applied voltage? Constant current loads do exist but they are quite expensive in high power ratings - not something you would use without good reason.


----------------------------------

Sometimes I only open my mouth to swap feet...

 

[rob43]

Was thinking something like that, although not that clearly. I ran a test with a stable power source and the load. the amps measured at 10.

Voltage on the known source was 15 and that was the voltage reading. Guess I assumed that the volts would go up with the rpm's but thought the current would be stable.

The motor output fluctuates (probably due to turbulence) and does not seem to be as smooth (is that the right term?)as the power coming out of the known source.

Thanks again ScottyUK.

 

[waross]

If it is a Permanet Magnet motor, try using a reostat instead of a power supply. You can adjust the load by adjusting the reostat. voltage and current should be almost directly proportional to the speed. Simple theory says that the voltage will be directly proportional to the load, but in the real world some of the components exibit less than perfect linearity, and the current will heat the conductors which will result in slight changes in resistance.
You will be able to calculate the power output by multiplying volts times amps. In very small motors the rotor resistance will create an error if not added in. In larger motors, the rotor resistance is often low enough that the error is negligible.
respectfully

 

[rob43]

Motor's about 2.5 inches. Wind tunnel is only 1.5 meters and we need a fair amount of clearance for the test rotors to avoid any forward overpressure so it limits the size of the rotor and thereby the motor.

Thanks waross