Another using motor as generator question and power output

[rob43]

In ref to "using motor as generator..." Jan 5th:

We had placed a small 15V .5A incandescent bulb in front of the main resistive load, just for a kind of visual cue.

Noticed when testing circuit with a stable 15V DC power source there was a differnence in bulb luminosity. Took almost 24V to acheive the same luminosity with circuit connect to the wind rig both with a flow in the wind tunnel and hooked as bootleg genset.

Does this have something to do with amplitude or frequency of the output from the motor, and, if so, does that need to be figured into calculations of power output.

Just as a note, we are mainly looking for relative power output of different rotor configurations and profiles on the same motor, but it would be nice to compare results generally.

 

[waross]

A light bulb is very non linear. The resistance change with temperature may be as much as 10 to 1. Re the different voltage readings at the same luminosity; There are several hidden factors in this seemingly simple circuit. One factor is wave form. The waveform output of the small motor will be a ripple voltage superimposed on top of the DC output. The magnitude of the ripple will diminish as the number of comutator segments is increased. The frequency of the ripple will increase as the number of comutator segments is increased.
Even though the output wave form of the stable power supply is propably fairly pure DC. we don't know this for sure. There may be a ripple on the output.
Test meters. Many of the older DC meters indicated average values. With a severe ripple there may be a significant difference between average values and RMS values.
I suspect that you have a fairly clean DC supply and a lot of fairly high magnitude ripple from the motor/generator.
This will give a metering error. Add to that the non linearity of the light bulb and your discrepancy is understandable.
respectfully

 

[itsmoked]

Change to a bulb across the load and use that as your indicator. It will still be highly non-linear though.

Better would be a voltmeter.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rob43]

Thanks Waross

Meters are new. They are RMS DMM's. Light bulb load is relatively small portion of overall load especially on amp side. Bulb is rated at 300ma.

How far off could the DMM volt and amp readings be? Is it +/- 5%, +/- 20%?

Also does it make a difference if all I am comparing is results from different configurations using the same motor?

Is there some output point at which the ripple effects the output to a greater degree?

Is there a way to smooth the ripple?

 

[waross]

I apologise that I'm short of time tonight an am going out of town for a few days. There are some details that I don't have time to go into right now.
The ratio between the average of a sin wave and the RMS of a sin wave is 1 to 1.1. Different waveforms havew different form factors. (The ratio of average to RMS).
You can smooth the ripple out somewhat in a motor with a free wheeling diode. That is because the applied voltage and the back EMF oppose each other. The freewheeling diode gives the back EMF a path to flow and smoothes the current. In a generator all you have is the back EMF. When you are using the power supply the internal diodes may be acting as a free wheeling diode.
How about some help here Gunnar. Am I in over my head?
It is not clear to me how you are using the power supply as a load.
How many segments does your comutator have?
respectfully

 

[rob43]

Sorry for so many questions Waross.

Not using power source as load. I must have been unlcear in the post.

Load for the generator (motor) is a combo of the bulb and a variable carbon pile set at 10 amps.

Hooked the load onto the 15V DC power source to make sure the meters were in line with input from a known stable power source and the load.

Motor is part of a small wind rig we are testing with different rotor configurations in a wind tunnel. Output from the motor in tunnel is variable given a change in the wind speed in the tunnel.

Rotors have to be tested across a number of different speeds as a particular blade profiles's efficiency will vary with speed.

Am trying to make sure I am comparing apples to apples with regard to power output as that will give the blade efficiency coefficient relative to total power available in the flow.

Thereby I can track efficiency at different speeds for different blade profiles and configurations.

Not sure how many segments the comutator has.

Anyway thanks for your help. Have a good trip.

 

[ccjersey]

I think your approach of using a set amperage is limiting your capability to test. I would adjust load to maintain voltage and amperage within the constraints of the motor rating. You may find you have to get another motor to handle the input of rotor under all conditions so you don't burn it up midway through your test program. the motor being in the airstream can possibly allow higher power generation than rated, because of increased cooling.

If you like having the bulb in the circuit, just measure motor terminal voltage and amperage through the whole circuit: wire, bulb and carbon pile put together. Your bulb may need to be a higher voltage to withstand the peak voltage encountered.
In any event, calculate apparent power by V x A.

I don't know if there's a good way to correct for the losses in the generator which should increase slightly as power generated increases. this should be constant across all the rotor profiles, but true power values may not follow exactly the apparent values you calculate.

The DC supply results will have no bearing on comparisons between rotors. However you could always use the variable DC supply to power the load circuit to the same amperage after you get done with the rotor/motor combination and make a correction to the voltage measurement.

 

[Skogsgurra]

I am very confused here! What ARE you discussing? Seems you jumped into something everyone heard about - except me.

Putting a 15 V bulb in series with load - and generator outputting 15 - 24 (or so) volts seems to be a bad idea in the first place. Shouldn't the load take most of the voltage? Then you say that it takes different voltages to get same luminosity from bulb. How do you know that? Do you measure luminosity? Or is it what you think? The eye has an extemely wide range of sensitivity and can easily adapt to many decades of light intensity. So it is a very poor judge when it comes to luminosity.



Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[rob43]

Sorry ccjersey & Skogsgurra this is a follow on question to an earlier post.

"using motor as generator..." Jan 5th

The DC supply (used an AC>DC converter) was just used to test the load and DMM's. It had nothing to do with testing the rotors.

We are ordering more motors with higher ratings this week, but we are limited on size by the wind tunnel dimensions for the moment.

The main question I had on this post was this:

It seems (subjectively) that it takes more power (v * i readings) for the indicator bulb to light (I videotaped it so I could readings and indicator concurrent) when everything is attached to the rotor/motor than it does when everything is attached to the 15V DC converter.

Was wondering whether this had something to do with the charactersitics of the power (waross had mentioned ripple) produced by the rotor/motor vs. the DC converter?

And if so are the volt and amp readings with the rotor/motor an accurate measure of the power regardless?

Based on answers this would not effect test rotor to test rotor comparisons, but it would have an effect if we wanted to calculate a rotor coefficient relative to the total power available in the wind at a given speed.

Thanks for previous answers.

 

[ccjersey]

Gunnar,
Here's the link to the original thread.

http://www.eng-tips.com/viewthread.cfm?qid=174657&page=2