Why did this student's windmill not make higher voltage?

[gophert]

Here is a question from another person that degenerated quickly on another forum so it deserves a fresh start here. The OP on the other forum was trying to do an autopsy on why a classroom wind turbine project didn't generate as much electricity as expected. Just an added note, peak voltage is the measurement of merit for grading (comparing) projects. It us a bit of a BS project but common challenge between classmate in various physics classes, Engineering technology classes, or even robotics clubs (high school of college level). Here goes:
(Note: first sentance from original post was removed for clarity)
===================================

This happened a long time ago I'm just wondering if I'd tried all I could.

Student comes into the lab, Group project was a windmill with a generator on it. Teacher gave him the "generator" (I'm pretty sure it was some motor). Nice windmill, he made, like a squirrel cage fan out of glued up posterboard. So I take him to electronics,
Hook thing up to bench multimeter in DC, spin. Two volts.
Bench multimeter in AC, spin. Like, no volts.
Tried another meter, same thing. Student insists it should make a bunch of volts.
I put some resistor across the "generator" and put an o-scope in series. I just wanted to make sure this thing wasn't putting out AC or something silly. Only amplitude changed by some mVolts, did not look like ac.
I really wanted to see what kind of motor it was but they'd buried it in wood and I couldn't see it. It spun free so it wasn't like a stepper motor, but certainly nothing real high torque.

The windmill was very easy to spin by hand and my thinking is that if it made any power worth making it would have been harder to turn... Probably so hard that their design would need a gale to work.
My question is what would you have tried?

 

[gophert]

Two options were discussed -
1) poor/slipping mechanical connection between squirrel cage and motor shaft.
2) An AC induction motor will not generate.

 

[MikeMl]

I would be inclined to use a machine that has a permanent-magnet rotor and a fixed stator windings. Alternatively, I would use a machine that has slip-rings to a wound-rotor, and fixed stator windings, like an automotive alternator. If rectification is needed, then put it between the stator an the load...

 

[alec_t]

My question is what would you have tried?

Check for signs of damage, poor connections.
Ascertain how many wires/terminals it has.
Measure the resistance between all accessible pairs of terminals.
Try running it as a motor to assess rpm as a function of applied voltage.

 

[gophert]

Thanks MikeMl and alec_t

Just to clarify, the wind turbine is long gone and the OP was mostly asking, what other testing methods could have been used or problem should have been looked for to help the student raise the output voltage on a wind turbine that appeared to built much better than the competitors' wind turbines.

 

[Externet]

My question is what would you have tried?

I would had applied power to confirm electrical/mechanical connections fitness performing as a blower motor.

 

[MikeMl]

Were all students in the contest provided the same "motor"?

 

[gophert]

Were all students in the contest provided the same "motor"?

That is assumed to be true but errors are possible (looked the same but actually had different specs).

 

[JimB]

Last night I was "looking over the fence" and saw the post you are referring to.

In my opinion, the OP was a bit like a "rabbit in the headlights".
With out going back and reading it again, my reasonably un-interested memory tells me that there were:
No firm details of the "motor".
No details of the impeller.
No details of how motor and impeller were coupled together.
No details of what was providing the "wind".

So how could anybody give any sensible advice?

Just another day on AAC and ETO.

JimB

 

[gophert]

Last night I was "looking over the fence" and saw the post you are referring to.

In my opinion, the OP was a bit like a "rabbit in the headlights".
With out going back and reading it again, my reasonably un-interested memory tells me that there were:
No firm details of the "motor".
No details of the impeller.
No details of how motor and impeller were coupled together.
No details of what was providing the "wind".

So how could anybody give any sensible advice?

Just another day on AAC and ETO.

JimB

Right, the OP said the chassis was too enclosed to examine those details. He was looking for ways he could had diagnosed the reasons for the low output.

 

[MaxHeadRoom78]

As I had posted I would try the quick BEMF trick, if there is any significant field in the motor any attempt at spinning the rotor with the output shorted will result in very high braking effect, dependant on rpm.
No significant BEMF then no significant field!
Unless the motor is relying on residual to generate/start its own field, as in wound field. In which case he may be missing residual.
Max.

 

[gophert]

Here is the original line of questioning. - continue debate whether or not this is "homework", please continue that conversation over there. If you have suggestions on the topic, please post here.

https://forum.allaboutcircuits.com/threads/why-did-this-students-windmill-make-low-voltage.121309/

 

[shortbus=]

My thought on the problem are that the speed of the 'turbine' wasn't fast enough. A squirrel cage 'fan' works better at moving/pumping air than being moved by air flow. Any squirrel cage I've ever seen takes the air in at the center and blows it out of the outside of the vanes. A brushed motor as a generator, voltage out is linked to the RPM it is driven at.

 

[dr pepper]

If it is a squirrel cage then it will not generate without assistance.
A squirrel cage motor is a form of rotating transformer, the squirrel cage being the secondary, very low voltage very high current flows in the rotor producing the required magnetic field to turn the motor, without any power applied this obviously will not happen.
A small squirrel cage motor like that from a washing machine pump would probably start to generate if you 'flashed' the windings with a 9v battery, once generating and feeding a load it will do so untill it stops rotating fast enough to generate.

 

[strantor]

Man that AAC thread is a train wreck. I guess the power trip train is picking up more riders.

If the OP wants to know what he could have done better, the only answer thst comes to my mind is rip the thing apart and see what's in it. It's probably a worthless shaded pole fan motor with no chance in hell of being a generator. If not, then it's probably screwed up inside. Whatever the case, you can't fix it from the outside and you can't know what the problem is without seeing the inside. And if it doesn't work, it's worthless and pointless, so what do you have to lose? Unless the points are awarded for aesthetic design, in which case put some pinstripes on it and call it art,

 

[MaxHeadRoom78]

If it is a squirrel cage then it will not generate without assistance.
A squirrel cage motor is a form of rotating transformer, the squirrel cage being the secondary, very low voltage very high current flows in the rotor producing the required magnetic field to turn the motor, without any power applied this obviously will not happen.
A small squirrel cage motor like that from a washing machine pump would probably start to generate if you 'flashed' the windings with a 9v battery, once generating and feeding a load it will do so untill it stops rotating fast enough to generate.

I think the original reference to Squirrel Cage was the type of fan impeller, NOT the rotor as in an induction motor.
Max.

 

[dougy83]

The generator voltage is proportional to the RPM, so if the goal is to have a high O/C voltage, then you should go for a propeller design that gives the highest rotation speed. If you have a potentially slow turbine (with high torque), such as a savonius or [I assume] a squirrel cage, then you'd want gearing to increase the RPM.