Backwards Dynamometer

[toygasm4u]

Hey guys,
My counterparts and I are digging in to what it would take to test rotor assemblies for VAWTs, with the target end results being measured in kW/RPM. I'm assuming that what we'll see is an increase in RPM and Torque, up to a certain point where RPM will start to level off due to loading if I can get this working. We already have an inline torque transducer (brushed style) and a RPM/Rate Meter from FUTEK and OMEGA that are left overs from other projects. I'm kind of stuck in how I want to load this thing.

On the shelf in the shop, I have a PM 3 phase AC motor, with a theoretical 1HP operating ceiling(630 in.lbs @ 100RPM). I would like to use this motor as my variable torque load. I've read on a thread or two here about using different sizes of light bulb, but this seems extremely impractical. I've also looked into tandem rheostats, and load resistors with choppers.

I'm wondering if I can't just get a fixed 3 phase 1kW resistive load, and then use a dimmer circuit like for lighting loads. Making matters worse, we're a little tight on budget, which rules out any Hysteresis Brakes I've read about. (They look a little pricey). Any advice is much appreciated,

~Mark

 

[itsmoked]

VAWT = Virtual Alligator Weight Training?

How about a big rheostat and a wattmeter?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

I'm glad to hear that parts are again available.
I have Vintage Abstract Widget Trammel that desperately needs a new, tested rotor.
Hook your motor windings in as parallel as you can. Connect a 12 volt automotive battery and measure the current draw so as to calculate the effective resistance. Use that information to design a single phase Variac and bridge rectifier circuit.
Use that to dynamically brake (break?) the motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

There was a good thread a while ago on using a regenerative drive as a brake on a dyno. You might not get a full regernative drive at such a low power rating but a drive with a brake resistor would be practical at 1kW. That said if a 1kW heater is pushing the budget then maybe you need to consider how badly you want to do this. I can pick up a 1kW fan heater for under £10.


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If we learn from our mistakes I'm getting a great education!

 

[GregLocock]

We use electric stove elememnts, or radiant fire elements, as absorbers. They have the advantage that they are cheap as chips and absolutely bullet proof.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[waross]

Sorry, I didn't notice that you had a PM motor. If you can find an induction motor you can load it with DC in the windings.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[toygasm4u]

I can't believe you guys have never heard of a Variable Axis Wallaby Tilter.

My apologies for the Acronym; Vertical Axis Wind Turbine

Lol. IDK what came over me.

Waross, If I reconfigure the 3 phase stator so that the windings are in parallel, won't that prevent me from having a kinetic load that's variable from 0-100%? To me, that would seem to be setting up a magnetic tug o' war between windings.

I've never had a reason to mess with dynamic breaks, so pardon my ignorance here. If I have an old VFD with dynamic breaking as an option, then I will connect the drive to utility, install the appropriate brake resistor on that drive, and I can use a squirrel cage motor as a dynamic load? Better yet, find a brake chopper and a resistor connected to two phases of a three phase squirrel cage motor?

Am I on the right track in saying this?

 

[toygasm4u]

..in addition, I guess the only thing about the DB setup is that I will be using it at 100% duty cycle. I should probably go ahead and dig out my safety glasses.

 

[LionelHutz]

Care to share the output of this generator? Voltage, configuration (DC, 1-phase or 3-phase) and expected current would be nice.

If this is say a 3-phase output and you're going to connect to a rectifier and a battery the best load is a rectifier and battery with a dump load controller over the battery to maintain battery voltage. The final customer is really only interested in the output vs rpm (where rpm needs to be related to wind speed by the blade manufacturer) in the as installed configuration.

A permanent magnet generator output voltage is fairly linear with rpm. So, it won't really do anything useful until the output voltage (after the rectifier) is higher than the battery voltage. Once that voltage is reached, the output power goes up quickly, since the rectifier output voltage is then being held constant meaning the further increase in generator output voltage is basically dropped across the stator windings making the current rise quickly.

 

[waross]

As I said, I missed the PM designation on your motor.
I think that Scotty and Greg are on the right track. Load your PM motor with cheap stove elements.
How are you going to be testing? Do you need dynamic control or can you use a fixed load at each load level?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[toygasm4u]

When we connected my PM motor as a basic generator, this is what we got for performance data:

http://i35.tinypic.com/no8pol.jpg

I would like to us this as my dynamic load for the rotors under test in the dyno setup. The more I talk about this, the more I feel like I've got it licked; 3 phase rectify this PM motor, and just dump it across an appropriate load resistance / rheostat. I've seen a few rheostats that have a 60 second drive motor attached, that will slew it through the range. This fixed time base would be advantageous for correlating my data polled from the other sensors, as I will be polling data at 1/2 to 1 second intervals.

 

[itsmoked]

And map the V and I right?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[2dye4]

Get some water heater elements and stick em in water bath and switch em with relays.
The first load has one heater.
The second has two
the third had 4
the fourth has 5

either parallel or series. to suit the load level
With these ratios you can select 2^n levels of load where n is the number of combination's by switching them in binary.

good luck

 

[sreid]

If the budget will allow you to purchase a PM DC brush type motor, 1 HP pretty inexpensive), you can use a variac and a full wave bridge to get DC current into the motor armature This makes a pretty good electrical brake. Use an amp meter to measure the current and you'll get a pretty good reading of the torque.

If you can afford a current amplifier, you can simply dial up the torque load independent of the motor speed.

 

[Skogsgurra]

I am very confused - or is the rest of the world?

The OP mentions a three phase induction motor and everyone starts talking about DC. Why?

Toygasm then mentions the correct way of doing it (putting an inverter before the motor and adding a chopped braking resistor to the DC link).

That is, in my view, end of discussion. Set the torque limit (the "braking power") either by using an analogue input or from the panel. Use a set of incandescent lamps for the braking resistor and let the chopper do the switching of the resistors. That's what it is there for.

You will then have a brake with a settable braking torque, which will remain constant across your speed range. Your speed setpoint should, of course, be zero RPM.

Lamps are not impractical. They are very practical. They are built to dissipate energy as heat and light radiation and they are cheap. Just make sure that you install enough watts and that your DC link voltage and the lamps' rated voltage do not differ too much. Use series connected lamps if necessary. Undervoltage does not hurt the lamps.

Yes Toygasm, you are on the right track.

Gunnar Englund

http://www.gke.org

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

 

[itsmoked]

I thought "[red]Permanent Magnet 3 phase AC motor[/red]" s are not used with standard OTS VFDs.

Or am I confused?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

They can very well be used. I have run many on standard VFD's. Especially well suited in a constant torque application like this, where speed is determined by the input shaft.

You may be confused, Smoked, but it doesn't show

Gunnar Englund

http://www.gke.org

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

 

[Skogsgurra]

Yes, Keith. I also missed the PM part. But, luckily, PM motors can be used here. It depends somewhat on what VFD the OP has available.

Gunnar Englund

http://www.gke.org

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

 

[sreid]

And is it not also true that the PM AC motor must have a shaft position encoder that is phased properly and compatible with the VFD position input?

 

[Skogsgurra]

No, not at all. I have run up to 11 MW synchronous motors without any encoder or rotor position indicating device.

Some CAN get quite unstable if you run them in vector mode. Scalar is usually possible and torque or current mode is (in my experience) always possible.

Textile drives are usually PM motors and there are hundreds of them running off a common VFD. No encoders, no nothing.

Gunnar Englund

http://www.gke.org

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