brush less motor as eddy current eng Dino

[dwilfong]

I am trying to use a small brush less AC motor to load a eng.
What options are ther to make a field to load the rotor.

David

 

[dwilfong]

http://www.tppower.com/sort.asp?class_id=4&news=57

Here is a link to the motor that I will be using.
I will turn this motor with a nitro powered 1 CID Eng that will run at 28000rpm to test the power out put.
the motor will be in a rotating cradle with a load cell attached to get ft.lbs. readings.
There are various ways to load the field just what is the best way?
Feed DC voltage to the winding on two poles.
Invert the 3 Ph out put and load with Resistance bank?
Feed AC to the windings?
Any help would be much appreciated.

David

 

[benta]

Your PMDC motor used this way is just a PM synchronous generator.
Just attach a variable three-phase resistive load.

Benta.

 

[ijl]

So, you have an (internal combustion, IC) motor that drives the brushless motor at the mentioned rotating speed. The brushless motor acts as a generator in order to load the IC-motor. You want to measure the torque (and rotating speed) in order to get the power of the IC-motor. Did I get this right?

The simplest way to load the generator is to connect equal resistors in each of the wire-pairs of the generator. That is, three resistors connected in a triangle, and the generator wires connected to each corner of the triangle. There are some problems with this approach, however: the size of the resistors in Ohms, and in terms of the power they can handle.

You do not mention the power of the IC-motor, but the data of the brushless motor indicates that several kW are available. The resistors should be "powerful" enough to handle this. In addition, cooling of the resistors is needed with a fan or water, for example.

The second problem is that the resistors (the value in Ohms) very much determine the loading. This means that you should have a set of resistors in order to find the maximum power. You are after the maximum power, aren't you?

 

[OperaHouse]

I'm impressed. I was all set to talk about the problems taking a typical motor and running it at 28K. Who knew something this esoteric existed in the hobby world.

 

[waross]

Probably the the cheapest way will be to buy replacement elements for a clothes drier to use as resistance. These are coils of nichrome wire that you may cut or tap for the values you need.
Your power dissipation will depend on the square of the voltage which will depend on the speed. You should plan on some instrumentation to indicate the power produced. Don't forget to add the internal loses of the motor.

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

 

[LionelHutz]

I don't see enough data on those motors to really be able to predict what the expected voltage and current would be under load.

If doing it starting with nothing, I would rectify the 3-phase output into DC and then feed a PWM controller dumping into a resistor. The only issue with this idea which might cause issues is that the DC voltage will vary with rpm. However, it might be possible to get a pwm controller with input voltage compensation. In other words, the controller will automatically vary the PWM duty cycle to maintain a constant power output even when the input voltage varies. You might also find you want the load to increase as the rpm increases and this really isn't an issue.

First, I would investigate if a RC controller could be used. I'm sure there must be one that does regen braking. You could just connect it to a big battery and it'd dump the energy into the battery. Switch a big load onto the battery to dump the energy.

 

[benta]

LionelHutz, the idea with rectifying and PWM modulating into a load is really nice.

A non-isolated step-down converter would probably be simplest. If the efficiency curve of the converter is nicely smooth and horizontal (is it not at low load, but who cares in this application), output power into the resistive load would be proportional to the input power coming from the BLDC generator.
Using heating elements for the load is the way to go.

I like it.

Benta

 

[dwilfong]

I have explored the Scenario of rectifying the AC to DC then resistors for the load.
Looked at kiln heater coils and also a custom made salt water resistor.
The problem is adaptability over a span of different RPM ranges and load conditions.
the voltage will change a lot with a 780Kv motor. All the different loads and RPM's will make it hard to have one size fits all. this will be a test stand so You do not really know what you will have till you run it.
I am interested in trying to put a DC current to the field coils on two legs of the Y wind or isolating the polls and running DC to all of them in a series parallel circuit.
The load dose not need to be measured the load cell will do that.
But the load dose have to be adjustable over a span of RPM ranges and different loads.
Dose any one have any thoughts on putting a DC current to the stator to load the rotor?

David

 

[dwilfong]

The Eng's I will be testing will be any where from 5 to 10HP.
I will be testing different size IC Eng with this setup. So it has to be flexible with its load adjustment.
I am using this small motor to keep the power absorption from the rotors inertia to a minimum.

David

 

[mikekilroy]

dwilfong
1) The problem is adaptability over a span of different RPM ranges and load conditions. All the different loads and RPM's will make it hard to have one size fits all.

2)I am interested in trying to put a DC current to the field coils
Dose any one have any thoughts on putting a DC current to the stator to load the rotor?

David


David, did you post wrong link to your motor? why do you insist on trying to supply a field when the link showed non?? You posted a link to a low voltage high current PERMANENT MAGNET FIELD motor. 3 wires coming out. so why the consistent talk about a field winding if you have none?

your link describes a typical pmdc V & I rating for GOLF CARTS. Google golf cart controllers and check your options there; I paid like $ 175 for 400 amp 36v pwm drive for my modified GE Electrac riding lawn mower with pmdc motor. You can then set torque level to value to match the load on your dyno at any time.

 

[dwilfong]

Will the winding in the stator become a opposing field when energized with DC current?
That is the Question I am posing.
If I use the speed controller from say a golf cart. how will this load the motor by back feeding to a battery? Or just using the breaking circuit in the controller?
then you need a programmer to adjust it?

 

[mikekilroy]

put dc into the motor and you make a humpy torque as each magnet goes by. being hi speed, i assume these are 2 pole motors so put dc in and you make a steady magnetic field that will attract both magnets as they go past; 2 humps of torque per rev; in between the humps = no torque. not much of a dyno....

there are ac model gold cart drives that operate in 4 quadrant mode; meaning yes, you dial in how much torque you want and if the motor is in generator quadrant it puts it back in batteries. w/o a drive i do not see simple way to make a pure resistive infinitely variable resistor capable of multiple kw load.

when we build kw dynos for servo motor load testing, we have supplied sophisticated VFD drives and run induction motors. control of dyno is our world has always simply been analog speed command potentiometer dialing up 0 - max torque load. Controller is the mn standing there changing the pot. More sophistication then is plc or controller using some other feedback to modulate it if that is appropriate.

 

[waross]

You may buy two identical motors. Connect the leads from one to the other. Now when one motor is turned the other will lock on and turn with it. Put an aluminum disk on the second motor and use a DC coil to make an easily controlled eddy current brake on the disk. The second motor and disk may be located a safe distance away.

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

 

[LionelHutz]

You can't put DC into the motor. Even if it would produce a load (which it won't), applying DC to the motor would mean all the test motor power (10hp = 7.4kW) would be dissipated into this electric motor which would most likely quickly burn it up. This is why you need a controller and a load - so the power gets safely dissipated into the load bank.

I still think investigating a suitable controller capable of regen or 4 quadrant operation is the first thing you should do.

I didn't think about this last post but there are dump controllers for wind renewable energy applications. This is a module you basically connect across your battery bank and the module begins to dump energy into a resistive load when the battery voltage reaches a certain level. the basic theory of operation for small wind RE is that wind turbines can't be unloaded so once the wind turbine has charged the battery bank the generated energy gets dumped as heat instead. Use a motor controller into a battery with a dump controller to prevent over-charging.

The controller requirements shouldn't be that big a deal. 780kV or 780rpm/V isn't listed for any of those motors you linked. Still, you're looking at 36V give or take or 3 x 12V batteries in series. There should be a RC controller and dump load controller available for 36V.

 

[OperaHouse]

Won't you need a couple of caps on the motor to get it to gen before you rectify it.

 

[dwilfong]

The motors are 6 poll design. as far as heat the motor will be in a water cooling jacket to take the heat out of the motor.

http://kershawdesigns.com/TPPower.htm

This is the motor I am thinking of using.
There are many motor to choose from any KV and polls you can think of.
Most of the high end motors are very expensive $700 or more.
I am trying to keep the cost down on this build for the first tests to see what is what.
That is why I was thinking of the DC system. easy to build and simple in design over a wide range of RPM and loads.
At 20000+ RPM I don't think the notching will be a concern.
Was thinking of a square wave generator as in a model train transformer to energize the coils.
I have worked on self propelled personnel lifts for 30 years. Have maintained and repaired many DC drive systems in the lifts. the first ones where SCAR rectifiers with GE EV-10 modules in them. the newer ones are like the golf carts and need programmer to adjust them. thy do not have much adjustment as thy are designed for each application.
Need a easy adjustable system to change the load as I am running the ICC Eng in testing.
Not looking for a preset load that can not be changed while running the test.
Need to adjust the load at RPM to see what the Eng can run.
I know this is not the norm as most systems are made to do just one thing.
That is my reason for posting here to get some input from a different perspective.
I am a mechanic with many years of electric over hydraulic power system experience not a engineer. I know my limitations.
I thank you all for your interest and input it is all good.


David

 

[LionelHutz]

That motor will not create a load if you apply DC to the windings. You need another solution. Try something already posted.

 

[waross]

A single phase load on a delta generator will safely load it to 2/3 of the generator three phase capacity. How about an auto-transformer on two leads with a resistor bank on the secondary. You can base load with fixed resistors and trim the final part of the load with the auto-transformer.
Opera, these motors have permanent magnet rotors. No need for caps.

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

 

[MikeHalloran]

Further to waross's idea, dispense with the two linked motors and just fit an aluminum disk to a jackshaft, and brake it with a DC coil.

http://en.wikipedia.org/wiki/Eddy_current_brake

Mike Halloran
Pembroke Pines, FL, USA