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]

There are some trying to use disks from a hard drive with the permint mag in the drive to build a load. moving the mags in and out on the disk will change the load.
Inertia is the reason for not using a large mass on the rotating assembly as this mass will absorb the power. A low mass rotating assembly will replicate the conditions the Eng will see when used in the application it is intended for.
The Eng are used in RC boat racing. We race at speeds close to 100MPH on a course that will fit in side a foot ball field. the boats turn 180deg in a span of 50' or less at speeds of 75+ mph and entry speeds close to 100 MPH.
We burn 50 to 65% nitromethan mix in the Eng for racing. a 1 CID can make 8+ HP at 28000RPM depending on the tune.
The high RPM is the problem with testing this in a Dino. what ever you use has to handle the High RPM at a load.
the smaller Eng .45cid will turn 30000RPM and the .21 cid eng will rev 35000rpm.producing 3.5+ hp.
Waross can you explain you idea a little more?
Delta wind is a option when specking these motors.
Auto-transformer? single phase load?

David

 

[waross]

Yes David I can describe it, then tell us if it will fit your plans.
Assume an 8 KW generator/load.
Build some resistor banks from nichrome heating elements. say 1 KW, 1 KW, 2 KW, and 4 KW. Arrange these to be switch connected.
Now add an auto transformer and load of about 2 KW. On single phase,this will generate the same heat and take the same generator capacity as a 3 KW three phase load. Pre-set the base load and fine tune it by adding load with the auto-transformer.
I see a problem here and a possible solution. Many electrical and quite a few mechanical breaking schemes are speed dependent. As the speed drops, the generated voltage will drop and the dissipated energy will drop be a square factor.
As an example, we want to look at the performance of a motor at 10,000 RPM and 28,000 RPM
Calculate the resistance needed to load the motor at 28,000 RPM and set up a resistor bank and auto-transformer setting combination that will match power output at 28,000 rpm.
Do the same for 10,000 RPM, but subtract the amount of resistance in place for 28,000 RPM.
Now, with the motor running and loaded at 28,000 RPM, switch in the resistance block for 10,000 RPM. This will create an overload condition which will slow the motor until the speed and load stabilize at around 10,000 RPM.
Switch out the added resistance and the motor can be expected to accelerate to 28,000 RPM.
You may use an auto-transformer for each speed, or if time constraints permit, use the auto-transformer to determine the resistance required and make a fixed resistance for that setting. Ni-chrome elements are easy to to connect and tap.
Of course this may end up pricing above controllers, but if you don't do the numbers you don't know.
If it fits your plans and you are testing a group of similar motors, consider a fixed setting and reporting both HP and speed.
One motor may be reported at 6 HP at 28,000 RPM and the next motor reported at 6.3 HP at 30,000 RPM
Stated another way, you may set the test for a given HP at a given RPM and then see if the motor under test meets or exceeds the test points.
Reporting both over HP and over speed may be a more accurate description of the motors expected performance, or maybe not. You tell me. grin


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

 

[dwilfong]

What you explained is how I had it planned at the on set. but I was running the 3 ph AC thew a bridge to change it to DC. your plain eliminates the need for a bridge. Great idea!
I have done all the calculations for resistance to make the loads with varying voltages as the RPM change with rotor speed already.
I am a little fuzzy on the way to wire the load system this way.
Lets say the 3 wires coming out of the motor are referenced a1,a2,a3 this is three leads coming from a Delta wind motor.
Make a load bank of whatever connecting a1 and a2. now make a variable load from a2 to a3 ?
I have looked in to making water resistor dummy loads from vinyl tubing with copper lug at each end using copper sulfate as the restive medium in the tube.
Then coiling the tube in a 5 gal bucket and running water thew the bucket to keep the heat under control.
Then for the fine tuning using a large Biddle rheostat or a jagabi carbon pile rheostat.

 

[dwilfong]

One more question what would the calculation be to figure the load on one ph at the lower voltages that the motor will produce.
Say a 15a 120v auto trans would build how many watts at only say 40v?

 

[MikeHalloran]

Are you sure about the quoted RPM figures?
I'd believe that a small piston engine could go 14,000 rpm, but 28,000 rpm is a bit less believable.
Did someone perhaps measure the exhaust frequency and assume they're 4-strokes, not 2-strokes?



Mike Halloran
Pembroke Pines, FL, USA

 

[dwilfong]

http://www.hobbysupplies.com.au/CMB Engines.HTM

Here is a link to one suppler that shows specks on the Eng I run in stock form.
I do all the modification to my own Eng and thy make more than the stock figures. also the stock HP and Rpm ratings are on 20% nitromethain not modified eng running 60% nitromethain.
Theses eng are the cutting edge of IC eng design. that is why I run RC boats.
To push the edge of the HP envelope.
I build different variants of these Eng from stock parts to gain even more HP.

 

[waross]

Examples.
You want 300 Watts load. Calculate the resistors for 100 Watts load. Make three resistors and connect one from a1 to a2, one from a2 to a3 and the third resistor from a3 to a1. Now you have a 300 Watt load. Now put a 100 Watt resistor on the auto-transformer and connect it across any two leads. a1 to a2 will be fine. Now you have a 300 Watt base load and and an additional 100 watt variable load. or 300 Watts to 400 Watts. The internal heating of the motor/generator will be the same as if a 450 Watt load was connected. The capacity of the generator must be calculated for 150% of a single phase load. That is the advantage of a three phase base load. If the entire 400 Watt load was connected single phase you would need 600 Watts generator capacity rather than 450 Watts.
More information later if you need it, but the 100 Watt resistor on the auto-transformer (set at 100%) will give you a 100 Watt load. AS you dial the AT down and reduce the voltage on the resistor to zero, the 100 Watt load will reduce to zero.

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

 

[dwilfong]

Bill
Can I use 3 transformers to change the lower voltage output of say 40v on each leg to 120v
and use a variac one pot on each transformer to adjust the load.

http://www.ebay.com/itm/181004336490?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

Here is the one I was looking at.

 

[waross]

I'm a little tired just now so please forgive any foolish typos.
For illustration, and to make it easy to grasp lets take a motor/generator which develops 50 Volts at 50,000 RPM.
At 28,000 RPM it will develop 28 Volts.
At 14,000 RPM it will develop 14 Volts.
Sizing a transformer: The winding will have to be capable of carrying the full current, Voltage and frequency is more flexible.
A six pole motor looks like an 1800 rpm @ 60 Hz motor. At 18,000 a 12 volt winding will take 120 Volts without saturating. (A 6 Volt winding would be better if available. So, over-voltage is not a problem as far as saturation is concerned.
Example: we want to dissipate 5100 Watts at 14,000 RPM. The voltage will be 14 Volts.
The current per phase will be: 5100W/1.73/14V=210 Amps.
A 12 volt winding to handle 210 Amps will be rated at 210A x 12 Volts = 2500 VARs.
The secondary will put out 14V x 10 = 140 Volts
At 28,000 RPM the voltage will be 28V and the 120 Volt secondary will be putting out 280 Volts.
When you look at the price of dry type transformers (figure 3 x 5 KVA or 3 x 7.5 KVA) you may want to go with resistors sized for the out-put voltage and forget about transformers (except for the auto-transformer).
Time for bed, more tomorrow.

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

 

[OperaHouse]

Everyone always says, I just want this. But really you are going to want a lot more capabilities
than just a simple load. This is an engineering forum and we should be going for more. You seem
competent enough for what I am about to propose.

Three phase rectify the output from the motor and filter that voltage with several large electrolytics,
say three 4700uF 63V. Monitor the voltage with an Arduino UNO ans drive three or more FETs with the PWM
output pins. Each FET will have its own load resistor. Load resistors are easy to make if you don't go
commercial. An adjustable resistor into one of the A/D inputs. Isolated ACS712 current sense boards are
only about $4 that will deed directly into another A/D input if you want to calculate power too. The UNO
has nice monitoring capabilities. You can skip exotic LCD programming by just using the USB port of a
laptop. A copycat version of the UNO is less than $15 and the development system is FREE to download and
try. "Conversational logic", if this and if that is straightforward and easy enough for anyone to understand.
Map functions easily convert data into engineering units. Standard PWM outputs operate at 490Hz which makes
driving a FET straightforward. With all this capability you can capture peak events, monitor temperature and
have a nice record of the test. There are lots of dump load discussions at fieldlines.com where this voltage
and much higher currents are common. Many of these logic functions have been discussed there. You could always
wimp out and use a GHURD controller and TURNIGY 130A to collect data. I think you would like using the UNO.
There would be interest in this dump project at fieldlines and you would get help there.

 

[dwilfong]

I have a Arduino Nano that I was planing on using as a eng control system for theses eng.
That is the reason for building this Dino to run simulation on the bench.
I have a data acquisition system

http://www.eagletreesystems.com/#

that I use now to log the RPM, head Temp and GPS speed in the boat as it is running on the water.
I have also logged EGT with this system.
This is what I am planing on using to run the Dino.
There is a analog to digital inter face card made for this system to gather the load cell info.
Every thing will be logged on one graft and also in Excel spread sheet form.
I can log RPM,HEAD TEMP,EGT(exhaust gas temp) and load cell readings at the same time.
That is why I am looking at a easy cheep fully adjustable load system for this motor.
I need to start some where before I jump into the Arduino system.
I am not very proficient at code writing or under standing the logic circuits involved with the Nano.
I need to walk before I can run.