Grid Resistor 1

[newenergy]

We are developing a 1,000 kW permanent magnet generator using an older conventional generator as our starting point. We will replace the old rotor with a PM rotor. In order to test this "new" generator we are going to need a grid resistor or load bank to simulate a resistive load. To preserve as much of our budget as possible, we'd like to build a grid resistor ourself. Desired specs: max 1,250 kW load with min. 100 kW steps. We intend to monitor the resistor power consumption, temperature and incoming voltage.

Any recommendations on how to build such a unit and inputs on how to monitor these values would be greatly appreciated.

 

[bacon4life]

Check out thread238-142149.

 

[waross]

Interesting! Can you share with us your scheme to increase the magnetic flux to compensate for the internal voltage drop due to loading? Any time I have had occasion to measure the excitation loaded and unloaded I have found about a 2:1 ratio of field strength from no load to full load.

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

 

[itsmoked]

I would highly recommend the saltwater rheostat load. This would provide nearly infinite load adjustment.

Do a little work to allow safe adjustment and to prevent the two electrodes from ever directly touching.

This system will have a large power capability and will not cost much. When you're done with the whole thing you won't be stuck with a resistor.

You'll already have ammeters and voltmeters to guide any adjustments.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[newenergy]

I checked out the referenced thread and have three questions: (1) how do you determine the volume of saline solution that you require? (2) are we referring to standard table/road salt or a special type of salt? and (3) for our three phase application are all six electrodes immersed in the tank simulataneously?

We expect a drop in magnetic field strength between a zero and fully loaded condition between 1.5 to 2; however that's (unfortunately) only theory right now. We'll have to wait until we actually get the unit running. We have compensated for this drop by incorporating a mechanical adjustment in the genrator to allow use to vary the air gap from 8 mm to 5 mm when the unit is stationary. This provides a facility for detemrining the optimium gap for the application.

 

[itsmoked]

1) You do some scratch math on how much power and the resulting evaporation rate it will cause. You don't want too small a volume or you will get to much roiling etc. Think dumpster sizes. You could even rent one. Then return it never filled with garbage.

2) Any old salt at all. You can set up with tap water and see what you get then toss in a quart scoop of salt. Wait a little and see what transpires. You can tailor the concentration to what you desire.

3) You mean all three.. Yes all three in the same container and hopefully equidistant.

I think waross has done more of this on bigger machines than I and can lend some more details.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

A variable air-gap generator? Sounds like an idea who's time hasn't come. How are you going to vary the air-gap on the fly? And on a timely manner? A wound field and constant air-gap sounds a whole lot more reliable.

 

[newenergy]

Thanks for clearing up my questions on the saltwater rheostat load.
The variable gap mechanism can only be adjusted when the rotor assembly is stationary and is intended only to be used on the first generator to allow the optimisation of the air gap size. We have already successfuly built a small scale modle of the system and it works quite well. The variable gap mechansim consists of 84 individual permanent magnet assemblies mounted on a nylon track assembly running along the inner circumference of the stator assembly. Each NdFeB magnet is attached (glued) to a ferous steel assembly, which in tern is mounted on the track via height adjustable bearings. The assemblies are connected to one anothor with a nylon holder assembly. To vary the air gap, the four bearsing mounted on each PM assembly are manually adjusted and the nylon holders are replaced. The gap between the PM assemblies increases through this adjustment, which leads to a step in the generated voltage, however the mechanism is purely to better determine the optimum air gap for this application.

 

[davidbeach]

What is your output voltage regulation over the range of no load to full load?

 

[newenergy]

We have calculated a no-load voltage of 460 volts. The output is frequency stable and instead of a rectifier/inverter system, we will use an external voltage regulator to ensure that the output power meets local grid specifications. While it is an expensive solution, it is the only effective one that we have been able to come up with for a PM generator.

 

[waross]

I would be interested to see the economic justification for this arrangement. You can buy an off the shelf conventional generator with a PM option which will do what you want.
The losses from the excitation system are quite low. In fact the losses are probably much less than the losses you may experience with an external voltage regulator. The cost will probably be less.
A brief description.
A conventional generator has a wound rotor. The exciter is mounted on the same shaft and the AC output is rectified by a diode set that is also mounted on the shaft. The inductance of the field winding supplies enough filtering to smooth the voltage ripple. The AVR (automatic voltage regulator) controls the stationary field of the shaft mounted exciter. The AVR senses the output voltage of the generator and controls the excitation to keep the output voltage within a few percent. The AVR also takes its power from the generator output, sometimes on the same terminals as the power supply.
A PMG (permanent magnet generator as applied to a conventional generator) is a small permanent magnet generator mounted on the back end of the generator shaft to supply the power for the AVR. The excitation circuit voltage drop with varying excitation load is compensated for by the AVR.
You seem to be trying to convert an induction generator into a conventional generator the hard way. No problem. You are already learning a lot about generators and will learn more.
However, may I suggest that at the end of the day, you use a conventional generator (with or without the PMG option) as a benchmark against your own machine.
By the way, it is hard to tell from your description if you have provided an iron return path for your magnetic circuit. If the inner ends of the magnets are not connected by an iron path the space between the inner ends of the magnets will also be part of the air gap.

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

 

[newenergy]

The points you have raised are all valid. We have used a conventional generator of the same design as a baseline (benchmark) and are intending to build a prototype PM generator by replacing the induction (conventional) rotor with a PM rotor.
The whole purpose of this exercise is to determine the merits of using either a IG or PM generator for a new type of drive system that we have developed over the last four years. We have proven the effectiveness of this system on the induction generator already. Sorry can't say more about this system in the forum.
You are right about the iron return path. That has already been incorporated into the magent assembly design. Essentially each of the 84 magnets is mounted on a rectangular shaped iron plate optimised for the magnet footprint. The magnet itself is delta shaped to ensure a smooth voltage curve as the magnet package passes over the stator.
By-the-way picked up an excellent text "Distributed Generation - Induction and PM Generators" ISBN 978-0470-06208-1, which is helping to quickly fill in my knowledge gaps. Thanks for your assistance.