The efficient Dyno

[CopperCone]

Well if you wanna do it right the best way would be to attach a heated aluminum block to a cold plate. Then you bore holes in the block and usse swaged self sealing cartridge heaters to act as resistors if your frequencies are low. Then you can gang em using a switch or relay bank.

A cheap way to do it is instead of the machinework to take an aluminum cooling block and stick heating strips of correct impedance on both sides then heavily seal it using sticky sided polyurathane foam. Add some epoxy atound the lead wires, cover it in paper and dip coat for water proofing.. You can make a 1kw heater this way in a few hours.. Key is to degrease the shit out of everything so the strips can adhere good and to uze gloves and a really clean work surface when using the stuff.

If you need better frequency responce then i gues you need to use electronics grade resistors.

Thehrating strips are actually decent because they are thin, a power resistor is something like 3 mm of cement, an aluminum housing and then your grease and then your block. The resistance material is in a helix coil in a resistor wheras in a strip its a flat zigzag maze like structure

A heating strip is flat, so you have say 50w distributed over 5 square inches seperated from the aluminum by something like 0.4mm of heat conductive rubber, so the thermal transfer characteristics are not too bad.

The swaged heating cartridge is the best because it will make a tight seal with the block by way of thermal expansion, it butterflies out like an arrow heat. If you calculate its expansion factor and bore performance you may get nice results but idk if you can heat up water cooled metal enough to properly make the seal.. But anyway if designed right no grease and easy replacement at room temp. And its something like 0.4mm or so of steel and the thermal transfer medium is highly compressed magnesium oxide so it will be decent despite its thickness.

From experiance something like a 12x6 inch heat plate covered with those strip heaters and foam can easily dissipate 600w with like 2-3 gpm flow of water while only rising in temperature by a little bit. This was with a half decent aluminum cold plate though that had various features for increased thermal transfer.. I think a 12x12x1.5 inch cold plate covered with heating strips would do real nice industrially even if cheap plates were used. Just beware of clogging and put thermostats on it to disable if overheating.

And ground it

[CopperCone]

The biggest problem is clogging.

You can make a fully servicable heat exchanger using swagelok parts if you are good at machining. You make a bunch large diameter pipes connected by t junctions. Then you basically make em into a zigzag.

 So you have a lenght of pipe connected to a t junction. On the perpendicular connection you put a 45 degree bent pipe that basically reverses the direction of water flow.

On the opposite end, you stick in a piece of coated polished star shape milled tube stock with a cartridge heater in it.

So the water flows in the tube that has a star shaped heat exchanger that water flows past, and changes direction to another section, then you make a ladder network.

Very expensive but no welds or threads in this, it can be fully swaged and disassembled by the simplest tools and all components besides the heat exchangers can be quickly obtained from manufacturers. You can do a periodic full clean or even internal polish and your heat exchangers can be removed for recoating or replacement. Fu boroscope inspection using only 1 and a quarter turns.

I will be god damned if anyone builds it though.

And it would also work at ridiculous pressures.

If you trust tig welding and can do the inspection you can bend a stainless snake out of tubing and drill holes on the side then weld on sections of pipe (maybe pure silver or gold braze would b good) to eliminate the need for all the t junctions. You would wanna line all the shit up in a vacuum kiln though with the correct spacing for brazing, i dont know if welding a section of bent pipe with a tig is a good idea, imo it creates a good place for corrosion to occur. But it will have sharp internal angles that are difficult to polish and inspect and clean. T junctions would be more expensive and less reliable but cleaner and more servicable


Now i guess you can also cut a heat exchanger in half and rely on a gasket but low pressure and periodic maitence is really necessary. And you could not use machine tools to repolish and inspection is difficult compared to removable heat exchangers

Also you would need to be wary of pressure drop, high pressuremeans more errosion but cost will go nuts with large diameters.

[max_torque]

If you're going to passively rectify the AC "dyno" motor output to DC, buffer into some caps (small capacity just for voltage spike smoothing), and then PWM into a "load"  you may as well BOOST through an inductor and back into the supply (and prob some more caps) feeding the test motor drive.  Hardly any extra effort (needs 1 more load switch) and an inductor, but as space isn't an issue that's an easy thing to make (just hand wrap copper wire around an old large transformer core.  You don't really care that much about efficiency, a slong as most of the power (lets say 75%) goes back to the supply, the heat released is fairly small and probably a small fan blowing on the induct or and power switch.

Voltages are all reasonably small so no exotic power components needed

[Simon]

OK well there is no budget here and i aw having to convince them to let me do this so it's a zero budget.

A water cooled resistor would be erm.... cool.... . It would make it more movable as the system would be closed but would require a header tank. Yes I'd stick the radiator outside that is the whole point to move the heat out of the room.

A cheapo diagram pump will move 8l/m and have a few bars of pressure so it won't go down much from peak flow.

[Simon]

If you're going to passively rectify the AC "dyno" motor output to DC, buffer into some caps (small capacity just for voltage spike smoothing), and then PWM into a "load"  you may as well BOOST through an inductor and back into the supply (and prob some more caps) feeding the test motor drive.  Hardly any extra effort (needs 1 more load switch) and an inductor, but as space isn't an issue that's an easy thing to make (just hand wrap copper wire around an old large transformer core.  You don't really care that much about efficiency, a slong as most of the power (lets say 75%) goes back to the supply, the heat released is fairly small and probably a small fan blowing on the induct or and power switch.

Voltages are all reasonably small so no exotic power components needed

Yea i guess why not, like you say anything is better than nothing. I'm no SMPS expert I'm just worried that we can control the load from low rev's when the voltage out is quite low. the input is always 20-30V if we are running slow and can't make enough of a boost voltage to match the input voltage we have no load. Obviously i have no idea what the output voltage will be, I guess it is linear with speed ? 20% is a reasonable minimum speed we would run at and I guess a 5X boost is not unreasonable.

[IanB]

A water cooled resistor would be erm.... cool.... . It would make it more movable as the system would be closed but would require a header tank. Yes I'd stick the radiator outside that is the whole point to move the heat out of the room.

There are electric immersion heaters for domestic hot water tanks. Could you construct a fixture with one or more of these installed in a big water tank? You could even plumb in the circulation to the outside radiator to keep the system fully sustainable?

[max_torque]

For "zero" budget, i'd still go with a s/h car alternator, some kettle elements for the load (can be slung straight across the 3ph output of the alternator, removing std rectifier pack), and a cheap ebay H bridge to pwm control the rotor excitation current.  Simple magnetic Hall sensor to measure rotor speed and a current sensor around the phase output to measure "torque".  Std alternators go to around 18krpm, so plenty of headroom, or the ability to run a small belt drive if you want to leverage some gearing (ie generate more dyno torque)

I think you could have it up and running on the bench in a day, for under £200

[Simon]

Well 3 phase motor or car alternator (1KW) is pretty much the same thing isn't it? I was thinking that with 2 identical shafts it will be easier to join them together and we already know they are matched, baring motor/motor inefficiencies we get the same out as we put in. What sort of speed can an alternator do? if we start doing pulleys and stuff it will never happen which is why i wanted to keep it really simple, 12mm tube heat-shrunk on or drill a hole in each shaft and pun a pin in. Once it is rectified we can decide what to do with the power. making a crude boost converter would be nice as it makes the whole thing more compact and portable and minimal heat output.

[cellularmitosis]

On the topic of dummy loads, Mike goes over a few of his, including just a bunch of magnet wire in a bucket of water.

https://youtu.be/WECW88rJYrE?t=1m53s

[CopperCone]

oh yea with my idea, so long you can make heat exchange rods,, there is no reason you can't make it out of shitty PVC pipe if you want to keep down costs, the exterior surface does not need to be metal for any reason then pressure, but it can melt and be a heat hazard, so i would recommend some kind of scaffolding made of metal on the outside.

then you have something along the lines of a adjustable power load if your voltage is constant, otherwise you would need a dc dc converter before it

[IanB]

One "thinking outside the box" possibility, if you want to avoid dumping heat into the room, is to consider a power storage scheme. For example, use an alternator to charge up a battery bank (through a rectifier and charge controller). You can then use a power inverter to recover the stored energy and make use of it later.

I know this is big, expensive, and complicated, and maybe not reasonable in this case, but it is one way to avoid generating lots of waste heat that you have to dispose of.

[Ian.M]

Well 3 phase motor or car alternator (1KW) is pretty much the same thing isn't it?

Not necessarily.  If its a PM BLDC motor, you've got a more or less fixed relationship between volts and RPM.   The alternator lets you set that relationship by the field current, fed through the slip rings to the rotor which will give you much better control of loading over a wide speed range.

Coppercone's heat exchanger ideas sound like a *LOT* of work.  If you've got access to a milling machine, it would be a lot simpler to just mill two mating serpentine half-channels in two slabs of aluminum, then bolt them together all round the edges with a gasket, drill and tap the edge faces where the two channel ends emerge to bolt on flanged pipe couplings again with gaskets, drill and tap blind holes in the faces for the metal cased resistors and run ordinary automotive coolant through it and the radiator to control corrosion.

[BrianHG]

12mm tube heat-shrunk on or drill a hole in each shaft and pun a pin in.

?  Are you sure you know how much torque is involved in braking 1kw?
Car alternators usually need a 12v excitation voltage applied to it's magnet generating coil.

Just adding a metal or copper disc, or pulley/flywheel on your motor shaft + some neodymium magnets mounted near that flywheel and you can generate a nice smooth braking force, no circuitry, no wires, no boost switchers.  But holding back the torque of an excess of 1 horse power (figure 2hp to be safe since you are going over 1kw and you might have an engage jerk reaction to deal with) will require sturdy mounting whether you use a generator or simple magnetic braking.

Your generator 1kw alternator, especially a car alternator, will probably make more heat than your resistors which will be liquid cooled.

My 500w and 1kw neodymium low RPM 3 phase PMG generators are around 4 times bigger than a car alternator, and driven at full load for 1 hour straight, they do get toasty.  A car alternator with an excitation coil & 3 phase output will easily pass 100 degrees Celsius under the same load, even if rated at 1kw.  Unless you find an expensive PMG car alternator, you also need a 12v supply for the excitation voltage which goes up in current with load unless you tap the output voltage, capacitor filter and regulate it, and feed it back into the excitation coil like what is done in a car.

Attached low RPM, 0 cogging, 500watt/1kw neodymium PMG.
BTW, short the 3 wire on the output on these guys, and the shaft wont budge at all.

[BrianHG]

You want a cheap 3 phase PMG generator, low RPM, high voltage, if you have access to a junk yard, many use these washing machine PM BLDC motors as generators, though, they use normal magnets, but their large diameter still gives them a lot of power at low RPM.
https://www.google.ca/search?biw=1289&bih=724&tbm=isch&sa=1&ei=fospW62sBsvCjwS_h4No&q=washing+machine+motor+3+phase+BLDC+generator&oq=washing+machine+motor+3+phase+BLDC+generator&gs_l=img.3...9900.11146.0.11874.5.5.0.0.0.0.122.372.3j1.4.0....0...1c.1.64.img..1.0.0....0.23TMxcFeiCQ

[CopperCone]

Well 3 phase motor or car alternator (1KW) is pretty much the same thing isn't it?

Not necessarily.  If its a PM BLDC motor, you've got a more or less fixed relationship between volts and RPM.   The alternator lets you set that relationship by the field current, fed through the slip rings to the rotor which will give you much better control of loading over a wide speed range.

Coppercone's heat exchanger ideas sound like a *LOT* of work.  If you've got access to a milling machine, it would be a lot simpler to just mill two mating serpentine half-channels in two slabs of aluminum, then bolt them together all round the edges with a gasket, drill and tap the edge faces where the two channel ends emerge to bolt on flanged pipe couplings again with gaskets, drill and tap blind holes in the faces for the metal cased resistors and run ordinary automotive coolant through it and the radiator to control corrosion.

Well if you do it just with a mill you wont have good thermal transfer because of surface area. For flats you want broached or cut grooves, you can double/triple surface area this way, which is why I said go with star

you also need coating for aluminum as mold preventers and stuff will destroy it and corrode, or impurities, especia;lly if you use an evaporator

closed loop distilled water or coolant will be alot more mild, but you won't get the concentrated cooling power of an evaporator, you need giant radiators and cooling fans to blow on them, which means more moving parts, or really really giant radiators that don't have moving parts, and you need to worry about sunlight control, etc.. hard to do outdoor radiators I think without moving parts.

A cooling tower is nice because the pump does all the work and its fairly low surface area so you don't have the sun turning it into a water heater so easily.

I mean a regular wet cooling tower that uses its own water to cool, not something more advanced, I think they are the cheapest and intermittent operation means you can get away with the downsides. I would recommend a closed loop one that uses air exchange only if its constant operation to keep down humidity, I think that at that point its worth investing in your facility to have more advanced infrastructure.

Some of the smaller ones you could also probably fit on a cart of some sort so you can keep it in a storage room when not in use to keep it clean. Bigger ones might require a fork lift or some shit.

Otherwise, you can use it, splash it down with soap afterwords using a garden hose, then clean it out ,maybe give it a gentle pressure wash if it gets filthy

[BrianHG]

We are getting to the point where you can say, just screw the generator idea, attach a water pump to the motor & use adjustment valves to control the water pressure/resistance.  Let the water circulating into the pump keep the system cool.  0 electronics, and, the system is designed to operate with water... 

[CopperCone]

that may result in dead heading the pump which may be bad for it, the design would have to be considered carefully. You would need to consider the pump curve.

[NiHaoMike]

We are getting to the point where you can say, just screw the generator idea, attach a water pump to the motor & use adjustment valves to control the water pressure/resistance.  Let the water circulating into the pump keep the system cool.  0 electronics, and, the system is designed to operate with water... 

Why even bother with water? Compress air and release it through some sort of restriction.

[CopperCone]

I can see someone getting greedy and trying to store all that air in a cheap manner for later use. Then kaboom.

I wonder if you can make some kind of pnumatic vehicle to drive a few miles on stored air so you can run errands for the company, maybe some kind of scooter that runs on old scuba tanks or something

wow they are real
https://en.wikipedia.org/wiki/Compressed_air_car#Developers_and_manufacturers

ahhahaha

https://www.greencarreports.com/news/1096772_tata-airpod-compressed-air-car-to-launch-in-hawaii-this-year-report

imagine one of those rolling by with rap music blasting

[jbb]

As you can see, water cooling is inconvenient. And could get pricey - remember to conside your time. So don’t discount the possibility of doing a loop back.

Can you sketch your system? What voltage level do you feed into the motor driver? Can you feed DC power into the motor driver? How tightly do you need to control the load torque? Do you need constant torque during rapid changes in speed? Does your generator have Hall effect sensors fitted?

Maybe an ODrive in regenerative braking mode would be sufficient? They only go up to 48V DC bus, though.

[CopperCone]

cooling system is fairly bullet proof so long the pump is good and the plumber knew how to tighten it and did not use the wrong materials. A drive etc can break and cost alot of money, its a fairly complicated device, especially with more features like regenerative breaking, different sensors, etc.

Also since its for testing its like, how do you calibrate something strange like that?

[IanB]

We are getting to the point where you can say, just screw the generator idea, attach a water pump to the motor & use adjustment valves to control the water pressure/resistance.  Let the water circulating into the pump keep the system cool.  0 electronics, and, the system is designed to operate with water... 

Unfortunately, physics gets in the way of this. If you put sufficient load on the pump to make it draw 1 kW of electrical power, you will be dumping 1 kW of power into the circulating water. It will get hot and start boiling just as surely as if you submerged a 1 kW heater within it. The circulating "cooling" water will rapidly heat up the system and accelerate its demise...

[BrianHG]

We are getting to the point where you can say, just screw the generator idea, attach a water pump to the motor & use adjustment valves to control the water pressure/resistance.  Let the water circulating into the pump keep the system cool.  0 electronics, and, the system is designed to operate with water... 

Unfortunately, physics gets in the way of this. If you put sufficient load on the pump to make it draw 1 kW of electrical power, you will be dumping 1 kW of power into the circulating water. It will get hot and start boiling just as surely as if you submerged a 1 kW heater within it. The circulating "cooling" water will rapidly heat up the system and accelerate its demise...

You would be pumping the water through the outdoor radiator...
Simon's original design needed a generator, water submerged resistors, wiring, separate water pump and radiator.  I simply got rid of a whole lot of electrical junk in that equation.

[Simon]

Well 3 phase motor or car alternator (1KW) is pretty much the same thing isn't it?

Not necessarily.  If its a PM BLDC motor, you've got a more or less fixed relationship between volts and RPM.   The alternator lets you set that relationship by the field current, fed through the slip rings to the rotor which will give you much better control of loading over a wide speed range.

Yes that is true, if we can directly couple them it will make the build easier. We could machine an adaptor to screw onto the alternator having removed the pulley so that the adaptor shaft is the same as the motor. It would indeed save a lot of power electronics.

[Simon]

The other thing with two back to back motors is that we already know something about the setup. Win = Wout - inefficiencies and we know that these motors are efficient. An alternator is 50% so yea, still plenty of heat and sod knows what is going on.

Also the motors have nice flanged fronts so we just use long bolts and we have all of the fixings that take torque done easily. The idea of this getup is to measure the rotor speed and power output to measure the performance of the motor driver.