The efficient Dyno

[Simon]

So, for test purses of some equipment I am going to connect two identical motors together, drive one and load the other which will be a generator once turning to mechanically load the motor being driven by the driver. These are brush-less motors so essentially the output will be a 3 Phase voltage that will never exceed the input voltage to the driver particularly at low revs. So i take it that at 1.1KW I have no hope of getting out enough voltage to put back into the input rather than just feeding it into power resistors as any form of SMPS jiggery pokery is well beyond worth while.

[max_torque]

As long as the dyno motor inverter is synchronously rectified, then the inductance of the dyno motor itself will drive current back into the supply when a lower average voltage is applied (lower than the Bemf at that speed).  There are certain mimimums depending on the current, due to resistive voltage losses & switch losses etc.


TBH, at just 1.1Kw it's generally not worth going regenerative as the cost of the inverter outweighs any energy saving (unless the test is run for hours and hours)

A cheap option is to use a large automotive alternator as the load, simply connect across some big water cooled power resistors, removed the std voltage regulator and PWM control the rotor excitation current to control the load.

[Simon]

I was just trying to find an alternative to 1KW of heat, once you actually work it out that is a lot of resistors, heatsinks and fans that make noise almost as annoying as driving the the original load. The thing with the motors is that they are repeatable, ideally we need to make 2 or a second one long before the first one fails so that we can transfer the "calibration/characteristic" to the second set or we defeat the object due to manufacturing tolerances.

Dumping the heat into water is probably the best idea.

The consensus with the motor manufacturer was 3 phase rectifier - smoothing cap and PWM the resistors so that the mechanical load can vary from 0-100% of that speed. I was hoping to do something like we discussed with recovering the power but no chance really I guess, bucket of water it is.

[C]

The consensus with the motor manufacturer was 3 phase rectifier - smoothing cap and PWM the resistors so that the mechanical load can vary from 0-100% of that speed. I was hoping to do something like we discussed with recovering the power but no chance really I guess, bucket of water it is.

This is a buck power supply with resistor load.
An option is
 a boost power supply with motor drive DC bus load.

[NiHaoMike]

What about some incandescent light bulbs to dissipate the power?

[jbb]

So i take it that at 1.1KW I have no hope of getting out enough voltage to put back into the input rather than just feeding it into power resistors as any form of SMPS jiggery pokery is well beyond worth while.

The SMPS jiggery-pokery may not be as hard as you think. 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.

[BrianHG]

I use 12 50watt 12v halogen spot bulbs.  (I only need to drain 500 watts max, wired in series + parallel to get the voltage I wanted), + I have a PIC's PWM driving a N-Channel mosfet to set a programmable load VS generator RPM.  I use the RTC input pin with series cap and resistor to on one of the generator's phases to acquire the RPM.

I use 6 schottky diodes + CAP to create the DC.  Use a super huge cap value if you want to electronically simulate a huge fly-wheel.  This may be useful for wind and water power generation as long as you load your power from the DC cap.

Note that without a socket, these bulbs are a hassle to solder wire to.
100w versions exist.

[BrianHG]

A bunch of these would work, but, they will also make a nice space heater & though they don't need it, I would still recommend a fan:
https://www.aliexpress.com/wholesale?catId=0&initiative_id=SB_20180618170743&SearchText=300+watt+high+power+ceramic+tube+resistor
https://www.aliexpress.com/wholesale?catId=0&initiative_id=SB_20180618170842&SearchText=500+watt+high+power+ceramic+tube+resistor

If you use the 6 diode bridge to get DC with cap, you can use the PWM mosfet trick with a smaller resistor than you need, you can simulate different larger resistor values.

[BrianHG]

As for dumping the heat into water, you can get a 120v kettle and use it as a 10 ohm resistor, though, once the water boils away, your in trouble.  The 1-2$ halogen lamps are the cheapest solution, but, if you leave them on at full power each continuously, they will burn out after around 3000 hours of use.  The big green resistors from aliexpress, or, equivalents from digikey are designed to radiate a ton of heat and last much longer. 

Here are Virshay 300 watt and 1Kw resistors from Digikey.
https://www.digikey.com/products/en/resistors/chassis-mount-resistors/54?k=resistor&k=&pkeyword=resistor&v=274&FV=8041f%2C80439%2C80486%2C80081%2C8050b%2C80082%2C8082c%2C8082d%2C8082e%2C8082f%2C80830%2C80831%2C80832%2C80833%2C80834%2C80835%2C80836%2C80837%2C80838%2C80839%2C8083a%2C8083b%2C8083c%2C8083d%2C8083e%2C8083f%2C80840%2C80841%2C80842%2C80843%2C800d7%2C80022%2C80023%2C80214%2C80215%2C8023d%2C8027f%2C80291%2C802bd%2C80315%2C8033f%2C803d4%2Cffe00036&quantity=0&ColumnSort=1000011&page=1&stock=1&nstock=1&pageSize=25

Note that for 1Kw, 75$ is not bad, however, the resistor is designed to go beyond 350 degrees Celsius!  You need to mount it in the clear.
With a fan, you can go a bit beyond 1kw.

[Amper]

This kind of setup is very possible and used when testing large bldc systems. I have worked on a 250kW system, where it was done like this, total loss was just about 5kW at 150kW mechanical. Though a very good control software is needed not to get everything to oscillate with loops working against each other.

If you want to just dump it into a load there is large 1.5kW power resistors that cost just around 50 bucks new and dont need heat sinking, just maybe a small fan for comfort.

EDIT: here is one for example https://www.mouser.de/ProductDetail/TE-Connectivity-CGS/TE1500B1R0J?qs=sGAEpiMZZMtbXrIkmrvidIFFm%252butUMKCrJSE%2fDccfJU%3d

[Simon]

Putting it into water will mean not heating the room. We usually work in small enclosed rooms and currently testing turns those into hot hell holes very quickly.

[Benta]

Incandescent (or halogen) bulbs are not really optimal for this, as they are very non-linear. Of course, if you monitor output voltage and current it could work. At least it's cheap.

[Simon]

I don't want to heat the air in the room.

[DTJ]

I use 12 50watt 12v halogen spot bulbs.  te the DC.  Use a super huge cap value if you want to electronically simulate a huge fly-wheel.  This may be useful for wind and water power generation as long as you load your power from the DC cap.

Note that without a socket, these bulbs are a hassle to solder wire to.
100w versions exist.

I've used that style lamp in the past. Initially to connect we used some ceramic screw terminal blocks, later we bought the proper push on connectors - probably from RS or Farnell.

[BrianHG]

I don't want to heat the air in the room.

If your water is flowing continuously from a cold outside source, then, it wont heat the room, otherwise, you will just boil a liter of water in less than 10 minutes and you will be steam heating the room.  Without external water-flow, the water will eventually just begin to heat the room.  The heat you are generating doesn't vanish just because you placed your resistive element in water, it just takes a little more time to heat up the physical matter which is the water.  Once the water is hot enough, it will heat your room.  Once you turn off the power, that hot water with all that stored heat will stay hot continuously heating the room until the water cools down.  You haven't solved your heating problem with water cooling, just delayed it.  And with 1.5kw, you can boil 10L of water in around an hour and create a steam bath unless you have a continuous source of cool water coming in.  A metal heatsinked resistor with tubed air vent to outside of the building may be cheaper to deal with.

Note that some of the glass ceramic style 300 watt large virshay/dale resistors can be left in water to keep them cool, however, I'm not sure about contact corrosion.

[Ian.M]

A 20L electric tea urn would give you an 1H 35M run time from 25°C to boiling.

A 230V 2.2KW element will have a resistance of about 24 ohms, so your rectified generator output needs to be over 165V on load to get 1.1KW dissipation.

If you don't have that  DC bus voltage, then you'll need to parallel 120V 1.5KW immersion heater elements at about 9 ohms each.  Three would get you down to 60V.  Folded ones are about a foot long, so could easily be mounted through the base of a 20L unheated urn.

With a low level switch above the elements, and a thermistor probe above the fill level to detect steam it should be fairly idiot proof.

Lag it with an insulated jacket so it doesn't heat your room too much.
After an hour and a half of testing, its tea break time!   Don't forget to dump the hot water what's not used for tea down the drain.

[Simon]

We would not be testing for extensive periods so water is a good compromise. With the amount we use changing it half way through the day in not unthinkable or we can run a pipe outside to a radiator capable of dissipating many KW so it might not even need a fan.

The bog standard aluminium power resistors should take water ok. Aluminium case, filled with resin at the ends and tinned terminals.

[CopperCone]

Can you redirect theheat into another room or enemy bathroom etc using a duct?

The right way to do this would be to get a evaporator / cooling tower, put it outside and build a house water cooling system. If its infrequency you just need rubber hose, big pump and a cooling tower. Some have build in reservoirs so you dont need a seperate tank and stuff. I think 2000$ would cover the power levels you have used, less if you have pumps.

Doing anything else sounds like a troll or one time gimic. I cant imagine a corporation in their right mindmaking some giant red hot wall, leave those things for satan.

https://m.ebay.com/sch/i.html?_from=R40&_trksid=p2380057.m4084.l1313.TR12.TRC2.A0.H0.Xcooling+tow.TRS0&_nkw=cooling+towerP

The only thing is that you probobly want to, if in frequent use, add some kind of blowers or directors or place it away from the building to prevent humidity increase, the large one will billow but i have seen the small ones used in something that could practically be considered residental.. I dont think there is a problem putting it against the side of a building so long its not running alot.. And maybe drain after use

Otherwise cook turkeys at work to make a bit of side money or feed the homeless?

[Simon]

erm i think we just need 20-50L of water with the load in it. a 1/2 inch hosepipe, self priming diaphram pump and a reject/returned radiator that does not leak, we supply them for BIG machines so will have something who's thermal mass alone will be a benefit before it needs to loose some heat.

[NiHaoMike]

I think 2000$ would cover the power levels you have used, less if you have pumps.

With that sort of budget, it should be pretty easy to get a grid tie inverter to use as a regenerative load.

[Ian.M]

The bog standard aluminium power resistors should take water ok. Aluminium case, filled with resin at the ends and tinned terminals.

Don't bet on that.  It only takes a tiny seepage through the seal as they cool down and next time you switch on it will puke its guts as the droplet of water that got inside turns to steam. 

If you *MUST* use aluminium case resistors,   the sane thing to do would be to mount all the resistors to a heavy slab of aluminum as a heat spreader and interface that to the bottom or side of a rectangular stainless tank (think catering equipment suppliers) with thermal grease, stainless bolts fitted from inside the tank with gasket paper washers coated with hermatite red (or potable water jointing compound if you want to make tea!) under their heads, nuts on the outside directly to the tank, then oversize holes in the aluminum slab, big, lightly greased penny washers to bear on the aluminum, then a stack of Belleville washers and a locknut to provide contact pressure without rigidly fixing the aluminium so its free to slide slightly against the stainless to allow for its differential expansion and contraction.

However if you can import 120V water heater elements within your project timescale, you can almost certainly do it more easily with less machine shop work.  They just need a deburred hole in the tank bottom or flat side + as they are usually intended to be mounted in a NPT threaded boss, you'll need some stainless nuts with the right thread to go on the inside. 

Another idea - if the frequency range is reasonable, boost the voltage with transformers then use a three phase Variac for power control and run a cable to bathroom heaters mounted on the exterior of the building, with a small roof over them to shield them from direct weather.

[Simon]

I think 2000$ would cover the power levels you have used, less if you have pumps.

With that sort of budget, it should be pretty easy to get a grid tie inverter to use as a regenerative load.

Possible but we do want to vary the load at various outputs o would become complicated unless we are simply torture testing.

[BrianHG]

erm i think we just need 20-50L of water with the load in it. a 1/2 inch hosepipe, self priming diaphram pump and a reject/returned radiator that does not leak, we supply them for BIG machines so will have something who's thermal mass alone will be a benefit before it needs to loose some heat.

The radiator still moves all the heat, you just have the option of placing the radiator portion outdoors.  Anyways, with a 2k$ budget, just use one of these water cooled high power resistors.  The center is just a water pipe and it's approved & designed for water cooling:
The resistor will be a fraction of your budget.
Google 'Water Cooled Braking Resistors'.  These guys go up to the megawatts in some cases, and they have 3 phase resistor contacts designs with wiring and tubing built in for the 3 resistors in some designs.

[Gregg]

A tank less electric water heater has most of the necessary plumbing done for a water cooled resistance load bank.  It should be fairly easy to wire directly to the heating elements and connect water with garden hose.  Just make sure it stays full of water and you don’t have any air pockets around the heating elements when powered and adjust the flow so that it doesn’t boil.

[BrianHG]

A tank less electric water heater has most of the necessary plumbing done for a water cooled resistance load bank.  It should be fairly easy to wire directly to the heating elements and connect water with garden hose.  Just make sure it stays full of water and you don’t have any air pockets around the heating elements when powered and adjust the flow so that it doesn’t boil.

They have automatic over-heat cutoff and you cant purchase the desired resistance you require, but, I guess it's possible.