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Home Built Water Brake Dyno's

Home Built Water Brake Dyno's

Home Built Water Brake Dyno's

Hi, I was just curious to see if anyone has any info on the design of water pumps that are used as power absorbers on dynos. My understanding is that it is basically a torque converter with a variable load valve that dumps more water on the stator(?) to create load. This is the major piece I haven't solved yet.  You can buy all sorts of torque sensors and lots of software, but a reliable and practical method of creating load is my problem, without spending 6-25 K on someones dyno. On small engines a large brake rotor with caliper works and I even thought of an oil cooled submerged rotor  (like a wet clutch) but that seems to high maint. I have thought of using a large generator as a load device with a variable load panel but it seems that it would self destruct at higher rpm. What I quess I am looking for is a water turbine to run backwards.
Thank you for any help,

RE: Home Built Water Brake Dyno's


What is the max H/P & RPM you want to test.
Inertia dyno's are great for small low output engines, but be (WARNED)!!!!! you have to consider the peripheral speed of the flywheel,drum etc etc. Eg If you made your flywheel or drum, say out of 4140 you would have to limit your peripheral speed to around 140 M/sec or else your in big trouble.

RE: Home Built Water Brake Dyno's

If you are testing automotive style engines how about an automatic transmission with direct clutch mechanically locked. Torque arm at the output shaft. Regulate the torque converter oil filling. Cheap and doable.-------Phil

RE: Home Built Water Brake Dyno's

For those kind of HP #'s the torque absorption capabilities would exceed most automotive automatics. Why not rent dyno time if high capitol investment is the drawback?-----Phil

RE: Home Built Water Brake Dyno's

Can't see why buying a second hand water brake would be a problem if your playing with that sort of H/P & revs.

RE: Home Built Water Brake Dyno's

There is a small company in Salem, NH  Land and Sea the web site is www.land-and-sea.com
They were making variable pitch props of marine applications and found a new use for them, dynos this may lead you in the right direction

RE: Home Built Water Brake Dyno's

willeng, head me in the right direction for an affordable used unit.

RE: Home Built Water Brake Dyno's


Not sure where you are?

Dyno-Dynamics of MELBOURNE have been advertising second hand dyno's. Trade-ins etc.

I picked up an old Telma eddy-current retarder for under a grand out of a truck at the wreckers. SCANIA trucks have these fitted as well as some tourist coaches. Some of these units are very strong to 2400 ft/lbs.  
The retarder i bought will brake 1000 ft/lbs but needs to be geared a little to keep the rpm reasonable.
These are good for acceleration testing
I am currently building the engine bed etc now.

RE: Home Built Water Brake Dyno's

On the West coast of USA.  I have seen some industrial engine dynos on the market before. But they don't handle the RPM's. It seems anything for auto engines is way over priced. I don't see that there is that much work in making them. That they should be priced like that. It's like anything else make them affordable and people would purchase them.

RE: Home Built Water Brake Dyno's


I think like you,that it's not that difficult to make a cheap brake especialy with the materials we have today.
I think people often misunderstand what a brake has to do.
I was talking to some people a while ago about breaking engines, they were thinking that you need super strong frames huge braking cells etc, they nearly fell over with embarrasment when i explained that with one of our current V8 supercar engines 530ft/lbs of torque all you need in reality is a 10ft lever & hold 53lbs on it.
It would pay to look up the inventer of the water brake
William Froud this was in the late 1800s would you believe. He was braking navy ships with 116,000ft?lbs----about 380 small block chevs. RPM was a consideration then because of materials but now we haven;t got that problem.
There is an old article by David Vizard that i have in Hot Rodding Basics 26/june/1982---might be of interest.

Also there are papers that i have from Heenan & Froud LTD---worcester-England Publication No 526/2 shows the old basic design of the brake. I have other diagrams of the old principle workings of these if someone could tell me how to put them on the (bloody) computer i have a scanner.
The reason i picked the eddy current retarder is that at the time I couldn't work out how to do transient response testing with a water brake--not smart enough. I have recently worked out how this can be done--it's easy.


RE: Home Built Water Brake Dyno's

You could directly measure the torque with a torqometer type torque wrench also. I saw a photo of a diesel engine being tested that way. The biggest hurdle is getting the power absorber. The measuring is the easy part.

RE: Home Built Water Brake Dyno's

Thank you for thinking for me! I had to draw it on paper to 'prove it' to myself. I do agree with the flaws on roller dyno's (they are good for tuning) and their inherit lack of consistance. I always have to laugh at(no to their face) people who talk to me about 2hp changes they had. One manufacturer solved this buy driving the pump off the axle directly. It is limited by because of the torque multiplication from the gear ratio of the axle. It is actually using one pump at each wheel (2 or 4 wheel drive)which adds complexity but also increases load capacity. Speed is less of a factor because of the vehicle's gear ratio. I was wondering if stepping the speed down through a 4:1 or 2:1 gear set and run multiple stages of pumps to handle the torque.
Thank you all for keeping me on track

RE: Home Built Water Brake Dyno's


Seems as though there are posts missing so i'm not sure where we were up to, or if you've solved your dyno problems yet. If not i may be able to assist you in building your own water brake. Online or E-mail


RE: Home Built Water Brake Dyno's

Thank you, I dont know what happened to half of this thread but I have found some promise in a product I stumbled across. I will keep you posted.

RE: Home Built Water Brake Dyno's

I am in the process of doing something very similar to test kart engines.  I am going to attempt to build a water brake and test stand like the land and sea one.

If you have any info that would help it would be great if you could share it.


Luke Haddow

RE: Home Built Water Brake Dyno's

Whats your email address.

RE: Home Built Water Brake Dyno's


What type off telma do you have, did it come off a scania buy any chance, if you do not mind me asking how did you set it up.
I am looking for some ideas on setting one up I have one, which I also got from a truck wreckers in Melbourne

RE: Home Built Water Brake Dyno's

A very easy way to build an eddy current brake is to just use any ac induction motor, and feed dc current through the windings. It only works for low power engines though, because the solid steel (squirrel cage) rotor does not dissipate heat well.

But if you are experimenting at home with very small engines, it is a very practical ultra low cost way to do it.

A three phase motor will have three terminals, just feed a dc current into any two terminals, and ignore the third. The rating plate will have a current rating for the motor, which should not be exceeded.

Only a low dc voltage is required because the resistance of the windings will be quite low. Also the braking torque will be extremely high, even at currents far below the maximum on the nameplate.

With a given braking current, the required torque to turn the motor increases very rapidly with speed. A given motor will brake full rated torque at its slip rate, which might be only 150 RPM.

Just to explain this, if the mains frequency is 50Hz, the magnetic field inside the motor rotates at 50 x 60 = 3000 RPM. The motor speed might be rated at 2850 RPM, so the slip will be 150 RPM.

If you try to spin it faster than 150 RPM with full field current, it will readily sink far more torque, so a small electric motor can easily hold almost internal combustion engine. But the big problem is heat buildup in the rotor. It will get very hot very fast, so that is what limits the power absorption, not torque sinking ability.

So, a single or three phase induction motor would make a nice home chassis dyno for a cart or small bike, provided you were careful not to cook it. A large  external blower or submerging the whole thing in oil might also work.

It is all a bit Mickey Mouse, but some of us are not that wealthy. Short power runs are all you really need with long cool down periods for home use. Also, excessive RPM and heat might cause it to fly apart, so a bit of caution is required.

Try it yourself, get a quarter horse or larger single phase motor, and while turning the shaft by hand, apply 12v dc and see what happens.

RE: Home Built Water Brake Dyno's


You may be able to help me, I need to control the current to my retarder, what is the best thing to use. I need a rotary control.

Originally it was 12 volt dc.
I have been told that you can wire the coils in series & use 240 volt ac, how much current would be required if used on 240 ac?
I have only single phase 240 volt supply, enough for welders etc.
My son has asked this question in ELECTRICAL/ELECTRONIC FORUM under the heading dyno control, but as yet has no useful replies.
I set one up a few years ago but only with ancient outdated controls.
We have a 250 amp ac/dc converter can this be used also???.
I build engines & need help with electricity.

RE: Home Built Water Brake Dyno's

The first thing is to determine the resistance of each of the coils fitted, and I assume you have six coils ?

Are you absolutely sure the original unit was supposed to work on twelve volts ? Some trucks work with 24 volts.

If you can get this information, then we can work out how many dc volts and amps you need, and figure out a simple way of doing it.

While the unit will operate in a fashion with ac, I strongly recommend dc current be used. It is difficult to explain, but there will be eddy currents in all sorts of unwanted places and lots of heat created, even when the retarder is not even turning.

RE: Home Built Water Brake Dyno's


The resistance in each coil is 0.945 ohms.
There are 8 coils either side of the stator = 16 in total.

Normally they are operated in 4 stages, 2 coils either side for stage 1 & 4 coils either side for stage 2 & so on.

For the dyno i will wire all the coils together & apply power to all coils at once. Would a rotary motor controller,a high voltage potentiometer or a Rheostat do.
Do they even make potentiometers that big or are these for AC power ?????

The retarder can be wired to suit 12 or 24 volt DC, i have wiring diagrams-spec sheet.

To keep it on 12 or 24 volt DC, i was going to use truck batteries unless there is a better way.

Max current draw for 12 volt is 196 amps & for 24 volt is 98 amps.

Braking torque is 1450 Nm or 1066ft/lbs.

Would AC power make the retarder vibrate or chatter because of cycles per min?.

As i take it DC is smooth & AC comes in cycles, correct me if i'm wrong.
I mentioned before we have a 250 amp AC/DC converter that i use for tig welding can this be used instead of batteries??
I know a little about eddy currents about 8 yrs ago i built a retarder ,wound the coils & used cast iron brake disc rotors that i machined to suit. It was an experimental project,it works good for small engines but it has to much resistance  per coil, wire to small for better engines. I controlled this--playing-- through a 10 amp AC rotary controller that i use for honing into a stick welder wound up full & then through the AC/ DC converter. Basically all i proved is, that it's easy to MAGNETIZE an entire workshop full of tools in one fowl swoop not to mention still having to comb my hair down every 10 mins.
                                  Thanks for your help

RE: Home Built Water Brake Dyno's


what type of retarder is yours?

RE: Home Built Water Brake Dyno's


I noticed you purchased a retarder, what is it (Make & model).
Have you got any specifications on this, if not i might have some let me know.
My retarder is a Telma:

RE: Home Built Water Brake Dyno's


Thany for your reply.

Mine is a telma unit from a 1996 143 scania, the thing about it is I can not see any way to mount it.  I got this unit sight unseen as I am in Queensland and it was in Melbourne and they were the only place that had one.

I am after some ideas on how I mount it.  It doesn't appear to be a self contained unit.  Everything was sent to me, the coils, 2 big rotars, all the wiring, relays and switches.  

I am only looking at testing engines under 100 HP.  Any help you may be able to give me would be muchly appreciated.

RE: Home Built Water Brake Dyno's


Can you post a picture of what you have so i can try to be of some help.
It could be any one of there models, there is normally a compliance plate on the stator with the model etc.
The comliance plate is on the bottom of the stator sometimes.
Are the rotors aluminium or cast iron, are the coils copper wound or aluminium.
What is the outside diameter of the rotors closest to the coils.
If it is not one complete unit you have a lot of work to do.
Send me some info.
What is your E-mail address
How many ton capacity was the truck it came off?

RE: Home Built Water Brake Dyno's


I will have a look for the compliance plate.  The rotars are cast.  For what I can see they are copper wound.  Being a 143 it would be off a semi I would think.

They sent me everything off the back of gearbox, and I mean everything.

You can email me at roslynb@hypermax.net.au.

I am not sure now to post a picture here.  Once I have received an email from you I will reply with some pictures.

RE: Home Built Water Brake Dyno's

Willeng, hey I am back! Sorry for the delay my ISP died and I have been unable to access the internet until now.

Now back to business. The Telma retarders I have seen are used on heavy trucks, and are rated to sink 1,000 Ft/lb torque, and some up to 4,000 Ft/lb torque, according to their website. They do this at truck type tailshaft speeds, so in a truck, I don't know but maybe 1,500 RPM or something might be typical in a mid gear going down a hill?

Now with an eddy current brake, the faster you try to turn it, the more torque it takes. What I am trying to say is you might not need to sink 4,000 Ft/lb of torque continuously at 1,500 RPM in your application.

You might not actually require anywhere near the 196 Amps at twelve volts. It might be more like 19.6 amps at 400 Ft lb at 1,500 RPM, and even less current at higher RPM to hold the same torque.

The way I would do it, would be to find myself an old style heavy duty industrial battery charger. It could be 12v, 24v, or 48v. something with plenty of grunt. It will come in an earthed metal box with an ammeter, and would be ideal. These things are used for recharging electric forklifts, and telecom applications an so on. You might get lucky at a machinery junkyard or in the Trading Post.

The sixteen coils can be connected in a variety of ways to suit any suitable operating voltage. I have no idea what voltage your TIG runs at, or how happy it would be running at a continuously high dc current, but it might well be o/k.

Once you have a suitable transformer and rectifier, you will need to control the output. The easiest way is with a VARIAC. These are a continuously adjustable output transformer where you can adjust the output voltage from zero up to full mains voltage by turning a large knob through about 270 degrees of rotation.

You just connect the variac up to 240 volts, and run your battery charger from the output. In this way the dc output of the battery charger can be adjusted from zero up to full rated output at the turn of a knob.

Variacs are rated by current and come in many sizes, so you can get for example, a 2.5 amp variac, or a 20 amp variac and so on. How big it needs to be, really be depends on how much power your battery charger draws. The rating plate on the back will tell you. So get the battery charger first.

I am in Melbourne by the way, and my e-mail is:



RE: Home Built Water Brake Dyno's


If you are still watching this thread can you please send me an email as I can no longer find your address.

Luke Haddow

RE: Home Built Water Brake Dyno's

Hi guys, I have been following your threads if that is what you call them and may be able to help. I have a vane 4000 dyno which uses a telma retarder and also have a telma control opperation maintenance leaflet.
I am attempting to build a water brake load unit which will hopefully absorb 500 ft lb at 2000 rpm. any help would be appreciated.
Would willeng be able to email me at audiofurn@iprimus.com.au so we can catch up directly please?
Many thanks

RE: Home Built Water Brake Dyno's

rickbar, and willeng, et. al.,

I just found this thread, and have some thoughts.  I hope you are still reading this thread.

To the one who had just the two rotors, and the bank of coils, it sounds like you bought a Telma Focal type of retarder that mounts directly to the "nose" of the ring gear and pinion housing of a heavy vehicle rear axle, or the rear housing of a transmission, where the flywheels mount directly to the flange where the driveline attaches.  If I am right, the flywheels will match together at their respective centerlines.  This type of retarder will be difficult to mount as you intend to use it, unless you use a transmission itself as part of your dyno.

The type of retarder you need has a through shaft, with the rotors mounted on this shaft, and supported on the shaft bearings.  Then all you have to do, is mount and support the stator, which on that type of retarder has mounting holes for that purpose.

Regarding the electrics, I have no opinion whatsoever on AC power for the coils.  I have messed with these things for 20 years, in all the voltages mentioned above, and never considered hooking up AC power.

However, for DC, it has been correctly surmised that they can be wired in various combinations for 12, 24, 48, or even 96 volts DC.  Electrical people will have to tell you how to get these voltage levels past the more common methods of heavy duty battery chargers connected to banks of heavy duty batteries.

There are people who make devices to vary DC, but I have been out of contact with them for years.  The retarder you have is designed for a four relay type control, each relay putting 12 or 24 volts to their respective set of coils.

Regarding an eariler post that I don't want to track back and find, this type of retarder has a torque curve, as well as a horsepower curve.  The torque curve peaks at a value at or near 1000 rpm, while the horsepower (absorbed) curve just keeps climbing with speed.  

This is while the retarder is cold.  As the retarder heats, the sloope of this horsepower curve (as well as the peak of the torque curve) falls off substantially, to a level of less than 50% of original (cold) value.

This is due to three reasons.  The resistance to eddy current flow in the rotors increases as they get hot, red hot in some cases, and the resistance in the coils increase with heat, due to I^2 x R losses.  The flywheels also are designed in the "spoke" portion of the rotor to allow the rotor to expand with the (red hot) heat of the eddy current absorpion, and they expand in such a way so as to allow the air gap between the rotors and the stator to open up.  I have seen these air gaps change from 1.8mm cold to 10mm hot.

The air gaps are set with shims, and the performance can be "juiced" by narrowing the air gaps.  Be careful, used flywheels have some warpage that might make this a dangerous suggestion.

I hope these comments have helped you gain some insight into what you have, and maybe some help with how to use them to accomplish what you want to do.

I have seen this type of retarder, when installed in a truck tractor, with the rear driveline to the rear end disconnecetd used as a sort of "dyno" in the truck frame.  This was to test and verify the retarder installation, not as a horsepower test for the engine, but it did demonstrate that the powerful engine in the truck could not pull the retarder in the highest few gears.  It was too powerful.

Good luck on your projects.


RE: Home Built Water Brake Dyno's

In the past I was trying to make an engine dyno out of an used telma retarder from some scania truck.
The telma has four positions
Total weight 348 kg
Rotor weight 82 kg
Stator weight 266 kg

Maximum moment it can hold is 3300Nm
Max rpm is 3500rpm
Voltage 24v

So in :
position 1 it consumes 43A
position 2 it consumes 86A
position 3 it consumes 129A
position 4 it consumes 172A
All by 24V

Here are my questions what do you think?

1) Because it is an engine teststand and not a dyno. I have a problem. Maximum rpm = 3500, I want to keep it on the safe side and decide to go for a maximum of 2000.
Maximum speed of my engine will be 12000 rpm. So I need to a ratio of 6. How should I best do this? Use gears, use a toothed belt. Any idea

2)The stator should it be mounted fixed or should it be able to move some degrees around the axis of rotation. The meaning is to put a lever on it with a force measuring device. I don't know the english word.

3) The current is a bit high for me. Therefore I am gonna put all those coils in serie => 24 * 4 = 96V => Therefore my current will be lower => cheaper electronic parts. I have 8 coils how should they be wounded? Any suggestions.


RE: Home Built Water Brake Dyno's


I am going to answer some questions now, and others later, after I have had time to think about them.

First, your retarder, if it is a Telma, should be 16 coils, eventhough they might appear to be mounted on a common back plate so as to make it look like 8.  If I am right, there will be 2 wires for each coil, for 32 wires and 32 connections.  If you are right, there will be half that number.

I am more familiar with the two coil model of retarders, although, I have plenty of experience with (actual) 8 coil models, and 16 coil models.  I will have to look at a 16 coil wiring diagram to remember how to recommend to you to up the voltage to 96 volts, and get the coil pole North/South orientation correct.  I will post back when I look at them.  It could be a few days.

The stator should be free so that your torque measuring device can resist it's movement to measure the torque.

Depending upon the diameter of the flywheels (rotors) you can get more rpm's out of these type of retarders by changing the taper roller bearings out to ball bearings.  You don't want to get the tips of the flywheels over sonic velocity, however.  That would be your limiting factor.

Telma in my country uses these type of retarders with ball bearings for engine dynos, but I doubt they are getting all the way to 12K rpm.

I would suggest a gear mechanism, as the retarder at higher speeds can absorb some real wickedly high horsepower, which would take a substantial belt system, not that it is not possible.  A gear box would let you select the correct range of retarder speed to match your engine horsepower.  Belts would be difficult to vary the retarder speed on very quickly, unless you had an expensive mechanism to do so.

That is about all I can cover off the top of my head for right now.

More later.


RE: Home Built Water Brake Dyno's

Has anybody been able to build his dynamometer with the eddy current brake?

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