20kW Generator Load for a dyno. - Page 1
Forum section: electronics / projects

author: Cmcada
author_rank: Newbie
timestamp: August 13, 2015, 02:57:44 pm
content: I am in the process of producing an EMU using an ARM and a 125cc 15hp petrol motor.To gain accurate data and test the system I have to put the engine under load and measure the torque/power it produces.The ideas I have had to accomplish this have all been through cost benefit analysis and the only method that seems viable is to drive a 20kW motor from an agricultural setup that we have laying around as a generator and apply a known load to the motor, the resulting RPM along with other measurables will give us some qualitative data to work with.Can anyone think of a cheap measurable way to do this?We are currently looking at using heating elements from scrap kettles in a vat of water.I have designed a high power grid to deliver the power generated consistently so any load will suffice as long as it is cheap and relatively sensible :p

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: August 13, 2015, 04:49:00 pm
content: Are we talking about a load for 240V AC @ 20kW?If so: 18.1m of standard 2mmøConstantan wire (Copper/Nickel alloy), wound with plenty of space between turns on a DIY insulating coil form made from strips of glass fiber board. Dunk coil in large container filled with relatively pure water (tap water will probably do, distilled if it doesn't). Presto. 2.88 ohm resistor at silly power rating.Double check my math or pipe up if I misunderstood the problem.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 13, 2015, 04:53:26 pm
content: Are you doing motor break-in or something that requires you to have a constant load? Otherwise you could consider an inertia load. You could also look at using a water brake or an eddy current break. Your power levels are so low, you have a lot of options.

author: Cmcada
author_rank: Newbie
timestamp: August 13, 2015, 04:55:43 pm
content: Exactly that, I got a similiar result on the back of a fag packet.The issue is I am looking to vary the load dynamically to simulate various conditions.Perhaps some form of potentiometer of a similiar form would do.

author: Cmcada
author_rank: Newbie
timestamp: August 13, 2015, 04:59:48 pm
content: I have looked into water brakes however I didn't know enough about fluid dynamics to give an estimate to the size of the vessel/load on whatever fans/prop I would need plus I could not think of how to alter the load other than removing some water.Eddy currents have been simulated and look promising, machining the rotor was the only real constraint, probably the best bet to be fair.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: August 13, 2015, 05:09:17 pm
content: May be too crude due to waveform distortion of the load current, but:Run AC from generator through full wave diode bride. You are 'only' talking about 83Arms, so this should be manageable. Raw, rectified DC waveform sent through the 2.88 ohm resistor and then through either a big SCR or IGBT. Make a small circuit, which delays the trigger for the SCR/IGBT after the AC waveform zero crossing. Result: One 20kW variable load (with current waveform distortion). Some tweaking probably required.

author: TheElectricChicken
author_rank: Frequent Contributor
timestamp: August 13, 2015, 06:25:58 pm
content: The scrap is the best idea. Space heaters are good, and you can wire straight past their controls and thermostat to the actual heating coil. Water heaters are good because they have very high power heating elements, 3 or 4 kw each one. They are also scrapped for reasons other than the element like a leaky tank, so should be cheap and easy to find the elements, or the tanks themselves. You can use ones that leak at the top, which is common.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 13, 2015, 10:32:48 pm
content: Quote from: Cmcada on August 13, 2015, 04:59:48 pmWater brakes are very common. For an eddy current setup, you may be able to find an old truck brake from a scrap yard.QuoteI assume you would run closed loop then with constant torque, constant speed, constant power modes. I think what you are going to be missing is a way to motor the engine. Post what you come up with.

author: Circlotron
author_rank: Super Contributor
timestamp: August 13, 2015, 11:27:08 pm
content: Quote from: Cmcada on August 13, 2015, 04:55:43 pmYour generator is a form of magnetic amplifier, so you can use this fact to your advantage. After you have gotten the actual load sorted out, instead of putting some kind of controlling element between the generator and the load, instead, simply adjust the generator excitation via some kind of PWM setup. You will only be dealing with a current level of about 1/20 to 1/50 of the output current and DC too.Also, don't bother trying to measure power output by the electric output of the generator; way inaccurate. Instead, mount the generator on some kind of on-axis bearing setup so that it can rotate, but have the frame secured by a vertical radius arm that pushes against a load cell. Multiply the torque reading of the load cell by rpm to get power output.Edit -> If a load cell is too expensive, maybe use a spring balancehttp://i01.i.aliimg.com/img/pb/538/244/421/421244538_796.jpgand put the tachometer beside it and record both with a movie camera as you do a run, then replay the movie frame by frame and plot your torque and rpm points into a spreadsheet.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 14, 2015, 05:58:48 am
content: Maybe get a scrap car engine (one that still turns but is unusable for other reasons) and convert it into an air compressor? Then your "load" can just be a restriction on the outlet.

author: max_torque
author_rank: Super Contributor
timestamp: August 14, 2015, 01:03:52 pm
content: It's going to depend on how long you want to be able to absorb 11kW for!if the tests are short and you can afford to wait for a cool down period between runs, just getting some big truck or van alternators would do it, using the field windings to control the load. You can buy a roll of 6mm or 1/4" copper tubing, uncoil it onto a wooden former and run a hose pipe through it to cool it.Alternatively, large industrial hydraulic pumps(lorry power steering etc) and a basic ball valve make a simple absorber, fit a large rad and #lecy fan to remove the heat generated.

author: LaserSteve
author_rank: Super Contributor
timestamp: August 14, 2015, 01:46:40 pm
content: You could use the old US Navy Method for large generator load tests, lower very large steel plates into an infinite tank of sea water with variable plate spacing or variable depth. This is still used and is known as a "Liquid Rheostat" load. Keep sufficient personal distance from said load, so that the released chlorine and hydrogen is not an issue. Ignore the environmentalists while running the test at 5 Megawatts... ( I'm Joking but it works!)If it were me I'd use the electric water heater elements, and measure I,V, and R(hot), and if I have them in a tank, do calorimetric measurements as well. Much easier then setting up a mechanical Prony brake... We have those in a mechanical engineering lab on campus, and making one rated for ~30 HP one would not be easy without a serious machine shop. In the US, large water heater elements cost 9-12$ each, come with a convenient male plumbing thread, and are available to astronomical power levels. I've used them to make a 2.2 Ohm 20 kW resistor before at a cost of less then 200$ counting the piping.Just remember that R(hot) of a water heater element is quite a bit different then R(cold). But that is easy enough to measure. You can get used current shunts on Ebay to make the I measurement for next to nothing.Your only fudge factor is one multiplier for the efficiency of the alternator. However those numbers are widely published. If possible and if I had budget, I'd then use a shaft torque sensor some place in the system. I like the idea of controlling the field.So I got curious and put in Prony Brake and Water Dynamometer into Youtube, and you can get a really good education doing that....Steve

author: Berni
author_rank: Super Contributor
timestamp: August 14, 2015, 02:38:38 pm
content: If you don't mind running at the same rpm all the time you can connect the generator to the mains and send the power out in to the grid. You will need to first sync your generator with the 50/60Hz of the mains before you switch it on, otherwise you are going to have a bad day. If its just a async motor then you just need to be in the right ballpark of rpm when switching it on.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: August 14, 2015, 02:41:41 pm
content: Quote from: Berni on August 14, 2015, 02:38:38 pmJust ... no.

author: LaserSteve
author_rank: Super Contributor
timestamp: August 14, 2015, 03:16:31 pm
content: It has to be said...No, do not ever run power back into the grid without local authority permission and the proper metering, phasing, and shutdown systems used to sell power to a utility. That is a real easy way to kill someone... Especially the lineman who will come to troubleshoot the issue....Steve

author: Circlotron
author_rank: Super Contributor
timestamp: August 15, 2015, 02:24:52 pm
content: An induction generator is a safe way of pumping power back into the mains. If the AC supply fails, the generator quits generating. All you need is a bog-standard induction motor, preferably 3-phase. When connected to the 50Hz mains for example, they will run no load at 1500 rpm.If you try and slow it down from it's natural speed it will slow proportional to load. Full load is when it slows by typically 60 rpm. It is converting mains power to mechanical power.If you try and speed it up from it's natural speed it will speed up proportional to how hard you try and speed it up. Full power is when it speeds up by typically 60 rpm. It is converting mechanical power to AC mains power.It resists being slowed down or sped up. Safe, docile, cheap. Check legality in your area.https://en.wikipedia.org/wiki/Induction_generator

author: johansen
author_rank: Super Contributor
timestamp: August 15, 2015, 02:31:56 pm
content: Quote from: Berni on August 14, 2015, 02:38:38 pmdon't do this if you have a smart meter, but it might work otherwise.only problem being you can only dump power at 1525 or 1850 rpm.

author: Jeroen3
author_rank: Super Contributor
timestamp: August 15, 2015, 05:28:10 pm
content: Do you want to test once or more often? Because you might just be able to rent a setup for this somewhere with proper measurement tools instead of testing the testbench with the device under test.

author: zze110
author_rank: Contributor
timestamp: August 15, 2015, 06:33:08 pm
content: this will probably sound silly but here goes...How about building a weighted cart for the motor and attaching a motorcycle wheel directly to the output shaft. Then find a motorcycle dynamometer and rent it out for the day.

author: Berni
author_rank: Super Contributor
timestamp: August 18, 2015, 06:58:03 am
content: Quote from: ElectroIrradiator on August 14, 2015, 02:41:41 pmThere is nothing bad about feeding power back out. Most solar power installations do that since its easier to sell the power than to store it in a battery bank for when you need it (Also in a lot of countries you get more money per kW/h that way rather than not buying that much power from the grid).Most AC motors send power back in to the grid if you try to spin them faster than they want to spin. The mains voltage wont rise or anything (Well it will a little due to cable resistance) and even if you could make more power than the whole street is using it can still flow backwards trough the distribution transformer (Might raise an eyebrow of someone in the remote control center but thats about it)The bad situation that can happen is if you put out more power than your main circuit breakers can handle because when those open up the house becomes disconnected from the grid and because now the power has nowhere to go the voltage would shoot up and fry stuff.

author: AlanR
author_rank: Regular Contributor
timestamp: August 18, 2015, 08:02:51 am
content: There is a technique I use quite often to build load banks for industrial battery discharging while still able to maintain constant current sink with relatively few active devices. The trick is to use shunt resistors to "shunt away" some of the current from the current control element. Be sure to "tuck" your shunt resistors into the current sense loop, rather it be sense resistors or some other method of current sensing. What will happen is that the active element current sink device will do the "fine" current adjustment for if you need ones, 10's, 100's, etc... position adjustment of the overall current.For instance, I designed a 25 amp constant current sink discharge load bank for 12 volt lead acid batteries that uses two, 1 ohm, 200 watt resistors wired in parallel to get .5 ohms and I tied this circuit into the current sense loop, which in my case was 3, 0.3 ohm resistors wired in parallel. The "fine adjust" current sense element, which is wired parallel to the shunt resistors, was a low side N-channel mosfet being controlled by an Opamp that was referenced to a 2.5 volt voltage reference. It would keep 25 amps all the way to the cutoff point, which I set at 11.25 volts.Some things you should keep in mind:Be sure the current sense element can handle the worse case current draw, which occurs at your lowest voltage cutoff point you select.This load bank only works well with voltage sources that go down in voltage under load.If you are using AC voltage then you are interested in keeping the same RMS current, this is not a "current sink" in the sense, but should be doable if you can time the firing angle to devices that let a portion of the AC waveform through. It's a bit more complex to design something like this.EDIT: I just had a light bulb moment that may work.You know about shunt wound or sepex EX DC motors? Here is what you can do: Build up your resistive load bank and hook it through a current sense element and then hook this up to the motor's armature connections which should be labelled A1 and A2. Then you want to make a variable current source for the field circuit of the motor and connect this to the E1 and E2 connections, which is the field circuit. You need to build your feedback loop such that you maintain constant power or constant torque by way of changing field current such that current stays the same on the armature output of the motor or power stays the same by measuring voltage and current output of the motor's armature. You looking for power or constant torque?The main issue is you will need to come up with the motor's field map since motor manufacturers seem to be very protective of this data for some reason.

author: LaserSteve
author_rank: Super Contributor
timestamp: August 18, 2015, 04:18:00 pm
content: \There is nothing bad about feeding power back out. Most solar power installations do that since its easier to sell the power than to store it in a battery bank for when you need it (Also in a lot of countries you get more money per kW/h that way rather than not buying that much power from the grid).End Quote.In the USA, this is very much illegal unless you have approved safety systems to prevent powering the grid, when the grid itself is down. You can sell back power to a utility here, but you must have their permission to do it.Part of this is due to how we troubleshoot local power, and US utilities can often easily disconnect small neighborhoods for troubleshooting or fault isolation. . After many technicians were killed, the law was changed to prevent this. Private generators are very common here for blackouts.Steve

author: leblanc
author_rank: Regular Contributor
timestamp: August 18, 2015, 05:34:29 pm
content: You can also check out hysteresis brakes. They can give you constant torque. So you can start with high torque (which will limit your rpm), then ease off on the torque slowly. From your "torque vs time" and "rpm vs time" curves, you can get your "rpm vs torque" curve, which is probably your most useful curve.Or you can get a four-quadrant dyno if you want a more off-the-shelf solution (with pretty decent data acquisition software). This will give you great results, but they can set you back $100k or so. Magtrol makes some pretty nice setups (http://www.magtrol.com/motortest/dynamometers.html). Again, pretty expensive stuff.If you just want a load for dissipating electrical energy, what I've seen done is a bunch of toasters connected in parallel (you can get them for under $10 each). Alternatively, you can get these load resistors with heatsinks, for a more legit solution.

author: leblanc
author_rank: Regular Contributor
timestamp: August 18, 2015, 05:49:11 pm
content: A low cost way of measuring your torque vs rpm curve is to attach a massive flywheel (with known moment of inertia, which should be as large as possible) and simply measure the rpm. This won't be very accurate, since you neglect the air friction, as well as moment of intertia of the engine (you can try to account for them if you want).From an encoder you will have an angular position vs time curve, which can give you a angular speed vs time curve, which can give you an angular acceleration vs time curve. Since torque = J*(ang acceleration), you now have a torque vs time curve. You can then get your torque vs rpm curve.If you'd like to include the effects of friction, an easy way to do this is to do a spin-down test with the flywheel. You get your engine at max speed then stop driving it. You will see the rpm slowly drop. Using the same method, get a torque vs rpm curve. Subtract this curve from the first torque vs rpm curve. That will give you a better idea of the torque that the engine can generate at various rpms.I have been pretty successful at this method. However, it was done on a relatively small (and electric) motor. This method does not need a brake, torque sensor, etc. You only need the engine, flywheel, and encoder.

author: Cmcada
author_rank: Newbie
timestamp: August 20, 2015, 03:09:31 pm
content: leblanc,I was part of the way through the flywheel/encoder method and hit a bit of an issue with balancing the flywheel.The budget was about £100 give or take, a 10" 6mm side wall pipe was purchased and the plan was to fill it with the cheapest heaviest material we could find which was concrete at the time, this was to be braked using a standard brake caliper from a car and a solenoid for some level of control.The issue was that concrete is non-homogeneous, when this 95kg+-50g cylinder was placed into its bearing housing and span up to the test speed of just 500rpm using a drill, the system started to shake itself apart, a few Megajoules of energy in a solid object spinning off its mount may sound like a hilarious Sunday morning however it was akin to a scene from armageddon.Without a sizeable budget and access to a large machine shop a purely mechanical solution to the problem seems out of reach.The final plan at the moment is to mount the motor in a frame free to rotate on a bearing. An arm from this frame will be attached to a strain gauge giving us our values for torque.To dissipate the energy produced we have built a controlled pressure vessel of 100L and are using a heating element to boil distilled water within it. This should keep the price and size of the system to a minimum, lets just hope my welds hold up. The motor is to excited with a controllable current from a 1kw power source. I think with enough safety devices in place this should do the job.Accuracy is not important here, we just need a controllable load for the petrol motor for mapping purposes, I don't fancy riding up and down an air field for weeks on end trying to track down an occasional misfire or lean condition caused by a poorly developed algorithm, this way I can gather all the data I need in a single day and spend the next month letting Simulink do all the dirty work.I appreciate all the responses! They have certainly given me a lot to think about and an idea or two for my dissertation next year.

author: leblanc
author_rank: Regular Contributor
timestamp: August 20, 2015, 05:04:41 pm
content: I've had trouble balancing our flywheel as well. It actually fell off the shaft at 10-12krpm. It sounded like a orchestra of power tools.Ok so maybe those two tiny setscrews (pressing on a smooth unkeyed shaft) weren't adequate.

author: Berni
author_rank: Super Contributor
timestamp: August 20, 2015, 05:31:47 pm
content: Quote from: leblanc on August 20, 2015, 05:04:41 pmIf anyone had a camera handy i want to see the video of that.