Author Topic: The efficient Dyno  (Read 7307 times)

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Online Zero999

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Re: The efficient Dyno
« Reply #50 on: June 20, 2018, 08:49:24 am »
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.
What do you want to do with the 1kW of power?

If you were able to achieve the suggestion, made in your initial post, of putting it back into the motor's input, if would make the motor look like an it had no mechanical load at all, to the driver, which I would defeat the purpose of the test.

I suppose it is realistic to put the power back into the input of the driver circuit, which should be possible with a boost converter, or even just regenerative breaking on the alternator side, which would involve using the motor's inductance as part of the boost converter.
 

Online SimonTopic starter

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Re: The efficient Dyno
« Reply #51 on: June 20, 2018, 09:30:12 am »
Yes the power has to go back to the DC supply of the driver. If we use the generators own wingdings don't we need to do all of it 3 times and then use isolation to control all 3 phases the same? probably simpler to just rectify it and then boost it.
 

Online Zero999

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Re: The efficient Dyno
« Reply #52 on: June 20, 2018, 10:13:20 am »
Yes the power has to go back to the DC supply of the driver. If we use the generators own wingdings don't we need to do all of it 3 times and then use isolation to control all 3 phases the same? probably simpler to just rectify it and then boost it.
Both of those methods would work.

Using a six diode bridge to rectify the output and a separate boost converter would also work, but it would require a large inductor or transformer to perform the boost function, unless the alternator's field could be increased to raise the output voltage.

Three half bridges could be used to do regenerative braking, which would use the motor's inductance to boost the voltage. The controller would need to be more complex, but it would avoid the need for a separate inductor and is probably more efficient, although the latter is probably not the most important thing, in this case. It's quite likely an existing motor controller IC/module could be used but it would need some changes to the suggested/standard schematic.
 

Online SimonTopic starter

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Re: The efficient Dyno
« Reply #53 on: June 20, 2018, 10:18:36 am »
The "generator" is a permanent magnet motor. Being identical to the driving motor we can anticipate the output voltage with RPM. It is a delta configuration. Unfortunately the current driver does not do regen breaking this was in fact the original suggestion max_torque made when we discussed it.
 

Offline max_torque

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Re: The efficient Dyno
« Reply #54 on: June 20, 2018, 05:14:09 pm »
As i mentioned, the car alternator idea is simple because of the following characteristics:

1) it revs to around 18krpm (easily fast enough)

2) it is easy to couple too, either directly with a bellows coupling, or via a polyV belt, meaning you can adjust the speed / torqur relationship if required

3) Load control is done simply by the rotor excitation, meaning you can control the brake torque with just a 5S bench top CC supply (get one with RS232 or GPIB etc and you can automate everything)

4) Car alternators are tough!  They run at up to 160 degC in the native application, have no permanent magnets to demag, and even come with a built in fan to help cool themsleves

5) the input shaft has very good bearings, because those bearings normally take the FEAD belt loads, so they are super robust to any input shaft loading (from out of balance DUTs etc)

6) CHEAP!  Ebay, gets you one for literally a few £, if you somehow break it, then just buy another

7) you could do with a load at all, just short the phase windings together, all current just gets turned to heat in the windings themselves.  This would need a seperately driven cooling fan to extended operation possibly, but the high thermal inertia and high operating temp mean it would almost certainly run for 5 to 10min un cooled (especially at reasonable rpm where the built in fan starts to work)

8) The overall set up is both simple, and can be quickly "trialed" not the case with a system that needs complex power convertors etc



Those factors make it a winner for me by a fair margin!
 

Online SimonTopic starter

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Re: The efficient Dyno
« Reply #55 on: June 20, 2018, 05:49:28 pm »
Yes as discussed the alternator is going to be easier to get going. Someone will just have to make us the metalwork. The motors run up to 12'000rpm so an 18'000rpm alternator will be fine directly coupled. We can measure the current being put back into the DC bus of which we know the voltage (and can get it from the PSU or measure it) so we know what load we are putting on taking into account the efficiency of the alternator. In this way minimal stress and variation while running and we make around half the heat. The last thing we want is to be running in the summer at 35C and be putting more heat into the environment the testers work in.

We will of course need a 24V alternator unless we are safe to take a 12V and push it up knowing the rectifier can handle it.
 
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Offline C

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Re: The efficient Dyno
« Reply #56 on: June 20, 2018, 09:50:44 pm »
You might want to think about a torque arm so you can sense the mechanical torque being applied.
 
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Online SimonTopic starter

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Re: The efficient Dyno
« Reply #57 on: June 21, 2018, 06:55:41 am »
We won't go that complicated. I believe that if we know the power and the speed we can make a calculation. The main aim is to characterise a motor driver and the be able to compare it to others on the same device. Absolutes are less important than a like for like comparison.
 

Online Zero999

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Re: The efficient Dyno
« Reply #58 on: June 21, 2018, 07:46:29 am »
Yes as discussed the alternator is going to be easier to get going. Someone will just have to make us the metalwork. The motors run up to 12'000rpm so an 18'000rpm alternator will be fine directly coupled. We can measure the current being put back into the DC bus of which we know the voltage (and can get it from the PSU or measure it) so we know what load we are putting on taking into account the efficiency of the alternator. In this way minimal stress and variation while running and we make around half the heat. The last thing we want is to be running in the summer at 35C and be putting more heat into the environment the testers work in.

We will of course need a 24V alternator unless we are safe to take a 12V and push it up knowing the rectifier can handle it.
24V might be more efficient too, since there will be less losses in the rectifier, which I presume will be silicon. Anther thing which could cut the losses further would be to change the the silicon rectifier for a Schottky one, which would halve the rectification losses.
 

Online SimonTopic starter

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Re: The efficient Dyno
« Reply #59 on: June 21, 2018, 08:24:03 am »
I'm not too bothered, we could if it makes much difference. it will be easier to have the 1KW load with 24V although if half of that goes in heat anyway then meh! but we need to match the supply voltage anyway which is 20-33V and there is nowhere else sensible to dump it.
 

Offline max_torque

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Re: The efficient Dyno
« Reply #60 on: June 21, 2018, 08:47:29 am »
I guess a "two stage" approach is also possible with an alternator based dyno.

For the first runs, where you just want to prove the concept, just remove the rectfiers and short the phases together, and concentrate on the mechanical arrangements (mounting and shafting), and on how to drive the excitation current into the rotor (uC to measure rotational speed via a simple hall sensor and simple pwm output to drive current through the rotor, with a PI loop to do basic speed control)
 When that's all working you can add a the rectifiers back in (or replace with lower loss ones) and build the boost convertor to re-cycle the power pack into the supply.  That could be a simple voltage control loop as well, using any number of OTS boost convertor control IC's  (ie, say you target the output of the phase recifiers to be 5v, that fixes your step up ratio to  be around 5 times) Of course, that voltage setpoint also sets the minimum speed at which your system can apply load.
 

Online SimonTopic starter

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Re: The efficient Dyno
« Reply #61 on: June 21, 2018, 11:48:26 am »
Why do we need a boost converter? if we use the alternator we can drive it to the same voltage as the supply or in effect regulate on current output. We could regulate to produce a speed or regulate to produce a desired output/load and then take the speed measurement to see what speed we get with that load.
 

Online Ian.M

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Re: The efficient Dyno
« Reply #62 on: June 21, 2018, 01:31:16 pm »
Yes,  though you may want to add torque sensing as 'C' suggested, and also if you want to improve the low speed capability, a cooling fan forcing air into the back of the alternator + a contactor to short its phase coils before the diode plate, so at speeds too low to get the load you want at your DC bus voltage, you can essentially use it as an eddy current brake  controlled by its field coil.

Since vehicle alternator efficiency can be as low as 60%, it should have no problem running in 100% dissipation shorted brake mode with forced air cooling as the total shaft power at speeds too low to generate effectively will only be a small fraction of its rated power.l

If you are going to do any sustained full load tests, you'll need to over-size the alternator considerably to avoid overheating.   1.1KW at 28V is 40A, so I would  suggest something in the 80A to 120A range.
 

Offline C

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Re: The efficient Dyno
« Reply #63 on: June 21, 2018, 03:39:27 pm »
As lan.m stated
Slow speed is a problem.
Normal minimum speed for a vehicle alternator is engine idle. And at this speed often do not produce full output to prevent heat build-up.

lan.m's post has a lot of great points that need to be covered.

In addition to the electrical load created, the vehicle alternator will also be creating a fan load.

I would think that torque sensing would be the easy way to get the total facts that make up the actual load.

Today a lot of vehicle alternator's have their own regulator built in. If you do not want to hack this an open a can of worms that could destroy the vehicle alternator, you could connect the output to a bidirectional buck-boost converter..
Some of todays vehicle's are using a 48 volt bus with a bidirectional buck-boost converter used to supply the lower voltage loads.

C
« Last Edit: June 21, 2018, 03:46:32 pm by C »
 

Online SimonTopic starter

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Re: The efficient Dyno
« Reply #64 on: June 21, 2018, 05:54:17 pm »
I don't know what power we get from the motor and driver at low speed and if we could get the full 1KW at minimum RPM. I would not be too bothered with an output below 20% (2000rpm).

We could short the windings at low speed and switch to dumping into the supply at higher speed. I am not familiar with torque sensors but yes that makes sense as our testing rig's characteristic is more defined against something real than some condition we registered once and need to see matched.
 

Offline BrianHG

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Re: The efficient Dyno
« Reply #65 on: June 21, 2018, 09:38:43 pm »
I don't know what power we get from the motor and driver at low speed and if we could get the full 1KW at minimum RPM. I would not be too bothered with an output below 20% (2000rpm).

We could short the windings at low speed and switch to dumping into the supply at higher speed. I am not familiar with torque sensors but yes that makes sense as our testing rig's characteristic is more defined against something real than some condition we registered once and need to see matched.
Shorting the windings for 1kw load at low RPM means 1kw of heat will be coming out of the alternator itself.  The alternator will warm up your room.

To support low RPM, you would be better off using a low RPM neodymium PMG where you get over 1kw and over 200 volts, over 85% efficient already at 500rpm.  Note that units exist designed for higher RPMs compared to the type/model charts I've attached.   Note that these are direct drive wind generator PMGs.
« Last Edit: June 21, 2018, 09:43:42 pm by BrianHG »
 

Online nctnico

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Re: The efficient Dyno
« Reply #66 on: June 21, 2018, 09:58:51 pm »
I'd mount the resistors on PC water cooling hardware and just run the water from the tap. Cheaper then buying a pump to recycle. Look at the WDBR series resistors from TE. The alumium cased resistors can't handle much power. The WDBR resistors can when mounted on a heatsink and offer better value for money. Be sure to include over temperature switches though no matter what solution you choose.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline BrianHG

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Re: The efficient Dyno
« Reply #67 on: June 21, 2018, 10:15:10 pm »
Have you considered an alternator with 2 sets of windings for each phase?  You switch to series for lower RPM to get a voltage boost and switch to parallel at higher RPM to halve the voltage.  An old fashioned mechanical relay may be used for the switch.

Also, since the output is 3 phase AC and you are already rectifying the signal, you can use a simple capacitor voltage doubler on each phase & switch to before and after the cap double for that 2x or 3x voltage boost.  (3x can be done by adding a second doubler on your negative rail.)  To get the most out of a car alternator's low voltage at low RPM, I would recommend trying the new field effect rectifiers from ST-Microelectronics. https://www.eevblog.com/forum/chat/take-a-look-at-these-new-field-effect-rectifiers-from-st-microelectronics/msg1428355/#msg1428355
« Last Edit: June 21, 2018, 10:26:22 pm by BrianHG »
 

Offline LaserSteve

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Re: The efficient Dyno
« Reply #68 on: June 21, 2018, 10:40:44 pm »
Um,   You folks know that commercial water cooled resistors are off  the shelf parts right? They are made for machine tool regenative braking systems and are common.

Steve
"What the devil kind of Engineer are thou, that canst not slay a hedgehog with your naked arse?"
 

Offline CopperCone

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Re: The efficient Dyno
« Reply #69 on: June 21, 2018, 10:44:46 pm »
what are some specs of high end water cooled resistors? I am really curious.

I see like VPG has nichrome in water ones, http://www.vishay.com/docs/32548/dcrf.pdf , which may not be ideal compared to fairly well isolated ones, they don't seem to work without very pure water which can be a problem to maintain without ion exchange resin
« Last Edit: June 21, 2018, 10:50:06 pm by CopperCone »
 

Offline BrianHG

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Re: The efficient Dyno
« Reply #70 on: June 21, 2018, 11:12:09 pm »
Um,   You folks know that commercial water cooled resistors are off  the shelf parts right? They are made for machine tool regenative braking systems and are common.

Steve
My post from page 1! https://www.eevblog.com/forum/projects/the-efficient-dyno/msg1617676/#msg1617676
 

Online Ian.M

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Re: The efficient Dyno
« Reply #71 on: June 22, 2018, 12:14:49 am »
There is absolutely no way the driving motor can output 1.1KW of shaft power at low speed if its rated for 1.1KW at several times that speed.

Shaft Power = 2 * Pi * n * T

where n is the speed in revs/s and T the Torque in Nm.

Neglecting frictional, windage and other losses, Torque is proportional to current in the driving motor, which has a finite upper limit above which its coils will burn up and/or its core will saturate.    Therefore if the motor is operating at 20% of the speed its rated for 1.1KW at, if its producing over 220W of shaft power its overloaded and will probably burn out if the overload is sustained.

Assuming a 24V 80A alternator, (which will actually be designed for about 28V),  its maximum output at rated speed will be about 2.2KW.  With forced air cooling, it will easily be able to handle 10% of that operating as a shorted brake, especially if the shorting contactor is directly across the stator coils so the diodes on the diode plate aren't being stressed.

Talk to a local alternator rebuild shop to find out which alternators are both readily available and  easy to mod for all phases + both field brushes brought out.   

There's little point in using external rectifiers.  Yes it may be able to improve efficiency (especially if you go to synchronous MOSFETs) but it pushes up the cost dramatically.  If the heat output from mk.1 is excessive you can always retrofit them on Mk2 without further alternator mods - just  hook up your external rectifiers to the stator phase wires you've already brought out, and move the output wire  from the terminal stud on the back of the alternator coming from the diode plate to your external three phase bridge.

I doubt you'll gain much from external braking resistors or voltage doubling - but that's easy to test - just try charging a large, 60% discharged 12V Lead Acid battery instead of regenerating into your motor drive DC bus, or patch in a bunch of wire in three buckets of water.  I suspect the speed range over which you can draw useful power at 14V but cant at 28V will be rather narrow.  Braking resistors shift much of the dissipation out of the stator, but they also push the minimum speed for a particular power upwards due to the voltage across them.
« Last Edit: June 22, 2018, 12:37:22 am by Ian.M »
 
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Offline C

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Re: The efficient Dyno
« Reply #72 on: June 22, 2018, 12:36:52 am »
The vehicle alternators I have seen are three phase.

If you were to connect some outputs to the junction of winding to diode you could get an AC output. You could then use transformers to change voltage/current

The internal regulator will sense the DC output.
A load change to DC output will cause a mechanical  input load change.

To protect the alternator you need to keep max current equal or below rated output. A failure here is burnt windings or blown diodes.
You also need to monitor temp.

If you are building your own motor drive electronics add a half-bridge with a resistor load to keep regen from causing an over-voltage of supply bus.

C
 

Offline BrianHG

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Re: The efficient Dyno
« Reply #73 on: June 22, 2018, 01:29:46 am »
Unless Simon is getting his 1.2kw alternator for less than 50$, it will cost less to buy a 125$ 1.2kw neodymium permanent magnet core style alternator.  He wont have to worry about the darn excitation supply and 12v through 70v versions exist, with and without output diodes and regulators, and he will get the full 1.2kw down at 1000rpm.  All he would need is a simple non-isolated switching step down regulator to recover most of the power.

example: https://www.amazon.com/Permanent-Alternator-Wind-Turbine-Generator/dp/B0755PPMPF

Full 1kw just over 1000rpm on this alternator's shaft.
Additional, be careful about car alternators, they specify their output power based on engine RPM, not alternator RPM, but 1kw 12v units exist though they aren't as efficient.
« Last Edit: June 22, 2018, 01:39:56 am by BrianHG »
 

Online SimonTopic starter

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Re: The efficient Dyno
« Reply #74 on: June 22, 2018, 04:39:54 pm »
A large 24V alternator with modified control will do the trick. Given the not terrific efficiency a 1KW output alternator will be fine as it will be at least 1.5KW in mechanical power and the input to the motor controller is at most 1.2KW and then you have the looses in the controller and the motor.
 


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