# Workshop Dynamometer using BLDC motor

#### Irving

Joined Jan 30, 2016
2,308
The more I think on this, given the situation, the more I think @LowQCab is right. It is a simpler approach. The question is now how to instrument it to give you the output needed. On at least 1 of the stator windings you need to measure current and generated voltage; ideally do it on all 3 since they wont be perfectly identical. The easiest way to measure the current is with a 75mV shunt, eg https://www.ebay.co.uk/itm/164828952645 inserted in one of the shorting link

You also need to measure the voltage generated, from the centre join to the shorted connection, like this...

Then you can measure current & voltage across the cycle and calculate Power Out. From that and revs you can calculate torque load.

But you will need to calibrate it,. One way is with a pulley and dropped weight.

#### LowQCab

Joined Nov 6, 2012
1,308
Measurement of Current is not necessary.
The Number that is needed is Pounds/Feet of Torque.
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#### Irving

Joined Jan 30, 2016
2,308
Ah, yes of course, I'd forgotten that option. I once built an eddy current dyno for small to medium electric motors, using a 20cm (8") spinning aluminium rotor and 2 large Chinese 50kg pull electromagnets mounted on a concentrically pivoted 0.5m arm and a force gauge.

Come to think on it, that's probably an option here 'cos the motors I was testing then were 5 - 8Nm stepper and servo motors.

#### shortbus

Joined Sep 30, 2009
8,955
I want to eventually be able to do repeatable transient run
And that can be done with the prony if you use a little imagination. Using a stepper motor to control the application of the friction on the motor shaft.

Using an alternator like is being suggested is opening another whole can of worms for you.

The alternator in a car is over driven, meaning that you have a ~8" pulley on the crankshaft and ~2 1/2" on the alternator. This is there to get the alternator to give an output at lower engine speed, like around the town driving. You will need some sort of belt or other drive from your motor to the alternator, and have to figure any slippage and power consumption into your test numbers.

With that done you have to be aware that alternator output isn't linear. Quote: " alternators have greater output at higher speeds. This increase in output is not linear but is actually a curve. This means that as rpm changes, the alternator will increase and decrease its output at different rates. " From - https://www.motortrend.com/how-to/0206sr-understanding-your-alternator/ So you will also need to factor that curve into your results.

Where as a prony brake doesn't have any of those constraints. I know I keep bringing up the prony brake, but that is because it's still the simplest way of doing what you want. Its worked for 200 years, this 2021 is it's 200th birthday after all. just modernize it with a little electronics on the scale and power application and make life easy.

#### LowQCab

Joined Nov 6, 2012
1,308
""
Using an alternator like is being suggested is opening another whole can of worms for you.

The alternator in a car is over driven, meaning that you have a ~8" pulley on the crankshaft and ~2 1/2" on the alternator. This is there to get the alternator to give an output at lower engine speed, like around the town driving. You will need some sort of belt or other drive from your motor to the alternator, and have to figure any slippage and power consumption into your test numbers.

With that done you have to be aware that alternator output isn't linear. Quote: " alternators have greater output at higher speeds. This increase in output is not linear but is actually a curve. This means that as rpm changes, the alternator will increase and decrease its output at different rates. " From - https://www.motortrend.com/how-to/0206sr-understanding-your-alternator/ So you will also need to factor that curve into your results. ""

You evidently are missing a variety of factors .........
When using a Mechanical-Friction-Brake, You still have to dissipate the same ~1640-Watts of Heat.
This could probably be done with a Disc-Brake off of a small Motorcycle, along with a Cooling-Fan.

Most Fiction-Brakes will require more or less pressure to generate the same Torque-Value
as the Temperatures increase with time, and the amount of Power absorbed.
A Water-Pump-Brake would be substantially more consistent than a Friction-Brake,
That's why they use them in full-scale, Commercial-Dynomometers,
although, some of the best "High-End" Dyno manufacturers currently use
Alternators instead of a Water-Brake.

https://dynomitedyno.com/tech-corner/absorber-comparison/

Nobody uses a Leather-Belt, wrapped around a steel drum, which is what a "Prony-Brake" is.

Your quote regarding "Alternator-Output" flies out the window when running the Alternator in
a "shorted" condition, rather than the Alternator trying to maintain a fixed Output-Voltage.
The shorted Windings make the Alternator much more linear, and in any case,
the desired outcome is NOT Voltage-Generation, but Torque-Generation.

An Alternator with shorted Windings will generate its maximum Torque at speeds as low as ~1000-RPM.
The Engine in question here generates its maximum Torque at around ~4000-RPM,
and generates its maximum Horsepower at around ~6000-RPM,
and makes useful Power up to around ~8000-RPM.
The Engine Idles between ~1000 and 1500-RPM,
at that speed the Alternator will easily produce enough Torque to stall the Engine,
so no increase in RPM with Pulleys or Gears is necessary at all.
The average Alternator can comfortably operate at ~20,000-RPM without damage.

No separate Brake actuating devices and associated Linkages, Motors, Gearbox, etc. are required.
just a standard Bench-Power-Supply, and an accurate Electronic-Scale, or Electronic-Pressure-Transducer.

The Torque generated responds to the Voltage applied to the Rotor-Winding fast enough for
an automatic, "Closed-Loop", feed-back scheme to be used for real-time-RPM-control.

The Voltage or Current that is generated is completely irrelevant, and does not need to be measured.
The only factor that should be monitored is Temperature,
which is the case no matter what means of generating Braking-Torque is employed.

1640-Watts is not a trivial amount of Heat to be handled,
it requires a large mass,
and a large surface-area for Heat-Dissipation,
and a Cooling-Fan to remove the excess Heat.
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#### Irving

Joined Jan 30, 2016
2,308
1640-Watts is not a trivial amount of Heat to be handled,
it requires a large mass,
and a large surface-area for Heat-Dissipation,
and a Cooling-Fan to remove the excess Heat.
That's one reason I like the eddy current dyno. The spinning ally disc is it's own heatsink.

Friction brakes always have problems as the friction coefficient varies with temperature and it's hard to maintain a constant load.

As you say, electromagnetic brakes have an almost linear load to excitation relationship and a fast enough response to be part of a closed loop setup.

My only concern with the alternator approach is the stability of the rotor winding at high rpm. As I have no experience of that I'll take your word on it. The eddy current brake is easy to make and a spinning disc has few issues with high rotation speeds (and it's easy to calculate size to be sure).

This is a good read:
https://mechanicalengineering.blog/dynamometer-introduction-types/

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#### LowQCab

Joined Nov 6, 2012
1,308
Eddy-Current-Brakes are the superior solution, no doubt,
but will require a substantial outlay of Cash, and or, Precision-Machine-Work-Construction.

Small Eddy-Current-Brakes are readily available commercially, but are quite expensive,
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The only alternator you will get at a junk yard, or at least one around here, is one that is burned out, and even they are worth more than that as rebuild cores. I have a friend that has a business buying andselling core and they are worth to him much more than $20. , , , , , , , #### shortbus Joined Sep 30, 2009 8,955 Your quote regarding "Alternator-Output" flies out the window when running the Alternator in a "shorted" condition, rather than the Alternator trying to maintain a fixed Output-Voltage. The shorted Windings make the Alternator much more linear, and in any case, the desired outcome is NOT Voltage-Generation, but Torque-Generation. You still need a load cell or scale to measure that torque. So that in effect still makes it a Prony brake. , , , , , , Thread Starter #### ElNormo123 Joined Nov 17, 2021 21 So your saying that a ~1600W load will stall a 8KW motor? The only alternator you will get at a junk yard, or at least one around here, is one that is burned out, and even they are worth more than that as rebuild cores. I have a friend that has a business buying andselling core and they are worth to him much more than$20.
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It doesnt need to stall 8kW my engine only goes up to 1.6kW.

I've already found a few alternators for around £30-£50 here in the UK which is within my budget so should be fine

#### shortbus

Joined Sep 30, 2009
8,955
I've already found a few alternators for around £30-£50 here in the UK which is within my budget so should be fine
All of the talk of shorting alternator windings may work out for you, but from my years of working on cars, when I've found an alternator with a shorted winding you know that it is shorted because the insulation on that winding is burned off, due to the heat in it.

Exactly how do you plan on measuring the torque from the shorted alternator? Or for that matter how did you plan to do it if you still used the original BLDC motor?