Workshop Dynamometer using BLDC motor

[ElNormo123]

Hi all

I am trying to make a brake dynamometer for a small 50cc engine (torque and power curves attached). The dyno must be capable of holding the torque of the engine at all speeds. I was thinking of using the following BLDC motor http://shop.dualsky.com/ga6000s-single-shaft-edition-giant-airplane-series-for-e-conversion-of-gasoline-airplane_p0253.html

From what I can tell it can get up to the speeds required for the engine and provide a suitable torque. I am planning on electrically braking the motor by varying the resistance in the coils of all 3 phases as shown in the attached picture. To do this I will put some form of variable resistor in each wire coming from the motor and solder them all together. I am not very confident with motors so Im not sure if this will work. My theory is that as turning the BLDC with the engine will generate AC current, I should be able to regulate the resistance in each coil to regulate the current and hence braking torque.

I have a few questions about this.

1. When the engine is turning the motor will it generate sufficient current to create the necessary back torque? All the stats on the motors website give torque ratings with 48V across the motor, I assume I will not have 48V when im generating. My understanding of it is that I could use the RPM/V specification to calculate the volts is this true?

2. If the above is true, I have attached a picture of calculations using the max throttle specifications from the companys website (also attached as a screenshot), If these calcualtions are correct I should have more than twice the torque needed to hold the engine. I did the same calcualtions at various speeds and this remains true, I just want to check if the calcualtions would actually represent the true torque or at least close to it.

3. Last question is how to actually control the resistance. I have used PWM controlled Mosfets before with DC but as this will output an AC current I assume I cant still do this, does anyone know of a way to regulate the current affordably? If my calcualtions in Q2 are true I might be making a maximum current of 130A but as the resistors will only be in 1 coil each I assume I can have a resistor with a lower amp rating than this.

Any help would be greatly appreciated

Thanks

Andrew

 

[fechter]

1. Yes, the voltage generated will be a function of RPM you can calculate using the kV. You'll get slightly less due to losses.

2. Yes, the torque will be a function of the current, so as long as you can control the current, you can keep the torque where you want it. Measuring the current will tell you the torque.

3. If you use 3 separate load resistors, the load will be equally divided. That's still going to be a lot of current. Since it's AC, you could possibly use SCRs or triacs like a light dimmer. You could also use back to back MOSFETs or use a bridge rectifier first to make DC and a single MOSFET PWM stage to control the current.

 

[ElNormo123]

Hi fetcher

Thanks for the response

1. brilliant, glad to know it does work like that, hopefully I'll be ok with losses as Ive picked a motor capable of making double the power I need.

2. so my main assumption in my attached calculation was that by using kV I could get the voltage then I assumed the current would be directly proportional due to V=IR. My main assumption was R (resistance) would be the same when Im using the motor as a dynamometer as it is when it is used as a motor, and therefore the current and torque would be the same. Someone has told me about impedence being a factor that can change the resistance as now I will be working with AC whereas when being used as a motor the Electronic Speed Controller will put DC across the coils. Will this have a large effect?

3. Thanks for those suggestions, I'll have a look into them and see what I can find!

 

[fechter]

The frequency is low enough that impedance won't be a significant factor if your resistor is purely resistive. V=IR will still apply. Things may change a bit if your resistor is a coil of wire and you get enough inductance to change things. A wirewound resistor will have less resistance at higher frequencies due to the inductance.

The resistance of the motor windings is very low compared to the load resistor, so the kV will be essentially the same for being a motor vs. being a generator.

If your load resistor is fixed, the current (and therefore the torque) will be a linear function of rpm.

Have you looked into dynos made from car brake disks? Maybe not as fancy but probably cheaper and easier to make. Another style I've seen is an eddy current brake, where you make a big disk out of copper or aluminum and put a big magnet on the side. The magnet can be moved in or out to vary the load. The heat all goes into the disk, which needs to be cooled.

 

[ElNormo123]

ok that all sounds quite promising, I'll try to use a bridge rectifier and some mosfets for the current control, which if I've understood you correctly should not effect kV?

So all in all do you think the motor will provide sufficient torque, as per my attached calcualtions?

 

[fechter]

I'm sure it will provide enough torque. The limiting factor will be heat buildup. As I mentioned earlier, the generator is going to need a fan to keep from overheating if the runs are very long. If your tests are short enough, you can get away without cooling. A temp monitor might be a good thing.

The kV doesn't change other than you lose about 1v in the bridge rectifier. That thing will get really hot too. It will need a big heat sink.