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-

... 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 file containing 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

 

[waross]

The voltage will be proportional to the speed.
The current will be proportional to the voltage.
You may have to inject an EMF in order to develop adequate torque at low RPM.
On the other hand, the motor may develop sufficient torque at a lower speed than you expect.
(Motor effective resistance equals the difference between applied voltage and back EMF. The difference, or effective voltage is typically very much less than the applied voltage. Calculations are fairly straightforward for DC motors.
With a DC motor you can determine or measure the resistance to use in your calculations.
With AC, the resistance term is replaced by the impedance. A component of impedance is inductive reactance, and inductive reactance is frequency dependent.At low speeds, resistance predominates, at higher speeds the inductive reactance will become dominant.
To make it more fun, there is a quadrature relationship between resistance and inductive reactance.
"Try it and see" is by far the quickest and easiest.
There is a good chance that the speed at which the torque is insufficient is below the minimum speed of the motor under test.)

Mosfets for AC?
Use bridge rectifiers and control the resulting DC.



--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[ElNormo123]

Thanks for the response waross

So when I use this as a dynamometer, impedence will impact the current and hence the torque as i will be generating AC? Would I be able to predict this from the motors specifications? I get a bit confused as when its being used as a motor the Electronic Speed Controller will put DC through the coils not AC and all the specifications online are for when it is behaving as a motor. As its fairly expensive I want to try and calcualte if the motor I buy is capable of doing what I need it to do before buying.

Because of the impedence changing the resistance like you say will the volts not be proportional to the rpm/V specification, making my attached calcualtions wrong?

For Mosfets, I was more asking if there was an AC equivalent to a mosfet, something that will help me regulate the current that isnt too expensive. Ive read that a rectifier will cause a voltage drop, would this impact the torque capabilities of the motor when being used as a dynamometer?

 

[waross]

Keep the current below the rated current.
At low speeds, the cooling may be less so beware of full rated current at low speeds.
The EMF in Volts will be proportional to the speed.
The current will be proportional to the Volts divide by the impedance. That is the entire impedance, internal as well as external.
In some applications the internal impedance will be negligible. The lower the external resistance, the more significant the internal impedance becomes in determining the current.

Your calculations will be usable at higher speeds.
If your results are not as expected at low speeds you may know why.
The voltage drop across a rectifier will probably be negligible.

In your sketch, use resistors suitable for the lowest operating speed. Use rectifiers and mosfets or some other device to short them together.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[BrianE22]

Andrew -

Did you come up with a way to use MOSFETs for the AC voltage? I believe a bridge rectifier with diodes would do the trick for you.

Sounds like you are using torque (current) for control . Are you using closed loop? As the current flows the current/voltage relationship will change for the resistors and probably for the motor as well.

 

[LionelHutz]

Personally, I'd want to do a dynometer load that has speed control. That way, you run the engine full throttle and ramp the speed with the load controller as you do the measurements.