Brushless motor to be employed as passive load - electrical engineering

Other Hardware Motors, Mechanics, Power and CNC
1

I'm going on with my engine test bench project...

all seems to be ok, all sensors give their correct value, mechanical components are almost complete, so now it's time to add something unseen before!

Now I'm working on an inertial test bench, some details of my project could be obtained on http://arduino.cc/forum/index.php/topic,66130.0.html

Inertial is not the best, pratically I have a large flywheel and it's accelerating with engine at full throttle. This make some problems, like very fast transitions with powerful engines and very low transitions with s**t engines. Add that I couldn't perform endurance test at full/partial load, or partial RPM test, and that bench isn't so helpful at all.

It's better than nothing, and on the market I even couldn't find neither better nor almost like it, this is the reason that make me messing around Arduino.

Ok, now the real trouble I wanna make it static, like classic design for bike & car test bench (in Italy we have Borghi&Savery producing them). Now the way I've found are two: the first are electromagnetic parasite current brake (eddy current brake), very expensive, difficult to tune, and hard to find, the second is a 3-phase brushless motor coupled to an inverter in which frequence is imposed by operator or controller (Arduino), so it acts like a generator and not like a motor. This second system would be the best for me, permitting complex curves, acceleration and last but not least even STARTING the engine!

But, there's ever a but, working as a generator it produces a lot of current that I'm supposed to waste somewhere. In industrial application, with asincronous three-phase motors, I used a battery of resistance wires (acting like an electric stove) to avoid burning both stator and rotor coils.

Does someone know which could be the suggested ratio engine power / motor power to avoid brushless stator burn-out? should the solution of resistance battery be applyed even with brushless(anyone tested it?)? for reference my engines are all above 1,5 kW.

Many thanks to anyone who could help me

2

Why not run all that power in an AC inverter and put it back out on the mains lines? Help reduce your electric bill.

3

I think this isn't so easy... but it's the last problem, first I have to validate this principle of working... If brushless couldn't be used as a variable RPM brake there's no way to came out! Now I'm looking for vectorial asincronous three-phase motors, seems cheaper and stronger than brushless and more versatile than a std. asincronous motor

4

I've done something sightly similar in the past but to test the electrical characteristics of a large DC motor (acting as your engine). I used another large DC wound-field motor acting as a generator to apply the brake load . The armature of the generator was connected to a large (low) resistance capable of absorbing the full rated power of the "engine". Power (watts) = V*V/R so knowing the power of the engine and the voltage capability of the brake generator it's easy enough to work out the required resistor. Resistors are generally power rated to operate in free air so one way of increasing the resistor power rating is to immerse it in a bucket of running water. By varying the field current to the generator you vary the output voltage and hence vary the braking load applied to the engine.

By this means it is possible to perform a complete performance test of the engine at all speeds and torque loads, including if required the ability to start the engine.

5

Thank you Jack!

I didn't thought to a DC motor because I dunno know the sistem to regulate its RPMs... basically it's proportional to voltage, am I wrong? so using a set of resistors, have I to vary the load by connecting/disconnecting 'em in parallel to gain the desired current value? the waste power is RI², or like yours V²/R, but the DC "generator" should have a voltage output which is something linear if compared to RPM, I'm not sure...

6

A single resistor of a value to suit the rated generator voltage and amps at full engine revs is all you need. The variation in capability is accomplished by varying the field excitation. Yes a generator output is proportional to speed at each an every value of field flux : if there is no flux then there is no output. So varying the flux at each and every speed will vary the output voltage and hence the absorbed power into the fixed resistor. You simply need to measure generator output voltage and amps to give output power to the resistor. On the basis that field flux is independent of engine speed it is a reasonable approximation to assume that electrical power output is the shaft power input.

7

jackrae:
The variation in capability is accomplished by varying the field excitation.

sorry for my ignorance, I'm a mechanical engineer so I'm not so expert about DC motors... how could I vary the field excitation?

8

Most readily available DC motors (to use as a generator) use fixed field permanent magnets, so you need to ensure whatever you use has wound field coils. Similarly if you get hold of a DC generator you need to ensure it's not a PMG type. (The PM stands for Permanent Magnet) You ideally need a variable DC power supply, but it all depends on what the generator requires. You might get away with a couple of car batteries and a suitable rheostat (a potentiometer used as a 2-terminal variable resistor). 50watt halogen bulbs make good field control resistors, you can switch these in series and/or parallel to give various currents. However these get very hot (most of the 50 watts) so you need to ensure they are kept off flammable surfaces. Until you find a generator it's impossible to specify exactly what you need. I used a large battery charger capable of providing 20 amps with 2-volt switched steps from 2 to 36 volts that I built when I was an apprentice - and it's still going strong some 50 years later. This also had a series rheostat in its output leg so as well as stepped voltages I could also trim the output current.

9

An automotive alternator would allow you to vary the field excitation. As I understand it you just add a rheostat to vary the field current -- I don't think you even need a regulator. And of course you'd need to sink the generated power into your heater.

...wait a minute, as an engine guy aren't you supposed to know this? XD

EDIT: Here's the scheme of things.

10

Chagrin:
...wait a minute, as an engine guy aren't you supposed to know this? XD

Hi Chagrin, thanks for the info! but NO!, I'm not supposed to know... :blush:

I'm a mechanical enginneer and my work is primely on automation, but after the choice of the actuator the rest of control system is generally on customers' shoulders. I also have a wide experience on sheetmetal forming and progressive dies, but when we're talking about electrons I must show white flag! I've only a very little know-how on electronics, and almost nothing on power controls, in my work 3-phase motors are excited via inverter, in my dimensioning I have only to calculate torque, power and for only few cases the reduced inertia

Another question... as I understand your solution is open-loop, I have to vary manually the load by the variation of excitation field, and then by this variation the engine will speed up or slow down if the torque will be respective less or more than the one required from generator. Speed will be in some verse undeterminated, I have to continuously adjust the field excitation. Is there any device that could self-regulate excitation so RPMs will be constant?

11

I'm not aware of any off-the-shelf methods of regulating the excitation based on engine speed. With any generator systems I'm familiar with the only regulation of the engine speed is controlled by the engine's governor.

12

Chagrin:
I'm not aware of any off-the-shelf methods of regulating the excitation based on engine speed. With any generator systems I'm familiar with the only regulation of the engine speed is controlled by the engine's governor.

I agree, but it's the inverse of what I need...

in a generator usually the engine vary torque to meet generator required one (by closing carburettor or adjusting injection), elseway I have to vary load to keep my engine at an imposed speed or to make a imposed progression RPM/time, I need something like a control board to be interfaced to arduino