Pitch trim speed control

[jrevens]

I know this has been discussed before, but I'm at the stage of building where I'm thinking about pitch trim speed reduction. Can anyone who's used a resistor for a considerable period of time (several hundred hours) tell me if there have been any problems with that approach? I'm assuming that the acuator motor still has sufficient torque with the reduced voltage to do the job, & that it is not harmful to it - is that correct? I'm wondering if a pulse width modulator (PWM) would be a better approach, as the voltage to the motor remains normal & the same for the torque. Here is a link to a possible device costing less than $10 with free shipping (although I think I'd do the registered mail for an additional $2.30) -
http://www.suntekstore.com/goods.php...gbus&u=1623592
This looks like a very small unit. I like the thought of a simple resistor & switch, but the PWM would give complete variable speed control & possibly be better for the motor. I'd appreciate any comments.

[Dorsal]

I have ~200 hrs with a resistor and it works fine. I can also state with some confidence that it is a misnomer that the PWM supply provides any more torque for a given speed than any other approach. A true speed controller requires feedback from the motor and will supply whatever voltage is needed to achieve that speed. This is not the case for the device shown which is a switching power supply which has the advantage of being more efficient than a linear supply but of no consequence to the motor. One possible word of caution with any switching supply is they can be electrically noisy and should be well shielded. The principal advantage using a variable supply (switching or linear) over a resistor is the ability to change the response in flight (though I suppose you could have a switch that bypasses the resistor for slow flight).

[jrevens]

Thanks Dorsal - that's what I was looking for. I hadn't thought about the possible electrical noise issue either.
It was my understanding that a resistor, lowering the voltage, would decrease the torque output of the motor while causing it to run hotter.
The PWM feeds full voltage to the motor - it's just in short, variable length pulses.

[jtpitkin06]

http://www.teamkitfox.com/Forums/showthread.php?t=2652

Here's the thread on the device I installed.

John Pitkin
Greenville, TX

[jrevens]

That's great John... thank you! I'm sorry I didn't look through the archives more carefully before starting this discussion again with a new thread. My only question is, did you detect any electrical noise from this unit? Did you mount it in a grounded box? Thanks.

[Dorsal]

Thanks Dorsal - that's what I was looking for. I hadn't thought about the possible electrical noise issue either.
It was my understanding that a resistor, lowering the voltage, would decrease the torque output of the motor while causing it to run hotter.
The PWM feeds full voltage to the motor - it's just in short, variable length pulses.

That may be where the misconception stems from, yes a PWM switches between 0 and full voltage though it does this 13000/sec which is filtered either by the output choke/cap or the inductance of the motor but the result is a constant voltage at the motor. The information provided by the manufacturers of these circuits is misleading at best. In all cases you are lowering the speed by lowering the voltage/current and lowering torque.

[Dorsal]

One more note, a PWM supply is a good choice for running cooler (the supply not the motor) and will draw less current but in addition to shielding it you should use a shielded wires back to the motor.

Yet another point of correction; the unit John is using only switches at 100 hz which should cause less RF problems though still in the audio band so I would still recommend shielded cables. As to whether this is a low enough frequency to cause PWM of the torque I can not say, but the average torque will be unchanged.

[jtpitkin06]

I’ve had the impression a PWM speed control could deliver more torque than a motor run at the same speed with a resistor circuit. Until now I never tested this belief; but, there seems to be some question.

So I went out to the shop and did a few experiments. (It's experimental aviation, isn't it?)



Here's what I did and the results. Please don't shoot the messenger. I'm just reporting what I did and what I observed.


I connected the trim motor with resistors in the circuit to slow the motor down. 12.5 Ohms slowed the motor to where the stab traveled from stop to stop in 14 seconds. About 1/3 of full speed.

I then ran the stab to the full down position and added a ten pound weight to the leading edge.


Power was applied to run the stab up. The trim motor drove the loaded stab full up in 29 seconds. More than double the time and the motor was almost stalled.


I then took the resistors out of the circuit and connected the PWM speed control. I adjusted the speed control so the stab without the weight would run from stop to stop in the same 14 seconds when using the resistor circuit.

I placed the ten pound weight on the stab leading edge and the motor ran the stab full up in 19 seconds.


The test was repeated several times and the results were the same. The trim motor delivered significantly more torque at low speed when using a PWM speed control than when using a resistor circuit.



Just my observation.

John Pitkin
Greenville, TX

[jtpitkin06]

100 Hertz is way above my long shot hearing range. Anything at or above the female spousal unit vocal frequency is shot from 40 years of jet engine assault.

Sometimes that's a good thing

JP

[Dorsal]

John, cool test, always good to have real data. The behavior is consistent with the switcher providing constant voltage and the speed slowing proportional to load. When the load increases on the resistor the current goes up and the voltage drop increases further slowing the motor. Your supply will definitely be more consistent under variable loads. The only data I have is that I don't notice much speed change from ramp use to in flight use suggesting limited load variations (or limited observation skills) . I do like the idea of being able to change the response in flight as it is a little slow in the pattern but I'm used to it now.