Electric Trim Motor - Speed Control

[jtpitkin06]

In the quest to slow down the pitch trim motor I started looking for parts to solder up a pulse width modulated (PWM) speed control. I stumbled across a completely assembled unit suitable for DC motors up to 15 amps. It runs on either 12 or 24 volts jumper selectable. Delivered price under $30. Perfect for the Kitfox!

It will control the speed of the trim motor from 0 to maximum. At maximum speed, the trim actuator went from stop to stop in 5 seconds. I was able to regulate the speed down to 15 seconds, stop to stop, with a 100 pound load on the leading edge, without stalling the motor.

I measured the unloaded current of the speed control at 38ma at 12 volts or just 4.56 watts. This means you can wire the unit in just before your trim switch and have it powered anytime the battery is on with only a 5 watt load.

If you want to have it completely off until trim is commanded, you will need to replace your trim switch with a triple pole double throw type switch or add a double pole double throw relay.

Revised: If anyone needs a schematic of how to wire it up, go to post number 3 for a jpg.

The controller is from QKits.com Price is $19.95 They are located in Canada and will ship to the USA for $10.

http://store.qkits.com/moreinfo.cfm/MX033

The only negative I find is the heat sink is not firmly mounted to the PC board. Prior to installation, I recommend potting the heat sink to the PC board with some epoxy to prevent vibration from shaking it and the MOSFET off the board.

Once you find a good trim speed, I doubt you will need to change the setting. The unit can be located anywhere. Wiring is simple spade connectors.

I usually get questions on motor speed control asking why I don’t just put in a resistor to slow the motor. Well, you can, but the voltage drops, the amps drop, the torque is lower. You might get less than 30% of the torque. A PWM controller will deliver full voltage, full current, and give near full torque at reduced speeds.

John Pitkin

[MotReklaw]

I would like to nominate John's post as tip of the month.
I've been wondering about this for quite a while.

Gonna order it next week.

Thanks John!

[jtpitkin06]

I'm getting many requests for the trim speed control schematic. The private messages do not allow attachments so I'll post it here. If you can't make it out. Send an email to me directly, not via forum private message and I'll send you the file.

john@cedarcreekrun.com

[chefwarthog]

John I have put that one in my Favorite, and will install that little gizmo on my panel.

And for the wireing, you drawing is very clear.

[Dorsal]

For what it is worth I timed my trim system today and it is ~9-10 seconds full span with a ~2.7 ohm resistor. Given the mechanical advantage of the jack screw the system is not even close to stalling. What I do like about John's approach is that you could make it in flight adjustable or two state, one for cruise and one for take-off and landing. Other than that a single resistor in series does the trick for me.

[Esser]

Glad I ran into this tip. It makes me ask the question though, does anyone run an trim switch interrupt on their plane? Maybe I am over thinking this but in my mind I can think of all the terrible things that would happen if the trim switch ever screwed up and did not stop turning the trim motor.

[Dorsal]

In my experience you can run the trim motor to its stops without breaking anything and you can fly and land the plane at any trim setting (not convenient but doable).

[HighWing]

One thought. I wonder if the (loose) heat sink is really necessary. I suspect the heat sink allows a continuous rather than intermittant use of the controller. I wonder if a fifteen second run (stop to stop) with mostly momentary inputs would generate enough heat to be a concern. I had a commercial trim motor controller that was about the size of two nine volt batteries. I had to think of why one was so robust and the other so small. The intermittant trim use of the little and the continuous design of the big one one was the only thing I could come up with.
Lowell

[Tom Waid]

When I took a demo flight with John McBean I asked the same question. He trimmed all the way nose up and let me fly it for a while. He then let me fly with the trim all the way nose down. The plane was completely manageable in both conditions. (Not very comfortable though.)

[jtpitkin06]

A runaway trim with a shorted switch or wire is probably going to be a momentary condition. As soon as the trim starts to run away, you will usually trim in the opposite direction. That will short circuit the speed controller and pop the breaker.

A trim cutout switch is a nice feature, or you could simply pull the circuit breaker. Iprefer the simple approach.

The most likely scenario is a trim switch or trim motor fail where the trim doesn't run at all.

The aircraft is flyable with the trim run to the up or down limit. It's just going to be a pain if you have a distance to go. If the trim runs away nose up you can use the flaps to relax some pressure. If it runs nose down, you'll be hanging on for a while and landing without flaps.

JP

[jdmcbean]

OK... didn't want to let the cat out of the bag just yet as there is some more testing to do..

We have designed and are testing a switch that does several things.. mainly it is setup so that when the airspeed exceeds 80 the trim motor speed will automatically reduce to a much slower rate (thats what is being tested now) then when you return to speeds 80 and below it will operate at full speed. There are some other enhancements like time limited operation to reduce the possibility of a runaway trim.

Hopefully it will be available in the real near future.

[Dorsal]

Sounds cool, please keep us posted.

[Esser]

John, that sounds great. The fact that I know the plane is still flyable with the trim all the way up or down makes me feel better about not putting a interupt switch. Thanks for all the great answers