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.