"stiff" motors on a twin set up

[David perry 1]

My son has set up a twin motor model and finds one motor is stiff to turn, not jammed but almost sticky. Im sure this is due to back emf or something like that (it does not occurs when at least two wires are disconnected). It only affects one motor. At full chat both motors seem to be giving the same thrust but at partial throttle the "stiff" one lags.

As far as I know there is no brake selected.

It wouldnt bother me me if it werent for the different throttle curves this is causing

Anyone any ideas?

David

[Simon Chaddock]

David

It sounds to me like one of the speed controllers (ESC) has its 'brake' set. The braking is achieved by the ESC short circuiting the motor wires when the throttle is off. This make the motor stiff to turn and is an important feature if you are using a folding prop. It also explains why the stiffness disappears when 2 of the wires are disconnected.

The stiffness should also go when the battery is disconnected although it may take power a minute or so before the ESC capacitors become fully discharged and the brake feature is cancelled.

The brake is instantly disabled as soon as the throttle is opened so the motor should otherwise perform normally.

It would make a difference if the plane(?) is gliding power off as one prop will windmill and the other won't.

It is possible to switch off the brake using the transmitter but you will need to know the specific instructions for that type of ESC.

If you post the the details of the plane it is quite likely somebody will know..

Edited By Simon Chaddock on 03/04/2017 17:59:03

[MattyB]

Is it sticky at a specific position in it's rotation, or the same all the way round? If it's the former it could be a loose/thrown magnet...

[David perry 1]

Thanks guys. No, the thing hasnt thrown a magnet, its a constant stickiness and feels very much like back emf.

The motors are "sunny sky" chinese ones in a homebrew twin. They dont feel tight with no cells attached or until armed....thereafter the sticky syndrome come up. I think its a brake function even though my lad says its not.

D

[John Emms 1]

With the cables disconnected, a motor that has resistance to turning or "sticky" is a sign of burnt windings. The fact that the motor turns freely when disconnected suggests this is not a motor problem. You could change over the motors or controllers as a check.

Regards,

John

[Andy48]

I'd be worried about diced fingers. Hopefully no props attached whilst checking.

 

Edited By Andy48 on 03/04/2017 21:13:27

[Simon Chaddock]

David

The way to check if the brake is 'on' is to run up both motors to full speed (prop off) and then slam the throttle shut. A braked motor will stop almost instantly as the braking effect is proportional to the motor rpm.

Do you know the 'brand' of your speed controllers?

[David perry 1]

No,no props attached!! We arent exactly new to this game

Not sure on ESC brand, probably something from HobbyKing.

Ill let you know what I find

[David perry 1]

Problem solved...unbeknownst to me the lads had used two dissimilar ESCs! p[roblem solved

D