Revving brushless motors
 

I read somewhere that you can damage a brushless motor by revving it with no load. Truth or untruth?

Jimmy

Of course it's true. I miss my Velociti 5.5 :cry:

By revving it with no load, the rotor spins at crazy speeds and breaks. Fact.

Never heard that before:eh?:

It wont break it, but it will get hot a lot quicker;)

It could break, if the rotor spins faster than it's designed to do, it could very well shatter.

not being a leccy wizard i'm now confused i had heard rumours of motors dying when revved hard without load

but from a theory point of view how can it ?

if it has a certain amount of windings it must pull a certain amount of current and develop a certain RPM pro rata per volt applied

i can see if more voltage is applied than recommended then it would overspeed and cause damage but that isn't a motor failure

interested for a specialist to put us right pugs and imull please take centre stage :thumbsup:
fid

I know someone who blew up a new lrp x12 doing just that. He was running lots of boost though which wouldn't have helped

Torque is inversely proportional to speed, i.e. the lower the load the higher the speed. If you rev the motor too fast for too long it will get too hot and you will damage it.

It's nothing to do with heat it's purely mechanical - just as it was with modified brushed motors.

If you throttle up off load, there is significantly less current draw on the motor. However, it is able to spool up significantly faster also. With a "dynamic timing" ESC, the timing is generally increased as the rpm rises. Because the RPM rises off load quicker than it would on load, it adds more (and more) timing.

If there is no load on the motor, it is able to achieve significantly higher RPM than it would when loaded. The rotor can then mechanically explode - which creates a big mess!!

I wonder if somebody has done that and put it on You Tube yet? If not......

:woot:

Jimmy H

you dont see anything the rotor fails and the motor simply stops, Matt is correct as i did one!

It doesn't happen in stationary mains powered applications. I'd have thought it was quite possible in this instance because your esc can generate much higher Hz than domestic applications. Normally stationary AC 3 phase squirrel cage motor (which is basically what brushless motors are) do not runaway or overspeed, this is because there maximum operating speed is controlled by the 50hz supply frequency. With these esc's the Hz isn't constant and changes through acceleration, by increasing the Hz I imagine it is quite possible to spin the rotor past the point it can maintain its integrity. Usually overspeeding and motor 'runaway' is assosiated with dc shunt wound motors, this generally happens when the field windings fail which causes the rotor (in theory) to accelerate to infinity and overspeed. The other overspeeding issue is with dc series wound motors like starter motors in cars, these must have a load applied or the characteristics of the motor let it accelerate until self destruction. Luckily in rc nearly all brushed motors are parrallel wound so should not overspeed!

I've seen it happen 2 or 3 times. The result was a shattered rotor and the motor stopping instantly.

It's got nothing to do with your esc and a over speed from the motor, if you have a fast-ish motor around 7.5 or lower and you rev the motor to full throttle with no load your rotor can get damaged. The motor will be able to reach a higher RPM than normal.

I'm not saying that every motor will have the rotor fail but I wouldn't want to try it.