Is flying at partial throttle with a sensorless brushless motor bad? - RC Groups
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Dec 11, 2001, 04:36 PM
Mitch G's Avatar

Is flying at partial throttle with a sensorless brushless motor bad?

A rather accomplished electric flyer told me today that he had heard from one of his electric flyer buddies that flying a sensorless motor at partial throttle ends up heating up the motor too much and is not a good thing.

Since I had never heard such a thing, and it just didn't seem to jibe with what I thought I had read here on the Ezone, I figured I would ask.

So, is it bad?

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Dec 11, 2001, 05:52 PM
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Keithw's Avatar
I don't believe it is bad, but it does generate more heat in the controller, & probably also in the motor, so make sure you have good cooling.

At full throttle the controller Fets are basically turned hard on, a state at which they have low resistance, & therefore generate little heat. (altho they do actually have to be turned on & off in sequence in order to change the magnetic polarity to make the motor run !)
At lower throttle settings, they are being turned on & off for shorter periods,& it is the switching that generates more heat.

As far as the motor goes, at full throttle, the long On periods & therefore fewer On/Off switchings cause less inductive (Back EMF) oscillating currents than a motor at part throttle that will be seeing lots more On/OFF switchings, & therefore more inductive currents.

these things have all been taken into account in the design of the motor & controller, so all you have to do is make sure you have good cooling.
Dec 11, 2001, 06:21 PM
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Gerald's Avatar
I've never heard of this before. but I suppose it is somehow possible. It would entirely depend on the controller. The only reason a sensorless motor would run hotter than a sensored one would be due to differences in the timing. It is the controller that establishes the timing for a sensorless motor whereas the sensored motor's timing is fixed through all rpm ranges by the sensors themselves.

The sensorless controllers needs to be able to 'deduce' the rotation direction and rotor position of the motor based on back EMF generated by the motor. It then sets the timing for changing the phase currents accordingly. Incorrect timing could certainly cause unnecessary heating of the motor, however for this to be due to part throttle operation it would mean that the timing was different for part throttle versus full throttle.

Usually part throttle operation lets the motor run COOLER but the ESC can run hotter due to additional switching losses. This is true whether sensored or not.

My own experience with a sensorless motor is limited to a Jeti 15/4 which seems to run perfectly well over the entire speed envelope. The only gripe I have is that the minimum 'idle' RPM is higher than I'd like. My Aveox sensored motors and controllers have a much better low idle speed.
Dec 11, 2001, 09:32 PM
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gkamysz's Avatar
This may have come from some of the thing being said about the Jeti SL ESC's which have a fixed timing advance. When comparing a given motor to many other ESC's the Jeti's tend to pull more current and be less efficient, particularly at part throttle. I have not heard of many failure so I don't think it's too much of an issue.

Dec 11, 2001, 10:49 PM
Registered User
Andy W's Avatar
Exactly. The timing on the Jeti is fixed and quite far advanced, and as such, it's less efficient at partial throttle than other models. Were talking percentages here, however - it works just fine, and it does not damage the motor or controller unless you load them up so that it is exceeding specs.
The FET's in Brushless controllers are NOT on all the time at full throttle like brushed motor controllers are. They are switched at all throttle settings.
Dec 12, 2001, 01:02 AM
small electrics r BIG FUN
I've had my brushless and sensorless Kontronik 400/22 for about a year now. I would guess about 150 flights or so. Most of my flying is part throttle and motor or controller barely get warm even at putt-putt settings. I have noticed that things do get warm at full throttle.
Dec 12, 2001, 03:15 PM
Registered User
BEC's Avatar
Sensorless has nothing to do with it. The only case part throttle is really an issue with the MOTOR is when using a frame-rate controller on a brushed motor or a standard high rate controller on the little coreless mtors used in some micro flyers. In these cases the motor does heat up more because the controller is switching slowly enough that the motor is constantly accelerating and decelerating.

Part throttle, above caveat aside, is a controller heat issue. It seems to be worst at about 70% duty cycle. And as Andy noted (sorry, Keith) the MOSFETs in a brushless controller (sensors or not) are being swithced all the time, even at full throttle, as part of the commutation function. (In a conventional motor the brushes do this switching.)

Some controllers are better at handling part throttle than others. MOST are just fine part throttle settings as long as the full throttle current is at or below the unit's rated maximums (I devoted a portion of one of my Ezone columns to this). Some brushless controllers a few years ago cautioned you not to run at extended part throttle settings, and schulze, in particular, have special versions with heat sinks added (the -Ko series) especially to cope with extended part throttle running.

But, to cut to the bottom line - prop so the full throttle current is within the manufacturer's continuous current rating and give the controller some airflow (even just lots of space around it helps) and all will be well.

It can't hurt to make sure the motor has a little airflow over it as well .