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Author Topic: Testing an unknown motor  (Read 362 times)
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3view
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« on: December 23, 2018, 02:30:45 PM »

Hi

I've had this unknown motor kicking around in the spares box for years, the label had fallen off so I didn't have a clue what it was!  After testing, I've discovered that it is quite a good, usable motor that shall find a home in a FF scale model of about 42" span.

First off, instead of a prop, I fitted a piece of balsa to test the motor with no load.   No Load RPM is   8070  at   11.48V.

***Now this is the important part!***   You must load the motor so that the rpm drops to about 75% to 80% of the no load rpm.  Don't load the motor any further as efficiency falls and the motor will get hot.

In the case of this test motor it needs to rev at 6000 to 6500 rpm.  This is the sweet spot where the motor is producing a decent amount of power at a good efficiency.  Time to try a few props.

9x4.5  6750rpm   66 Watts   this prop is a bit small!
.
9x6     6420rpm      77 Watts   getting there.

11x4   5970rpm      94 Watts   motor getting warm but not hot.

Conclusion.......  This unknown motor is a 700 kV  100W motor that will suit my application perfectly using a 10x5 or 11x4 prop.

Hope this might be of interest to others out there.

Steve
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Testing an unknown motor
« Last Edit: December 23, 2018, 02:46:29 PM by 3view » Logged
Yak 52
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« Reply #1 on: December 23, 2018, 03:50:21 PM »

Thansk Steve, this is helpful.

***Now this is the important part!***   You must load the motor so that the rpm drops to about 75% to 80% of the no load rpm.  Don't load the motor any further as efficiency falls and the motor will get hot.

I have to test some brushed motors soon - does this rule hold good for them too?

Thanks
Jon
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alset
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« Reply #2 on: December 23, 2018, 04:07:21 PM »

My understanding is that if the motor is "bad" that it will ruin the ESC. If this is true, how can one confidently do a motor pre-check before hooking it up?
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« Reply #3 on: December 23, 2018, 04:35:38 PM »

Yak, if that rule of thumb method is the one you want to use, then yes, should hold true for brushed as well as brushless. Checking the motor temp is the main important part. There's also Motocalc's method of characterizing the motor which require's more work, plus Motocalc.

Alset, not sure what you mean by motor being "bad" but I've had several motors go bad after building flying the model, or get disconnected in one wire from vibration and yet not ruin the ESC. It all depends on luck and what the actual problem is.

A straight short is probably the most likely bad to cause an ESC burnout. A more common cause of wrecked ESC's (and motors) is somebody putting too big a prop on a motor for the ESC's current rating, or not enough air getting to the esc buried in a foamie.
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« Reply #4 on: December 23, 2018, 04:41:55 PM »

Thanks VT - that gives me a prop RPM to aim for.

Alset - it's worth looking for a short by checking continuity between the wires and the case with a multimeter.


Jon
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3view
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« Reply #5 on: December 23, 2018, 04:47:52 PM »

My understanding is that if the motor is "bad" that it will ruin the ESC. If this is true, how can one confidently do a motor pre-check before hooking it up?

Never tried it myself, but some spin the motor with an electric drill like a dynamo and measure the voltages between each of the three wires. They should be the same

Steve
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« Reply #6 on: December 23, 2018, 05:31:58 PM »

BTW probably best to also think of your motor not as a "100 watt" motor but as a motor capable of absorbing the commensurate amount of current.

It's only a 100 watt motor at your 11.5 volt test. It's better to think of it as 100W/11.5V in current rating, ie. about an 8-9 amp motor.

That's a reasonable max current that its windings will take, no matter what the voltage, or number of cells.

It may also be a 150 watt motor on 3 cells, or a 50 watt motor on 1 cell, more or less.

But it's a 9 amp motor (max) on any of them.
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« Reply #7 on: December 23, 2018, 08:24:16 PM »

Alset - it's worth looking for a short by checking continuity between the wires and the case with a multimeter.
Good place to start. 1) In my motor there is NO continuity between the lead wires and the case. 2) There IS continuity between the wires themselves (but there also is between the wires of my new working motor)

So that brings up the question: How does one check an ESC with a multi-meter? Presumably the main ESC failure event is trying to draw too many amps.
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« Reply #8 on: December 23, 2018, 10:32:56 PM »

Personally, I would check it against a known good motor within its current capability with a suitable prop.

It might be easier to help if you named the ESC, its current rating, and the motor and prop in question.
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