Thanks Kloner......
BTW - I just came across this item and thought I would share on MRF..... cut and pasted it from a pdf found some time ago on the Internet and made a point of keeping it safely stored away..
Dry Testing" brushless motors (Update Sept 10 2008)
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Some of us may have run across a brushless motor that has questionable operation, perhaps after overheating it, or after crash damage. It may even have come from a swap shop. The question is "Do you want to connect it to an ESC" and risk possible damage to the ESC? (I'm a retired Service Tech with 45 years experience in troubleshooting, and repairing electronic controls for high power 38,000 volt circuit breakers.)
Electrically, these motors are not really complex, and can quickly be checked out for problems. All that is required is a variable speed battery operated drill, and a AC volt meter or digital multimeter. These brushless motors are three phase motors, or perhaps a synchronous three phase motor with a variable speed electronic drive, the ESC units we are familiar with. In fact, with a four channel Oscilloscope, and three resistors (star) wired in to provide a neutral connection for the scope, you can look at a three phase voltage waveshape signal from any of these brushless motors.
For what it's worth, this is how I've checked out various motors and automotive alternators that I've wound up or rewound. (Ten years ago, I rewound an automotive alternator to put out 70 Volts at 5 Amps. It was used along with a 3.5 Hp Briggs and Stratton engine as my design peak charger for the 38 Nicad Cells used in a 1/4 scale Piper Cub model airplane. This model was powered by an Astroflight 90 geared motor with a home made gearbox.)
First:
Disconnect the motor (ISOLATE ALL THREE WIRES FROM EACH OTHER!) from the ESC, chuck it into the drill, and slowly wind it up while holding the stationary part with the three lead wires. The rotation should be very smooth, with no rattling, virtually no noise. Also, their should be very little drag. Run the motor with the drill at full speed (1000 RPM or higher if possible) for a minute or two. Nothing inside the motor should get warm or hot.
If so, you've got a shorted winding or two, you've got a real problem. Or, a magnet has come loose, dragging against the stator inside the motor. Also note if the "bell" of the motor wobbles while running the motor. If so, it can result in the magnets hitting the stator. (The Hacker 40, 50 and 60 series outrunner motors have a third ball bearing to prevent this from occurring)
Second:
Keep the three motor wires isolated from each other, and label them #1, #2 and #3. Connect your voltmeter to an AC range, and connect to terminals #1 and #2. Wind up your drill to full speed, and write down the AC voltage. Then repeat this test on terminals #2 and #3. And again on terminals #3 and #1. All three voltage readings should be near identical, providing your drill is running at the same speed on all tests.
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Third:
Short circuit all three wires together directly at the ends of the motor windings. Then SLOWLY wind up the variable speed drill while holding the stationary part. Be careful, you will have considerable drag where the motor will try to twist out of your hands. This drag will be very even, with little variation, and no "Chugging". As a comparison, only short two wires, and try again. This test will be very obviously much less smooth than when all three wires are shorted. Don't run the motor more than 5 or 10 seconds, as the windings will get warm doing this. (Don't use jumper wires, we are generating 20-30 Amperes or more of current at very low voltage, jumper wires won't work.)
Four: 06/03/08
Ron van Sommeren suggests checking for short circuits between the windings and the stator. Good idea. Since the windings are all connected to each other, check for any connection between the motor stator laminations and the windings. Look for bare metal somewhere on the stator for one connection to the meter, and the meter windings to the other. If no steel bare spots are available, you will need a needle point on one of the meter probes to punch through the coating of the steel laminations. The meter should show an open circuit, or infinite reading. Again, any connection is bad news, and is cause to scrap the motor. (Or if you've wound the motor your self, unwind, and do it again, but more carefully.)
Five: 06/05/08
Coro reports that it is a good idea after the motor passes the above tests, to check the motors "no load current", and compare it with the manufacturers specifications. This also will potentially identify "shorted turns" in the motor. FYI, a shorted turn is a single winding, (or more than one single winding) that "has shorted out". What this does, is that shorted turn becomes a very high current generator when the motor is running at full speed. Since the winding is shorted, the motor develops short circuit current through that shorted turn winding. This results in very high temperatures of the shorted turn, and will lead to serious motor failure. This would place your ESC at risk.
Only way to repair a shorted turn, is to replace, or rewind the motor. One possible cause of "shorted turns" is running the motor beyond its maximum current rating, overheating the motor to the point where the varnish insulation on the motor windings is damaged.
I've worked with "Double Formvar" magnet wire before I retired. This stuff is rather impressive. It will pass a high voltage hipot test directly on the surface of the varnish and the wire directly underneath of 12,000 VOLTS. This double formvar wire is what I used to wind my go-brushless motors.
6/5/08
Brian Click Wrote:
Another tip (I'd add it to the WF forum but haven't signed up there yet) is to use a DMM and terminating resistor (or small lamp) connected across a winding phase to measure the generated current...
Spin up the motor using a cordless drill and measure the voltage (AC of course), and note the reading. Then do the same for the other two phase connections, and compare readings.
All 3 measurements should be the same and if they're not - you have a shorted turn or missing / extra turn. Or your drill battery is dying between runs
You should also touch one wire to the motor's frame (to check for shorts to the pole piece) while doing each phase test; there should be NO change in the reading.
If you have an oscilloscope, you can use that as well, to look for distortion and/or amplitude variations in the waveform. Distortion indicates a shorted winding.
(Kyleservicetech: Note, the wave form I've observed is not a really pure sine wave. Depending on the motor brand and so on, you are likely to see the top of the sinewave a little flat, along with other minor changes. BUT, all three phases should show identical
TESTING BRUSHLESS MOTORS
waveshapes! If you have access to a four channel scope, get three resistors, about 270 to 1000 ohms each, connect all three together on one end. Take the other ends of the three resistors and connect it to the three motor wires. The common (wye) center of the resistors is your connection to the scope ground, connect the three channels to "phase A, phase B, phase "C" respectively. Then spin the motor! You will see a three phase voltage signal.)
Amplitude variations indicate turn imbalances between poles and can even reveal weak magnets in the rotor and/or spacing (magnet to stator clearance) variations or a (badly) out of round rotor, assuming turns are known to be correct.
Dennis / (Kyleservicetech)
I've got one Hacker A30, one Hacker A40, and two Hacker A50 motors plus a half dozen hand wound go-brushless motors. The Go Brushless motors were checked per above before connecting to an expensive ESC. Found a few problems such as shorted turns with this procedure that saved the ESC.
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And, lastly, before connecting the questionable motor to your ESC, you may want to put a temporary fast blow type instrument fuse in series with the battery and the ESC. Typical fuse size would be on the order of 10-15 Amps for an ESC of 40 Amp rating or higher, and 5-10 Amps for the back yard flier type.
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Be certain to slowly wind up the motor without a prop with the small in line temporary fuse during testing, since quickly winding up the motor can blow the fuse due to high starting currents.
06/05/08
Other responses in this thread indicate that current limited power supplys should be used carefully. Don't know if this is an issue or not. I suspect that anyone that has access to a current limited power supply like this would also have access to a good oscilloscope which can be used to monitor the DC voltage output of the power supply. The scope would be used to watch for any wild DC output voltage fluctuations while running the ESC and brushless motor.
For testing, I use my home made variable DC power supply that has a rating of 25 VDC at 25 Amps. Output capacitor on this supply is 0.2 FARADS! The capacitor is a special industrial type with very low internal impedance. (The power transformer and reactor on this supply also were hand wound!)
Note that only outrunner motors should be run with no prop, inrunners could shed a magnet with no prop, when turning at 20,000 RPM or more. (Read the motor manual on this!)
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