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Motor testing using a PSU

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I have run into problems trying to test brushed motors for free flight E30 and am hoping that somebody will be able to help my faltering understanding of matters electronic.

The pic shows a rudimentary thrust rig with a ball race slide reacted against a gramme scale for thrust. Motor speed is via a 3mw laser tachometer. In the past I have run this via battery power but splashed out on a cheap  5a variable voltage PSU to eliminate this variable. The test motor is a 12mm GWS brushed in a Feigao 3.5 :1 S2 gearbox. This should be pulling about 1.8amps at 3.5v and 5000 rev/min, but shows a miserable 0.32 amps at 1800rev/min. Note that the CV (Constant Voltage)LED is lit. I suspect that the PSU is current limiting and tried the following checks.

I have put a 1.5farad capacitor across the output…same low current, just takes a few seconds to stabilise.

I have changed the inductive load for a pure resistive one using a 3.4ohm 20W resistor and measured 2 amps at 6.8v…...as expected. The PSU is capable of delivering but does not like my motor load.

Any idea how I can resolve this issue or does it serve me right for buying a cheap Ebay PSU?

 

thrustrig.jpg

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Jef Ott

I have just had an idea that may work Ian.

Discharge a 2-Cell LiPo to the voltage that you want to run the motor at, then set the PSU for that value, and connect them all in parallel so that the PSU replaces the voltage lost in the operation of the motor, and the battery smoothes the voltage to the motor.

Very best regards,

Jef

 

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martynk

It may be that the surge current (an inductance is effectively a short circuit until the back EMF increases) may be causing the PSU to fold back (short circuit detection. Try putting the 3.4 ohm resistor in series with the motor and see if it starts spinning - if it does then simply link out/short out  the resistor and then see what happens.

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Guest

Jef, I have about 30 motors to test to sort out the better ones. The differences will be 2-3%. Different battery states will mask these small effects and I will have to condition the battery for every test.......a good idea but too much of a faff unless all else fails. How is your SK3 doing?

Martyn, Thanks for you post. I have already done this, but the time constant of the cct is about 5s (which I forgot) so in whanging the voltage knob about with abandon saw 5.0 out climb to 9v and then the 5v capacitor cooked. I think that it worked as the motor was screaming and I saw 3amps before the smoke fairy intervened and the current sagged off to an amp or so.

Have ordered 4 off 2.7v capacitors which I will series connect (10.8vcapable?) across the PSU with the 3R4 in series with the motor. I will be more careful to let the voltage stabilise and am confident that  yr explanation is on the ball . Apologies for my electronics ignorance....mech engr by trade.

Regards Ian

 

 

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martynk

Just be careful. The surge current can be quite high if you are putting the caps across the output terminals, You may want to current limit it with another 3.4 ohn resistor in series with the capacitors. And ideally, if you are connecting capacitors in series - remember that the the total C value will be 1/4 of the common value (for 4 caps if they are all the same), you should also put resistors across the capacitor terminals as well to allow the current to balance properly - I would say about 10k ohm each.

If its not clear I'll draw you a little circuit. 

Martyn

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Jef Ott
7 hours ago, boffin said:

Jef, I have about 30 motors to test to sort out the better ones. The differences will be 2-3%. Different battery states will mask these small effects and I will have to condition the battery for every test.......a good idea but too much of a faff unless all else fails. How is your SK3 doing?

Martyn, Thanks for you post. I have already done this, but the time constant of the cct is about 5s (which I forgot) so in whanging the voltage knob about with abandon saw 5.0 out climb to 9v and then the 5v capacitor cooked. I think that it worked as the motor was screaming and I saw 3amps before the smoke fairy intervened and the current sagged off to an amp or so.

Have ordered 4 off 2.7v capacitors which I will series connect (10.8vcapable?) across the PSU with the 3R4 in series with the motor. I will be more careful to let the voltage stabilise and am confident that  yr explanation is on the ball . Apologies for my electronics ignorance....mech engr by trade.

Regards Ian

 

 

If you leave the battery connected to the power supply, and the supply set at the same voltage, then there won't be any voltage variations, will there? Got to be worth a try with digital Multimeter monitoring variations.

SK3 is holding up well. 272m in 30 seconds dragging a 2.8kg, 3m twenty year old design glider. Had to go to 4S1P to achieve it, 58A max is too constricting at 3S1P, Will be attending my first eF3B session on 11th Nov. Can easily reduce the power back to 700W, if the CD deems a lower launch height or longer time limit are ok. 

Very best regards,

Jef

271m That is enough.jpg

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Guest

Martyn,

Methinks that this is going beyond my comfort zone and a sketch might well save me from making more smoke.

Much appreciated , Ian

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martynk

Here you go. If unsure please ask.

Good luck. However, I like Jef's solution better

 

Scand00136.jpg

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Guest

Martyn,

Thanks for this...will try this  tomorrow...on lunch duty today and flying dlg this afternoon.

Regards Ian

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Guest

Jef,  Thanks for the battery idea, I did not go with this as I wanted to test at various voltage inputs so I could calc kv. So with Io and R , I could model the performance on paper using equations I found in a FF quarterly and attempt to see what the motor might be capable of with mods. I appreciate that this is very nerdy and OTT for a power unit of a mere 7Watts but the winter is on us and If I cannot fly in the rain and fog I may as well learn as much as I can.

Martyn, Thanks for your wise words. Your ideas worked and I started testing motors in earnest this afternoon.

Best regards Ian

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Jef Ott

Ian,

Glad that Martyn's expertise in this area gave you the answer that you were looking for. :thumbsup:

From our previous conversations, I remembered that you are ultimately opposed to connecting the mains (via anything) to a non-Lead-Acid-type battery, so I knew that you would not go with the battery smoothed supply, if there was another way. I held off posting my idea for a while in the hope that someone would come to your rescue.

With such knowledge within our forumite base, the BARCS site is a very rich environment to pose all of our questions.

Best regards,

Jef
 

 

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