See if you can help me with this

[PatMc]

BEB, we don't know the brand of ESC, not all ESC's require that the Tx needs to be calibrated.
By changing out the motors in order to eliminate the ESC from the fault finding process David has already demonstrated an understanding of fault finding. I think he knows how to catch fish.
Why not just stick to the problem reported in the OP ?

The fact that the AXI motor doesn't run or respond to the ESC by "bleeping", whereas both other motors do points to an open circuit between the motor connections.

[PatMc]

Posted by Gordon Tarling on 29/08/2018 10:34:27:

Some good replies here, but my suspicions lie with the motor connectors which were fitted by your clubmate. It is possible that the motor wires have been shortened and the insulation not fully removed before soldering the connectors. David - is it possible for you to post a close up photo of the motor connectors? To check with your multimeter, set it to the ohms range, short the two meter probes together and ensure that gives a zero reading. Then, with the motor fully disconnected check the resistance between the pairs of wires. All three readings should be roughly equal. Report back here with your findings.

Edited By Gordon Tarling on 29/08/2018 10:35:10

Gordon, I agree with what you've posted except that I think "All three readings should be roughly equal." is misleading. The resistance between the connections will be in milliohms [The winding resistance of the AXI 2820/10 is .039 ohms] , only a specialised meter would give a reading that would show any difference.
To avoid possible confusion I think it;s worth saying that the average multimeter will effectively show a short circuit between each winding.

[Dave Hess]

The easiest way to check the motor is to put a voltmeter set to a low voltage AC scale between any pair of the motor wires, then flick the propeller round. You should see it generate something. Repeat for the other two pairs. If one connector or coil is open circuit, 2 pairs will not generate. If all three pairs don't generate, something serious is wrong, you have a faulty meter or you have the meter set incorrectly.

You need a special meter to measure the low resistance of the windings, and checking the continuity is also not conclusive because you could have a short somewhere.

[Nigel R]

DD

You really must get a wattmeter!

regarding the Axi, I would concur with the first port of call being to check connectivity between the motor connections. Possibly a bad solder joint or wire broken within its insulation.

[Dickw]

I would agree that checking winding continuity is the first thing to do, but it seems to me that the soldering of the connectors must be suspect at this stage - particularly if there is any chance the wires were shortened.

In that, case as well as checking the continuity via the connectors, I would remove some insulation and scratch off the enamel from each wire and check continuity on bare shiny copper.

Dick

[David Davis]

Posted by Nigel R on 29/08/2018 13:10:57:

DD

You really must get a wattmeter!

I've ordered one from Hobby King.

[MAD Dave]

My take on David’s question 1 (For what it’s worth!).

Continuous power is measured in Watts; Volts x Amps in the basic sense. For batteries, we of course have a time factor, hours, because they are limited in the total amount of power they can supply.

For example, a 4000mAh 3s battery (~11.1V) stores a nominal 44400mWh power charge; 44.4Wh. Similarly, a 2000 mAh, 6S battery (~22.2V) at will also hold 44400mWh.

All electric motors have a maximum continuous power rating in Watts before the inevitable occurs; should they be asked to deliver more!

If a 630kV motor uses the 3S battery above, the maximum rpm would be expected to be around 7,000.

Using an in-line Watt meter, one chooses a propeller that will cause the motor, at max throttle, to draw the maximum power as specified in the suppliers guide – or maybe a little less to be on the safe side!

If a 6S LiPo was then substituted, the motor would then try to turn the same prop at around 14,000rpm and probably emit a little smoke before it got there – which it didn’t of course!

With the same Watt meter arrangement, one would chose a smaller prop that would present the motor with a reduced load/rpm yet retain the same max power consumption at max throttle, 14,000rpm in this example.

More S’s, smaller props! Q.E.D!

One point alluded to earlier is to bear in mind is that for the same power delivered through the cables, a higher voltage reduces the current that flows.

The advantages of reduced current is that there will be reduced volts losses at connectors (due to their contact resistance – V=IR etc. more volts for the motor, but not a lot necessarily) and the ESC will not need to be so “Ampy”! Also the desired maximum discharge current rating of the battery will be reduced, saving funds for the next project and perhaps extending the life of your battery too!

In the examples above, minimum Amp spec’s for a 550W motor would be approximately 25Amp for a 6S battery and 50Amp for a 3S battery; on the safe side, perhaps 30amp and 60Amp respectively.

So, in order to choose how many S’s provide optimum performance, do we now start a debate/thread about propeller efficiency with respect to rpm, diameter, pitch, etc, etc…….?!

[Mike Blandford]

I disagree regarding to using the motor power rating. You should use the motor CURRENT rating as the limit. Regardless of the voltage, the heat generated in the motor is current squared times the winding resistance. Exceed the current rating and the motor will overheat.
Note also that many motors show a "headline" current limit, but closer inspection reveals this is limited in duration (often 10 or 60 seconds). Look for the continuous current rating.

Mike

[Peter Beeney]

As I remember there are answers to the smaller prop conundrum back in the depths of threads past, but maybe one reason is that it’s tied up with electrical principles and the way the motor works with respect to something called the motional or back emf. Also one other important principle is the fact that if you double the voltage applied to a given resistance you quadruple the power (heat) it has to dissipate. We are not actually doubling the voltage here but we are increasing it. Applying a lighter load, or smaller prop, allows the motor to spin faster thus generating a greater back emf to oppose the applied voltage and thus the amount of heat the motor has to dissipate.

This is all very simplistic but what it means is that if you still wanted to turn the larger prop at the higher revs then you may have to find a suitable (larger?) motor to do it.

Considering a fictitious motor for a moment with a kV of 1000 revs per volt, an internal resistance of 50 milliohms or 0.05 ohm and a supply battery of 10 volts; that gives us an unloaded speed of 10,000rpm. Some current has to be flowing to create the magnetic field to enable the motor to run and also overcome the motor’s inertia, this is sometimes given in the spec along the lines of: Unloaded current - 1 amp at 10 volts. (Supplied). My brackets. This equals just 5 watts of heat dissipated. If this were our motor, say, then the back emf generated would be 9.95 volts, allowing just 0.05V to be applied to the motor to obtain the necessary 1 amp. If we then prop it with a smallish prop the motor will slow slightly, the back emf will then also decrease proportionally, therefore the applied voltage will increase and more current will flow to increase the torque. Crucially though, the watts generated in heat dissipated by the motor are now also staring to rise. Hang a larger prop on and and the same thing happens over again but the heat is now increasing at an even greater rate.

Let us consider fitting a prop such that the back emf is reduced by one volt, now we have a rpm of 9,000. The prop has reduced the revs by 1,000. Now 1 volt would be applied to the motor and the current flowing will be 20 amps and the watts needing to be dissipated would be 20. A larger prop dropping the rpm to 8,000 will result in 2 volts being applied and 40 amps flowing; that’s a twofold increase, but the watts situation increases to 40; that’s a fourfold increase. So the heat output has already increased 4 times.

Now you are getting in front of me perhaps, continue to take this down to to a point were the motor is actually stationary then the full 10 volts would be applied, 200 amps would flow but the watts have now increased to 2,000. This is all heat so hence the possible desperate smoke signals for help…

If we now fictitiously add a cell to increase the the battery voltage to 15, then all the figures change. The unloaded rpm = 15,000. Now our same previous prop that reduced the back emf by 1 volt would not be able to do the same here, the total inertia that the motor has to overcome is greater so let’s say the revs reduce by 1,500. Now 1.5 volts are applied to the motor and the watts dissipated are up to 45. The larger prop requires an even more turning effort so maybe the revs are down by say 4000. Now for the same propeller the watts are up to 320. With the motor at a standstill the watts equal 4,500! That’s fifteen hundred watts greater than the ubiquitous 3kw mains heater!

This is all hypothetical of course, I’ve just made these figures up to nice round numbers, but it might be some sort of an answer to the question: Why is it that when describing electric motors that suppliers recommend using a smaller propeller with higher voltage packs than they do with lower voltage batteries?

This well might nicely lead us on to one other valid point as well. In theory we could go on applying a higher and higher voltage to a motor and it would just turn faster and faster. However, from the above it can be seen that eventually the effort to overcome the inertia of even the unloaded motor will be so great it would not be possible to fit any size of propeller at all. A rule of totally diminishing returns, perhaps… And this just might happen sooner rather than later, too…

PB

[PatMc]

As Mike Blandford has stated "Regardless of the voltage, the heat generated in the motor is current squared times the winding resistance."
Whatever voltage is applied if a prop that loads to within the motor's current limit is used the heat generated by the motor will also be within limit, so long as the manufacturer got the current limit correct in the first place.

Any motor will generate the same heat when running on 2x volts @ y amps as when running on x volts @ y amps but, ignoring propellor efficiency etc, the power will have doubled.

Edited By PatMc on 30/08/2018 00:31:49

[David Davis]

I carried out the resistance check yesterday afternoon. With the meter probes shorted, i.e, touching I got a reading of 0.6 ohms, with them not touching. 1.0 ohms.

Connecting the red cable to the white cable produced a reading of 1 ohm. I got the same reading with the other pairings, i.e, red and black, black and white. I then hit upon the idea of pushing the terminals into the output sockets of the multimeter. They were not a tight fit as they are 2mm plugs going into 3mm sockets but by pushing them over with my free hand I got readings of 0.5 ohms for the red and white combination and 0.4 ohms for the other two.

I repeated the exercise with my second brand new multimeter and got the same readings, i.e, 0.5 ohms for the red and white pair and 0.4 ohms for the other two pairs.

I don't know whether this takes us very much further but earlier this morning I was making up a test rig from a piece of plywood which was once a part of my flight box. I had planned to fit the Axi to the board together with other electric motors and to test them with a new speed controller and battery. However, once I'd bolted the motor to the board I found that the can, which was always somewhat "notchy" to turn, something I put down to the power of the magnets, was much harder to turn than it usually was, so wondered if we may have a mechanical problem rather than an electrical one. I started to dismantle the motor but found that I could only undo one of the grub screws. Whatever I did, including dressing the end of of an Allen key to ensure a better fit, just resulted in the tool turning round in the grub screw without moving it.

I have a lot to do over the next few days, including several hours instructing today, so will not have the time to devote to this motor. I have ordered another motor, a Protronik DM 2815-1100, for the Junior 60 and will fit that. If I never get the Axi to run, it's no great loss but I'm grateful for all of the help which you have given me in trying to get this motor to work.

I will of course inspect the soldering that my club colleage has carried out but I doubt that his workmanship is at fault. Anyone who can research the aircraft in which his grandfather was killed in the Great War, a Farman 11, draw up a plan in CAD copying the aircraft's original aerofoil, build the model using traditional and modern materials and then fly the thing, is capable of soldering three connectors to the motor. I'll concede that even experts make mistakes occasionally.

For information the Junior 60 uses a Gens Ace 2200 3S LiPo and a Black Mantis(?) 50 Amp speed controller.

 

Edited By David Davis on 30/08/2018 07:20:41

[Martin Harris - Moderator]

A mechanical problem shouldn't prevent the motor windings vibrating to give you the arming beeps so it's unlikely that that is the problem.

As I don't know the accuracy of your meter it isn't really possible to draw any meaningful conclusions but I would have expected to see less resistance. Were you actually reading 0.05 and 0.04 ohms?

It might also be worth checking that the windings aren't shorting to the motor frame - check each lead to an non-anodised part of the frame - there should be no circuit. The windings should also have a fairly bright coppery appearance...if someone has stalled it in the past and damaged the windings they may look dark brown or black.

[Engine Doctor]

Last night at our club meeting a  very similar  problem was raised. A club mate bought a cub at our swap meet . The previous owner had brought it to the field earlier this summer after testing motor etc at home . At the field the motor wouldn't run , no arming beep or anything but all servos worked fine . he took it home changed the speed controller and checked it out and hey presto the motor spun albeit without a prop as he was in his workshop . Move on a few months and Swap meet comes around owner decides that electric is not for him so sells it . New owner takes it to field after checking it out . Recommended battery, speed controller and prop for motor etc. Starts take off run and after a few yards a large puff of white smoke came out of the motor . On inspection all the laquer on the windings has bubbled up so suggests a short in the windings Had this happened loger into the flight then we would suspect that the motor was overloaded but this happened after a few seconds and all specs were correct ?

You say the peed controller etc works fine on other motors So suspect the Axi motor . If you can get it to run give it a thorough run on the ground as it could cost you a model . Personally I would dump it and fit a good motor 

Edited By Engine Doctor on 30/08/2018 09:17:45

[Dave Hess]

If a motor is rough to turn by hand or harder to turn than what you expect, there's a high chance that the cause is a short in the windings. Take any three phase motor and bridge two of the wires to see what I mean.

If your motor only exhibits rough running or is hard to turn when connected to the ESC, but not when disconnected, it's normally a sign that you've blown a mosfet in the ESC. I can only guess how many people feel the resistance in their motor and dismantle it to try and find out why it's so rough, only to be disappointed when they can't find anything.

As I said before, the easiest way to check a three-phase motor is to spin it by hand and measure whether it generates a small voltage in each pair of connectors.

[Dickw]

Posted by Dave Hess on 30/08/2018 09:16:40:....................................

As I said before, the easiest way to check a three-phase motor is to spin it by hand and measure whether it generates a small voltage in each pair of connectors.

I just checked continuity on a random working motor in my workshop with a normal multi-meter and get >1000 k-Ohms open circuit and 1.3 Ohms when connected. For the moment we must assume your 1.0 ohms is actually 1000 k-Ohms like mine. However, as others have said, any beeps are from the motor windings not the ESC, so if you get them with other motors and not the Axi then there is still a problem with the Axi other than mechanical.

As Dave suggests, try connecting two of your motor windings at a time to your multi-meter (AC setting) and spinning it (the motor – not the multi-meter ). You should definitely see a voltage output when spun if the motor windings are OK.

Just being pedantic for a moment for the sake of accuracy, these are electronically commutated DC motors not “three-phase motors” – but I am sure you knew that Dave.

Dick

[Mike Blandford]

Posted by David Davis on 30/08/2018 07:19:57:. . . I started to dismantle the motor but found that I could only undo one of the grub screws. Whatever I did, including dressing the end of of an Allen key to ensure a better fit, just resulted in the tool turning round in the grub screw without moving it. . . .

For now, you shouldn't need to undo the grub screws. At the "other" end of the motor you should see a circlip. Just remove this, then the whole "bell" and shaft should slide out. Because the magnets are very strong, you will need to pull quite hard. Be careful when replacing the bell and shaft, the magnets pull everything together very firmly, make sure fingers etc. cannot be trapped.

Mike

[Gordon Tarling]

David,

If you'd care to send me the suspect motor, I'd be happy to check it out for you and repair it if I can. No charge, save the postage each way.

[David Davis]

Thank you Gordon. Please pm me your address.

[David Davis]

I've just sent off a pm to you Gordon.

[Gordon Tarling]

Reply sent.

[Gordon Tarling]

David, just sent you a PM regarding your motor.

Gordon