Electric motor temperature sensor

[60wattParticipant @60watt]

Barzo,I’ll repeat my conclusion that, if you stick with that motor,you must increase the prop until you have between 1/4 and 1/2 the stall current and then alter the voltage.

If iron loss and copper loss is so much smoke then I practiced the black arts when all day long I designed windings and cores.I can do this in my sleep.

 

If you remove a flux ring you do the opposite of adding a pair of Neodym magnets to boost the field of the motor magnets.It makes the no load speed increase,it raises the peak of the efficiency curve and shifts it to the left.It then suits a smaller prop.You could do that but it would only nudge you in the right direction.

 

You will have to ask Mark the source of his own data.I am using the Graupner data and I can make all their figures add up.As I said in my last post,the key points that make the graphs are obtained graphically.

 

First published data given was they were different motors and hint from Dave was not to run the Ebay vendor(Graupner ones) above 12 volts.Modelboatmayhem posts recently state they were exactly the same motors from the same source.That I believe.

 

[TelstarParticipant @telstar]

Wow

 

Iron loss, from the magnetic paths including air gaps etc. and copper loss from length and area of windings and commutator, are fixed in the design and construction of a motor (hopefully not a pipe dream) So having bought a motor there is nothing the modeller can do about it. So forget it.

I tend to think motor manufacturers/suppliers should give the correct parameters for motors they supply (I should know better the number of times it’s been proven otherwise).

But if you want to get the maximum power out of a permanent magnet model motor, in general you set the rpm band by selecting the working voltage, the select the load to about halve the no load rpm. Since this will run the motor in an inefficient manner you then have to manage the losses in the form of heat (water cooling??)

 

Some time ago ?Mark Beard? a forum member send me a program on which you can insert a motors parameters (either from data sheets, or measured by test) and the program produced analysis like the Mabuchi graphs. I tested these against practical tests (on a dynamometer) It is quite good.

 

 

Oh by the way DC motor formula if you want to work it out

 

T = 11.73 x (Z / paths) x x poles x IA x 10-10

where:

	T = torque in ft-lb
	Z = total number of conductors
	= flux per pole in lines
	I = current in amperes

 

Edited By Telstar on 07/10/2011 23:47:42

[60wattParticipant @60watt]

No I did not refer to Mark Beard.

I am sure your Mark will be able to get Graupner figures to tally up nicely even if he adopts crudest measures to account for things like temperature and magnetic properties.

He will be able to confirm the 900 that the efficiency at maximum power is a tinkle over 40% and not the KISS version of 50%.The reason down the very first point I made in this now tired and dog-eared thread….significant iron losses in this motor which is what Barzo is getting with his small props.

I also did tests of many d.c. motors and found the results to match the manufacturer data.That is why I implicitly trust results from MFA,Graupner etc.

Joe Public might use a DVM reading of the motor free running and with a locked rotor and think he has got Mabuchi’s Io and Istall. figure.That does not provide meaningful information and is why it is not used.

Telstar I designed cores and windings all day.I know what I am talking about even if you don’t.I don’t need it misinterpreted.

Edited By 60watt on 08/10/2011 01:31:07

[TelstarParticipant @telstar]

Hi 60watt

Telstar I designed cores and windings all day.I know what I am talking about even if you don’t.I don’t need it misinterpreted.

you really must get over yourself and don’t get personal

I think that a BSc Elect Eng , and a working life in engineering should allow me to comment on DC Electric motors.I don’t use cheap DVM (I do know what the abbreviation means) but I also try not to scare fellow modellers by “flashing my genius”

This thread (apologies to Barso for hijacking his thread) was a request for advise on cooling an errent motor, I passed comment initially on scoop position, and only commented later when ‘technical experts’ began clouding the issue. For the average modeller selecting the motor/prop setup can be a challenge I don’t think it helps when ‘experts’ vociferously voice their expertise.

One reason for ME mentioning Mark Beard’s program is that I found it useful and user freindly for simple analysis of motor performance. (not for the experts though)

The reason I refer to motor speed, not load current, to find the ‘sweet spot’ is that a lazer tachometer is much cheaper for the average modeller than one of my high range DC ammeters and it is easier to use at the pond side If you check you will find your statment

‘you must increase the prop until you have between 1/4 and 1/2 the stall current you may find these values ‘ these currents are quite close to the values of rpm I quoted for the same points (ie max effy. and max power out) My main bone of contention is the average modeller does not need to know the finer designe points of motors, but if some expert can come up with some form of simple test/mesurement to help find the best combination motor/prop, then I’m all for it

 

May your design be with you Tom

[BarzoParticipant @barzo]

Gentlemen,

 

I have found the various responses very helpful, if mainly en passant.

 

My first query was about cooling motors and to some extent I feel that I am still left with that problem.

 

However I do think that I have moved forward in my understanding of the way to optimize motor/prop combination and that discussion has been vibrant – no flame wars here please!

 

I will obtain a tachometer of some sort and beef up my ammeter to handle the bigger currents.

 

Maybe I will try at some time to parallel my batteries and see what happens with only 10volts, but first I must find out the prop size that will give the recommended amps at the recommended rpm’s.

 

Thanks again

 

Barzo

 

 

[Gareth JonesParticipant @garethjones79649]

Unfortunately I have never designed a motor core or a winding in my life – I am beginning to wonder what I have missed. I have tested a number of motor/ propeller combinations and looked at motor performance curves and I think the simplest and most useful tool to use is a lazer tacho, available for around £10 on the web. Don’t worry too much about amps and volts.

 

Measure your motor’s free running speed at full throttle with the propeller removed and the boat out of the water. I have found the best place to put the reflective sticker for the tacho is on the flexible coupling between the motor and prop shaft.

 

Next measure your motor speed in the fully loaded condition with the propeller fitted and the boat static in the water. If this loaded speed is between 65 and 75% of the unloaded speed your combination is pretty well spot on.

 

If the loaded speed is less than 50% of the free running speed your propeller is bigger than optimum and if you spend a lot of time at high power, your motor will get hot. Solution – fit a smaller diameter propeller.

 

If your loaded speed is greater than 90% of the free running speed your propeller is too small and you wont get much thrust. Solution – fit a bigger diameter propeller.

 

There are lots of other subtle factors you can consider like the type of boat, how you want to sail it and how much friction there is in the drive train, but in my experience its best to keep this analysis simple.

[TelstarParticipant @telstar]

Hi Eliquently put Mr Johns. I will endorse your method as a practical answer to find the correct prop sizes.. Keep It Simple .

 

Returning to Barzo’s original query. A number of modellers at my local club has the same overtemp problem (for whatever reason) The most popular solution is use of cooling coil very much like the one in Barzo’s pic. Some also use water pumps (scoops don’t work at low speed or when going astern). A popular pump is a car windscreen washer pump,(apparently resonably priced at Halfords)

I have borrowed one to play with (sorry evaluate)

On 12V (its rated voltage) it pulled 3.5A approx when pumping water, and moved a lot of water.

However on 6V it pulled 1.2A when pumping, and still had a good flow. Also it did not need to be primed to pump. Even with the pump some 50mm above the water level and dry the punp was able to suck water up (through a pipe obviously) and operate almost immediatly.

 

Hope this info helps

Cheers Tom

 

 

[Gareth JonesParticipant @garethjones79649]

Thank you for those few kind words Telstar.

 

As another option for Barzo, in my current build project, Shemarah II, I am planning to fit a small water pump, not to cool the motor, but to simulate the water flow you see from several points in the side of the hull. I guess the largest of these flows may be engine cooling water.

 

The pump I am planning to use has been robbed from my model aircraft flight box where it was used to transfer fuel to between a 5 litre bottle and the aircraft tanks. Judging by the number on the pump it is part number MFA 650 and although its many years old, it is still available from Sussex Model Centre for £10.99 (other sources probably exist) where it is advertised as a fuel/water pump.

 

I have tested it in the sink and at 12 volts it produces a very healthy flowrate and takes approximately 0.65 amps. It is a gear pump and self priming at least up to 6 inches above the water level (couldn’t get any higher as that’s the limit of my tube length). Its a bit noisy and the flow at 12 volts is greater than I need so I am planning to run it from the main 12 volt battery but via a 4.8 volt BEC power supply unit. At this voltage is quieter and still seems to give enough flow for my needs.

 

Regards

Gareth

[Dave MilbournParticipant @davemilbourn48782]

Gareth

At the risk of going off Alan’s original post for a minute, you may find one of these useful to regulate the speed of the pump motor (top unit on the page) http://www.mfacomodrills.com/speed_regulators/voltage_regulators.html.

Just set the required speed/flow with a screwdriver and leave it to get on with the job. Note the 3A continuous current limit, which is fine for the MFA pump but might be marginal on other types, especially if run at higher voltages.

And a little note to Mr 60 Watt –

If I had spent as much time agonizing over the theory and hydrodynamics of the Fairey hullform when I designed the Modav/Precedent Huntsman in 1972 then the kit would probably not have reached the shelves even by now………….. Get out a bit more, matey – it’s only a hobby.

And for Alan Barzo – why don’t you stick a 65mm prop on the damn thing and run it on 6 volts, if only to prove or disprove the point?My guess is that it will plane like a narrow boat.

Dave M

Edited By Dave Milbourn on 09/10/2011 17:57:06

[Gareth JonesParticipant @garethjones79649]

Dave,

 

Thanks for that. I think I will try one of those, it has the advantage of being adjustable which might be useful in the final installation.

 

Gareth

[Mark Beard 1Participant @markbeard1]

I seemed to have missed the excitement when this thread was current!

 

For what it’s worth, iron losses and temperature rises do have real effects on motor performance. But the calculations I’ve put out there have used the 80:20 rule, to get 80% of the way to a useful answer by applying 20% of the calculations.

 

60 Watt: My calcs were based on first principles, including T=KtI, E=KvN, Kt = (Kv in v/rad/sec) and Ohm’s Law. They match manufacturer’s published curves almost exactly. If you’re interested, PM me an email address and I’ll gladly send you my spreadsheet.

 

Barzo: IMO I think you’re cooking your motor and would do well to run water cooling 100% of the time.

 

Here are some suggested golden rules for motor-prop sizing, for those without a laser tachometer!:

 

1 – Use the recommended motor for the model you’re building, or if it’s a scratch-build then pick a similar hull which performs well and use a similar motor.

 

2 – Run the motor at, or below, the rated voltage, but NEVER above it, unless you’re racing and know what you’re doing.

 

3 – Run your motor at full throttle with the boat out ot the water, (no-load) and listen to the pitch of the motor sound. Now run the boat full throttle on the pond. If the motor pitch on the water is more than one octave below the no-load pitch then you’re running on the EVIL side of the torque curve, your motor is over-loaded, is overheating unnecessarilly and delivering less shaft power than it could. In this case, choose a smaller prop or one with a finer pitch. That smaller prop WILL make your model go faster, cook your motor less and make your battery last longer!

 

4 – For a non-planing scale hull, the best compromise between efficiency and performance is when the shaft is loaded so that on the water it’s running at 75% of the no-load speed, or three full-tones below in pitch. (If no-load the pitch is uper C with the throttle wide open, then on the pond the tone should be G below). If not, adjust the prop size or pitch to suit. This will give you around 90% of the maximum motor efficiency and the best battery life. If the performance is inadequate, change to a larger motor and repeat.

 

5 – For a high-performance planing hull, you need to accept short battery life and a cooking motor. Choose the best water cooling arrangement you can, as hot motors not only have short lives, but deliver less power. Select the prop to give a water pitch just less than an octave below the no-load pitch. (If no-load the pitch is uper C with the throttle wide open, then on the pond the tone should be middle D). If the performance is inadequate, change to a bigger motor as you have already got the most from the one you have.

 

I’d be delighted to hear how these rules apply to real models in practice. If your scale model runs with a motor pitch more than three tones below the no-load pitch, does your motor get hot?

[BarzoParticipant @barzo]

Mark,

 

Thanks for the contribution.

 

Little did I think that my ABRSM grade 3 music theory, would ever be conjoined with my model making!

 

Fortunately my prop selection does not now rely on my ‘ear’.

 

I purchased a laser tachometer.

 

The latest position is this:

 

I freshly charged the two batteries to 11 volts, no load.

 

The boat was floated in the bath.

 

The free running rpm recorded was 11091. With my 3 bladed 45mm brass prop., it recorded 6254rpm flat out.

 

I have run the boat with this configuration, in fact my second youtube video is just that.

 

I will be able to re-evaluate the configuration with an extended run, probably next weekend.

 

I will PM you for your spread sheet .

 

Cheers Alan

 

PS Glad to see you have got rid of that nom de plume.

 

Cheers Alan

[Mark Beard 1Participant @markbeard1]

Hey, you’ve got a 44% speed reduction in the water, (albeit static and not moving through the water), which is close to perfect for a performance hull. IMO, you’re now getting the most from the 900 motor so if it isn’t fast enough now then you need a higher power motor.

 

Best regards,

 

Billy!

[John wilson 18Participant @johnwilson18]

Have you ever been here to find a sensor?

[Fred wooleyParticipant @fredwooley76212]

Having tested MMB motor to destruction on the orders of Mark afew years ago and tested that very motor with a perkassa on various voltages form 12 up to 48 on Marks orders…it actually blew up on 48 after 45 mins of flat out playing…..Right I would highly recomend you ditch the 3 bladed prop and get a 2 bladed 55mm one you should get better performance let alone slow speed as well….The Motor will run a lot colder for you…..I used to get well over an hour flat out running on 24v and it barely got warm

[Malcolm FraryParticipant @malcolmfrary95515]

It is a very old thread, but some things are as true today as they were then – the maxim that if the motor is getting too hot you need a bigger motor is still true, as is the one about not having too much prop for the motor, either diameter or blades or both.

Motors in boats tend to have a harder life than if they were in cars or planes – cars tend to get intermittent use giving a bit of a cool down cycle, planes tend to get lots of air cooling.

[ashley needhamParticipant @ashleyneedham69188]

As above. A well matched boat/motor/prop should not get that hot. Very warm perhaps but not to the overheating stage.

Ashley

[Dave MilbournParticipant @davemilbourn48782]

Mr John Wilson 18

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