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[Doca]

I have a problem with starting speed of a brushed motor. Motor is Mabuchi RS550SH and when I slowly raise throttle lever on Tx, motor starts buzzing and it draw 0,5-0,6Amp and continue drawing more and more as I push throttle lever toward middle of the range. Somewhere about that middle position motor suddenly starts turning and just before it starts it draw over 1,2-1,3Amp. When it starts it is turning quite fast… it is not starting slowly and then gain speed as I push lever further, but starts abruptly with medium RPM. Once it starts I can slow it down nicely with puling back throttle lever on Tx. What is the problem (if any… i.e. maybe that is normal behavior?) and how to achieve nice slow start with low RPM ?

[Tug-Kenny RIP]

This is not normal behaviour.  Have you tried it by itself on the bench ? It sounds as if there is an obstruction somewhere. Does the shaft spin freely ? Is there any drag when turning it by hand ?  Is the prop shaft catching ?

Let us know, as the 550 is supposed to be a smooth motor with many brushes for steady power.

ken

[Martin (Admin)]

Also, what speed controller are you using?

[andrewh]

Hi, Doca

What you describe is completely normal behaviour for any motor and ESC - But normally no-one sees it in the detail you have described it

First: you are using a 3-pole motor - so it only rotates in three "lumps"
Second: Mabuchi RS550SH only tells us the maker, poles number and core length (55mm).  It says NOTHING about the wind or magnets so it could be wound for 1.2V for a power drill or 96V for industrial equipment
Third it sounds as if you have a fairly low frequency ESC

The resistance of a non-rotating motor is just the resistance of the winding - and this is usually very small, so the initial current is very high (up to 10 times the running current)
So you have (eloquently) described what happens when you try to pass a high current into a stationary Dc motor with the current switched on and off at the "growl" frequency

Suggest you:
  Find out what motor you have
Run it direct on the battery and estimate the speed - look it up in the tables
And/or use a 5-pole motor (mabuchi RS555) (or lots more poles!) - this will ALWAYS help the starting since it goes round in 5 nibbles (or more) instead of 3
Use a higher frequency speed control
Disconnect the prop shaft and try it running light - if it still growls at you it is an unhappy motor - possibly with bent shaft or wound for very low volts
andrew
 

[Doca]

I designed and built my ESC based on ZN409 IC and it is working perfectly. I am relatively familiar with electronics and that shouldn’t be a problem, but motors I forgot. Model building and RC is my OLD love from yesteryears, but I haven't touch anything in this area for almost 40 years!

All tests I mentioned in my first post were done on the bench, no propshaft with prop, no any load imposed to the motor. Motor is new and designed for 6V (engraved on the case). I can feel some mild resistance when I try to turn it by fingers (free motor, detached from wires and electronics) like I am fighting against several small magnets around the circle. That is not related to bearings or damage of any sort I would say, motor is new! I can’t see any obstruction. I also tried to wire it directly on DC PSU with continuously variable output voltage, and it behave more or less equal as on electronic controller. 

“… 550 is supposed to be a smooth motor with many brushes for steady power.”
No, there are only two graphite brushes and three teeth-poles inside with winding on them as well as three contact copper plates on collector.

[Doca]

But normally no-one sees it in the detail you have described it

Yes, I know I am a weird one! 

[andrewh]

Doca

Yes, I'm old enough to also use the 409 - it works
But I believe its effectively a "frame-rate" controller so its about 650hz rate.  Most ESCs use a much higher rate to smooth the power delivery, and this would help the starting, I would think

Yes the 550 is 3-pole, and has 2 brushes
What you are feeling as you turn it is magnetic "cogging" and the fact that you feel it (and can clearly describe it) means that the motor has good magnets - this is not what we meant by being stiff or damaged

the 6V rating is interesting, and rules out some of the 2 turns of fencewire winds but does not tell us WHICH 6V 3-pole mab 550 it is

As you say, since its new it is unlikely to be bent or deranged
It IS normal for these motors to be delivered with the brushes only approximately bedded onto the commutator - the normal procedure for flyers was/is to run the motor on 1.5V U2 battery (the big one) iuntil the battery was dead.  This settles the brushes without overheat.

What do you want to do with the motor?  Does the behaviour you are descrbing make it impossible or undesirable?

andrew

[treeboa]

i always thought the 6volt was the 545 and five pole ??

[John W E]

hi there

I noticed you are using the circuit from a book by Ken Ginn - its an 'older book' and you have missed the relay and reversing side of it off.   If you are still using the FETS BUZ11 to drive the motor - I think these are not up to it much, unless you have the modern day equivalents.

aye
john

[CF-FZG]

It IS normal for these motors to be delivered with the brushes only approximately bedded onto the commutator - the normal procedure for flyers was/is to run the motor on 1.5V U2 battery (the big one) iuntil the battery was dead.  This settles the brushes without overheat.

You forgot the 'dunk it in a glass of water first'  one 'C' cell does two motors this way too

(don't forget to 'run it dry' afterwards though)


Mark.

[tr7v8]

hi there

I noticed you are using the circuit from a book by Ken Ginn - its an 'older book' and you have missed the relay and reversing side of it off.   If you are still using the FETS BUZ11 to drive the motor - I think these are not up to it much, unless you have the modern day equivalents.

aye
john

According to this data sheet the BUZ11A is 26A forward current so 4 should be more than up to it.
http://www.datasheetcatalog.com/datasheets_pdf/B/U/Z/1/BUZ11A.shtml
The relay is probably to deliver reverse only

[Doca]

Well, you are an excellent bunch of guys here willing to help it seems! Thanks for your effort and time up to now anyway.

Yes, controller is from Ken Ginn’s book and I believe four BUZ11's are more than up to a job with one RS550 never to be pushed too much?! They are on a smallish heatsink and together capable of about 30- 40Amps I suppose. I am building a bait-boat for carp fishing because that’s my favorite pastime for more than 40 years. There are ready made bait-boats from various manufacturers but “that is not that”… they are not good enough if I didn’t built them! I don’t need relay and reverse polarity for motor, hence, all circuitry related to reversing is omitted. Even if it works as it is now, it’s still quite usable but it will be MUCH better to have very slow speed as well. Whole vessel is not supposed to run particularly fast – if it reaches 100m in 2 minutes that is more than OK. RS550 is about the best (and only) I was able to source here and I can only dream about 5 pole types. That is probably much easier to find in your part of the world! It took me almost 6 months to find this one! I actually sourced two such motors, one new-unused and one damaged beyond repair. I opened that faulty one and count 22 turns of 0,7-0,8mm enameled wire on each pole. There is engraved - impressed (not printed) on the case “6V  RS550SH”.

If I understand properly, my only option is to try to change "frame-rate" of the controller?

[treeboa]

that controller looks very similar to the one used on the dynacraft Mk2

[John W E]

hi there, if you have a look at the pic you will see that the first output transistor is charred and burnt.  This is one of the set of three BUZ11s that are supposed to handle 20anps (as they say in theory what is supposed to work dos not in practises ) by the way the speed controller was driving a 540 in a model boat 50inches long with a 30mm prop and is the same one as the circuit  diagram further up the page only this one has the reversing relay in it simples.

 
Aye
John e
bluebird
 

[malcolmfrary]

A 409 works at the frame rate generated by the transmitter, so no real possibility of a change there.  Moving to a higher frequency will just cause the motor to whine rather than growl.  Careful tweaking of the pots will change the response, but since the output pulses will always start off by being too short to move the motor armature round to the next pole, the motor will always growl, buzz or whine until the pulses are long enough to move it on.  When that happens, very likely there will be some inertia built up, and the next pulse will have an easier time kicking the armature round the next bit, so the motor will set off enthusiastically.
At low frequencies, the armature might settle back to its start position at the end of a short pulse, so it has to start all over again with the next pulse.  At high frequencies, the armature might not have time to settle back, and so gets a head start with the next pulse.
Bedding the motor in will help, but the motor will always growl a bit when the power setting is not high enough to ensure rotation.
Magnetic cogging is a good thing in most motors - holding the motor by the shaft and rotating the case, counting the resistance pulses until a full rotation, then dividing by 2, is the best way to get the number of poles on a sealed motor.  All the pulses should feel the same in a healthy motor.
Although a power transistor might have a rating of many amps according to the datasheet, careful reading usually reveals that that figure requires a very considerable heatsink, just the tab or a small sink requires considerable de-rating.

[Doca]

Well, ESC and motor are working acceptably and it seems there is no other reasonable way to improve that. I can’t find 5 pole motor here in my country, and it seems no modifications of ESC will give any improvement. Thanks to everyone for your help!

[More Coffee]

there was a lady here that rewound an armature to give the motor a little more UMPH..

I dont know but maybe rewinding the armature with a finer wire and more turms may eliminate some of the issues you have.

Just throwing it out there .

[wombat]

Hi Doca,

Just had a look at the spec for the BUZ11A = I think the problem is that the supply voltage of 6V is really too low for this device - it doesn't really switch on properly until the gate voltage gets above 6V. I don't think that the output transistors are turning on hard enough and so the resistance makes it difficult for the motor to start. This is why it just grunts until you get to half throttle. It would also explain the burn up of the device - the channel resistance is just too high and so the traansistor dissipates a lot of heat and goes inot meltdown.

Wom

[Doca]

Well, tonight I tried something else. I found another motor “MAXON DC motor” and it is way different from RS-550. I open it and I never saw such motor before. RS-550 is 3 pole motor with 3 soldering lugs for ends of windings and consequently 3 copper “contacts” on commutator. This MAXON has 13 soldering lugs and 13 “contacts” on commutator. It has two rectangular graphite bars about 10-12mm long and with some spring to push them and those are brushes. It looks that brushes lays on two adjacent contacts at any time.
With this motor running on 12V DC it is possible to start it with very low RPM. I can easily see every turn. There are also two curiosities (at least for me). Without any resistance to oppose turning, it draws about 70-80mA in a beginning and going to almost 100mA at full speed! If I try to slow it down by squeezing shaft with my fingers, no matter how hard I squeeze there is no way to stop it when voltage on the motor is about 6V and higher. When voltage on a motor is 12V it starts to burn my skin on fingers when I press hard but no way to stop it. Even with highest finger pressure it never draw more than about 400mA!!! Motor is almost noiseless even at full speed.
RS-550 and similar types of motors have 3 poles i.e. 3 “fingers” on a rotor around which the wire is wound. This one doesn’t have ANY “finger”… it is like some kind of cylinder OVER which is wire but lengthwise wounded – not around circular part.
What a hell is this???