Factors effecting startup speed (brushed motor)

[05Fronty4x]

I normally run a basic Axial 55T motor in my SCX10 which offers so much low speed control that I run a lot of negative exponential just to take advantage of it. It also has a ridiculously low start up speed.

Today, for giggles, I took an old 12T modified Reedy motor with high strength megnets and stand up brushes (supposidly tuned for off-road use) and swapped in a 27T armature from an even older motor from my touring car racing days. I timed it to spin equall speeds in both forward and reverse (I'm guessing this is 0 degrees?).

The result was a motor that seems more punchy and have more pure ripping power than I ever remember a 27T having. I also had to turn the drag brake on my FXR down several notches because it was much stronger. The downside is that the startup speed is much higher. At first I simply chalked it up to the higher kv of a 27T motor, but the amount of throttle I have to give before it starts spinning is also significantly higher telling me there is more at play here.

I realize there is probably numerous factors that effect the startup speed of a motor, but if any experts out there can shed some light on some of the basics, that would be great. I'm interested in what different things I can try to get more creepability out of it weather it be spring tension, timing, magnet strength, lay-down brushes as opposed to stand-up, etc.

There's no real point to this, I just love to tinker and experiment.

Thanks.

[Olle P]

The lower wind motor provides less torque.
The torque required to get the rig rolling is the same indifferent of motor, and you don't mention any change in gearing. That's why you need more throttle to get it moving.

Once it begins to move (at low speed) the demand for torque drops, and then the low wind motor instantly can (and will) deliver more speed.

To get more "creepability" out of it you need to gear down, decreasing the required motor torque to get it going. With half the pinion size of what you use for the 55T motor you'd get roughly the same behaviour.

[05Fronty4x]

Thank you. I understand this. But I'm talking about motor startup speed which I understand to be the point at whitch it actually begins to spin assuming no load at all. It's not that it takes more throttle to get the 27T to move the rig, it's that the 27T actually requires more throttle before it even tries to spin.

I'm looking more into the design of the motor itself, and reducing the current needed to start the motor up, or an explanation as to what exactly are some aspects of motor design that effect this. Let's assume the motor has no load at all.

[Ryan87zuk]

You could try narrow dead band on the transmitter. This should get the motor spinning sooner because less pot voltage when the switch comes in. Maybe try a lower fixed timming on the motor. This should help low speed drivability. In the end though that 12t arm just plain has a higher kv. It will always start out faster. The hot ticket seems to be a high turn (low kv) motor with higher voltage. Nice smooth start up and voltage for tire speed. My 2 cents

[05Fronty4x]

It's a 27T arm in a Reedy modified can that originally had a 12T in it. I wouldn't ever consider putting the 12T in my scx10 and expect anything but broken parts, haha.

I thought about deadband/neutral width, but I already lowered that to next to nothing on my FXR, and my transmitter doesn't seem to have much of one at all. It's almost like the motor requires a lot of juice to initiate movement, it's not terribly high and it's actually more torquey than I expected, but I thought maybe there was some tuning I could do on the motor to make it a little more crawler friendly. The lowest amount of throttle I can give it will easily get the rig moving, but your right, there's no breaking through the higher kv, it's just naturally going to move faster even at mininum throttle. I may try messing with timing and a can with weaker magnets just to see what happens though.

I understand a 27T isn't going to reach the amount of creepability a 55T has, and ultimately I'm putting the 55T back in because it matches my style so well, but I like little projects and messing with things. I never really got into motors in my racing days other than typical com and brush maintenance, but after recently getting into crawlers, motor design has really sparked my interest.

[EddieO]

I am gonna guess your 27t arm is out of a Trinity/EPIC based stocker. If so, those arms have very little steel compared to most arms you see in a crawler motor. The magnets in the Trinity Stocks were not as strong as the Reedy Mods FB9 magnets. You also turned the timing down, as your old stock motor was timed at 24 degrees.

The reason its jumpy is because the arm has to build up a lot to move in the field of those strong magnets......those arms pull a lot of amps, gonna pull more now trying to overcome it.

Trinity stocks were designed to provide pure power and this was almost always done with RPM. The arms contained very little steel against less powerful magnets. This is why the Reedy MVP stocks sucked compared to the trinity ones. They tried to use FB9 magnets in them, which did nothing but bog the arm down....we did all sorts of tricks to counter it, including using a torch on the can to weaken the magnets.


Later EddieO

[05Fronty4x]

Thanks EddieO, you hit the nail on the head there. The arm was out of a Trinity/EPIC P2K pro (still have another nearly brand new one) that was spec'd at nearly 30K RPM, haha. And the can was originally part of what was basically a tuned handwound Reedy Ti modified which I can tell has ridiculously strong magnets. There was no rhyme or reason to the part choices other than they had very little run time, so the bearings, brushes and com are all nearly new.

What you say makes perfect sense... the armature, aside from the wire fill, is a very lightweight minimal material design and takes a lot of current to initiate movement with those magnets. This also explains the strong drag brake. It sure is a punchy 27T though, hehe.

Am I right in assuming I found 0* timing by setting it to spin the same speed in both directions with a given voltage? Would advancing the timing some help or hurt me? Should probably just ditch the can all together?

My goal with this is to build a 27T for trail outings and keep the 55T for the @home rock garden.

[EddieO]

That is a workable way to set the timing....without fancy scopes and meters, along with a long funky process yer not gonna get it much better.....

I've never found the need to have the stuff go the same speed in both directions. Its a penalty in crawling and in scaling how often are you in reverse? I would rather have the extra HP from the extra timing. Only reason I set most stuff at 0 now a days is to prevent incorrect installation, which often leads to a blown motor.

Increasing the timing may work to help it....may make it worse. Worst you can do is try it....

I certainly would not ditch the can....the can is the good part. Its basically a slighty different version of what we use now. I would use it if I could still get them, as I feel its a better can than what we get now. The arm, while great in the racing days, just doesn't work well for what we are doing.....thats what you need to replace.

Later EddieO

[05Fronty4x]

Originally Posted by EddieO

That is a workable way to set the timing....without fancy scopes and meters, along with a long funky process yer not gonna get it much better.....

I've never found the need to have the stuff go the same speed in both directions. Its a penalty in crawling and in scaling how often are you in reverse? I would rather have the extra HP from the extra timing. Only reason I set most stuff at 0 now a days is to prevent incorrect installation, which often leads to a blown motor.

Increasing the timing may work to help it....may make it worse. Worst you can do is try it....

I certainly would not ditch the can....the can is the good part. Its basically a slighty different version of what we use now. I would use it if I could still get them, as I feel its a better can than what we get now. The arm, while great in the racing days, just doesn't work well for what we are doing.....thats what you need to replace.

Later EddieO

That's exactly what I'm worried about. I think with a flipped transmission in my SCX10 the motor has to spin backwards to go forwards, but I'm going to take a 24* motor, test which way is forwards, and then hook it up to find out. The crappy part is that all my 27T arms are from touring car motors that were all RPM tuned, so I could try them in this can but would probably get very similar results. My 2 modified motors are for torque, but they are 11 and 12T, I don't need to pull wheelies or burn my FXR .

I could just buy an el cheapo closed can 27T, but what's the fun in that? Hehe.

Thanks for the insight, you provided the exact information I was looking for.

[05Fronty4x]

I found an old Reedy Rage and it happens to have adjustable timing. Com looks great, could use new brushes, bushings are fine, but I have a couple questions. I'm going to try it out with 0* timing first and see if it has adequate speed, but if I wanted to advance the timing say 10*, but the motor spins the wrong way for my application, is there any harm in retarding the timing 170* instead and essentially running it backwards?

[Olle P]

Originally Posted by EddieO

... those arms have very little steel compared to most arms you see in a crawler motor.

Steel? That's the axle only, AFAIK. The anchor is supposed to be mild iron.

Originally Posted by EddieO

The reason its jumpy is because the arm has to build up a lot to move in the field of those strong magnets...

Isn't it the other way around?
Because of the strong permanent magnets it shouldn't take as much induced magnetism (from the armature) to move the axle, right?

Originally Posted by EddieO

...those arms pull a lot of amps, ...

At low speeds it's because of the shorter and thicker wire.
_______________________

I can't say exactly what determines the no load start voltage.
- Permanent magnets, strength and shape.
- Armature physical design.
- Wind design and such.
- Friction.
... are factors involved.

[EddieO]

Armature Laminations are made with Silicon Steel (sometimes called Electrical Steel). While it contains iron, they have used Silicon Steel since the 60s in every hobby style armature I know of.... Whoever told you they are made with mild iron clearly doesn't know what they are talking about.

Arms and magnets are a balanced field. When magnets overpower the arm, the arm has to build up more to move in the field. It doesn't magically build up this field without power. There is a reason powerful magnets can bog down an armature, thus causing the arm to pull more amps, to generate more field to actually spin.

A 27t stock arm surely doesn't have shorter or thicker wire really. Rules limited the size, which is MUCH smaller than what could be fit on the arm. Rules also required a minimum length back then, so the arms typically have extra wire on them....


There are a host of things that determine the startup voltage, but it all comes down to how easy you can make the arm move through the field created by the magnets. It's all a fine balance though....when you mess with certain things you can cause other issues that are undesirable...

Later EddieO

Originally Posted by Olle P

Steel? That's the axle only, AFAIK. The anchor is supposed to be mild iron.

Isn't it the other way around?
Because of the strong permanent magnets it shouldn't take as much induced magnetism (from the armature) to move the axle, right?

At low speeds it's because of the shorter and thicker wire.
_______________________

I can't say exactly what determines the no load start voltage.
- Permanent magnets, strength and shape.
- Armature physical design.
- Wind design and such.
- Friction.
... are factors involved.