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LED
2015.05.04, 12:53 PM
Hi guys, need some help understanding this.
I have 5 or 6 stock motors, most used, 2 only broken in.
they all are faster in reverse then in forward direction. Some up to 3500 RPM, measured with the RPM app. I will be using a motor tester later tonight to confirm.
First I tought it was a car problem, but all my cars (MR02 and MR03) have the same issue.
I have 1 brand new motor, and that one is he same in either direction.
So, it probably has something to do with the wear of the brushes?

Has anyone else noticed the same thing?

Would breaking them in again in reverse direction fix it, or just make it equally slow in reverse direction?

I am a little baffled with this....


Thx :-)

TheSteve
2015.05.04, 01:53 PM
I would assume it is related to motor timing. The stock motors may have timing retarded in the forward direction which means it is advanced in the reverse direction(more speed in reverse). Brushes can change timing a little as well but to me it is more likely they were built to be slower in the forward direction.

LED
2015.05.04, 04:06 PM
The motor tester didn't happen, something came up so I cant confirm it realy is a motor issue yet.

Why would they make the slower in the forward direction? Doesnt make sense to me :p
Thx for the input, I will post more info as i find out.
If someone else has another idea keep em comming :-)

TheSteve
2015.05.04, 04:18 PM
Less timing equals less current draw, less heat, more battery life, longer motor life. The higher speed in reverse is likely just an unwanted side-effect.

DMALMAD
2015.05.04, 07:30 PM
I think that this has been discussed a great length already on the forum but I am not sure but if I recal it is mostly due to the fact the the motors we use were not designed specifically for RC and as such they are built to specifications that suit other purposes which require more power in what we would consider the reverse direction. I am sure that someone that understands the motors more will chime in but generally speaking, unlike 10th scale the motors we use were not designed around the Mini-z.

LED
2015.05.05, 10:25 AM
Realy? I searched the forum but didnt find anything related. I also have been using the forum for 6 years now (with a pause of about 1,5 years) but I cant recall such a discussion.

I'll look again :-)

TheSteve
2015.05.05, 11:45 AM
I noticed that phenomenon right out of the box. It is due to the path of current being lower for reverse than forward under the stock-like FET configurations.

Also, I dropped the pull-up resistor at the gate of the P-channels to allow for more current, hence faster charge response and turn-on/turn-off times.

[

The first statement is 100% incorrect, the speed difference has nothing to do with the fets at all.
Changing the pull up on a fet like this will not allow for more current(fets are not bipolar transistors).

I cannot stress enough to please not follow this "advice" of changing the resistor.

cowboysir
2015.05.05, 12:57 PM
I would assume it is related to motor timing. The stock motors may have timing retarded in the forward direction which means it is advanced in the reverse direction(more speed in reverse). Brushes can change timing a little as well but to me it is more likely they were built to be slower in the forward direction.

I think that this has been discussed a great length already on the forum but I am not sure but if I recal it is mostly due to the fact the the motors we use were not designed specifically for RC and as such they are built to specifications that suit other purposes which require more power in what we would consider the reverse direction. I am sure that someone that understands the motors more will chime in but generally speaking, unlike 10th scale the motors we use were not designed around the Mini-z.

While your expansion of the electrical detail might be valid these two answers are in reality the reason why some stock motors are faster in reverse.

It's not a bad thing to have a set point to work with in stock motor as long as your competitors are all at the same set point.

I wouldn't recommend fooling with your electronics if you want your chassis to conform to stock rules.

TheSteve
2015.05.05, 01:55 PM
Jshwaa - Just as a bit of a bit of info. I don't come here to wage technical war with others or brag about my technical knowledge. I come to this forum because I enjoy Mini-Z's.

Now to address your reply...

I didn't go into further detail because your reply was off topic. This thread is about why a stock motor is faster in reverse then forward. Plain and simple it is related to the timing of the motor(the angular relationship of the brushes to the permanent magnets).

As you would like further info though related to my reply here it is.

Your statement "It is due to the path of current being lower for reverse than forward under the stock-like FET configurations." is still 100% not true. The motor drive of a Mini-Z is a standard H-bridge as your own diagram indicates. The total resistance of forward and reverse drive is the same as the current flows through both a P and an N junction no matter the direction of the motor.

As to your second point regarding lowering the gate pull up resistance on the P channel fet your reply was phrased such that it really seemed as though you were thinking in the bipolar realm. So assuming you understand that a mosfet type device is voltage driven and not current driven, then yes, turning a fet on faster will reduce the time it spends in the linear region. But really we should dig a little deeper shouldn't we? If you're going to lower that resistance will it really turn the fets on quicker? Well that depends on the transistor driving that resistance. It may respond slower if it doesn't have enough gain or has a high internal resistance on its own. Further if your specific board does perform better with a lowered resistance will all hardware variations of the Mini-Z controller perform better? Keep in mind the older hardware generally used cheaper/worse components as the technology/hardware used was spec'd 10 years ago.

Then there is the side effect of switching mosfets quicker - noise. Even the highest end electronic speed controls in the world purposely use higher value resistors to slow switching time to reduce the interference the speed control emits. This is pretty critical when it is just inches from a receiver - sometimes a very sensitive 27 MHz AM.

So even if your "mod" happens to work for you it is poor judgement to suggest others do it without understanding the potential risks. They could end up ruining the race for someone else because the car is now a random noise generator.

btw, the PTC (or polyfuse) that Kyosho adds has the same resistance no matter the direction of current flow so it doesn't come into play here.

And lastly let me add again that I don't come here to try to match technical wits with others or brag about my education or experience. I come here for the enjoyment of the hobby. I totally respect your sense of adventure and desire to modify, customize, improve etc. I also appreciate that you want to share your knowledge with others. Just keep in mind what seems like an improvement to you might not be for others or in the best interests of others.

Jshwaa
2015.05.05, 02:29 PM
Jshwaa - Just as a bit of a bit of info. I don't come here to wage technical war with others or brag about my technical knowledge. I come to this forum because I enjoy Mini-Z's.

Now to address your reply...

I didn't go into further detail because your reply was off topic. This thread is about why a stock motor is faster in reverse then forward. Plain and simple it is related to the timing of the motor(the angular relationship of the brushes to the permanent magnets).

As you would like further info though related to my reply here it is.

Your statement "It is due to the path of current being lower for reverse than forward under the stock-like FET configurations." is still 100% not true. The motor drive of a Mini-Z is a standard H-bridge as your own diagram indicates. The total resistance of forward and reverse drive is the same as the current flows through both a P and an N junction no matter the direction of the motor.

As to your second point regarding lowering the gate pull up resistance on the P channel fet your reply was phrased such that it really seemed as though you were thinking in the bipolar realm. So assuming you understand that a mosfet type device is voltage driven and not current driven, then yes, turning a fet on faster will reduce the time it spends in the linear region. But really we should dig a little deeper shouldn't we? If you're going to lower that resistance will it really turn the fets on quicker? Well that depends on the transistor driving that resistance. It may respond slower if it doesn't have enough gain or has a high internal resistance on its own. Further if your specific board does perform better with a lowered resistance will all hardware variations of the Mini-Z controller perform better? Keep in mind the older hardware generally used cheaper/worse components as the technology/hardware used was spec'd 10 years ago.

Then there is the side effect of switching mosfets quicker - noise. Even the highest end electronic speed controls in the world purposely use higher value resistors to slow switching time to reduce the interference the speed control emits. This is pretty critical when it is just inches from a receiver - sometimes a very sensitive 27 MHz AM.

So even if your "mod" happens to work for you it is poor judgement to suggest others do it without understanding the potential risks. They could end up ruining the race for someone else because the car is now a random noise generator.

btw, the PTC (or polyfuse) that Kyosho adds has the same resistance no matter the direction of current flow so it doesn't come into play here.

And lastly let me add again that I don't come here to try to match technical wits with others or brag about my education or experience. I come here for the enjoyment of the hobby. I totally respect your sense of adventure and desire to modify, customize, improve etc. I also appreciate that you want to share your knowledge with others. Just keep in mind what seems like an improvement to you might not be for others or in the best interests of others.

I hear you. I apologize about the static, however the war on technical wits never occurred until some time was spent sharing, under the request via private message by LED, what I could share on the topic only to be brushed off as having nothing to do with it. I'm sorry, but I take the electronics a little more seriously than whatever racing venue you are restricting yourself to.

You make it sound as if I have a sinister plan to have people break their stuff, or get them disqualified from a race where judges got their meters and magnifying glasses out, before handing out some huge trophy in a race involving strictly stock motors. I'm sorry if that would ever be the result from following any of my advice, whether it be on topic or not. My bad...

...but just curious how I could expect noise to interfere with someone else's elecs if it doesn't interfere with my own, all technical brilliance set aside...

And after all, I can agree with the timing effect being most dominant as it pertains to forward/reverse speeds, however I originally thought that to be a topic you guys would have beaten to death by now, and LED asking was an inquisition involving other factors never discussed here.

LED
2015.05.05, 04:31 PM
Well I can confirm one thing 100%, it has nothing to do with the car because I have been testing today with only a 4 cell battery pack directly to the motor and I have that same issue with all my motors.
So I will accept the timing explanation (thanks for that) and leave if with that. As long as its the same for everyone there isn't something wrong with what I am doing then

Thx for the input everyone!

DMALMAD
2015.05.05, 05:04 PM
Yeah it is the same for me too. All this debate made me go back and check my motors too and I can say after testing around 10 motors they all are faster in reverse. I used my motor run in feature on my charger with the same voltage and all 50t so to try and eliminate any other variables and I have come to the same conclusion as you:cool:

arch2b
2015.05.05, 05:27 PM
All bravado aside :). I'm great fun to all of you guys going on in minutia detail. It's certainly interesting reading, even if I don't follow it all. Nice to know there is the science behind the magic so to speak.