Is it OK to go over motor voltage rating?

[mvly]

I want to drastically increase voltage, but decrease current in the process to maintain the same power.

Say I take this motor
http://www.hobbyking.com/hobbyking/store/%5F%5F15199%5F%5FHobbyKing%5FDonkey%5FST3508%5F730kv%5FBrushless%5FMotor.html

It states that it should be used with 11.1v, but I want to put in 37V with really low current.

My questions:
1) will this burn out the motor if I keep the currrent low enough such that the power input is the same?
2) will this give me more torque?
3) Say I put in hall sensors, and build my own Controller to drive this, will it help to reduce the burn out?

 

[bane77087]

That motor sells for $8.95 and it is really really small - around 4 cm X 3 cm. It would have serious problems with heat dissipation and component failure with increased voltage I believe its life would be measured in minutes. Other guys on this forum can give you a better idea of what % you can generally overvoltage a motor safely but I'm pretty sure its less 100% let alone 300%. The voltage would increase the rpm beyond what the bearings could take and you would lose torque. Voltage = rpm and Amps = torque
I have no idea how you intend to use this motor. But if space isn't a problem I would suggest running 3 of these instead of one or moving to a larger motor. Its hard to say without an idea of what you are wanting to use this in can you provide more details? Chances are there is a cheap motor out there better suited to your application

http://www.hobbyking.com/hobbyking/store/__15199__HobbyKing_Donkey_ST3508_730kv_Brushless_Motor.html

RPM/v: 730kv
Dimensions: 41 x 28mm Max Current (A) 35
Shaft: 5.0mm Max Voltage (V) 15
Voltage: 11.1v~14.8v Max Watts: 330
Weight: 115g
No-load current: 1.9A @ 11.1v

 

[Kin]

That's a 730kv motor. So if you run it at 37 volts you're forcing it to go 27,010 rpm. Plus, since you said 37v I assume you mean 10s LiPo, which will be 41 or 42 volts off the charger...which is 30k rpm.

It is unlikely that would be feasible at all. Changes are the bearings would wear out quickly, or some other problem. That said, if you look around the non-hub-motor subforum there's a page [I was reading it recently, so page 1 or 2] about using tiny RC motors.They are possible in certain settings.

 

[mvly]

I am trying to use 2 of these to drive a friction drive scooter. Is there a high torque brushless outrunner motor that can safely handle > 18.5V?

I can go with in runner brushless, but i know they do not provide the same torque as the outrunner. I need something that is high torque, but low speed outrunner that can handle 10s Lipo?

 

[Kin]

You probably do want to post this in the non-hub motor ES subforum, although maybe enough people go here and there, but you'll want something in Wye (over delta) windings, and something that has a low Kv. I'm looking for the same (high torque low rpm), but trying to first learn about rc motors since I realized I dont know much about them.

 

[Hillhater]

I am trying to use 2 of these to drive a friction drive scooter. Is there a high torque brushless outrunner motor that can safely handle > 18.5V?
.... I need something that is high torque, but low speed outrunner that can handle 10s Lipo?

Contradicting statements there .. 18.5v ...10s lipo ?

Search these forums for the friction drive threads ..lots of info and kit ideas.
the turnigy 5065 and 6374 series outrunners , with KV down as low as 170, are popular for friction drives and will handle 0-50volts

 

[bane77087]

How big of a scooter? whats the weight without the batteries and motor? How big are the tires? Whats the top speed you want this thing to hit? Is it chain driven? if so how many teeth on the freewheel drive on the axle? Or is it belt driven? Does it have a motor on it currently that you want to upgrade if so can you put the specifics of the motor- manufacturer, model number and wattage that you want to replace. What batteries do you have?

 

[mvly]

How big of a scooter? whats the weight without the batteries and motor?

http://www.motorboards.org/index.php?title=Main_Page
not big. I think with Lipo, it is only 18lbs.

How big are the tires?

http://www.motorboards.org/index.php?title=Rubber_wheel_information
These look pretty small.

Whats the top speed you want this thing to hit?

10-12mph should be the max considering the weight and all.

Is it chain driven? if so how many teeth on the freewheel drive on the axle? Or is it belt driven?

Nope. Friction drive. It's not the best, but works.

Does it have a motor on it currently that you want to upgrade if so can you put the specifics of the motor- manufacturer, model number and wattage that you want to replace.

http://www.motorboards.org/index.php?title=Motor_Part_number
They use 2 motors, but I want to replace them with 2 outrunner motors. Most of the RC inrunner with sensors are 25mm mounting spacing between the holes. The one that comes with this board is 29mm, so If I want to use the inrunners, I will need to make some mod mounts.

What batteries do you have?

10s 5Ah Nanotech 25-50C Lipo. I also have 4s nanotech also 25-50C.

So far looking at what is available, I think I will be going with sensored inrunner motor
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=9821
these are suppose to go up to 4s (14.8V or 16.8V hot off the charger). Add 50% on top of that, I think if I keep the current low enough I should be able to use it at 5s safely.


The goal is to make something that is super high torque but low speed. I want zip around campus and be able to climb hills relatively easily without worrying about burning out the motor. Yet I don't want something large such that I cannot bring it in the classroom.
This is the initial reason why I went with an outrunner (high torque), but I think I might be better off using an sensored inrunner. That way my custom controller is easier to design. I can also get speed out of it vs the sensorless version.

I am also looking at these inrunner sensored brushless RC motor.
http://www.shopatron.com/products/productdetail/part_number=S352X/135.0.1.1.62703.0.0.0.0?pp=10&
these motor might be better since they are assembled in USA and also have longer body, so there should be more copper in it to handle more power. Of course I will be going for the high wind version (6.5T)

 

[Hillhater]

Have changed your mind about the low kV then .. ??
The HobbyKing motor has a kv of 3150 ! ...so on 5s (21V) it will be doing 66,000 + rpm !
how do you plan to reduce that down to a useable speed ?
dito the Novac !
did you bother to check those motors i suggested earlier ?

 

[mvly]

Have changed your mind about the low kV then .. ??
The HobbyKing motor has a kv of 3150 ! ...so on 5s (21V) it will be doing 66,000 + rpm !
how do you plan to reduce that down to a useable speed ?
dito the Novac !
did you bother to check those motors i suggested earlier ?

Yeah I am trying to get the lowest KV motor of the series that can support > 18.5V Ideally I want 37V, but if I can't, I will be good for 1/2

Also I will be needing 5mm shaft motor so, that means it will be small motor.

OK so the novak is out of the question.
The Hobbyking I chose is also out of the question, so is there a small RC motor that can handle > 18.5V?

Keep in mind that I will be using 2 of these motors to power the scooter.

would these work better? Say I will be running 5s instead of the 10s. so about 10K rpm. Keep in mind I will be using 2.

http://www.hobbyking.com/hobbyking/store/%5F%5F19610%5F%5FNTM%5FProp%5FDrive%5FSeries%5F42%5F58%5F500kv%5F1300w.html

 

[Kin]

You said that you are trying to do a "friction drive". I dont know if by that you mean you'll have a small circle pressed against your wheel, thus propelling the thing forward. But, you should calculate the speed that you will be going.

E.g here, you just showed me a 500kv motor.

At 5s, lets say nominal 3.7v. That's 9250 rpm = 154 rotations per second.

Your "gearing ratio" will be the diameter of the bit on the motor, divided by the bit on your motor.

I'm going to assume your motor bit will be 1 inch in diameter. I'm assuming the diameter of the wheel on your board is 3". So...You're talking about 51 rotations per second of a 3" diameter wheel. So that's moving 3*pi*51 inches per second. Or, 27 miles per hour [Uhm, in reality I didnt need to care about your wheel size. You can just base it off of the diameter of motor bit, since we're not talking about teeth or anything in this drive system]. . Even assuming your loaded speed is 80% of that, that's much higher than you want.

With all respect, I think you should be
A) reading up about RC motors
B) looking into similar builds on this forum and other forums. There are a lot of people who have tried to make skateboard/mountainboard EVs.

It doesn't seem like you're at the point to choose motors. Have you even figured out how you will control your throttle?

 

[bane77087]

I thought you were talking about a scooter like this one http://marketplace.urbanscooters.com/Motor-Scooters/E-Wheels-EW-550-Transport-Electric-Moped.html
The one you have is specced at 750 watts with 2 of them together. That should be able to get you 20 mph provided its geared right. Are both of the motors you have in it now fuctioning? And wow I just did the math and granted my math is horrible maybe some1 can check this but with a drive wheel with a 2.5 inch radius I think that little polyurethane wheel would have to do 1076 rpm to hit 20 mph. Thru the links you posted I found this

http://www.motorboards.org/index.php?title=Wheel_quality_and_mileage

I think his was around 10-12 mph.

with http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=9821 its 190 watts so even with 2 you're losing 370 watts, about half your power.
and the other motor is just 2 much money. plus 2 controllers or 1 rigged on 2 motors. Plus redoing your batteries I think you can find a 20 mph electric scooter for below 400$

 

[bane77087]

Or.... you could wait until december when the replacement for the turnigy 80-100 is released and put it on a scooter with a chain sprocket on the rear wheel. 7,000 watts and gear it for 35 mph. I myself am holding off on upgrades until that new outrunner is released.And putting it on a scooter you wouldn't need a double reduction drive

http://www.hobbyking.com/hobbyking/store/uh_viewitem.asp?idproduct=5142

Rumor has it that the ones from the new line will start being distributed in december
and the magnets and epoxy are supposed to tolerate 400 F without damage

 

[mvly]

You said that you are trying to do a "friction drive". I dont know if by that you mean you'll have a small circle pressed against your wheel, thus propelling the thing forward. But, you should calculate the speed that you will be going.

E.g here, you just showed me a 500kv motor.

At 5s, lets say nominal 3.7v. That's 9250 rpm = 154 rotations per second.

Your "gearing ratio" will be the diameter of the bit on the motor, divided by the bit on your motor.

I'm going to assume your motor bit will be 1 inch in diameter. I'm assuming the diameter of the wheel on your board is 3". So...You're talking about 51 rotations per second of a 3" diameter wheel. So that's moving 3*pi*51 inches per second. Or, 27 miles per hour [Uhm, in reality I didnt need to care about your wheel size. You can just base it off of the diameter of motor bit, since we're not talking about teeth or anything in this drive system]. . Even assuming your loaded speed is 80% of that, that's much higher than you want.

With all respect, I think you should be
A) reading up about RC motors
B) looking into similar builds on this forum and other forums. There are a lot of people who have tried to make skateboard/mountainboard EVs.

It doesn't seem like you're at the point to choose motors. Have you even figured out how you will control your throttle?

Here is how it's driven:
http://www.motorboards.org/index.php?title=FileSC06856.JPG

note the two cone shape metal piece on each brush motor. These will be pushing again the wheel's side. Out runner might not be the best choice since it might be putting pressure on the shaft bearings. But I will try once i get some time.

base on this information
http://www.motorboards.org/index.php?title=Motorboard_Controller_Source_Code_(Open_Source)
100 pulse per turn of wheel, 8 pulse per turn of motor

And again from this site:
http://www.motorboards.org/index.php?title=Rubber_wheel_information
125mm = Diameter. 2pi*125 = 785.4mm per turn of brand new wheels. It will wear down a bit after use. but this is worst case.

(100 pulses/1 wheel revolution)*(1 motor revolution/8 pulses) = 12.5 motor revolution/wheel revolution.

assuming 10Krpm for 5s = 166.6 revolution/1second (keep in mind this is no load rpm)

(166.6 motor revolution / 1 second)*(1 wheel revolution / 12.5 motor revolution) = 13.328 wheel revolution/second

(785.4mm/1 wheel revolution)*(13.328 wheel revolution/1sec) = 10468mm or 10.468m/s

using google to convert miles/hour to meter/sec = 1mph = 0.447meters/sec

(1mph/0.447meters/sec)*(10.468m/s) = 23.42mph.

Now that is assuming full throttle, no load. At 50% throttle as being the high end (need to keep current low) and taking account load, I think 10-12mph is doable as top end speed on flats and possibly slower on slopes. I don't want to be burning out wires.

What do you guys think?

Keep in mind that I have 2 motors. So the load will be divided between the two.

 

[bane77087]

I think you can buy a electric scooter that does 10-12 mph for 100$

 

[mvly]

Or.... you could wait until december when the replacement for the turnigy 80-100 is released and put it on a scooter with a chain sprocket on the rear wheel. 7,000 watts and gear it for 35 mph. I myself am holding off on upgrades until that new outrunner is released.And putting it on a scooter you wouldn't need a double reduction drive

http://www.hobbyking.com/hobbyking/store/uh_viewitem.asp?idproduct=5142

Rumor has it that the ones from the new line will start being distributed in december
and the magnets and epoxy are supposed to tolerate 400 F without damage

Yeah I was not looking at some high end scooter. This is solely meant for moving around campus. With the current setup and current controller/motor, it is fine. but it does not do hills well. I need something that can handle the hills well.

as for the $100 electric scooter, find one that is light enough to take in class and doesn't look like a kid's toy.

 

[bane77087]

$109 for 10 mph at wally world just ignor the picture of the 8 year old standing on it

http://www.walmart.com/catalog/product.do?product_id=8216520&findingMethod=rr
But if I got 1 I would want one that used unite motors so I could upgrade it easily and cheaply with a chain drive not friction

 

[Kin]

Hey, that is actually a pretty interesting friction drive. That said, I think you really need to consider the noise aspect. That's something I've realized as I picked up a $40 e-kickscooter off craigslist. I like this thing, to an extent, but for my e-kickscooter I'm really trying to see how I can *limit* the motor rpm and other noise parameters. You dont want to be running around campus on this screaming EV creature.