Cars Motor ratings

UnclePete

New Member
First I'd like to thank everyone who responded to my first post, was surprised with the amount of help i recieved.

Just have a quick brushless motor question, for example, a motor is rated at 3,500watts, 120amp max, and 38volt max.
Is it possible to exceed these amounts, or is that the maximum the motor will draw?
In the above case, if i ran 50volts, would that voltage get to the motor, or will the motor only accept 38v?
Thanks in advance guys.
 

pvolcko

Well-Known Member
Lets see if I've learned my lesson on this (I have a bad history of giving electronic advice that is wrong, usually corrected by someone else very soon after):

Voltage is driven by the source (batteries and esc). If the motor is rated at 38V max then you do not want to feed it more than that, so in a lipo battery that would be 10S. The ESC will adjust the voltage presented to the motor in order to control its speed, limited at the top end by the voltage supplied to the ESC by the batteries. Actually 10S fully charged is 42V, so you may actually want to go no higher than 9S. Ultimately you are limited further by the speed you want the motor to run at. 6S or 22.2V may well get you the motor speed you need (with some headroom) in order to achieve the prop or headspeed you desire.

Amps are drawn by the motor depending on load and voltage supplied. You want to make sure your ESC and batteries are capable of supplying the current that the motor will reasonably draw during flight at any given time. If you under size the ESC it may overheat, blow a component, or some will limit the current flow in order to protect itself, the batteries, and even the motor it is controlling. If the batteries aren't rated for the current draw it can lead to over discharge of the battery, overheating, puffing, etc.
 

Tony

Staff member
Just to correct the above post... I'm kidding, he is spot on hehe. Just had to poke at him for his reply.

Usually the numbers they post on the motors are the max that the motor will handle. I would not go over them. If I was close to them, I would be stopping every couple minutes to check the temp of the battery, ESC and motor just to make sure something is not getting too hot.
 

UnclePete

New Member
Lets see if I've learned my lesson on this (I have a bad history of giving electronic advice that is wrong, usually corrected by someone else very soon after):

Voltage is driven by the source (batteries and esc). If the motor is rated at 38V max then you do not want to feed it more than that, so in a lipo battery that would be 10S. The ESC will adjust the voltage presented to the motor in order to control its speed, limited at the top end by the voltage supplied to the ESC by the batteries. Actually 10S fully charged is 42V, so you may actually want to go no higher than 9S. Ultimately you are limited further by the speed you want the motor to run at. 6S or 22.2V may well get you the motor speed you need (with some headroom) in order to achieve the prop or headspeed you desire.

Amps are drawn by the motor depending on load and voltage supplied. You want to make sure your ESC and batteries are capable of supplying the current that the motor will reasonably draw during flight at any given time. If you under size the ESC it may overheat, blow a component, or some will limit the current flow in order to protect itself, the batteries, and even the motor it is controlling. If the batteries aren't rated for the current draw it can lead to over discharge of the battery, overheating, puffing, etc.

So your saying if one is to run 10cell on a 8cell max motor, the motor will blow?
I was under the assumption that the motor will only draw it's max power, and run at it's maximum wattage, no matter how many watts are delivered to it via the Voltage X currant from the batteries.....
 

Tony

Staff member
Yes, if you over volt the motor it will burn it up. The voltage is supplied by the battery and is always there. You can have voltage without current, but you can't have current without voltage. The motor will only draw what current it needs, but the battery is what determines the voltage.
 

UnclePete

New Member
Yes, if you over volt the motor it will burn it up. The voltage is supplied by the battery and is always there. You can have voltage without current, but you can't have current without voltage. The motor will only draw what current it needs, but the battery is what determines the voltage.

You said the motor will only draw what current it needs...
So if it detects V x I is too much, won't it just drop the current very low?
Then power will be lowered, no matter what the voltage is. True?

- - - Updated - - -

You said the motor will only draw what current it needs...
So if it detects V x I is too much, won't it just drop the current very low?
Then power will be lowered, no matter what the voltage is. True?

For example, if the motor only draws 10amps, then even at 12 cell (50.4V), it's only producing 504watts.
So how can the motor blow?
 

Tony

Staff member
This all falls under Ohms Law. And without getting too technical using a bunch of equations that even I don't understand, I will try to explain. Voltage is a constant. It's always there. If a motor has been setup for say 40 volts, then you do not want to put any more voltage than that to the motor. The battery will supply the voltage, but the motor will pull the current.

Think of it this way, you wouldn't take the headlight out of your 12vDC car and plug it into your 120vAC house socket, it would blow it. Even though the light will only pull the amps that it needs, the voltage is too much for the filament in the light (same for the windings in a motor) even though the wattage or amperage was relatively low. On the other side of that, you wouldn't take the light out of your ceiling fan and try to put it into your car when a light blows out. Not only is there not enough voltage to even light it up bright enough, but the amp draw would be too great for the wires that are supplying the power to the socket.

To put it bluntly, the wires in a motor can only handle so much voltage. I have always said that the motor will pull the power, but power is wattage, not voltage. Voltage is a constant and should be regarded when making a battery choice. If you go over their suggested voltage, then it's likely that the motor is going to smoke its self.
 

pvolcko

Well-Known Member
Just to correct the above post...

Thanks. :)


For example, if the motor only draws 10amps, then even at 12 cell (50.4V), it's only producing 504watts.
So how can the motor blow?

I = V/R. Voltage applied across a resistance results in a given current draw where current is directly proportional to voltage. If you apply beyond rated voltage it will cause the motor to draw more current than rated for and it will heat up excessively (or spin faster than rated/balanced and cause motor bearing failure or possibly catastrophic failure of the rotors in a heli). It may work for a little while. Using Tony's 12V light: if you plug it into 120V it will blow very quickly, but if you give it only 20v it may be very bright for a short time and then burn out. Same with the motor. If you go way over voltage it will burn out quickly. If you go a bit over, it will build up excessive heat and eventually fail and it is a crap shoot as to how long it will hold out.
 

UnclePete

New Member
Thanks. :)




I = V/R. Voltage applied across a resistance results in a given current draw where current is directly proportional to voltage. If you apply beyond rated voltage it will cause the motor to draw more current than rated for and it will heat up excessively (or spin faster than rated/balanced and cause motor bearing failure or possibly catastrophic failure of the rotors in a heli). It may work for a little while. Using Tony's 12V light: if you plug it into 120V it will blow very quickly, but if you give it only 20v it may be very bright for a short time and then burn out. Same with the motor. If you go way over voltage it will burn out quickly. If you go a bit over, it will build up excessive heat and eventually fail and it is a crap shoot as to how long it will hold out.

Yeah i do understand what's going on now.
Thanks again guys, this forum has alot of decent guys willing to help out.
Appreciate all the replys.
 

UnclePete

New Member
Thanks. :)




I = V/R. Voltage applied across a resistance results in a given current draw where current is directly proportional to voltage. If you apply beyond rated voltage it will cause the motor to draw more current than rated for and it will heat up excessively (or spin faster than rated/balanced and cause motor bearing failure or possibly catastrophic failure of the rotors in a heli). It may work for a little while. Using Tony's 12V light: if you plug it into 120V it will blow very quickly, but if you give it only 20v it may be very bright for a short time and then burn out. Same with the motor. If you go way over voltage it will burn out quickly. If you go a bit over, it will build up excessive heat and eventually fail and it is a crap shoot as to how long it will hold out.

Tho you can't compare it to a light, there's a speed controller regulating current.
And with our transmitter, if we only set it to say %20 throttle end, then there's no chance of blowing the motor right? Yet there's still too much voltage present then what the motor can handle.
Can u explain that
 

pvolcko

Well-Known Member
Speed controllers do not regulate current. Some will montior it and limit it if it goes over a threshold level, but in general they do not regulate current, they manipulate the voltage applied to the load (motor, light, whatever).

However, if you limit the throttle top end from the Tx to 50% or something, yeah you could potentially run it like that safely, especially if your supply voltage isn't that high above the rating for the motor.

I won't say there is no risk. Why? Previously we'd said that ESCs vary the voltage to the motor in order to control it's speed. That is not exactly true. They actually vary speed by varying pulse lengths (duty cycle) of full supply voltage to the coils of the motor. So in reality the full battery voltage is being applied to the motor many thousands of times a second (typical outrunner pulse frequency is 8khz). Due to the electrical properties of the motor the varying pulse lengths will filter out into a varying effective/average voltage between 0V and battery voltage. In some cases (changing load torque, for instance, due to going to 0 blade pitch or going into a dive with a plane) the motor's electrical characteristics will change, the filtering effect will be lessened (or increased). This is all to say that the effective voltage seen by the motor will vary based on external factors and if you intend to try to run a motor using a lower than maximum upper limit on throttle, you should plan for a good margin of error.
 
Top Bottom