Why do brushless ESCs get hot?

[GordonFreeman]

Why do brushless ESCs get hot? No speculation please, I want to hear from some EE guys. Or better yet, Novak engineers, Hacker, Lehner, Castle, ect. I guess what got me thinking about this again was when I put a 19T in my old SS+ and ran it (in a B4) and the ESC comes off cool. But with the 5800, the ESC comes off hot (and one guy even had it thermal before he vented the body).

So the same ESC runs hot with a brushless motor and cool with a brushed motor. I guess I just don't understand how FETs (is that the right term?) work. Aren't they just switching power on and off? It would seem to me that the power/duty cycle would be about even for both brushed and brushless. Say in brushed you are pulling 30 amps through three paths, so 10 amps/path, in brushless you might be pulling 30 amps through 1 path, but only for 1/3 of the time, hence 10 amps/path average.

Is it something to do with reversing current in brushless mode? In brushless mode do they try to do a synthetic sine wave and the partial power causes less effeciency?

Inquiring minds want to know.

[kufman]

There are a couple of reasons for heat in brushless. Brushless controllers are always switching, even at full speed when they are switching the fastest. Turn on and turn off of the FETs is not instantaneous. There is a finite rise and fall time where the FET looks like a voltage controlled resistor. The more times per second (higher frequency) that the FET is turned on and off, the higher the losses will be. A brushless controller is a doing a lot more work than a brushed controller. In a brushed controller, full speed is achieved by turning the FETs on and they stay on until you decided to go slower than full speed. In this condition, the FETs only losses are those from the series resistance of the device or the "Rds on" as printed in the data sheets. If you had a fet that had a 6m Ohm resistance and there were 5 in parallel, you would have the equivalent of 1.2 m ohms of resistance. So say you use a stock motor that draws 20A of current, you losses would be (20^2)*(0.0012) = 0.48W which is very small. So you 30A through one path for 1/3 of the time is not correct. It is 30A through 1 path for 100% of the time that you are at full throttle.

Losses in a brushless are harder to calculate. They are a function of switching Freq, load current, and RPM of the motor. In sensorless controllers, the losses are also a function of phase timing. If a sensorless controller fires the wrong coil it may look like a dead short due to the induced voltage by the rotor. Another reason for extra heat is the high inrush currents that result from switching between inductive loads. High current during turn on leads to higher losses.

Yes, brushless controllers do produce a AC voltage wave form in conjunction with the motor. It is basically a DC pulse of voltage that charges the inductive coil of the motor. By varying the duty cycle of the DC pulses, you can change the amount of time required to charge a coil and thus the speed of rotation.

[TBone77]

Wow. Nice explanation!

Of course, I don't understand a word of it... but Wow. Nice explanation!

[kufman]

Of course, I don't understand a word of it... but Wow. Nice explanation!

Good thing I didn't included the differential equations and such :eek:

[dave mac]

my gtb doesnt get hot

[GordonFreeman]

Nice explaination. A couple more questions. So is it mostly the "ramping" up and down that causes the heat? Whether it is the ineffiecencies of the FET device during transision or the nature of current flow (high inrush as you say).

Yea, throw some math in there, me likes math.

Excuse the kindergarden drawing, but hey it only took me about a minute. Do the brushless systems try to synthesis a sine wave (top drawing) or does is just switch them full on for 120 degrees (or whatever)

Drawing is supposed to represent an accelerating motor.

[chilledoutuk]

i think this animation explains a lot about 2 pole brushless control.

http://www.servomag.com/flash/2-pole/2pole-bldc-motor.html

[GordonFreeman]

Doesn't make any sense...until you find the step through control. ;)

[tdkclass]

One way to think about it might be the water analogy where you have some barrier that opens and closes really quick.
While it's completely opened or closed (no flow at all) there is almost no drag on the barrier, but while it opens or closes the water will push against it.

[kufman]

Here is an image to think about for a while

http://www.kufman.com/img/ave%20voltage.JPG

It is a corrected version of gordon's picture with one equation. It also shows the idea of a time average. The longer the pulse, the higher the average voltage. This is also knows as duty factor, D= t_on divided by t_period. A duty factor of 100% would be on all the time, a D= 50% would be on half the time and off half the time. The top sine wave picture shows a D=50% and the bottom shows D=25%. notice that the D=50% has a faster frequency and in our application the motor runs faster.

[DrMotor]

Why...

Reason 1:
Many (most?) brushless motors for RC are slotless, while most brushed motors are slotted --- slotted meaning that the magnet wire is wound on an iron frame. Slotless motors have low inductance. Low inductance needs higher switching frequency in order to maintain a stable current. Higher switching frequency gives higher switching loss. Higher switching loss gives warmer ESC.
In order to run low inductance motors at reasonably low switching frequency it is sometimes necessary to connect inductors in series with the motor (but I have not yet seen that done in RC).

Reason 2:
Many brushless RC systems have high power. High power motors have low resistance. During non-regenerative braking the kinetic energy of the vehicle is transformed to heat in the motor and ESC. Example: A 1 mOhm ESC will generate approx 10 times more heat during braking with a 10 mOhm motor than it will with a 100 mOhm motor.

[BobNovak]

Most of what everybody is saying here is correct but there is one fact that everybody is missing. All brushless controlers have an upper and lower set of Mos Fets in series with each coil because you need bi-directional current flow thru each of the three coils to make the brushless motor work correctly. Thus we always have twice the voltage drop across the Mos Fets as we have in a brushed speed controller which means twice the power loss for an equal number of Mos-Fets. To get the power losses down in a brushless controller we have to use a lot more Mos-Fets and this is why the controlers tend to be larger.
Bob Novak


Why do brushless ESCs get hot? No speculation please, I want to hear from some EE guys. Or better yet, Novak engineers, Hacker, Lehner, Castle, ect. I guess what got me thinking about this again was when I put a 19T in my old SS+ and ran it (in a B4) and the ESC comes off cool. But with the 5800, the ESC comes off hot (and one guy even had it thermal before he vented the body).

So the same ESC runs hot with a brushless motor and cool with a brushed motor. I guess I just don't understand how FETs (is that the right term?) work. Aren't they just switching power on and off? It would seem to me that the power/duty cycle would be about even for both brushed and brushless. Say in brushed you are pulling 30 amps through three paths, so 10 amps/path, in brushless you might be pulling 30 amps through 1 path, but only for 1/3 of the time, hence 10 amps/path average.

Is it something to do with reversing current in brushless mode? In brushless mode do they try to do a synthetic sine wave and the partial power causes less effeciency?

Inquiring minds want to know.

[ElectricThunder]

Most of what everybody is saying here is correct but there is one fact that everybody is missing. All brushless controlers have an upper and lower set of Mos Fets in series with each coil because you need bi-directional current flow thru each of the three coils to make the brushless motor work correctly. Thus we always have twice the voltage drop across the Mos Fets as we have in a brushed speed controller which means twice the power loss for an equal number of Mos-Fets. To get the power losses down in a brushless controller we have to use a lot more Mos-Fets and this is why the controlers tend to be larger.
Bob Novak
The GTB is still very small though, and the SS isn't too large either. I take it ya'll use smaller, high powered fets (size wise, like the ones found on Mini-Zs) and stack them or something? :confused:

[kufman]

Small, Low resistance and slightly lower voltage FET's. Generally, the lower the voltage rating of the FET, the lower the Rds On (Resistance Drain to Source when the FET is On).

[DrMotor]

Most of what everybody is saying here is correct but there is one fact that everybody is missing. All brushless controlers have an upper and lower set of Mos Fets in series with each coil because you need bi-directional current flow thru each of the three coils to make the brushless motor work correctly. Thus we always have twice the voltage drop across the Mos Fets as we have in a brushed speed controller which means twice the power loss for an equal number of Mos-Fets. To get the power losses down in a brushless controller we have to use a lot more Mos-Fets and this is why the controlers tend to be larger.
Bob Novak

I did not miss it, I just did not mention it because brushless ESC can run reverse (i.e. bi-directional), therefore you need to compare a brushless ESC to a brushed ESC with reverse. Brushed ESC with reverse need at least 4 FETs and will have the same ohmic loss as a brushless one with (6 of) same FETs.

During forward operation the brushless ESC distributes the heat over all 6 FETs while the brushed ESC dissipates the same ohmic loss in only 2 of it's 4 FETs. The heat is more distributed (and therefore easier to handle) in a brushless ESC than in a brushed.

Right?

[glassdoctor]

Sounds like you should be making your own controllers. Let us know when you have a prototype.

[kufman]

During forward operation the brushless ESC distributes the heat over all 6 FETs while the brushed ESC dissipates the same ohmic loss in only 2 of it's 4 FETs. The heat is more distributed (and therefore easier to handle) in a brushless ESC than in a brushed.

The advantage that the brushed ESC has is the fact that it is not switching when you are at full speed, so you only have the Rds On to worry about. The brushless is always switching, even at full speed therefor you have to take into consideration the switching losses at all times. Brushless motors also have higher inrush currents since there is very little to limit the current. This is especially true of air core motors like lehner basic motors.

[BobNovak]

I did not miss it, I just did not mention it because brushless ESC can run reverse (i.e. bi-directional), therefore you need to compare a brushless ESC to a brushed ESC with reverse. Brushed ESC with reverse need at least 4 FETs and will have the same ohmic loss as a brushless one with (6 of) same FETs.

During forward operation the brushless ESC distributes the heat over all 6 FETs while the brushed ESC dissipates the same ohmic loss in only 2 of it's 4 FETs. The heat is more distributed (and therefore easier to handle) in a brushless ESC than in a brushed.

Right?
This is true when you compare brushless ESC's with Reversable Brushed ESC'S. I think the person that asked the original question was comparing foward only brushed ESC's with brushless ESC's. I hope we all have answered his question.
Bob Novak

[dave mac]

Why does my battery get warmer then all other brushless batteries, these are lipos.

My enginge 155 after 15 minutes, esc never gets warm, battery very warm

[kufman]

What setup do you use??

[tcolesen]

Why does my battery get warmer then all other brushless batteries, these are lipos.

My enginge 155 after 15 minutes, esc never gets warm, battery very warm

Usually that would mean that the Lipo battery that you are using probably isn't up to the amp draw of your setup as much as your other batteries are. What's the amp rating on it?

[DrMotor]

I think the person that asked the original question was comparing foward only brushed ESC's with brushless ESC's.

You are right --- he asked about one of your forward only ESCs --- and forward only has much lower ohmic loss.
BTW: Have you tried to run a brushless ESC in reversible brushed mode (connecting only two of the 3 wires)?

[SpEEdyBL]

Any brushless esc? How will that work?

[kufman]

It would require a software change to make it work. Theoretically, this is how the hardware could be used to make a forward/reverse ESC out of a brushless.