pulse width modulation in controllers
- Thread starter fractal
- Start date
dnmun
1 PW
if the controller is controlling the current through the mosfets by turning them on and off a lot then there is a long time in which the mosfets are in the transition phase from on to off and back again.
but the mosfets continue to conduct current as they are being turned off because it takes a short but measureable time to turn off the channel.
during the period that the channel is conducting this current, the density of the conducting electrons in the channel is dropping rapidly and that lack of current carrying 'majority' carriers acts like a resistance to the flow of that inductive current still flowing through the mosfet.
heating in the channel follows the rule: power(heat in this case)=(I^2)xR
this heating causes the mosfet to heat up rapidly and damages the mosfet when the heat cannot be dissipated through the heat sink.
if the mosfet is turned on all the time, when there is no modulation of the length of time the mosfet is on, then the channel is always full of electrons, the majority carriers, and the current is conducted without the heating associated with a depleted channel. so the full power mode means less heat is generated during this transition phases and less risk of overheating the mosfet.
i am sure that one of the experts assigned to monitor what i write will trash this but that's the way it works.
but the mosfets continue to conduct current as they are being turned off because it takes a short but measureable time to turn off the channel.
during the period that the channel is conducting this current, the density of the conducting electrons in the channel is dropping rapidly and that lack of current carrying 'majority' carriers acts like a resistance to the flow of that inductive current still flowing through the mosfet.
heating in the channel follows the rule: power(heat in this case)=(I^2)xR
this heating causes the mosfet to heat up rapidly and damages the mosfet when the heat cannot be dissipated through the heat sink.
if the mosfet is turned on all the time, when there is no modulation of the length of time the mosfet is on, then the channel is always full of electrons, the majority carriers, and the current is conducted without the heating associated with a depleted channel. so the full power mode means less heat is generated during this transition phases and less risk of overheating the mosfet.
i am sure that one of the experts assigned to monitor what i write will trash this but that's the way it works.
dogman dan
1 PW
Makes sense to me, but I'm the idiot.
Riding in the desert heat as I do, I've learned to ride wide open throttle if possible, lessening the chance of overheating a controller. But sometimes it's better to back off and not overheat the motor.
It's one of the reasons I became a fan of slower winding motors. If 20 mph is max, then riding wot is never too fast.
Overheating controllers seems to happen seldom anyway, on less than 20 amp controllers, if the controller is getting some breeze.
Riding in the desert heat as I do, I've learned to ride wide open throttle if possible, lessening the chance of overheating a controller. But sometimes it's better to back off and not overheat the motor.
It's one of the reasons I became a fan of slower winding motors. If 20 mph is max, then riding wot is never too fast.
Overheating controllers seems to happen seldom anyway, on less than 20 amp controllers, if the controller is getting some breeze.
I've been wondering this for a while, but I was too embarrassed to ask...and I hoped someone else would ask soon. Thanks, fractal.
This encourages me to work out more of the details of how to give a motor some gears (3?)...several ways to do it. A BB-drive seems to be the most common. I am getting interested in how a geared hub would work for that (stoke-monkeyish). A 350W MXUS, or perhaps a 500W cell_man (are they the same size? time to whip out my Google).
This encourages me to work out more of the details of how to give a motor some gears (3?)...several ways to do it. A BB-drive seems to be the most common. I am getting interested in how a geared hub would work for that (stoke-monkeyish). A 350W MXUS, or perhaps a 500W cell_man (are they the same size? time to whip out my Google).
fractal
1 kW
- Joined
- May 10, 2011
- Messages
- 322
I'm asking this because I have a 100 volt bike, 24 fet controller, and 22 (outer diameter) inch wheels in a 9Kv(approx) cromotor/hubzilla. The thing is, my commute has some spots where i need to slow down because of traffic, pedestrians, bike paths. Can I damage my bike because of this? I would hate to bring the voltage down!
dnmun
1 PW
never slow down. hehe
i doubt if it is a problem.
if someone builds a 100V system and tries to ride it all the time at half throttle it may add up.
so tell the cop that the reason you were going so fast is to protect the controller.
i doubt if it is a problem.
if someone builds a 100V system and tries to ride it all the time at half throttle it may add up.
so tell the cop that the reason you were going so fast is to protect the controller.
bigmoose
1 MW
There have been some questions and dialogue about how going slow adds heat to the controller. Some of our members talk about sizing your battery so that you are running near flat out at your cruise speed.
Here is the results of an analysis using Qty(4) IRF4110 Fets at the conditions indicated. Enjoy and analyze... there is likely an error or two that I will catch with time. It gives some numbers to the intuition of our riding members... a 30% drop in waste heat going from 5% to 95% throttle PWM percentage.
This chart is for a 24 IRF4110 FET Controller:
This chart is for a 12 IRFB4110 FET Controller:
Here is the results of an analysis using Qty(4) IRF4110 Fets at the conditions indicated. Enjoy and analyze... there is likely an error or two that I will catch with time. It gives some numbers to the intuition of our riding members... a 30% drop in waste heat going from 5% to 95% throttle PWM percentage.
This chart is for a 24 IRF4110 FET Controller:
This chart is for a 12 IRFB4110 FET Controller:
TylerDurden
100 GW
How is the junction temperature changed? That appears to be the altered parameter...
bigmoose
1 MW
TylerDurden said:
You type a new value in!
It is set up to give a comparison for similar operating conditions at 2 different temperatures.
TylerDurden
100 GW
Ah... a simulator then, not data. (?)
bigmoose
1 MW
TylerDurden said:
Correct, it is an analytical model. The diode Vf is not corrected for temperature either at this time.
dogman dan
1 PW
Put a remote thermometer in that big controller, and see what is actually happening. Then add forced air ventilation if it's needed. If it's not, ride on.
Chances are, if you don't have to do the entire ride half throttle, you have no particular problem.
Chances are, if you don't have to do the entire ride half throttle, you have no particular problem.
To answer this original question:fractal said:
I would guess that at least some of what you've been reading concerns brushed motor controllers, rather than multiphase brushless. In a brushed controller, there are times at full throttle and no current limiting when the FETs can be simply turned on and left that way.
In a multiphase brushless (BLDC, etc) the FETs will *always* be switched for at least commutation, even if not for current limiting or voltage (throttle) control.
So with brushless even if you aren't actually using PWM itself, you're still switching the FETs on and off to change the current path thru the phases to make the motor spin.
cal3thousand
10 MW
So what difference if any does using a current limiting switch (like those available on cell_man kits) have on this heat effect?
Current limiting is done by PWMing the voltage to the motor, so....
TylerDurden
100 GW
Even then, it depends on the power going through.dogman said:
Controllers chop battery current - if the motor is not pulling a lot of current, 50% duty-cycle should not be a problem. That's part of the picture when choosing a controller.
So, it again comes back to picking the right motor, then a controller for that motor, then a pack to make it go fast/far enough.
Has anyone discussed "conformal coating" the controller insides, and then half-filling it with oil (or 3/4 full?)...possibly with the controller positioned so the FETs are on the bottom half?
I apologize in advance if this idea is nuts...
I apologize in advance if this idea is nuts...
TylerDurden
100 GW
I'm not sure coating would be needed.
IIRC, some here have discussed oil cooling. But like with motors, heat dissipation is secondary to sizing properly in the first place (IMO).
My little Currie/Ananda controller was entirely potted... but it only had to deal with a 500W rated motor.
IIRC, some here have discussed oil cooling. But like with motors, heat dissipation is secondary to sizing properly in the first place (IMO).
My little Currie/Ananda controller was entirely potted... but it only had to deal with a 500W rated motor.
Alan B
100 GW
Nice spreadsheet Bigmoose. Good to see some numbers. Are those typical for the types of controllers we are using?
bigmoose
1 MW
Alan, depends what the switching period (PWM Period) is and the transition time for the FETs. It's in the ballpark, I would say -0% + 25% or so.
I run a brushed set up, with an ebay controller, its never hot and find that my motor often runs hotter at partial throttle, than full throttle. Why? one would think the motor would run cooler with less load. The controller runs at 4.6 kilohertz is that it ?
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