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Old Feb 22, 2002, 05:25 AM
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paulaac's Avatar
Johannesburg, South Africa
Joined Jan 2002
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ESC switching frequency... high or low?

Greetings all...

I have been designing and building my own speed controls for some time now but have never had much clarity as to what the best switching frequency would be. I currently run my ESCs at about 1.5KHz and have recently modified the software to run at over 3KHz, but have not flown at this frequency yet. I fly speed 400ís and speed 600ís off 8 cells.

Does anyone have any ideas or information regarding this? Your input would be appreciated.


Regards...

Paul.
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Old Feb 22, 2002, 06:19 AM
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Bill Glover's Avatar
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I don't know for sure what a 'normal' switching frequency would be, if you look at ESC specs. on the major manufacturers websites you'd probably get a good idea. 1-3 KHz is probably about right.

I do know that the small coreless motors used on some indoor models are meant to be used with 'high frequency' ESCs - these are made by a number of companies and vary from 100 to 133 KHz.
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Old Feb 22, 2002, 08:37 AM
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Bosque Farms, NM
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Hello Paul:

One, and only one, of the considerations in the design of a device like an ESC is power dissipation in the output FETs caused by operation in the linear region. Ideally the FET should be either on or off, in either case it dissipates relatively little power. When the FET is off the current is 0 while the voltage is max resulting in 0 Watts dissipated in the FET When the FET is on the current is max and the voltage is low (the on voltage being current times the on resistance of the FET) for a relatively small powere dissipation. But, as the FET switches from on to off or vice versa, the FET goes through the "linear region" where it is conducting current and is dropping voltage across it. This results in relatively high power dissipation, consider 1/2 voltage * 1/2 current, which gives a much higher value than either the full on or full off states. Therefore much of the power dissipated by the FET occurs during on-off transitions. So, how does this relate to switching frequency? Since the on-off transition (technically the rise and fall times) is finite, as the frequency is increased the FET spends a larger and larger percent of it's time in the linear region and thus power dissipation increases. Therefore there is a limit (only one of several) on how fast we can switch the output FET.

Kevin

You speak your truth, I will speak mine.
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Old Feb 22, 2002, 08:49 AM
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From the Electric Flight FAQ (link at top of page) :

"Q. What is the best switching rate for a hi rate ESC?
A. (Doug Ingraham, Lofty Pursuits)

There is little difference between a control that operates from about 1000hz up to around 5000hz. The exact best rate would depend heavily on the motor. For our hobby motors about 3000hz is near optimum. As the switching rate increases, the losses due to turning the switch on and off start to go up as the losses in the motor go down. The crossover point for these losses is about 3000hz for most motors."

Doug knows what he is talking about.

HTH - Steve
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Old Feb 22, 2002, 10:42 PM
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Colonial Heights, Virginia, USA
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what about the often posted ( in these forums ) "fact" that hi-rate ESC's are preferred and sometimes "requirred" on modern, core-less motors as being easier on the comm and reduced brush arcing....also..when any ESC is at Full ON , is there any effective switching rate at all ( full on = steady DC ) ?? .....kw
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Old Feb 22, 2002, 11:45 PM
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"hi-rate" in that context is in comparison the the first electronic ESC's which were frame rate designs - 50 hz. Anything over a few hundred hz is "hi-rate" in that context. The coreless motor issue is a special case, that switching rate is so high the ESC's are useless for any real current draw - say over 5 amps. On is on, there are no switching losses because there is no switching taking place.
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Old Feb 23, 2002, 12:39 AM
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Perhaps higher rate = less pulsing,smoother running motor.

If it was accelerating, deccelerating that would waste power.
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Old Feb 23, 2002, 03:21 AM
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As Chris (and the FAQ) say, for standard iron core brushed motors, we talk about high rate controls by contrast with the earlier and much less efficient "frame rate" controls which switch at about 50Hz. The best rate for these is around 1-3KHz. To get it exact you'd have to work it out for a specific motor as it depends on the loss characteristics of each motor.

The small coreless motors use a much higher switching rate. Because they have no iron core their internal losses are quite different and they can sustain a higher rate (125KHz is typical).

As for brushless motor controllers....

Steve
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Old Feb 23, 2002, 07:01 AM
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Hello Killerwatt:

Great handle. If, for example, you have a 5 pole brushed motor (and coreless motors have brushes) spinning 40,000 RPM you get 5 x 40,000 = 200,000 poles per minute or dividing by 60 you get 200,000/60 = 3333 poles per second. This means that the commutator segments are passing the brushes at 3333 Hertz. If your ESC is switching at 1000 Hertz at part throttle and say 25% duty cycle (on for 25% of 1 millisecond and off for 75% of 1 millisecond) a complete commutator segment may pass by the brushes and never receive any power at all. This may not be a good thing. When I looked at the data on coreless motors and did the calculations I found a situation like the above example existed. If the ESC frequency were raised to 100+ kHz each commutator segment was energized several times as it passed the brush. This is my humble opinion of why the coreless motors require high speed ESCs. And, as stated above, a full on ESC is not switching at all.

Kevin
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Old Feb 23, 2002, 12:23 PM
DNA
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NE Ohio
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Hi Kevin,

Good explanation on the ESC frequency. I'd never really thought
about it that way before. Even though 40k rpm seems a little high,
I suppose it's possible. What is still not clear to me is how this can
affect the brush life of a coreless motor. It has been stated before
that when using a coreless motor with a 3k ESC, when run mostly
at full throttle, the brushes last a very long time. But when run at
partial throttle, the brushes wear out very quickly. This problem of
brush wear seems to eliminated when the coreless is used with a
100k ESC. Now that there is a 9 pole, 9 segment comm. coreless
available, it seems the timing problem is even more exagerated.
Any thoughts on that.

In your previous post about the FET switching more frequently
requiring more power dissipation, how does a 100k ESC handle
what must be a lot more power dissipation?

Thanks
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Old Feb 23, 2002, 02:30 PM
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i'm still confused, assumming the only purpose of any motor IS to safely consume and convert dc Current into rpm and torque and Could continusiously be run at some steady voltage and resultant amp. draw within reason.....And the only reason for ESC is to safely Slow down this Same motor to some other rpm , usually by Removing Completly or Duty-cycle wise, some of the Steady DC into a time-shortned ON Pulse to ALL (preferred) of the Comms( 3,6,or 9) causing the "dead" motor to coast /average Down to a slower RPM, WHAT exactly is Wrong if the DC-pulse does infact miss a total comm segment as long as it doesn't result in an obvious "jerking" effect ????? and even so the heavy air/prop torque -inertia would smooth that effect out....Anyhow , Why would missing any comm segments Result in Any comm/winding burning damage as it Just Slows down the motor ????....IF infact a coreless motor is designed for continous DC running (no pulse) , How could Any pulse rate Hurt anything as the pulse Leading edges probable Don't cause current-flow or torque and at sometime even ESC motors are used at Full on during the latter flight -lower battery runs..... SO my question remains , how does slow ESC switch rate Hurt a coreless ????.................kw
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Old Feb 23, 2002, 02:51 PM
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Coreless motors have very low inductance so the voltage spikes of a switching ESC burn out the brushes. (They can be 10 times the average current) You can filter the voltage with a coil to provide inductance, but it would be very heavy. However, the higher the frequency, the smaller the coil.

The need for a very high frequency ESC then is to be able to use a small coil. Not really due to any motor properties.

An alternative is to use the FET's in linear mode to provide a change in voltage without switching. This is how the Astro Firefly control works. But since the voltage drop is due to resistance, it only works well at low currents (< 2 A).

That's my understanding,
Rick.
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Old Feb 23, 2002, 05:48 PM
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this sounds good but makes no sense to me...what ESC voltage spikes ?? there's no more or less voltage involved than the battery pak input of 6, 7, 9.6 V or whatever used, what "spike" ? or are you refering to,, the normal ESC out-put pulses ( which are just short spurts of the ESC input voltage ) ??? .....also , If referring to coil counter-EMF voltage as the "spikes" , then i would think for sure that the Much Reduced coil Inductance, resulting from the core(iron)-less design Would also generate much Less Counter-Emf (spikes ??).....i believe that it is the counter-emf effect that infact, when operated at a certain voltage and load condition, causes a particular ,normal , running amp - current to flow, that is Much Lower than the normal, ohms-law current expected for the Very low ( milli-0hm ) DC winding resistance ........Soooo , if someone could enlighten me on the ESC " spike " issue ....would appreciate........kw
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Old Feb 23, 2002, 06:29 PM
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in addition to my previous post, it would appear that , infact, even during steady,non-esc pulse operation ,(steady DC) that it IS the winding, comm,and brushes themselves that generate a normal counter voltage that when subtracted from the motors input DC voltage results in the Normal running current.....now, if somehow,the input voltage is of a type (?) that caused little or none of the normal counter EMF to be produced, than a much higher winding, brush-comm running current would occur, with expected damage......so if no current causes no damage, and steady input current also causes no damage ( no ESC or full ON ) than how can any on/off input duty cycle hurt ????...once the motor is turning, EMF is always being generated, regardless of in-put state, thus always " protecting" itself.......brushes and comm included...........kw
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Old Feb 24, 2002, 02:43 AM
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Sorry KillerWatt I'd like to help but I'm finding it almost impossible to decipher your messages.

Perhaps you could get access to an oscilloscope where you will have no trouble seeing the spikes on an ESC output and the differences between coreless and conventional motors. The effects of switching in an inductive circuit are well known and all motors have inductance. Not sure if that helps cus I'm not sure what you're not understanding, sorry.

Steve
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