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Old Jan 01, 2013, 02:56 PM
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United States, NJ, Frenchtown
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Question
50 % speed of a ESC, The losses are highest.

Why do people say they are the highest over the entire speed range ? What becomes so dominant that they state it is the roughest temperature point ?

Thanks
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Old Jan 01, 2013, 06:58 PM
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Switching losses in the FETs.

When a FET is either ON or OFF, it's very nearly ideal. When it's switching though, it passes through all resistances between 0 and infinite. As it does so, it wastes some energy through I2R losses.

At high duty cycles (near 100%) it carries high currents when ON, but doesn't switch very often. Losses when ON are very low compared to when switching though.
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Old Jan 01, 2013, 07:07 PM
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Bruce Abbott's Avatar
Hastings, New Zealand
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Obviously it won't be 'highest over the entire speed range'.

At zero throttle the losses will be minimal.

At full throttle the dominant loss is FET on-resistance.

At part throttle, a number of factors come into play.

1. The higher PWM switching frequency increases transitional conduction losses because the FETs are being turned on and off more often. It also increases the power required to drive the gates.

2. In most ESCs the high-side FET body diodes are used to recirculate motor current during the 'off' part of each PWM cycle. These diodes typically drop ~0.7V when conducting, compared to <0.1V drop when the FETs are turned on.

3. Peak motor current is higher because the back-emf is lower, and winding inductance is not high enough to completely smooth out current variations.

It's not easy to calculate the exact point at which ESC losses will be highest, as it varies depending on characteristics of the motor and load. However in most cases it will be somewhere above 50% throttle, because prop loading (and therefore current draw) drops rapidly as rpm reduces.
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Old Jan 01, 2013, 07:09 PM
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True, body diodes are evil things.

Having a bit more of a think, I have a strange feeling body diode conduction probably contributes to total losses more than resistance through the miller plateu...
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Old Jan 01, 2013, 08:12 PM
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You have found my free wheeling diode ?
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Old Jan 01, 2013, 09:01 PM
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Didn't know it was missing! Mine are generally in my sock drawer, or down the back of the couch...
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Old Jan 02, 2013, 04:08 AM
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Romania, Dolj, Craiova
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What about N-FET only ESCs ?
Mine are cold even if packed two ESC sharing a single heatsink, wrapped, and between two carbon plates, with no airflow, like in this picture.
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Old Jan 02, 2013, 05:25 AM
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n-fets have a lower RDSon than an equivelant p-fet, but that only affects heat when ON. During the switching or body diode conduction, there's no difference.
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Old Jan 02, 2013, 05:29 AM
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This is why the YGE or YEP ESCs have freewheeling.

And presto, I saw 5-10% more efficiency when I used YEP instead of Turnigy plush.
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Old Jan 02, 2013, 06:16 AM
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I was wondering how much effect that has.
I run two of them on my helis and one in an EDF, and am amazed at how much cooler they come down. I wasn't sure how much of an effect it'd have on the motor...
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Old Jan 02, 2013, 06:36 AM
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Clue me in... what is freewheeling?

Is it a ZVS or ZCS strategy?
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Old Jan 02, 2013, 06:46 AM
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My understanding is neither. Before the FET is turned on, the body diode will be conducting. It's just there to move the current flow from the diode to the FET channel.

Say phase C was just switched to floating. The field in the inductor begins to collapse, sending current through the body diode. Freewheeling is the action of the current running from phase C to phases A and B. Using the FET to do the freewheeling keeps them cooler, and puts that energy back into the motor.
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Old Jan 02, 2013, 06:47 PM
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Quote:
Originally Posted by jakestew View Post
Clue me in... what is freewheeling?
Perhaps this will help. Synchronous Rectification.
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Old Jan 08, 2013, 01:30 PM
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Finland
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Hi all

Big ESC's are pricy and I just had to do some data mining. I found this thread very interesting. I bought YEP after reading this. What I've heard it's copy of YGE with old software. Electrical engineering background helps but is not mandatory
http://www.helifreak.com/showthread.php?t=443322

Jyri
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