[TowerTalk] filter for rotator DC motor PWM amplifier

[Earl Morse]

Grant,

That's going to be the same filter you would see on the output of a SMPS like in a computer,  adjusted for higher voltage/current.  You need to pass 16 kHz but attenuate everything above 1 MHz.  If I understand that this is a DC motor driving a ring rotator and you are switching 180V off and on at 16 kHz to control speed.  Outputs are ground and switched DC.

Look at the square wave output with a scope.  You might be able to filter it down some if the square wave output has lots of ringing or overshoot/undershoot on the square wave.  They want to switch those transistors very fast so that they don't heat up and overshoot is the result.  Not to mention that the rise time of that square wave is going to determine how high in frequency those harmonics go.  However, rolling that off will increase heating in the transistors.

I think the big problem is that you probably have a 100'+ of cable going from the PWM box to the ring rotor and that the twisted pair might not be enough.  It might require shielded twisted pair, especially since you note that you can hear it on another tower a distance away.  Throw a big chunk of ferrite on the pair at the power supply end.  If you notice a big change, especially on the higher bands if you use the standard type 43, 850u, 20-200 MHz, ferrite then this is a common mode problem and not purely differential.  Put a separate chunk of ferrite on each lead if you think it is differential.  You want to make sure to have enough there so that it doesn't saturate under high current.

I have been seeing 5000u and 10000u ferrite showing up with ferrite vendors.  I have some samples of it that I am experimenting with for baluns.  Looks too lossy for RF power applications and the supplier is selling it for EMC suppression.  Wound some 9:1 beverage transformers up and need to get around to testing them.  Nice to be able to use 4-5 turns instead of 60 to make the transformer.  Have to see how lossy they will be.  The problem with these cores in your application I think is going to be the high current and saturation of the core making it useless as a suppressor, unless you have a big enough core.

Earl
N8SS

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Message: 3
Date: Wed, 26 Aug 2015 19:11:59 -0700
From: Grant Saviers <grants2 at pacbell.net>
To: towertalk at contesting.com
Subject: [TowerTalk] filter for rotator DC motor PWM amplifier
Message-ID: <55DE71EF.0 at pacbell.net>
Content-Type: text/plain; charset=utf-8; format=flowed

I'm seeking some guidance for the design of a PWM amplifier noise filter 
for a 180v 1/2 hp DC motor that drives a ring rotator.  My googling 
turns up a lot of information for 3 phase VFD filters but little for DC 
motor applications, so I'm hoping there is some experience/expertise 
among towertalkians.

The PWM amp is a commercial Minarik 16Khz switch and drives the motor 
via twisted pair.  The amp is in a RFI gasketed Hoffman steel enclosure 
and I have a Corcom 20a/240v line filter on the AC source.  However, I 
have hash everywhere and S9 noise on a separate tower 20m antenna.  The 
overwhelming source is the PWM amp, not motor brush hash. The box is 
grounded to the tower master ground plate.

I think a DC filter design is similar that of AC line filters, a equal 
current forcing/common mode balun followed by capacitors followed by an 
L-C filter in each leg.   My major question is about the practical 
parameters of a balun in this application.  A high perm toroid seems a 
likely choice for a parallel L1-L2 balanced winding.  I have some 
FT-193-JA cores, u=5000 and the question is what assumptions/parameters 
govern the balun winding design?  For the L in the L-C section, high 
current toroid inductors are widely available (e.g 50uH @ 10amps).

Any links to references would be appreciated.

Grant KZ1W


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