Dale,
I agree in some instances but the discussion here has been for motors of
significant size in appliances. Rise times when driving something such
as a motor with a large mass is not that important. I have done
considerable work driving fairly large stepper motors and assuming a
drive frequency of 5kHz (period of 200us) I could never measure any
performance differences when the rise time was .1us, 1us or, 5us. Yet a
5us rise time produced much less RFI. However a rise time of 50us would
seriously produce a different waveform and probably affect the
performance of the motor.
Every large appliance I have tamed was not fixable with clamp on ferrite
cores (even with multiple turns through the core) which only affect
common mode currents. In all cases I needed to insert (cut into wiring)
and put in a differential filter. The differential filter slowed the
motor drive rise times and the RFI was eliminated or greatly reduced.
My $.02
Larry, W0QE
On 8/16/2012 9:13 AM, Dale wrote:
> Larry,
>
> The issue of rise time may also be inflexible for some systems. While I have
> often encountered design situations in which rise time was considerably
> faster than required (the digit guys wanted "reserve speed"), there are other
> cases in which the interfacing devices (at either end of a signal feed)
> require very fast risetimes and the resulting steep "fronts" they present.
> You then have an RFI/EMC issue to resolve.
>
> 73, Dale
> WA9ENA
>
>
> -----Original Message-----
>> From: Larry Benko <xxw0qe@comcast.net>
>> Sent: Aug 16, 2012 9:48 AM
>> To: rfi@contesting.com
>> Subject: Re: [RFI] Motors/controllers and something to think about
>>
>> Jim and others,
>>
>> I have taken several EMC compliance classes over the years and they
>> always stress that there are 3 main issues that contribute to RFI. They
>> are in no particular order:
>>
>> 1.) Signal rise time
>> 2.) Signal current
>> 3.) Loop area of wire carrying the signal
>>
>> Signal current is usually the least flexible parameter but rise time and
>> loop area are quite easy to alter. Components can affect the signal
>> rise time and good construction can reduce loop area. Loop area
>> problems can be both printed circuit board issues and external wiring.
>> No earth shaking news here.
>>
>> 73,
>> Larry, W0QE
>>
>>
>> On 8/15/2012 11:40 PM, Jim Brown wrote:
>>> On 8/15/2012 10:30 PM, Roger (K8RI) wrote:
>>>> The issue has dealt with specific makes and models with and without RFI,
>>>> but as PWM is by its nature an RFI generator we need to stop and think
>>>> about how companies (any company) acquires the motors and speed
>>>> controllers.
>>> It is critically important to realize that it is EASY to build a PWM
>>> system without RFI if you simply pay attention to the fundamental laws
>>> of physics -- keep track of where the current flows, ALL of the
>>> current, including DC, baseband AC, and RF -- and design your product so
>>> that it flows in loops that have small cross sectional areas, and over
>>> wires that don't make good antennas. It's really as simple as that.
>>> The most recent edition of Henry Ott's classic text on EMC even has
>>> chapters devoted to switching power supplies and PWM motor controllers.
>>>
>>> 73, Jim Brown K9YC
>>> _______________________________________________
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>>>
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