Dear Mr Chong Sorry to be so long replying. Please forgive me if I'm not answering the right questions, or if I'm telling you stuff you already know.
Q: Finding the EM in a circuit using network analysis:
I'm no expert on finding EM in a circuit using network analysis (I'm
more of a hands-on guy) but there will be many people subscribing to
emc-pstc who will be able to help you with this.
Q: Why does EM happen in motors and drives?
Well, every time we use electricity for any reason at all we are really
using electromagnetic forces and fields which can never be entirely
contained and so leak out. Every electrical and electronic product has
its own EM signature for emissions and its own susceptibilities to EM
events in its environment.
Did you know that the first law on EMC was passed in the UK in 1897 or
thereabouts? It was called "The Arc Lamp Trimming Act" or something
similar, and concerned the very first attempts to provide an electrical
supply which various users could connect to. It turned out that if one
user did not keep his arc lamp electrodes trimmed correctly (as they
burnt away) it made his lamp splutter and this caused voltage
fluctuations on the common supply which made everyone else's lamps
flicker annoyingly. EM and EMC has been with us ever since electricity
was first used.
Getting back to the question....
Briefly considering the sources of electromagnetic emissions in motors
and drives we find the following:
AC Motors on their own, excited by pure sinewaves: These all have
slightly non-linear magnetic circuits which means that they emit
harmonics of their exciting frequency, as well as "cogging" frequencies
related to the number of poles and rotational speed. All these are low
frequencies, and usually only a problem where motor loading is a
significant proportion of supply network capacity.
Commutator-type DC Motors on their own, excited by pure DC: These have
sparks around their commutators caused by the fields in the rotor
windings collapsing and flying back as they become disconnected in turn.
These sparks emit very wideband energies, from the commutator switching
rate right up to well over 1GHz (probably continuing right up to
daylight), and this can be a significant emissions problem, usually
worst for smaller cheaper motors. It is not uncommon to find a small DC
motor in a children's toy emitting more VHF interference than a large
industrial DC motor.
There are DC motors available which don't spark (or don't spark very
much): these include some pancake types with "PCB rotors", and motors
with varistor discs connected to the windings on their rotors (e.g.
small motors found in some CD players). Some manufacturers also provide
DC motors fitted with internal filters to reduce their emissions.
Stepper motors: The drive waveform and current is the main source of
emissions.
Switched-reluctance motors: Again, the drive waveform is the problem.
Apart from the above, all motors and their cables will also emit
electric and magnetic fields related to their exciting voltages and
their currents, which is why motor drives are often the main problems
for EMC these days.
Adjustable speed DC and AC ("invertor") drives, and stepper motor
drives, all are naturally electrically noisy due to the high-power
rapidly-changing waveforms they use. A Fourier analysis of their output
waveforms, and their typical electromagnetic emissions (conducted and
radiated), shows that typical stepper and low-power DC drives can have
significant emissions up to 10MHz or so, while typical small AC invertor
drives can have significant emissions up to 1,000 times their internal
switching rate (which is usually 50 to 100 kHz, giving significant
emissions up to 100MHz in some cases). DC drives and steppers are not
usually so bad, mostly because their basic internal switching rate is so
much less (50Hz to 5 kHz or so).
Larger power drives tend to use higher-power semiconductors which switch
more slowly, so their internal switching rate is lower and the upper
frequency at which their switching harmonics cause significant emissions
also tends to be lower.
Most of the EMC problems in industrial systems these days are caused by
three things: AC supply waveform distortion (harmonics); mobile
radiocommunications (walkie-talkies and cellphones); and motor drives
(especially invertor drives for AC motors).
As far as EM immunity is concerned: motors are generally very rugged
indeed, although high levels of harmonic distortion on the AC supply can
cause AC motors to overheat and to wear out their bearings more quickly.
The problems with immunity are otherwise all down to the electronics in
the drives, and since even very high power drives are increasingly
likely to have their timing controlled by a tiny microchip they tend to
have all the immunity problems of tiny microchips (i.e. almost all the
immunity problems that exist!).
Q: Any specifications on EMC in motors and drives? or any references?
The European Commission had harmonised the IEC standard IEC 1800-3:1996
as EN 61800-3:1996, "Adjustable speed electrical power drive systems,
part 3: EMC product standard" and this is the only standard I'm aware
of which specifically relates to power drives. I understand that this
standard is not highly regarded by EMC Competent Bodies and Test Labs in
the UK as I understand it puts much of the onus on achieving compliance
with the EC's EMC Directive onto the user of the system, rather than its
manufacturer or supplier.
As for references: any good technical library will be able to find
hundreds of books on motors and their fields, but only some of these
will deal with EM issues so you might try searching for "motors" and
"emissions" or "EMC". The IEE library would be a good place to do such a
search for you Email [email protected], website:
http://www.iee.org.uk/Library/libgen.htm, although they would make a
charge for it. Ditto a search for drives and EM.
I attach a list of some books on EMC (emcbooks.doc in Word 6 format)
which may help, although I don't think any of them deal solely with
motors and their drives.
Tim Williams' book "EMC for Product Designers" is a good introduction
for the circuit designer, including designers of motor drives.
I hope this all helps!
Keith Armstrong
Partner, Cherry Clough Consultants
Cherry Clough House, Rochdale Road, Denshaw, OL3 5UE, UK
phone: +44 1457 871 605, fax: +44 1457 820 145, Email:
[email protected]
EMCBOOKS.DOC
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