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#127986 04/21/02 03:29 PM
Joined: Oct 2000
Posts: 5,389
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sparky Offline OP
Member
my new buzz word for the week...{pun intended} , .'Reflected Waveform' ...

apparently happens when a large load is shut down fast???

[Linked Image]

#127987 04/21/02 09:29 PM
Joined: May 2001
Posts: 160
C
Member
Sounds good to me.Reflected energy happens when the load is not properly matched to the generator/transmission line,thus causing "standing waves "on the line.This is seen quite often in higher frequency communication circuits,however it can happen at power line frequencies under the conditions that you describe.

Chris

#127988 04/22/02 04:19 AM
Joined: Apr 2002
Posts: 4
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Junior Member
Sparky,

It can also happen when "sudden draw" loads are switched on. Typically these include power supplies for home electronics such as computers and stereos. These surges are one reason I've always recommended that people *never* use a power strip to turn their computer and monitor on/off.

Scott

#127989 04/22/02 09:19 PM
Joined: Oct 2001
Posts: 597
E
Member
Another variation of a reflected waveform occurs on power transmission lines at the point that they go from overhead to underground.

A pulse, commonly from lightning, will propagate along the overhead line and will reflect off the impedance change to shielded underground cable. During the interval of reflection the voltage adds to itself, doubling the pulse voltage stress on the underground cable.

Al


Al Hildenbrand
#127990 04/23/02 04:00 PM
Joined: Mar 2002
Posts: 582
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Ron Offline
Member
I believe that is the problem that occurs when feeding a motor with the VFD installed >100' away.


Ron
#127991 04/23/02 08:50 PM
Joined: Mar 2001
Posts: 2,056
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Member
I know when inductive loads are de-energized, the magnetic field collapses and dumps the energy back onto the power line.
As far as I know, "Reflected Waves' occur on high frequency circuits such as the output of VFDs and are a result of a mismatch between the carrier frequency of the VFD (2-8 kHZ) and the impedance of the circuit. This can result in up to 1,600 volts appearing at the motor terminals, depending on the circuit length. Termination kits are available from most VFD manufacturers to help minimize the problem.

#127992 04/27/02 03:22 PM
Joined: Aug 2001
Posts: 7,520
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A reflected waveform, which in turn sets up a standing wave on a transmission line occurs any time there's an impedance mismatch between source and load.

At 50/60Hz powerline frequencies it's usually of little consequence, but it becomes very significant at radio frequencies, especially at UHF and above.

The ultimate is terminating an RF transmission line with a short or leaving it completely open circuit. For example, with a shorted line of a quarter-wavelength at the frequency in use, the source driving will "see" not a short, but an infinitely high impedance.

Such lines are often used in RF work as filters.

#127993 04/28/02 09:56 AM
Joined: Apr 2002
Posts: 53
C
Member
So, what happens when a standing wave occurs on a 60Hz line. I am familiar with the symptoms on PLC networks. (Nodes at any location randomly cease operating)
Does it harm appliances? Blow Breakers? Outlet stopworking with no other symptoms?
Are they caused by the inrush current?
Or are they caused by the collapse of the EMF field? Or both?

Just trying to add another layer to what I already know! [Linked Image]

#127994 04/28/02 12:17 PM
Joined: Feb 2002
Posts: 36
3
Member
A spike on the power or transmission lines will be described best by the common engineering formula, dv/dt. These singular occurances are considered high frequency events which can cause "ringing" into the entire grid infrastructure in accordance with fr= 1/(6.28*sq.rt.of L*C. There will be a natural electrical resonant frequency for ANY amount of inductance and capacitance, with the resistive portion tending to swamp (dampen)the ringing. This is never problematic for the relatively low frequency of 60Hz. A spike, however, will set off those natural resonances within the system, although I really don't see those being problematic either, as they are of short duration. Speaking of high frequency communication circuits, keep in mind the utilities do xmit and rcv MF (med.freq.)carriers on their xmission lines to monitor and relay curent line conditions in real-time......Burns

#127995 04/29/02 03:19 PM
Joined: Aug 2001
Posts: 7,520
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To look at why this affects higher frequencies more, just look at this in terms of wavelength.

At 300MHz for example, one wavelength is just 1 meter, or a little over 3 ft., so a short length of feeder can be significant. But at 60Hz powerline frequency, one wavelength works out at over 3000 miles, hence the effects are much smaller.

This is all tied in with the capacitive and inductive effects and the resonant frequency, as given by the formula above (in case anyone hasn't figured out the 6.28, it's 2 times pi). This is why a small capacitance of, let's say, 10pF will have virtually no measurable effect on a 60Hz power line, but can play havoc with UHF television signals.


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