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Re: (ET) Small E20 fire: What is it with MOVs?
It's doing what it is supposed to do !! It protects the relay contacts.
.... Walt
Metal oxide varistorThe most common type of varistor is the Metal Oxide
Varistor (MOV). This contains a ceramic mass of zinc oxide grains, in a
matrix of other metal oxides (such as small amounts of bismuth, cobalt,
manganese) sandwiched between two metal plates (the electrodes). The
boundary between each grain and its neighbour forms a diode junction,
which allows current to flow in only one direction. The mass of randomly
oriented grains is electrically equivalent to a network of back-to-back
diode pairs, each pair in parallel with many other pairs. When a small or
moderate voltage is applied across the electrodes, only a tiny current
flows, caused by reverse leakage through the diode junctions. When a large
voltage is applied, the diode junction breaks down due to a combination of
thermionic emission and electron tunneling, and a large current flows. The
result of this behavior is a highly nonlinear current-voltage
characteristic, in which the MOV has a high resistance at low voltages and
a low resistance at high voltages.
Varistor current-voltage characteristicFollow-through current as a result
of a lightning strike may generate excessive current that permanently
damages a varistor. In general, the primary case of varistor breakdown is
localized heating caused as an effect of thermal runaway. This is due to a
lack of conformality in individual grain-boundary junctions, which leads
to the failure of dominant current paths under thermal stress.
Varistors can absorb part of a surge. How much effect this has on risk to
connected equipment depends on the equipment and details of the selected
varistor. Varistors do not absorb a significant percentage of a lightning
strike, as energy that must be conducted elsewhere is many orders of
magnitude greater than what is absorbed by the small device.
A varistor remains non-conductive as a shunt mode device during normal
operation when voltage remains well below its "clamping voltage". If a
transient pulse (often measured in joules) is too high, the device may
melt, burn, vaporize, or otherwise be damaged or destroyed. This
(catastrophic) failure occurs when "Absolute Maximum Ratings" in
manufacturer's datasheet are significantly exceeded. Varistor degradation
is defined by manufacturer's life expectancy charts using curves that
relate current, time, and number of transient pulses. A varistor fully
degrades typically when its "clamping voltage" has changed by 10%. A fully
degraded varistor remains functional (no catastrophic failure) and is not
visibly damaged.
Ballpark number for varistor life expectancy is its energy rating. As MOV
joules increase, the number of transient pulses increases and the
"clamping voltage" during each transient decreases. The purpose of this
shunt mode device is to divert a transient so that pulse energy will be
dissipated elsewhere. Some energy is also absorbed by the varistor because
a varistor is not a perfect conductor. Less energy is absorbed by a
varistor, the varistor is more conductive, and its life expectancy
increases exponentially as varistor energy rating is increased.
Catastrophic failure can be avoided by significantly increasing varistor
energy ratings either by using a varistor of higher joules or by
connecting more of these shunt mode devices in parallel.
Important parameters are the varistor's energy rating in joules, operating
voltage, response time, maximum current, and breakdown (clamping) voltage.
Energy rating is often defined using standardized transients such as 8/20
microseconds or 10/1000 microseconds, where 8 microseconds is the
transient's front time and 20 microseconds is the time to half value.
To protect communications lines (such as telephone lines) transient
suppression devices such as 3 mil carbon blocks (IEEE C62.32), ultra-low
capacitance varistors or avalanche diodes are used. For higher frequencies
such as radio communication equipment, a gas discharge tube (GDT) may be
utilized.
A typical surge protector power strip is built using MOVs. A cheapest kind
may use just one varistor, from hot (live, active) to neutral. A better
protector would contain at least three varistors; one across each of the
three pairs of conductors (hot-neutral, hot-ground, neutral-ground). A
power strip protector in the United States should have a UL1449 3rd
edition approval so that catastrophic MOV failure would not create a fire
hazard.
High voltage varistor[edit] HazardsWhile a MOV is designed to conduct
significant power for very short durations (≈ 8/20 microseconds), such as
caused by lightning strikes, it typically does not have the capacity to
conduct sustained energy. Under normal utility voltage conditions, this is
not a problem. However, certain types of faults on the utility power grid
can result in sustained over-voltage conditions. Examples include a loss
of a neutral conductor or shorted lines on the high voltage system.
Application of sustained over-voltage to a MOV can cause high dissipation,
potentially resulting in the MOV device catching fire. The National Fire
Protection Association (NFPA) has documented many cases of catastrophic
fires that have been caused by MOV devices in surge suppressors, and has
issued bulletins on the issue.[citation needed]
A series connected thermal fuse is one solution to catastrophic MOV
failure. Varistors with internal thermal protection are also available.
There are several issues to be noted regarding behavior of transient
voltage surge suppressors (TVSS) incorporating MOVs under over-voltage
conditions. Depending on the level of conducted current, dissipated heat
may be insufficient to cause failure, but may degrade the MOV device and
reduce its life expectancy. If excessive current is conducted by a MOV, it
may explode inside the case, keeping the load connected but now without
any surge protection. A user may have no indication when the surge
suppressor has failed. Under the right conditions of over-voltage and line
impedance, it may be possible to cause the MOV to burst into flames, the
root cause of many fires and the main reason for NFPA's concern.[citation
needed] Properly designed TVSS devices should contain the flames,
eventually resulting in the opening of a safety fuse.[citation needed
-----Original Message-----
From: Pieter Litchfield [mailto:pieter_litch yahoo com]
Sent: Tuesday, March 15, 2011 10:03 PM
To: Elec-Trak Tractor
Subject: Re: (ET) Small E20 fire: What is it with MOVs?
This exactly describes my ET behavior. I have a very heavy E15 (with
bucket loader), and if I move from reverse to forward while the tractor
has ANY backward "freewheeling" (non powered movement) going on, one of
the relays will weld itself immediately, requiring replacement. I suspect
the MOV is long gone. The workaround for me is to apply the foot brake
and wait for a complete stop when reversing. But it can easily generate
enough current to weld the relays.
-----Original Message-----
From: David Roden (Akron OH USA) [mailto:etpost drmm net]
Sent: Tuesday, March 15, 2011 2:22 PM
To: elec-trak cosmos phy tufts edu
Subject: Re: (ET) Small E20 fire: What is it with MOVs?
I haven't looked at an E20 diagram yet, but as a first guess, I'm with
Rhett
on this one. Even with nothing more than residual field magnetism,
voltage can rise to truly astonishing levels in a spinning motor, and if I
understand aright, here the field was powered (even if weakened).
Hitting the pedal while rolling backward, even slowly, has released the
smoke from controller freewheel diodes in road EVs with series motors.
With
a sepex motor, where the armature current isn't reversed through the
field, I can see how rolling forward downhill at high speed could do
something similar.
Also remember that MOVs lose their ability to absorb surges as they age.
They're rather like sponges; eventually they sort of become saturated.
Disclaimer : I am not an engineer.
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