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Electric Coil Safety
Name: Alick B.
Status: student
Age: 17
Location: N/A
Country: N/A
Date: 12/20/2004
Question:
For my physics project i am using an experiment using a
ten thousand winding transformer coil. however I am concerned that the
back emf generated when the apparatus is switched off will be very large
(as dI/dt is proportional to the back emf?) Would you tell me whether
this experiment is safe to carry out, or what precautions i would need
to take to minimize the risk?
Replies:
Alick,
Your concerns are vary warranted. Unfortunately, we will need to know more
about the coil to FULLY answer the question. But in pretty much all cases
will have a RECOIL voltage caused by the collapse of a magnetic field. The
field energy MUST go somewhere. It will then manifest itself as a rather
high voltage present across the coil winding leads. I believe that the
voltage could be presented as v = L * di / dt. More precisely it would be
v(t) = L * di(t) / dt ...where v(t) and i(t) are both time dependent as this
is a transient state response discussion of a coils emf recoil.
I hope this helps.
Darin
You do not say the currents involved, but you should definitely be
careful, I think the word is "teacher supervision" etc. Do not touch
things that are live. Before you touch things that have been turned off,
touch them first with a wire that is grounded, to remove any excess
charge. If this sparks, then it shows that you have some power there,
and you might want to put a resistor in series with the grounding to
lessen the current spark. You might consider leaving a high ohm
resistor on the output anyway, just to never let too much voltage
accumulate.
If you put a diode across the input of the coil, an make it so that it
is reversed biases when the coil is charging, then the diode will
conduct if the coil bucks this voltage with any opposite polarity
voltage. So the diode can help remove opposite polarity voltage.
Steve Ross
I think you have a point there, Alick.
A good number of Neon Sign Transformers have probably ruined themselves,
punching holes in their own insulation, deep inside,
from being switched on and off instantaneously by simple mechanical switches.
Once there is such a puncture the coil has a short circuit, draws too much
current, and overheats itself badly.
That is when you usually notice it has a problem
The voltage impulse can make a longer-than-expected spark at the output
coil too.
The electrocution danger to anyone caught by this spark depends on the
size of the stored energy in Joules.
The maximum stored energy is proportional to the weight of the
transformer's iron core.
Most old neon sign transformers are large enough to be dangerous just from
the stored energy.
Newer, more compact ones might be less so.
There are ten-thousand-winding transformers smaller and less powerful than
neon sign transformers.
Automotive ignition coils, and the flyback transformers used in ionizers
might be in this category.
These may be a little less dangerous to themselves and to you at
switch-off, but they still need to be taken seriously.
There are a bunch of ways to reduce the dI/dt at the instant of turn-off:
- capacitors or series resistor-capacitor pairs
- voltage clamping:
- spark gaps,
- encapsulated air-gaps,
- neon bulbs,
- MOV's (metal-oxide-varistors),
- other semiconductor devices related to SCR's
- soft-switching the primary:
- Variac
- SSR with zero-crossing turn-off (they usually do that)
If it uses 120vac, run the transformer from a "Variac", an variable
auto-transformer.
Only turn the main switch on and off when the "Variac" is dialed down to zero.
Using a Solid-State-Relay (SSR's) to switch the power on and off is
quicker than a Variac,
but safer than the mechanical switch, because they never turn off except
at when the input 60Hz sine-wave crosses through zero current.
(Actually, SSR's might quietly get damaged by the transformer.)
The magnetic energy is stored in the core, which is surrounded by both
coils almost equally.
To a large extent, the stored energy can be relieved through either the
primary or secondary, interchangeably.
Often it is easier to find a proper clipping or filtering or
soft-switching device for 120vac than for 10 Kvac.
Having an MOV protection device (usually look like a large, paint-dipped
disc capacitor) is another good way.
For this you choose one which has a working voltage of 120-180vac, which
means its breakdown voltage is about twice that high.
So when you put it in parallel with the primary coil, it will stay safely
turned off,
except during those instants of surge voltage coming backwards out of the
transformer.
At those instants it turns on just enough to short circuit the excess voltage.
dI/dt is low, because the I is instantaneously unchanged,
it just starts smoothly declining at a rate dI/dt = V/L, where L is the
inductance of the primary
and V is either the clamping voltage of the MOV (~300V) or the surge
voltage from the transformer,
which can be considerably higher than that.
You will not find an MOV rated for 10KV, so you cannot put one on the output.
No problem, that is the range in which it is easy to make a spark-gap.
Make the gap about 50%-100% wider than what can barely spark over during
normal powered-on usage.
Once a spark starts, it can have almost zero resistance,
so there should be a fuse in series with the primary to stop large
currents after an occasional spark when powered-on.
Please have high-voltage working principles when adding these parts.
Left hand in your pocket, do not move a wire without turning it all off;
try to have a thick rubber mat under your feet; and work on a
non-conductive bench.
Jim Swenson
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