- CIRCUIT OPERATION
The Magnabend sheetmetal folder is designed as a DC clamping
The simplest circuit required to drive the electro-magnetic coil
consists of a switch and a bridge rectifier only:
It is to be noted that the ON/OFF switch is connected on the AC side
of the circuit. This allows the inductive coil current to circulate
through the diodes in the bridge rectifier following turn-off until
current decays exponentially to zero.
(The diodes in the bridge are acting as "fly-back"
For safer and more convenient operation it is desirable to have a
circuit which provides a 2-handed
interlock and also 2-stage
The 2-handed interlock helps to ensure that fingers cannot be
caught under the clampbar and the staged clamping gives a softer start
and also allows one hand to hold things in place until the pre-clamping
with Interlock and 2-Stage Clamping:
the START button is pressed a small voltage is supplied to the magnet
coil via the AC capacitor thus producing a light clamping effect.
This reactive method of limiting the current to the coil
no significant power dissipation in the limiting device (the capacitor).
Full clamping is obtained when both
the Bending Beam-operated switch and
the START button are operated together.
the START button would be pushed first (with the left hand) and then
the handle of the bending beam would be pulled with the
other hand. Full clamping will not occur unless there is some
overlap in the operation of the 2 switches. However once full clamping
is established it is not necessary to keep holding the START button.
small but significant problem with the Magnabend machine, as with most
electro-magnets, is the problem of residual magnetism. This is the
small amount of magnetism that remains after the magnet is turned OFF.
It causes the clamp-bars to remain weakly clamped to the
body thus making removal of the workpiece difficult.
of magnetically soft iron
is one of many possible approaches to overcoming
this material is hard to obtain in stock sizes and also it is
physically soft which means that it would be easily damaged in a
inclusion of a non-magnetic gap in
the magnetic circuit is perhaps the simplest way to reduce remnant
This method is effective and is fairly easy to achieve in a
fabricated magnet body - just incorporate a piece of cardboard
aluminium about 0.2mm thick between say
the front pole and the core piece before bolting the magnet parts
together. The main drawback of this method is that the
non-magnetic gap does reduce the flux available for full clamping.
Also it is not straight forward to incorporate the gap in a
one-piece magnet body as used for the E-type magnet design.
reverse bias field,
produced by an auxiliary coil, is also an effective method. But it
involves unwarranted extra complexity in the manufacture of the coil
and also in the control circuitry, although it was used briefly in an
early Magnabend design.
decaying oscillation ("ringing") is conceptually a very
good method for demagnetising.
oscilloscope photos depict the voltage (top trace) and current (bottom
trace) in a Magnabend coil with a 90 micro-farad capacitor connected
across it to make it self oscillate. (The AC supply has been turned off
approximately in the middle of the picture).
first picture is for an open magnetic circuit, that is with no clampbar
on the magnet. The second picture is for a closed magnetic
circuit, that is with a full length clampbar on the magnet.
the first picture the voltage exhibits decaying oscillation
(ringing) and so does the current (lower trace), but in the second
picture the voltage does not oscillate and the current does not even
manage to reverse at all. This means that there would be no oscillation
of the magnetic flux and hence no cancellation of residual magnetism.
problem is that the magnet is too heavily damped, mainly due to eddy
current losses in the steel, and thus unfortunately this method does
not work for the Magnabend.
is yet another
idea. If the magnet is too damped to
self-oscillate then it could be forced to oscillate by active circuits
supplying energy as required. This has also been thoroughly
investigated for the Magnabend. Its main drawback is that it involves
overly complicated circuitry.
is the method which has proved most cost-effective for the Magnabend.
The details of this design represent original work performed by
Magnetic Engineering Pty Ltd. A detailed discussion follows:
essence of this idea is to store energy in a capacitor and then to
release it into the coil just after the magnet is turned off.
polarity needs to be such that the capacitor will induce a reverse
current in the coil. The amount of energy stored in the
can be tailored to be just sufficient to cancel the residual magnetism.
(Too much energy could overdo it and re-magnetise
magnet in the opposite direction).
A further advantage of the
reverse-pulse method is that it produces very fast demagnetising and an
almost instant release of the clampbar from the magnet. This
because it is not necessary to wait for the coil current to decay to
zero before connecting the reverse pulse. On application of the
pulse the coil current is forced to zero (and then into
very much faster than its normal exponential decay would have been.
Now, normally, placing a switch contact between the rectifier and the
magnet coil is "playing
is because an inductive current cannot be suddenly interrupted. If it
is then the switch contacts will arc and the switch will be damaged or
even completely destroyed. (The mechanical equivalent would be trying
to suddenly stop a flywheel).
Thus, whatever circuit is devised it
must provide an effective pathway for the coil current at all times,
including for the few milliseconds while a switch contact changes over..
above circuit, which consists of only 2 capacitors and 2 diodes (plus a
relay contact), achieves the functions of charging the Storage
capacitor to a negative voltage (relative to the reference side of the
coil) and also provides an alternative pathway for coil current while
the relay contact is on the fly.
Broadly D1 and C2 act as a charge pump for C1 while
D2 is a clamp diode which holds point B from going positive.
the magnet is ON the relay contact will be connected to its "normally
open" (NO) terminal and the magnet will be doing its normal job of
clamping sheetmetal. The charge pump will be charging C1
a peak negative voltage equal in magnitude to the peak coil voltage.
The voltage on C1 will increase exponentially but it will be fully
charged within about 1/2 a second.
It then remains in that state until the machine is turned OFF.
after switch-off the relay holds in for a short time. During this time
the highly inductive coil current will continue to recirculate thru the
diodes in the bridge rectifier. Now, after a delay of about
milliseconds the relay contact will start to separate. The
current can no longer go thru the rectifier diodes but instead finds a
path thru C1, D1, and C2.
The direction of this current is such that it will further
increase the negative charge on C1 and it will begin to charge C2 also.
The value of C2
needs to be large enough to control the rate of
voltage rise across the opening relay contact to ensure that
arc does not form. A value of about 5 micro-farads per amp of coil
current is adequate for a typical relay.
details of the waveforms which occur during the first half a
second after turn OFF. The voltage ramp which is being
by C2 is clearly visible on the red trace in the middle of the figure,
it is labeled "Relay contact on the fly". (The actual
time can be deduced from this trace; it is about 1.5 ms).
As soon as
the relay armature lands on its NC terminal the negatively charged
storage capacitor is connected to the magnet coil. This does
immediately reverse the coil current but the current is now running
"uphill" and thus it is quickly forced thru zero and towards a negative
peak which occurs about 80 ms after the connection of the storage
capacitor. (See Figure 5). The negative current
a negative flux in the magnet which will cancel out the residual
magnetism and the clampbar and workpiece will be quickly released.
and Current Waveforms on Magnet Coil
above depicts the voltage and current waveforms on the magnet coil
pre-clamping phase, the
full clamping phase,
is thought that the simplicity and effectiveness of this demagnetising
circuit should mean that it will find application in other
electromagnets that need demagnetising. Even if residual
magnetism is not a problem this circuit could still be very useful to
commutate the coil current to zero very quickly and hence give rapid
The circuit concepts discussed above can be
combined into a full circuit with both a 2-handed interlock
pulse demagnetising as shown below
Figure 6: Combined
This circuit will work but unfortunately it is somewhat unreliable.
To obtain reliable operation and longer switch life it is necessary to
add some extra components to the basic
circuit as shown below (Figure 7):
Figure 7: Combined
Circuit with Refinements
This is a 2-pole isolating switch. It is added for convenience and to
comply with electrical standards. It is also desirable for
switch to incorporate a neon indicator light to show the
ON/OFF status of the circuit.
Without D3 the
latching of the relay
is unreliable and depends somewhat on the phasing of the mains waveform
at the time of operation of the bending beam switch. D3 introduces a
delay (typically 30 milli seconds) in the drop out of the relay. This
overcomes the latching problem and it is also beneficial to have a drop
out delay just prior to the onset of the demagnetising pulse (later in
the cycle). C4
coupling of the relay circuit which would otherwise be a
half-wave short circuit when the START button was pressed.
This switch has its housing in contact with
magnet body and it will go open circuit if the magnet gets too hot
(>70 C). Putting it in series with the relay coil means that it
has to switch the small current through the relay coil rather than the
full magnet current.
When the START button is pressed the relay pulls in and then there will
be an in-rush current
which charges C3 via the bridge rectifier, C2 and diode D2.
Without R2 there would be no resistance in this circuit and the
resulting high current could damage the contacts in the START switch.
Also, there is another circuit condition where R2 provides protection:
If the bending beam switch (SW2) moves from the NO terminal (where it
would be carrying the full magnet current) to the NC terminal, then
often an arc would form and if the START switch was still being held at
this time then C3 would in effect be short circuited and, depending on
how much voltage was on C3, then this could damage SW2. However again
R2 would limit this short circuit current to a safe value. R2 needs
only a low resistance value (typically 2 ohms) in order to provide
The varistor, which is connected between the AC terminals of the
rectifier, normally does nothing. But if there is a surge
on the mains (due to for instance - a nearby lightening
strike ) then the varistor will absorb the energy in the surge
prevent the voltage spike from damaging the bridge rectifier.
If the START button was to be pressed during
a demagnetising pulse
then this would likely cause an arc at the relay contact which
turn would virtually short-circuit C1 (the storage
The capacitor energy would be dumped into the circuit consisting of C1,
the bridge rectifier and the arc in the relay. Without R1
is very little resistance in this circuit and so the current would be
very high and would be sufficient to weld the contacts in the relay. R1
provides protection in this (somewhat unusual) eventuality.
Note re Choice of R1:
If the eventuality described above does occur then R1 will absorb
virtually all of the energy that was stored in C1 regardless of the actual value of
We want R1 to be large compared with other circuit resistances but
small compared with the resistance of the Magnabend coil (otherwise R1
would reduce the effectiveness of the demagnetising pulse). A
value of around 5 to 10 ohms would be suitable but what power rating
should R1 have? What we really need to specify is the pulse power, or energy
rating of the resistor. But this characteristic is not usually
specified for power resistors. Low value power resistors are usually
wire-wound and we have determined that the critical factor to look for
in this resistor is the
amount of actual wire used
in its construction. You need to crack open a sample resistor and
measure the gauge and the length of wire used. From this
calculate the total volume of the wire and then choose a
with at least 20
(For example a 6.8 ohm/11 watt resistor from RS Components was found to
have a wire volume of 24mm3).
these extra components are small in size and cost and hence add only a
few dollars to the overall cost of the Magnabend electrics.
is an additional bit of circuitry that has
not yet been discussed. This overcomes a relatively minor
If the START button is pressed and is not
followed by pulling on the handle (which would otherwise give full
clamping) then the storage capacitor will not be fully charged and the
demagnetising pulse that results on release of the START button will
not fully demagnetise the machine. The clampbar would then remain stuck
to the machine and that would be a nuisance.
The addition of D4 and R3, shown in blue
in Figure 8 below, feed a suitable waveform into the charge pump
circuit to ensure that C1 gets charged even if full clamping is not
applied. (The value of R3 is not critical - 220 ohms/10 watt
would suit most machines).
information about circuit components please refer to the Components
section in "Build Your Own Magnabend"
reference purposes the full circuit diagrams of 240 Volt AC, E-Type
Magnabend machines manufactured by Magnetic Engineering Pty Ltd are
Note that for operation on 115 VAC many component values would need to
Engineering ceased production of Magnabend
machines in 2003
when the business was sold.
The above discussion was intended to explain the main principles of the
circuit operation and not all details have been covered. The
circuits shown above are also included in the Magnabend
manuals which are available elsewhere on this site.
It is also to be
noted that we developed fully solid state versions of this
which used IGBTs instead of a relay to switch the current.
The solid state circuit was never used in any Magnabend machines but
was used for special magnets that we manufactured for production lines.
These production lines typically turned out 5,000 items (such
a refrigerator door) per day.
Engineering ceased production of Magnabend
machines in 2003
when the business was sold.
Please use the Contact
Alan link on this site to seek more information.