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Recently, Engineered Lifting Systems fielded a call from a
customer with production that includes the hoisting of
molten metal. The question was raised regarding the use of a
power circuit limit switch (upper limit) on a Magnetek
variable frequency drive controlled hoist.
Over the
years, it has been my experience that the majority of hoists
handling molten metal use an upper power circuit limit
switch as the final means of preventing a “two-blocked”
condition. Two-blocking is the term referring to the event
when the hoist raises the bottom block until the bottom
block applies force to the hoist unit itself, or the support
structure for the hoist.
You
cannot “break” the motor leads from a variable frequency
drive to a motor in operation without damaging the
drive. The customer asked
for an opinion on the application and how
to solve the problem.
To begin,
there are several requirements of OSHA to remember on any
hoist application. First, 1910.179(n)(4)(i) states:
“At
the beginning of each operator's shift, the upper limit
switch of each hoist shall be tried out under no load.
Extreme care shall be exercised; the block shall be "inched"
into the limit or run in at slow speed. If the switch does
not operate properly, the appointed person shall be
immediately notified.”
This
means that a limit switch must be available for the operator
to test at the beginning of ever shift. Typically, this
would be a rotary geared limit, set below (in elevation) any
upper paddle or power circuit limit switch.
Next, it
is important to remember 1910.179(n)(4)(ii),
“The
hoist limit switch which controls the upper limit of travel
of the load block shall never be used as an operating
control.”
Originally in ANSI Z241.2 (a standard which has been
discontinued) and now in ASTM (originally
known as the American Society for Testing and Materials)
there is a standard for molten metal hoists. The standard is
E 2349-05 and available directly from ASTM at their website
www.astm.org.
Section 9.5.4.1 (1)
“The
lifting mechanism of any hoist handling hot molten materials
shall be provided with an upper overtravel limit device to
stop lifting motion. This device shall directly interrupt
power to the hoist motor without the use of auxiliary
devices and shall not be used as an operating control. When
a limit switch is used as an operating device, a second
switch shall be used as the upper operating limit;”
If using
this standard, we suggest to users that they employ a geared
limit switch for testing at the beginning of every shift,
and a power circuit limit switch as a back-up when handling
molten metal with a hoist.
When
using a variable frequency drive, you do not want to break
the leads from the drive to the hoist motor while the drive
is controlling the motor. Newer
power circuit limit switches can be provided with a control
circuit element that breaks just prior to the power leads
being broken to the motor. These control circuit wires would
“base block” the drive just prior to the motor leads being
disconnected, thus saving the drive and motor from any
damage.
Below is
a drawing of a typical wiring arrangement for a variable
frequency drive used in this type of application,
protecting the load, and the equipment.
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