On 2014-01-25, Jim Wilkins wrote:
"DaveC" wrote in message
...
Looking to provide switching for 1-phase, 220 AC, 1/2 hp motor that
doesn't
include use of a contactor. I appreciate the need for precise
current limit
(LRA) that a branch breaker can't provide.
[ ... ]
Would 2 precisely-matched (ie, to the 1/4 amp), slow-blow HRC fuses
be
sufficient to protect this motor?
Thanks.
Why do you need two fuses for a single-phase motor?
In the UK, you don't, because 240 VAC comes with one side
grounded. However, in the USA, 240 VAC is normally supplied with a
grounded center tap, so if you have only one fuse -- or have two but
only one blows, you still have 120 VAC live in the motor's housing, and
potentially available for contact and personal zapping. :-)
Do you know the motor's actual i-squared-t overcurrent limit
requirement?
http://www.copleycontrols.com/motion/pdf/IsqT.pdf
When I ran a lab at Mitre an engineer kept asking me to buy him more
and more Polaroid film for his scope camera. I finally asked him why
and found that he was trying to discover the principles of how a fuse
blows. He looked really sheepish and subdued when I handed him a
Littelfuse pamphlet of current vs time curves.
:-)
http://www.littelfuse.com/~/media/fi...ofuseology.pdf
And I'm not sure why he feels the need for precision current
limiting on the motor. Most motors will handle a fairly wide range of
time vs overcurrent exposures. (An exception is a permanent magnet DC
servo motor which can be partially demagnetized by a sort spike over the
rated current limit.
Enjoy,
DoN.
--
Remove oil spill source from e-mail
Email: | (KV4PH) Voice (all times): (703) 938-4564
(too) near Washington D.C. |
http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---