Fuses in place of motor "heaters"? (induction motor protection)

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.
So, I choose to not have a contactor but instead, this, which provides the
power-fail protection:
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This has a relay (rated 1/2 hp) that drops out on power fail, and has
external connections for additional e-stop mushroom switch. But no provision
for motor protection.
Would 2 precisely-matched (ie, to the 1/4 amp), slow-blow HRC fuses be
sufficient to protect this motor?
Thanks.
Reply to
DaveC
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Why do you need two fuses for a single-phase motor?
Do you know the motor's actual i-squared-t overcurrent limit requirement?
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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.
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jsw
Reply to
Jim Wilkins
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When I worked one Summer at Bell Labs, one of the engineers told me he was trying to figure out how much detail the eye could see in color. What he didn't know (and I'd forgotten that I knew) was that this information was part of the design of the NTSC system.
Reply to
William Sommerwerck
As a chemistry student I was expected to learn a lot of practical detail and hands-on procedure that I later noticed new electrical (and some mechanical) engineers often lacked. jsw
Reply to
Jim Wilkins
The panel circuit breakers protect the wiring from that. The separate fusing is for a motor overload condition.
Reply to
Jim Wilkins
Fuses ARE heaters. They have thermal action and the slow ones are meant for motors. I would say that fuses are "overload heaters for the poor".
I spent some effort when replacnig electricals on my three phase bandsaw, and finally found a contactor with the exact heaters that the motor needed. Very happy about it.
i
Reply to
Ignoramus8074
"Jim Wilkins"
** Cos the AC supply is two phase.
The OP is an American.
They have spit phase power where 240VAC is split into a pair of 120V lines with a common neutral.
While 120V appliances use one or the other, some ( usually high powered ) ones are rated at 220 /240 and connect across the pair.
..... Phil
Reply to
Phil Allison
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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. :-)
:-)
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.
Reply to
DoN. Nichols
I assumed it was wired with a 20A double-pole breaker at the panel protecting from shorts and a smaller fuse sized (how?) to blow before the motor burned out.
jsw
Reply to
Jim Wilkins
Panel breakers are there to protect the upstream wiring from excessive load current, not to protect the load.
Matching the I-squared-t of a breaker, or fuse, to startup and running conditions of a motor is not trivial. Motors are subject to starting inrush currents sometimes tens of times the rated full-load current. Be guided by the data published by reputable manufacturers. There's plenty of it.
Reply to
Fred Abse

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