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
(Amazon.com product link shortened)
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?
Why do you need two fuses for a single-phase motor?
Do you know the motor's actual i-squared-t overcurrent limit
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.
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.
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
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.
** 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.
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.
On Sun, 26 Jan 2014 08:13:56 -0500, Jim Wilkins wrote:
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
"Design is the reverse of analysis"
I got this in an email from someone who wasn't able to post directly:
Obviously, I can't post to the newsgroup.
You have three options.
1 - Fuse block for 2 fuses. Slo-blow type.
2 - Overload Line heaters with control cicuit to kill the relay if either
one of the overloads trip.
To size the heaters or slo-blows you need a MAX AMP rating if possible, if
not then size it for the RLA or FLA times the service factor. Typically SF
1.15 If the motor is rated at 115/230 Volt 7.2/4.1 FLA and you are using
230 then use the 4.1 Amp times 1.15 or whatever the SF is. Size and buy the
heaters for the closest amp range above the amp rating of the motor. Fuses,
same thing, fuse at the rating or the next step above.
With the heaters you can reset them after they trip. (once you determine
the cause of the overamping.)
If your belt is too tight the motor can over amp. If your actual voltage is
200 not 220/230 get a buck n boost transformer. 200 is way to low, and with
cause motor failures, high energy costs and loss of HP.
By all means buy an amp clamp to verify operation especially after changing
belts. Many people over tighten their belts.
The third option, get a linecontactor, 2 pole with contacts on each pole,
and get the line contactor with an overload 2 pole heater block attached
directly to the line contactor. They are not that expensive and the control
circuit can also be routed through your switch.
Fuse blocks can be deadly as one might blow, but not the other, same with
single wire protection. With one line hot motors can go to ground on the
unprotected line and zap you into the next world ready or not.
This is not a place to take shortcuts.
The circuit breaker should be rated at least 20 % over the normal load on a
dedicated circuit. Do not fuse at LRA's I have seen motors get crisp at
half that rating, that is pretty much the same as no protection at all.
Next time post to an electrical Where you will find professionals in that
trade. I snipped the other NG's outside of woodworking.
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