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

Why do you need two fuses for a single-phase motor?

Do you know the motor's actual i-squared-t overcurrent limit requirement?

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.

jsw

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.

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

because it's got two live inputs, and either could develop a short to ground.

The panel circuit breakers protect the wiring from that. The separate fusing is for a motor overload condition.

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.

"Jim Wilkins"

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

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.

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

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.

Most motors are tough old buzzards. Why the op needs more protection is unknown, I would think some kind of thermal or time delay overload is sufficient which may already be in the motor.

Jeff

I got this in an email from someone who wasn't able to post directly:

Obviously, I can't post to the newsgroup.

Jeff Angus