Fuse for an AC motor 1

[williedawg]

this is probably a dumb question, but...

For an AC 120v 60hz, single phase motor (which is no longer being made); the tag on the 3M127B Dayton motor says 1.4 amp.
I'm assuming that means operating amperage at 112 in lbs max torque.

Motor is non-reversible.

So...what would be the rating for a fuse to protect the motor from overload?

Thanks

 

[jraef]

Although technically you are allowed to use fuses as overload protection, it's not a good idea. It's damn near impossible to get the fuse rating at exactly the right value to protect the motor. But a lot of 120V 1 phase little fan motors like that are what is referred to as "self protected", meaning they have their own internal overload device built-in. Look for wording to that effect on the motor nameplate. If it is, all you need to provide is short circuit protection, and that would be based on the wire size you run to the motor.

If the motor is not self protected, it's better to use an "overload relay" or a "manual motor starter" with one built-in. They are much better at protecting against overloads than fuses are, plus they are resettable, unlike fuses.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[lostlazy]

Or...If this is not a "code" installation, or if you just don't feel like wasting 40-100$ on a motor starter for a fan that probably cost less than 20...a 5A fuse will be adequate. Fuses aren't resettable, but easily replaced.

 

[ScottyUK]

lostlazy,

Explain in nice simple words how that would provide any form of overload protection. The motor could draw 5A indefinitely, or at least until it caught fire, which seems to defeat the purpose of overload protection. Following your reasoning, why not take it a step further and replace the fuse with a 1" diameter bolt. That gets around the problem of blowing the fuse entirely.

I hope you don't design, install, or maintain electrical installations for a living, but if you do please let us know where so we can keep well away from them.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[rbulsara]

As jraef said, use the manual starter with o/l protection. Here is one example from Square D catalog( should be available from a local electrical wholesaler).

Part Number 2510FF1 (Square D)
Description : Fractional Horsepower Manual Starter , Non-Reversing, 16A, 1HP(AC), 1-Phase, 1-Pole, Provides overload protection as well as manual ON/OFF control for small motors in a variety of industrial and commerical installations., Type: F, Operator Type: Toggle, Overload Type: Thermal - Melting Alloy, Reset Method: Manual, Thermal Units Required: 1, Single Unit Gray Flush Plate, Mounting Type: Flush, Depth: 1.44 Inches, Height: 4.50 Inches, Width: 2.75 Inches, 600VAC/250VDC

Price $52.00

 

[parlitu]

You shouldn't use a fuse for overload protection. Why, is very simple; when motor starts the current will be around 5-6*nominal current, and to avoid burning the fuse (and cut the motor supply) you have to choose a value more than 6*nominal current. That means a fuse of 5-7A. But overload means everything bigger than around 1.15%*nominal current. So, if you use the fuse it is possibly to burn the motor, because it does not protect your motor properly. Look for a solution as rbulsara or jraef advise you. But if you want to burn you mother-in-law take lostlazy solution.

 

[lostlazy]

#1 Scotty you need to relax. Don't let the shroud of internet anonymity get to you head.

#2 As for nice simple explanation that you requested,it is called a Class R fuse. Just do the math, and pick the right one.

 

[ScottyUK]

lostlazy,

Giving irresponsible and dangerous advice is nothing to be proud of, anonymous or not. A 5A fuse does not provide any overload protection whatsoever for the motor; your advice is that of an idiot. No offence.



----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[rbulsara]

A comment on fuses for overload protection:

Dual element fuses do provide separate overload and short circuit protection. For a 1.4A motor a 2A or 3A dual element class J fuse should work. Start with low and up it if required until the motor starts normally.

 

[Kiljoy]

rbulsara,

Ditto. As I read the code, the OL device should be about 115%. With Dual element, no problem. Fast blow fuses may need to go up to 125%.

 

[devojka]

guys, 125% means to ge beyond, what?
sorry but i don't know, i do thank you all,

Best Wishes,
devojka = (girl)

"women make such excellent scientists and engineers-because of their passion for learning and will-power to succeed..."

 

[waross]

Welcome to the forum devojka.
In the first post, the motor full load current is given as 1.4 Amps.
All percentages are percentage of full load amps.
A 5 amp fuse will give some protection to the rest of the installation after the motor burns out. A bolt won't, but these circuits are normally protected at 15 amps in the panel. When the motor burns out, the 15 amp breaker in the panel will race with the 5 amp fuse. Either one may win, or it may be a tie. (Both interrupt)
If you go with dual element fuses, the man who sells them to you may have a sizing chart for motor protection. If not, check the fuse manufacturers web site for the size.
I remember one such chart from many years ago when I was he head electrician in a Western Canadian lumber mill.
The book had pages of fuse sizes correlated to motor full load current. At the bottom of each page was a little notice:
For light duty use one size smaller.
For normal duty use the size listed.
For heavy duty use one size larger.
Then, rubber stamped by hand near the bottom of each page,
For sawmill duty in Western Canadian lumber mills, use two sizes larger. We were brutal to the motors but we were very very fast changing out burned out motors. (Practice makes perfect.)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[devojka]

waross, thank you for welcomming me and thank You for the explanation.


***** you are Sunshine.

devojka

"women make such excellent scientists and engineers-because of their passion for learning and will-power to succeed..."