Fuse for an AC motor

Yes, that's why a fuse is no good. You really need a proper thermal overload for the motor. It's possible to wire up a three phase motor starter to run a single phase motor but that's expensive. You should be OK with something like this

You can use a 6 ohm 5 watt resistor in series with the motor, to create a 2.22 volt drop under the normal condition, and a 2.94 volt drop when jammed. Rectify, filter and feed that into a 4N35 opto coupler circuit that disconnects power to the DPDT motor relay until manually reset. Something like this for the opto sensing portion:

|> | D1 AC ---o o---[Motor]---+--->|---+---+ RLY1-2 | | | Pot = 500 ohms | | P [6R] [47uF] O

Hi, Uriah. It does sound tough. For very small AC motors, this can be a difficult problem to solve with either a fuse or a circuit breaker. Have you considered a mechanical aid like a breakaway clutch? In the event of excessive torque, the cluch disengages from the load, saving the motor.

Cheers Chris

Bad idea. Have you analysed this circuit for each component failure and each combination of component failures?

is a better idea - they are a well known and well understood component and any electrician can maintain them.

Sure. If the resistor opens the cap, pot and opto die. So he's out $1.86, Mouser prices. If you count the resistors, they are 39 cents and 35 cents, bringing the total replacement cost to less than 3 dollars. And what is the probability either resistors opening? They are running at less than 35% of rating for the

So suppose you tell us why it is a bad idea.

But will it continue running the motor at .37 to .45 amps, and disconnect it when current rises above that? Nope. The CR101 is rated at 16 amps. This motor draws .37 amps, nominal. And what happens if it fails? The same scenario as above, either the motor won't run, or jam detection won't work. Except in your device, jam detection won't work in the first place. So it's a useless expenditure of money - and the expense will be relatively huge compared to proposed circuit. Parts for the circuit I proposed cost about $12.00, not including the junction box, receptacle, miscellaneous hardware. What does the device you propose cost?

Then you can throw in the cost of an electrician, which you imply is a good idea.

Ed

That creates the wrong impression. The contacts are rated 16 amps, but the tripping mechanism can be rated as low as .48 full load amps. Still won't work - you'd need to draw 125% of that to trip it, and the max this motor draws ia .49 amps, locked rotor.

Ed

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What happens if that motor fails? What are the safety implications? Are there fire or injury risks?

You fail to understand this device. As part of the installation, a small, carefully chosen, heater element is installed which automatically opens the switch if that motor has a sustained overload.

Plus $400 or more for development and testing. Plus $1,000 to get it passed by the inspectors (assuming it ever is). Plus $200 or more for repair on each failure since no electrician will touch it.

List Price: $38.00 UL/CSA listed.

It's in the first post. If the motor draws more current than the set point, the overload circuit I proposed disconnects power to it, and must be manually reset before the motor can be started again.

Stating it for the second time: tell us why the circuit is a bad idea. Support your contention - maybe you have something worth discussing.

It was spelled out in my post at 9:36 this morning. I'll repeat it: The CR101 will *not* protect the motor. the smallest CR101 heating element is rated at .48 amps, and requires a 125% draw to trip. The locked rotor amps on this motor is .49 amps. The CR101 will do nothing - it can't trip unless current drawn equals or exceed .60 amps, and the motor draws only .49 amps when it is jammed. That information appears in the GE catalog on page 164 at

Why are you attempting to twist the DIY project I proposed into a commercial product? Is it because you cannot support your contention that it is a bad idea, or are you saying that a DIY project (not the circuit) is a bad idea for the op when a commercial product is available?

That's 38 dollars, plus the electrician's fee, down the toilet, because it cannot protect his motor. Then you can add the cost of the replacement motor and installation. Plus there's whatever safety implications and fire and injury risks you had in mind in your question, and any associated costs. I call *that* a bad idea, and support my contention with the information on page 164 of the url:

The good news is that the (considerably more expensive at $99, plus $12 for the required enclosure, plus $27 for the heater because you have to order at least 3, plus installation cost) CR1062R at the url you posted will most likely do what he wants. You can get a heater for it rated at .37 amps, so the trip current would be .4625. That should work, except the CR1062 is designed for infrequent starting. That may preclude it - I can't say.

So, all that remains if for you to tell us what is bad about the circuit I proposed. Or is your point that he'd be better off to use the CR1062, which makes using a DIY circuit bad in your view?

Ed

Simple. You cannot install electrical items in any factory which do not comply with code and home made protection circuits are right up there.

I am a registered electrician AND an experienced electronics technician and I wouldn't put my ticket on the line for some home made gadget like that.

What happens when there is a fire? You really think they won't point fingers at the thing they understand least of all?

Thank you! Whether it's a factory or not, now I understand what you were thinking.

Ed