Fuse for motor protection?

A small 1/2 hp induction motor is turned on and off with a motor-rated toggle switch connected to a 208V 20A branch circuit. No other protection installed.

Obviously not enough protection so I'm planning to add a slow-blow 9A fuse (rating of the motor) to keep it safe.

Any objections?

Reply to
Mike Cook
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As in a resettable overload button? Don't remember, I'll check. Can you give me a couple of "what if's"? ("If it does, then..." and "If it doesn't, then...")

As in, the 9A may blow quickly and I'll have to adjust upward a little?

Reply to
Mike Cook

Any reason that you didn't bother to disclose all the important motor characteristics and details? Single phase or 3 phase? 1725 rpm or something else? The nameplate of your 1/2 hp motor should specify the operating current at 208v. What is the nameplate operating current? Extra credit for the maker and model number.

The problem is that starting current can be 3 to 4 times the nameplate oprating current. Your common 9A slow blow cartridge fuse isn't going to work if your motor operates at about 4A.

You have some choices. Try plugging into this calculator and pick the fuse type (non-time delay, dual element time delay, instant trip, or inverse time trip) that you find appropriate.

Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

And does the motor have to start-up fully loaded

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In the US the nameplate would typically say "thermally protected" and there would not be a manual reset. If the motor overheats the internal thermal protection will disconnect the motor and it will reconnect (and restart) when the motor cools off.

Starting current is often 6 x running current.

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No go right ahead :-) However I reckon you would be better off with thermal protection if the motor does not have it.

Reply to
Rheilly Phoull

How does thermal protection work? I understand the safety mechanism, but if a motor is heating up under load, isn't it drawing over-max current? Shouldn't the (carefully-chosen value) fuse blow?

I need to learn about this, I know...

Reply to
Mike Cook

The phase angle of induction motors changes with varying load. The current draw only changes a little with loads from zero to the motor's rating. So, if the motor is rated at 1/2 Hp, 208 V, the running current at rated load will be something just over 2 A. I = (745 W * 1/2 * 1.2 (efficiency))/208

At idle, the current may be 1.5 - 1.7 A with a very low power factor (phase angle near 90 degrees lagging). At full load, the current will be about 2.1 A with a phase angle of maybe

5 - 10 degrees. This, the motor draws more REAL POWER at full load, but the current doesn't change much throughout the range the motor is rated for. This is why external fusing for motors is to protect the building wiring from fire, it does not protect the motor from minor overload or overheating. Internal thermal protectors actually measure the temperature INSIDE the motor, and thus are a lot more effective.


Reply to
Jon Elson

There's usually a fair bit of heaving and grunting as a motor starts spinning - especially if it starts under load.

Even a slow-blow fuse that can handle the second or two of what is pretty much stall-current, will carry enough steady running current to let the magic smoke out if a fault/overload develops.

Reply to
Ian Field

A fuse is not a replacement for a properly-sized motor OLP overload protection device.

A fuse will protect wiring, as previously stated.. but you'll just keep asking the question until you get the answer you want, right?

Overload protectors and TP thermal protection devices aren't the same, and are intended for completely different situations.

The OLP will react rapidly to protect a motor in the event of a sudden machine jam or other fault.

The application was previously stated to be a bandsaw which previously had a smaller motor on it.

Generally, fuses aren't motor protection devices, and won't replace a proper OLP.

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All fuses are rated according to their impulse capability in terms of I^2t because their cold resistance varies with rating. But the true power calcul ation is I*2Rt.

Every protection device must have a time constant faster than the failure m ode it is protecting, which implies a smaller mass or accelerated by magnet ic or thermal forces.

I can remember blowing breakers due to induction motors starting up and wis h I had a better means to control the starting current at the expense of st arting torque or no-load waiting time while getting up to speed.

Other than a variable speed controller, a simple solution is s thermal brea ker which ought to be embedded in the motor assembly. In the early days a fan fail circuit that might overheat with s stuck rotor consisted of a heat er resistor and bi-metallic strip bonded together in line with air flow. N ow you can use a PTC which regulates to 85'C upon short circuit and thus re sistance and power depends on max sustained current for required voltage an d size.

In this case I suspect, you have two reasons;

1) to avoid false tripping due to over-sensitive slow-blow fuses 2) provide safety by reaching 85'C faster than any bad contact. This is ha rd to predict if your wiring has or will have any bad connections.

There are standards for thermal breakers for I^2t time constants and is you r best bet, albeit may be a little expensive for 2phase breaker.

Another alternative are PTC's which may be ganged in parallel if thermally coupled but exposed to air for rapid resettable air fuse characteristics. They regulate to 85'C on short circuit but run cool in normal operation due to several decade range in variable resistance.

You might consider two (2x) 4 Amp PTC's in parallel, where 4 Amp is the tri p current which is 2x the Hold current and trips in < 30 sec @ 5x hold curr ent rating. Verify specs if this works for you. Cost $2.84 each. 0.5" pitch radial capacitor-like thin body using metal oxide material in epoxy... I think this is what they use in DC motors for cars such as for power windows . Include voltage margin to rating.

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Reply to
Anthony Stewart

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