Fuses Keep Blowing

Look at he way that the motor is wired at the terminals and come back to the forum once it is checked out as correct.

it is ok wired excatly how the wire diagram shows already checked that when i turn on the disconnect u can hear ir trying to start but something is not allowing it to start up. could it be something in the junction box like a bad connection not allowing it to get the right amount of voltage??

Yes, it could be that the correct voltage is not getting to the motor leads.

In fact, it sounds like you may be "single-phasing" the motor. (Missing or low voltage on one phase.)

I would remove all power, lock the source out, then using a digital multimeter verify continuity from the starter output terminals through each of the motor windings by taking ohmic readings between all three phases. Make sure the readings are all balanced and of the same value.

If the readings are not balanced from the starter/contactor; Disconnect all three motor leads at the motor junction box and check the motor winding "T" leads going into the motor.

If the readings are balanced going into the motor the problem is between the starter/contactor and the motor which will be a bad cable or wire or if there are any splices in between, one or more of the splices are bad.

Be careful and make sure there is not any possibility for any of the circuit to become energized while you are working on it.

Is this a new install or has it been running for a while and now suddenly blowing fuses?

it brand new motor

Have you checked the amp draw? You might not have the proper slo-blo dual element fuses, or you could have a loose connection in the fuse holder, and it's heating up and melting the fuse element....

Good point on the slow blow fuses.

This motor should only draw about 2.5 amps, but the locked rotor current of start-up is possibly what is getting the fuses.

Or a bad start capacitor. Wonder if this was a replacement for a "bad motor."

3-phase motors don't usually have start capacitors, as they don't need them.

If it's one of those small round fuses, like this

a 22 caliber long rifle bullet will work as a replacement.

I'd recommend standing well clear of the fuse box on start-up.

_{"...a 22 caliber long rifle bullet will work as a replacement.}

_{I'd recommend standing well clear of the fuse box on start-up."}

Those are "audio-visual blown fuse indicators"!

The inrush current is blowing those fuses. Call the manufacturer and ask what they recommend for Slo-Blo fuses.

I'll just take a wild guess and say that the fuses blow due to an over current condition.

Is this delta or wye wired for operation?

Should this switch from wye to delta after starting?

Is the centrifugal blower spinning backawards and therefore not providing enough of a mechanical load?

Do you have the correct motor?

Have somebody else look at your wiring, you may not be able to see your own wiring mistake.

If in doubt, consult a qualified Electrician.

everything is NOT good, its shorted or grounded

When to the fuses blow? Immediately as soon as you apply power, or a few seconds or minutes into it? The difference is important.

Also, is the motor thermally protected (it would say so on the nameplate)? If not, do you have a starter on this with an overload relay and if so, what is the OL relay set to trip for, or (if it is an older NEMA style) what brand is it and what heater elements are installed?

You said "fan", but that could be almost anything. Can you describe the fan and how it is being used?

If you want help, try to avoid a game of "20 questions" and tell us everything you know.

Remember that the fuses are there to protect the wiring. The motor should be protected by a separate overload device. Selecting these items is called co-ordination. British Standard 7671 gives full protocols.

throw these in, guaranteed to last longer

Pity this didn't

CR4 Admin: deleted pictures/broken links

Ive struck Probs with duct fans in that with the fan de-energised, the fan revolves with natural draught, but in the opposite direction. As a consequence, when the fan is powered up, it has to first bring the fan to a standstill then take off in the right direction, with the net result that heaps of current is drawn.

...."heaps of current"...

So, why not install a brake mechanism?

Or a flap that seals off the duct when a "reverse" draft happens? Lightly counterbalanced.

A further method that works with any electric motor I have seen is to place a dead short across all motor windings/power connections, once the power is removed.

It may not completely stop the fan rotating backwards, but it will be very, very slow.....enough that fuses might not be blown.....

Dead easy to test too, to see if it will slow/stop the fan rotating.....!!!

this is all due to the starting current of fan..check the current at the moment of starting.if its under 10 -12 A,its ok..glass fuse are not capable for withstanding starting current..you can check the complete wiring by disconneting the leads of motor and checking voltage after switiching the motor starter.

In rush current for a motor that draws 2.5 amps, will be around 2.5 x 6 = 15 amps. Generally speaking that is a "middle" value. It could be more or less.....the manufacturer could help you further....

The current drawn is dependent on several things, one being motor mechanical load at start up.

I might suggest that:-

a) you need HRC fuses

b) try 10 amp ones as a good starting point......less might still give problems.

Make sure that the wiring is at least at code for current draw or better......

Also check electrical code for where you live.....

Daft as it sounds but have you tried that the motor spins freely? It doesn't say anywhere in the thread. I have had a partly seized motor from new/overhaul.

My chart says the motor should draw 3.0 full load amps but a 15 amp thermal-magnetic trip rated breaker should be used with dual element time delay fuses of 5.6 amps.

Good Luck!

All of this talk of it being due to starting current is ASSuming that the fuses blow instantly. Until Mr. (ASSumed) tccs responds further with details, additional speculative responses serve no purpose other than to see ourselves in print, which I know for some is their entire raison d'être.

Except, of course, for the humorous divergent posts...

Your signature of:-

** All I every really wanted to be, was... A LUMBERJACK!.**

Was that because of this song?

Lumberjack Song

Yep. I'm a big Monty Python fan.

As big a Monty Python fan as Brave Sir Robin?

Further humorous divergence...

As I pointed out when I first saw this in another forum, the device shown as the "130A" fuse replacement on the bottom may be a puzzlement for anyone under the age of 30, maybe even 40.

No penny?

Pennies are no longer copper, they are 97% zinc. It's a poor conductor now.

No pun intended.

I have this often with machines that I plug in. One of the sockets or pins has oxidized and doesn't get a good connection. A wiggle of the plug during startup cures this but this technique can cause an arc weld making removal difficult. It is better to clean the mating parts first.

Jim

Ensure

1.) if all phases are available at motor end. Might be there is phase failure or two phases / or a phase and ground gets entices en-route to motor.

2. Measure the load current - might be it is over shooting because of issues with bearings, loss of phase supply or may be any other issue - sat RTS.

3.) Calculations say the FLA is 2.255. Therefore your fuse is just sufficient for starting current ( for a common motor). Therefore any small isuue causing current/ load to increase may overload the fuse. If this is HRC type ( thermal acting - slow type for lower excesses) fine else will blow away. Check it.

I don't see any other possible issue in this case.

Please note:-

1) This is a 3 phase induction motor. No reduced current starter is mentioned.

2) Most people agree that starting such motors, depending upon the mechanical load and motor design, will be around 5 to 8 times normal running current.

3) Error to somewhere around 5 or 6 times in my very limited practical experience many years ago in what might be considered a "normal" situation.

So let us just say that 5 x 2.255 is a long way above the fuse size quoted....and it could be more!! The fuses are also NOT mentioned as being HRC type......but they might be HRC, but are probably too far under the start current to be able to survive many starts - just guessing there.....

In case you don't believe me, check here for "improved" 3 phase motor infos here:-

Why_is_the_starting_current_of_a_3_phase_induction_motor_higher_than_its_rated_current?

It is sometimes also referred to as "Locked rotor current"....I believe that is US wording.....I first saw that here on CR4.....it does explain the situation quite well I feel....

I hope that helps you further....

My personal guess is that HRC fuses of around 10 amps are needed.....but there maybe codes that require even higher. The motor manufacturer may be in a position to help further....