[guitarboyled]

I have fuses blowing with no reason in the starter of two identical motors

Last time it happened both fuses on the A phase blew.

Inrush current duration is well inferior to the time delay curve of the fuse but some reason it blows once in a while

Multiple measures have been taken at different times. Volts and Amps are normal at all times.

What could be the problem?

 

[99cents]

Are you using dual element fuses?

 

[Ultrafault]

Check for heat at the contacts. Put a logger on it if possible.

What are the size and type fuse you are using and the nameplate info of the motor.

 

[guitarboyled]

Are you using dual element fuses?

Yes, Dual Element/time Delay Smart Spot Amp-Trap 2000 Fuses

 

[guitarboyled]

Check for heat at the contacts. Put a logger on it if possible.

What are the size and type fuse you are using and the nameplate info of the motor.

Fuse: Smart Spot Amp Trap 2000, AJT 15A, Class J, Dual Elemement / Time Delay

It's a 10 HP motor but we are running it at approx. 7.6 HP

Efficiciency 3/4 load: 91.8%
Power Factor 3/4 load: 82.0%
Volts: 575
Phase: 3 / 60 Hz

Inrush : 82.5 Amps
FLA : 7.0 Amps (reached in 4.5 seconds)

 

[Ultrafault]

Assuming you have seperate overloads, you should size those to 20 amps.

 

[guitarboyled]

Assuming you have seperate overloads, you should size those to 20 amps.

Yes seperate overloads

We have since replaced with 20A with no failure yet

But on paper (according to tripping/melting curve) 15A should do the job

 

[JRaef]

...fuses blowing with no reason ....

Never true. There is ALWAYS a reason. You should ALWAYS assume that the fuse is doing its job, preventing a fire.

The reason for intermittent clearing will fall into one of two categories: Mechanical or Electrical.

Mechanical: occasional jamming of your load, failing bearings, icing, mechanical imbalance, too high of a duty cycle.

Electrical: failing winging/motor lead that only draws enough current under certain circumstances, loose connection, motor lead connection worn through insulation and shorting to ground against the frame on start, chattering contactor /failing coil /broken shading ring / bug or grit in contactor armature face, phase loss or intermittent low voltage on the supply side, the list goes on and on, but those are the most common.

Intermittent failures are difficult to find, but a great tool for that is a data logger of some sort. It will tell you what happened at the time and hopefully, right BEFORE the fuse cleared.

 

[99cents]

This where I have a problem with the CEC. They don't allow much for current limiting. You have sized the fuse properly according to the code book but a current limiting fuse like a Bussmann LPJ will conservatively allow you to increase to 20A, even more, according to Bussmann's published literature.

The CEC assumes all time delay fuses are the same. They are not. A properly sized dual element, current limiting fuse can allow you to go up to 300% of FLA. That doesn't come from me or CSA, it comes from the guys who actually make and test the fuses.

 

[Ultrafault]

Yes seperate overloads

We have since replaced with 20A with no failure yet

But on paper (according to tripping/melting curve) 15A should do the job

......

 

[Ultrafault]

I do know if you mail the fuse to bussmann they will do forensic analysis (xray) and give you a range of fault current it was subject to.

 

[99cents]

Never true. There is ALWAYS a reason. You should ALWAYS assume that the fuse is doing its job, preventing a fire.

The reason for intermittent clearing will fall into one of two categories: Mechanical or Electrical.

Mechanical: occasional jamming of your load, failing bearings, icing, mechanical imbalance, too high of a duty cycle.

Electrical: failing winging/motor lead that only draws enough current under certain circumstances, loose connection, motor lead connection worn through insulation and shorting to ground against the frame on start, chattering contactor /failing coil /broken shading ring / bug or grit in contactor armature face, phase loss or intermittent low voltage on the supply side, the list goes on and on, but those are the most common.

Intermittent failures are difficult to find, but a great tool for that is a data logger of some sort. It will tell you what happened at the time and hopefully, right BEFORE the fuse cleared.

The fuses are for short circuit protection only.

 

[99cents]

Here's where it gets interesting. The fuses, in fact, aren't there for motor protection at all. They exist to protect the conductors feeding the motor. The Canadian Electrical Code is clear about branch circuit protection for motors.

This is something we often forget. We are usually protecting the conductors when using a fuse or breaker, not the appliance, motor, etc. those conductors are supplying power to.

 

[Ultrafault]

Here's where it gets interesting. The fuses, in fact, aren't there for motor protection at all. They exist to protect the conductors feeding the motor. The Canadian Electrical Code is clear about branch circuit protection for motors.

This is something we often forget. We are usually protecting the conductors when using a fuse or breaker, not the appliance, motor, etc. those conductors are supplying power to.

However, everything jreaf said is true. I dont understand how what you are saying is related to his statement.

 

[99cents]

However, everything jreaf said is true. I dont understand how what you are saying is related to his statement.

He is talking about overloads. If the fuse is opening ahead of the overloads, then there is a coordination issue unless, of course, there is a short circuit.

 

[Ultrafault]

He is talking about overloads. If the fuse is opening ahead of the overloads, then there is a problem. Yes, the fuses are there to prevent a fire but a fire caused by short circuit and conductor overheating, not by motor maintenance issues.

Thats exactly what the point is if the fuses are opening there is a problem. Believe me those problems all can cause the fuse to blow ahead of the overload.

 

[BBQ]

He is talking about overloads. If the fuse is opening ahead of the overloads, then there is a problem. Yes, the fuses are there to prevent a fire but a fire caused by short circuit and conductor overheating, not by motor maintenance issues.

If there is a short circuit in the motor it will be the fuses that blow out before the OLs have time to open.

 

[99cents]

Thats exactly what the point is if the fuses are opening there is a problem. Believe me those problems all can cause the fuse to blow ahead of the overload.

Certainly, but the OP said the fuses were blowing for no reason and I agree with him or, at least, that they were blowing for very little reason. Replacing the 15A fuses with the properly selected 20A fuse proves that.

The primary purpose of the fuse is to protect against fault conditions. Certainly a locked rotor can cause an abrupt and rapid rise in current that opens a fuse. Most of the issues jraef was referring to, however, are motor maintenance issues that are protected by the overloads, not the fuses. The OP was not having overload problems.

 

[JRaef]

If there is a short circuit in the motor it will be the fuses that blow out before the OLs have time to open.

And often if there is a locked rotor for some reason, the fuses MAY clear before the OL trips.

Or if there is a severe mechanical vibration in the machine that can transmit to the fuses, the vibration can weaken the fuse links or fause the fuse clips to over heat, which transmits into the fuse itself and causes premature clearing.

Or if there is a control device that is causing or allowing too rapid of a number of cycles on the starter, the repeated inrush will blow the fuses.

Or if a bearing is failing and the rotor is crashing into the stator under certain load conditions,, there is an intermittent ground fault that clears the fuses.

Or...

99cents,
I was not implying that ANY of these things were related to tripping OLs. My mentioning of these issues is based on 30+ years of motor control experience. My point to guitarboyled was mainly in response to his saying that the fuses were blowing for NO reason. That is a dangerous assumption, as is increasing the size of the fuses just to keep them from blowing. There is ALWAYS more to the story, increasing a size is the LAST resort after exhausting all possible causes and it might end up only serving to increase the collateral damage next time it happens.

 

[Wirenuting]

If there is a short circuit in the motor it will be the fuses that blow out before the OLs have time to open.

Yup,
O/L's are for over load protection and fuses/breakers are for short circuit protection.
Big difference that people seem to overlook.
If everything is properly sized and not hacked over the years troubleshooting is easier.
Fuses don't open for no reason.