Motor Fuse Blow During Starting

• Please explain why the local Electrician has not already carried out motor insulation tests and cable insulation tests.
• Please explain why the local Electrician has not already verified that all connections are "clean, tight and bright".
• Please explain why the motors have not been swapped out already for their strategic maintenance spares.

1) This project is under commissioning, IR test done for cables and FAT report is available for motors

2) all terminations points are verified for clean and tightness, there is a checklist also for the same. im worried about the contact resistance in the switchfuse part where the removable hrc fuse is placed. i checked that part it is hot. but cables or any other termination parts are not hot.

3) Its new only and in commissioning stage still not handed over

thanks

Please explain why strategic maintenance spares are not available during the commissioning period.

do u mean commissioning spares? if so it is available and changed for so many, the question is not the availability of spares but the reason of fuse blow. if its happening on first time starting we can say its due to inrush, but it is happening after a period of operation. the selection of fuse is i think ok, for ex. 11kW motor fuse rating is 40A, full load current is 20A, so even the starting current is 12times it is 240A. GL type fuse catalogue says is rated for 6times current for motors.

If it isn't the motor, as proven by insulation test and by substitution of commissioning spares, and it isn't the fuse rating, as warranted by appropriate use of co-ordination schemes, then it can only be the wiring.

• Please explain why cable insulation testing has not been carried out since the problem first arose.
• Please explain what has changed since the time of the initial installation and the time that this problem occurred.

What is the motor FLA of each motor? What is the fuse nominal current rating for each?

its as per type-2 co-ordination chart provided by the switchgear manufacturer, for ex. for 3kW it is 16A, 11kW it is 40A like that

refer the link http://www.moeller.net/en/support/slider/motorstarter_en.jsp

Fuses are there to protect the wiring.

Technically, the "supply-side" or "upstream" wiring.

Not enough relevant information.

It sounds like the motor is undersized for the load, not enough starting torque....

It's highly unlikely tht the real STARTING current of the motor is that high, it is virtually impossible for the STARTING current of an AC induction motor to be 800-1200% of FLC, it should be 500-600%. But the MAGNETIC INRUSH current may in fact be anywhere from 700-2000% of FLC, depending on motor design, AND the phase angle at the exact moment the contacts are closed, because it takes place as the core magnetized, so there is nothing except winding wire resistance to slow the initial current inrush until the magnetic fields are established and impedance takes place. That MAGNETIC INRUSH current however only lasts for 1-1/2 cycles and generally is not a factor in fuse clearing, as long as you are using Time Delay fuses. The fact tht you are recording this value speaks only to the sampling rate of your meter, I suspect it is a high cost DMM that is set to Peak Hold and is capturing that magnetic inrush. The cure for this is to select the fuses based on the reality, not some chart.

...or that, and you have a loose connection in the fuse box...check the amp draw in real time, if it's not exceeding the fuse rating and it's just heating up and failing, you probably have a loose connection....

You are assuming that just because a fuse fits physically it behaves the same electrically, but IEC 60269 says otherwise:

• ..."The first letter is a if the fuse is for short-circuit protection only; an associated device must provide overload protection.
• The first letter is g if the fuse is intended to operate even with currents as low as those that cause it to blow in one hour. These are considered general-purpose fuses for protection of wires..."

While the short-time characteristics of a GL fuse may be similar to a AM fuse, the long time characteristics are not, as you found out when you noticed the overheating of the fuse cartridge. Spend the money for the proper fuse.

Yes, curve shows similar characteristics only, only but time delay is varying. Ofcourse olr is there. Only the thing confusing it doesn't blow in first time start /commissioning but if we stop it after 2days running and restart after 1minute.

Of course it doesn't blow after the first start, the internals of the fuse and its contact points are cold. On the other hand, after the fuse has been passing FLC for two days, and is much warmer than the ambient as a result, it is not unreasonable to suspect that the fuse may be more susceptible to blowing during the inrush period because it is already preheated.

When you do your due diligence you will discover that it is exactly this characteristic that marks the difference between the AM and GL fuses. Even though both are HRC (High Rupturing Capacity) body fuses, the internal construction and fill material is different. Use the correct fuse type.

you are right sir, this is what our conclusion also so we asked the switch fuse provider L&T to give the details of the switch fuse contact resistance, permitted temperature in the power contacts etc.

You may want to consider the capacity of the supply from which you are applying power to your motors.

A drop in line voltage at start time will increase the demand for current at that time. Are you sure your lights are not dimming when starting your motors?

You may need to consider how heavily loaded your substation is loaded. All too frequently people make the mistake of using too much of the rated capacity leaving nothing else for temporary demands such as start up currents, etc.

Fans can be high in rotational inertia, and therefore take longer to start. You may need to go to 30-second coordination rather than 10 or 20.

Fuses are to protect the distribution system against overheating caused by over-current.

Heating involves a time function.

Fuses melt because the heat does not dissipate fast enough for the current flowing.

To prevent fuses blowing, you must reduce the current and/or the time it flows.

The time characteristics of your fuses do not match the motors, or your motors take too much current (are they starting on-load?), or they take too long to run up to speed. The latter could be due to 'low' voltage (which matters as others have said).

There is no chance for voltage drop, because the cables are over sized and distance is below 30meters only, and the project is not yet commissioned fully so the power consumption is less than 50% of the project only. how ever we asked the switchfuse manufacturer to provide the permitted contact resistance of the switch fuse. the hrc fuses are rated for 50degree ambient temperature.

thanks to all

There is ALWAYS some voltage drop, even if there is only a couple of meters of oversized cable. Of course longer and smaller diameter cables will have more voltage drop than shorter and thicker cables. 30 meters is plenty of distance to provide a quite measurable voltage drop. With a quality AC voltmeter of an appropriate range, measure the voltage at the input to the switch when the switch is in the open position, and again when the motor is running, and there will be a difference.

Since you use the term "switchfuse", I suspect we have a language problem here.

A fuse, by the very meaning of the word, is a device that melts when it gets overheated due to excessive current which continues flowing long enough to reach the melting point of the link inside the fuse. Once the fuse has melted, it is useless, and must be discarded and replaced with a new one.

A circuit breaker is a device that contains a switch that will disconnect the load from the source when actuated by a magnetic coil and/or by a thermal device. It is vaguely similar to a fuse, in that both devices will disconnect the source from the load when excessive current flows for a significant time, but while a fuse can only do its job once, a circuit breaker can be reset and used multiple times.

A Motor Starter or Contactor is yet another device that normally contains overload protection. In this device, an electromagnet turns the switch ON, to operate the load, while the overload device will disconnect the electromagnet in the event of excessive current, and a spring returns the switch to the OFF position.

According to your link, I assume that the device that is failing is the latter. Is that correct?

If you cables are oversized then you could possibly increase the fuse size to the maximum allowed for the cable - taking account of the time factor for short circuit currents.

I am too out of touch (long retired) to be able to comment with any confidence what size fuse or type you need - that will an exercise for today's experts.

But fuses must be able to withstand the starting current, DOL or SD, (where in my early days (in the absence of fault conditions) fuses were 'upped' to a current that did not blow on starting) assuming that cables would accommodate the high starting current and consequently the ensuing short circuit fault current.

Fuses selected on this basis were always much too large to protect the motor and cables from prolonged running overload, for which reason motor starters with overload protection were/are used.

What has changed seems to be the available 'pre-science' attached to selecting fuses/cables/starters being translated into standards and regulations to match the motors - the finer details of which I am not familiar with.

But the underlying logic is the same. It is fairly easy to see that you could go on increasing fuse sizes until you found ones that did not blow. Whether these would be too large for the cable network is another matter.

At the moment your fuses are blowing because they are not 'big' enough to handle your current current.

The motor is consuming more current for too long a time due to the load. A tip I found was a hard starting grinder. In order to start the grinder without the breaker tripping, the grinding wheel was manually spun to overcome the initial inertia. Not only did this prevent a trip, it also reduced wear on the grinder. It may not be practical to do on a fan, but you could try the initial spin before starting to see if that solves the problem. A possible fix would be installing a centrifugal clutch between the motor and the fan.

What response have you gotten from Eaton Tech Support?

eaton confirmed that the GL fuses are suitable for the application and its type tested. they r not able to find out reason of fuse blow, its under investigation

You still haven't given the FLA of any of these motors and the nominal ratings of their accompanying fuses. No wonder you have technical difficulties.