What breaker/fuses to use? 3

[Bestboy]

I am doing a motor control upgrade.
The present installation is as follows:-
· 400V, 15 KW, 2pole, 3Ph sq-cage induction motor spinning a high inertia fan.
· Cable from sub-station isolating fuses to the starter and then the motor 6mm sq – a distance of 18 meters
· Fuse rating 80 A
· Starter – direct on line
The new installation will consist of:-
· A similar motor as above but 18.5kW
· 18.8kW VSD
The VSD installation manual specifies that I should use:-
· 60A eternal fuses
· 100A circuit breaker
· Cable size 14mm sq (6 AWG) R,S,T and 8mm sq (8 AWG) U,V,W
This cable sizes do not make sense to me. I am planning to use 16mm sq PVC armoured cable from the substation to the VSD on a cable tray – a distance of 10 meters and 10mm sq from the VSD to the motor a distance of 8 meters.
I am thinking of de-rating the fuses to 60A and use a 100A circuit breaker in VSD panel. Some one else suggested to me to leave the 80A fuses as they are.

Any comment? Assistance much appreciated. Thanking you in advance

 

[jraef]

You have a circuit breaker AND fuses? If that is the case, the fuses should be selected to protect the VFD, the breaker to protect the cables. But regarding the fuses, don't just look at the size, look at the type recommended by the VFD manufacturer as well because the acting speed of the fuse is just as important when trying to prevent damage to the VFD. When they make those recommendations, they take into account that many of the fastes acting fuses are not "time delay" fuses that can withstand short surges from normal inrush currents, therefore they recommend a slightly higher rating, then recommend redundant I²t protection for the cable. i.e. the circuit breaker. So derating the fuses may turn out to be a nuisance for you if they keep blowing.

Wire size recommendations from VFD manufacturers are always minimum sizes, you need to investigate and determine whether using a larger size is necessary on your application.

 

[ozmosis]

Always go with the manufacturers advice.
For our 18.5kW VSD we would recommend a 63A fuse and between 10~35mm sq cable (7~2 AWG) (both for input cable and output). 8mm would be on the small side IMHO.

 

[Bestboy]

Thank you jraef and sed2developer for your valuable advice.
Jraef, you sound surprised that I have both fuses and CB? I have seen a couple of installation done that way especially if the supply comes from a far source.
I thought I may escape the trouble of replacing the old 80A isolating-fuses (in the substation) with a circuit breaker. The sub is old and the cubicles are small and awkward to work in especially installing a circuit breaker. But your reasoning sounds very upright and wise to follow. I will just need to add that task to my critical path.
The way I understand you is that I should have the 63A fuses in the VSD panel (cubical) and a breaker in the substation. My desire is to use 16mm sq on both sides. But will 100A circuit breaker not be so big for the cable?
I am assuming that there will be no inrush current by using a VSD compared to the direct on line. And if there is it will be so small.

 

[ozmosis]

Unlike the DOL starter, the recommended method of starting the motor is to use the control logic on your VSD. The VSD will already be powered up and so ramping up the motor using the RUN command from the VSD will not incur an inrush in the same way a DOL starter will. You will have an adjustable ramp up (and down) time to accelerate the motor and therefore it is this that will determine the current demand on start-up. If you 'rush' the acceleration (short accel time) then the VSD will demand more current but will inherently limit it to 110~150% (typically) but for a high inertia fan then you can input a long ramp up time so this will mean your current at start-up should be fairly low <100%.

 

[jraef]

I may have appeared surprised that you had both, but in reality I usually recommend that. I just find that most people ignore the issue in favor of cost reduction, which in my opinion is imprudent in the long run.

As to the sizes, I am in the US so fuse or breaker sizing for conductors is different for me, I'll leave that to others more familiar with your wire sizes and ampacity ratings.

 

[Bestboy]

Thanks a lot for the replies.

 

[ScottyUK]

Cable sizes you're proposing sound reasonable. The 'recommended' sizes look strange - not regular sizes anyway.

80A fuses are appropriate for DOL starting an 18.5 kW motor. A 15kW motor is on the border between a 63A fuse and 80A fuse for DOL starting.

With the VSD you should be able to use a 50A fuse, but if the manufacturer is recommending a 63A then use that type. Why does your colleague think it necessary or preferable to retain the 80A fuses?


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Sometimes I only open my mouth to swap feet...

 

[Bestboy]

Thanks scottyUK.
He feels that the purpose of 80A fuses will be to protect the whole installation i.e. cable, motor and VSD. This is also good for discrimination purposes between the breaker and the CB. But this argument is very crude especially considering jraef's comment. Installing correctly rated and the right fuse in the in the cubical feel right to me because they will save the VSD in case of a fault. A minor disadvantage to this is instances where one needs to look for new fuses wherever there is a fault. But leaving a box of new fuses in the cubical is a small effort. Again faults are quite rare in a good installation that is regularly serviced.
The breaker in the Sub will be good enough to protect the cable. I like them because they break all the phases at the same time unlike the fuses where you may have one fused and two still supplying the load...depending on the severity of the fault.
Thank you jraef for the insight.
Who agrees with the following information given to us by a sales rep? "Circuit breakers normally degenerates to a point of needing replacement if they interrupt more than 3 major faults" If this is the case then one less star to CBs.

 

[ScottyUK]

Breakers definitely degrade if they have to interrupt a major fault. The rate of degradation depends on how big the fault is, how big the breaker is, and what type of breaker we're talking about. On a board with a fault level approaching the maximum breaking capacity of the breaker there is a good possibility that an MCB or one of the smaller MCCBs will be a write-off after one close-in fault.

An MCB which I knew had interrupted a 10kA fault would go in the skip - they are so cheap that it isn't worth the risk. I would be concerned about relying on a small MCCB which had interrupted a 50kA or 60kA fault and would at least want an internal inspection, and preferably a replacement. By contrast an ACB under the same conditions would suffer some damage to the pole faces which would justify an inspection and possibly replacement of the contact faces but the breaker itself would in all likelihood be fit for service.


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Sometimes I only open my mouth to swap feet...

 

[Bestboy]

Thankyou ScottyUK for the information.I have forgoten what ACB is. AutoCB,ArmatureCB? Sorry for a stupid question..?

 

[ScottyUK]

Not a stupid question at all.

MCB: Miniature Circuit Breaker
MCCB: Moulded Case Circuit Breaker
ACB: Air Circuit Breaker (I think the US uses a different name - LV Power Circuit Breaker perhaps?)


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Sometimes I only open my mouth to swap feet...

 

[jraef]

We use both.

Also, just to add to the treatise on breaker trip degradation: If it is an old thermal trip element it will build up a thermal memory and trip easier each time. So the "degeneration" is that you will start getting nuisance trips, not that it will fail to trip. Using solid state trip units almost totally eliminates that as a problem. That said, if your breaker trips 3 times or your fuses blow 3 times, chances are really good that your system design needs some attention somewhere.

BTW, what he was probably thinking of is that (in the US anyway), the UL testing for circuit breakers states that the breaker will trip and reset no less than 3 times. That doesn't mean it will be no good the 4th time, but if it gets out of calibration on the 3rd trip, it fails.

 

[Bestboy]

Great infor

 

[ozmosis]

Scotty,
The recommended cable sizes we provide are based on encompassing the AWG sizes as well as the metric sizes. The correlation between the two often leads to some odd metric sizes.