80A MCCB may also be permitted for 22kW motor. Is it now okay with you?
Post #9 deals with VFD size in relation with the motor it would control.
Then the motor to be up-sized with due care taken of remarks in post#9.
MCCB and Wire Calculation for VFD
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80A MCCB may also be permitted for 22kW motor. Is it now okay with you?
Post #9 deals with VFD size in relation with the motor it would control.
Then the motor to be up-sized with due care taken of remarks in post#9.
I already commented that I'd use a 63A fuse switch with fuses fitted to co-ordinate with upstream protection.
And I provided the design data that we are committed to for a couple of VSD systems of that rating currently going through our works.
Feel free to reciprocate with your data where you have designed and manufactured similar systems.
Just as I suggested in post #2 perhaps?
The VFD has in-built overload and over current protection and so a separate OCPD may not be necessary. The 80A MCCB suggested is for over current protection of the motor, when the VFD is by-passed.
But is this not so for your VFD's?
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I wonder if the input switch fuse 63A and Upstream fusing =40A would withstand the starting current of 22kW motor, when the VFD is by-passed.
Perhaps you used time delay fuses?
There is no bypass required.
Thank you everyone. I have been following the thread closely though some of the things are a little to complex at this stage at least for me to fully comprehend. (Not too sharp actually 
.
Besoeker you were right when you said that a 22kW motor should have been selected. I went to one of the seniors and he said that in actual a 22kW motor was used with a 30KW VFD and not an 18.5KW motor.(He had no idea why Mr. Rajesh told me that 18.5KW motor was used) After that I did some calculations again.
30KW VFD has an input current of 55A, now if i multiply it by 150% it turns out to be 82.5A. Now does it make sense to use a 100A MCCB
Now as for wire size I have the folliwng info and my calculations are as follows,
Type of conductor = Copper
Installation Method =
Distance = Not more then 70ft
Ambient Temperature = 30C
Full Load Current = Ia = 55A
MCCB = 100A
Current Rating for Cable = Iz = 1.25*100 = 125A
From the table I have its says that a 25mm2 has an amperage rating of 124A and 35mm2 has an amperage rating of 154A. So i use a 35mm2 4 core cable. Is this right??
I would appreciate if I can get some help on this as well. Sorry if my calculations are wrong. And yes I agree with "Jraef" that this forum is meant for NA based applications, but I just hope that you all can help me a little with your experience.
Kind regards,
. Besoeker you were right when you said that a 22kW motor should have been selected. I went to one of the seniors and he said that in actual a 22kW motor was used with a 30KW VFD and not an 18.5KW motor.(He had no idea why Mr. Rajesh told me that 18.5KW motor was used) After that I did some calculations again.
30KW VFD has an input current of 55A, now if i multiply it by 150% it turns out to be 82.5A. Now does it make sense to use a 100A MCCB
Now as for wire size I have the folliwng info and my calculations are as follows,
Type of conductor = Copper
Installation Method =
Distance = Not more then 70ft
Ambient Temperature = 30C
Full Load Current = Ia = 55A
MCCB = 100A
Current Rating for Cable = Iz = 1.25*100 = 125A
From the table I have its says that a 25mm2 has an amperage rating of 124A and 35mm2 has an amperage rating of 154A. So i use a 35mm2 4 core cable. Is this right??
I would appreciate if I can get some help on this as well. Sorry if my calculations are wrong. And yes I agree with "Jraef" that this forum is meant for NA based applications, but I just hope that you all can help me a little with your experience.
Kind regards,
I don't know why a 30kW VSD would be selected without some justification for doing so. The 22kW VSDs in production with us have an output current rating of 46A. For a 22kW fan/pump motor load that ought to be adequate. Unless the motor has an uncharacteristically low efficiency or power factor. But we seem to have no information to that effect.
Again, I don't know where the 150% comes from. You mentioned IEE or IEC standards. As far as I'm aware, there is no such requirement stipulated.
On that basis I'd still go for a 63A input MCCB or fuse switch.
Again, the 1.25 factor and the 150% are not factors that I know of that need to be applied.
Rating the conductors at nearly three times the steady state drive input current is not something I would do or have ever needed to.
I don't know what table you are using but I use BS7671 which is the IEE standard for wiring regulations.
I assume that, given the application, you would be using Multicore armoured pvc insulated so table 4D4A* would apply.
From that 10mm2 gets you 62A in a four core configuration for usual installation practice. That should more than suffice for an input current of 55A in a 30C ambient.
*This from an older version of BS7671 - my more up-to-date version has gone walkabout, last known whereabouts in my office at work. If I was an angry person I'd kneecap the culprit. IRA style.
But I'm a benevolent old chap. I'll just behead him......keeps the suffering brief you see......
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My guess:
Engineers who believe that anything worth doing is worth OVER doing... I see a lot of that.
It's either that or they have little confidence in what they are specifying, so by over specifying, they are hoping to deflect blame if anything happens at some later date. "Hey, I told them to use a larger drive, it's not my fault if it burned up..."
My guess:
Rio is misinterpreting the basic design specification for a Heavy Duty / Constant Torque drive that stipulates that the VFD will be capable of 150% output current for 60 seconds (as opposed to a Normal Duty / Variable Torque spec that calls for 110% for 30 seconds).
Rio, this has NOTHING to do with the sizing for the circuit components, this is a performance spec for VFDs, but given the time limitation, you do not change the circuit / conductor sizing based on that.
My guess:
1.25 may be coming from the US NEC requirement, Article 430-122:
No idea if there is an equivalent IEC spec that calls for that though.
Rio is Using British and IEE standards - he says.
The relevant publication is BS7671, the IEE wiring regulations. There is nothing, as far as I tell, the requires the conductors to be rated at a greater current than the VFD input.
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