MCCB and Wire Calculation for VFD

[Rio]

Hello Everyone,

I am currently studying Electrical Engineering (Completed my first year) and these days working as in internee with an HVAC organization. My supervisor has given me an assignment to work on involving VFD. I will list all the details I have.

1. Vertical Discharge VFD operated Condenser Pump for Hot Water Absorption Chiller
2. Actual Flow Rate = 825 USGPM
3. Actual Developed Head = 90 ft
4. Pump Efficiency = 0.75
5. He said the motor they will use is a 18.5KW IP55 Class F Insulation.
6. He said the VFD will be a 22KW
7. Supply = 380V, 3Phase, 50Hz


He has asked me to calculate the MCCB and wire size. Now this is what I have done,

1. In order to calculate the Hp we will use the following method;

Hp= (USGPM * Head * Specific Gravity)/(3960 * Efficiency of pump)

Hp= (825*90*1)/(3960*0.75)
Hp= (74250)/(2970)
Hp= 25

1Hp = 746W , Therefore 25Hp = 18.5KW motor. Is this right??? Is this how he sized the motor to be 18.5KW?? And for the VFD, they used the next size up ? Am I right?

As for the VFD, I had no information which model to select, so I did some research and found a danfoss VLT HVAC Drive. As per the technical information the rated amps are 44A.

As for the MCCB, What I have learned is this that for VFD operated motor we use the maximum rated amps of the VFD and not the motor. What ever the maximum rated amps of the VFD are we multiply it by 125% or 150%. ( I am confused here, as some articles say it should be 125% and other say it should be 150%) Any how if I multiply by 125% than I get a value of 55A so I guess we can use a 60A MCCB and if I multiply 150% than I get a value of 66A so I guess we can use a 80A MCCB.

Now when I asked my supervisor, he said I am wrong. He said they used a 100A MCCB. Now I am confused what to do. I know I am wrong and I am missing something very important.

I would appreciate, if anyone can help me with this. And also how to calculate the wire size, the info over the internet is way too cloudy.

Kind regards;

 

[Besoeker]

Hello Everyone,

I am currently studying Electrical Engineering (Completed my first year) and these days working as in internee with an HVAC organization. My supervisor has given me an assignment to work on involving VFD. I will list all the details I have.

1. Vertical Discharge VFD operated Condenser Pump for Hot Water Absorption Chiller
2. Actual Flow Rate = 825 USGPM
3. Actual Developed Head = 90 ft
4. Pump Efficiency = 0.75
5. He said the motor they will use is a 18.5KW IP55 Class F Insulation.
6. He said the VFD will be a 22KW
7. Supply = 380V, 3Phase, 50Hz


He has asked me to calculate the MCCB and wire size. Now this is what I have done,

1. In order to calculate the Hp we will use the following method;

Hp= (USGPM * Head * Specific Gravity)/(3960 * Efficiency of pump)

Hp= (825*90*1)/(3960*0.75)
Hp= (74250)/(2970)
Hp= 25

1Hp = 746W , Therefore 25Hp = 18.5KW motor. Is this right??? Is this how he sized the motor to be 18.5KW?? And for the VFD, they used the next size up ? Am I right?

As for the VFD, I had no information which model to select, so I did some research and found a danfoss VLT HVAC Drive. As per the technical information the rated amps are 44A.

As for the MCCB, What I have learned is this that for VFD operated motor we use the maximum rated amps of the VFD and not the motor. What ever the maximum rated amps of the VFD are we multiply it by 125% or 150%. ( I am confused here, as some articles say it should be 125% and other say it should be 150%) Any how if I multiply by 125% than I get a value of 55A so I guess we can use a 60A MCCB and if I multiply 150% than I get a value of 66A so I guess we can use a 80A MCCB.

Now when I asked my supervisor, he said I am wrong. He said they used a 100A MCCB. Now I am confused what to do. I know I am wrong and I am missing something very important.

I would appreciate, if anyone can help me with this. And also how to calculate the wire size, the info over the internet is way too cloudy.

Kind regards;

I don't know where you're from. The 380V 3ph 50Hz might be European yet your calculations are in Imperial units.
Anyway, I agree with your calculations more or less - I made it 18.7kW so I would have chosen a 22kW motor.
The 44A rating of the Danfoss VFD is most probably the output current. The input current will usually be a bit less.
We normally fit a fuse switch for input isolation/protection. In this case I'd use a 63A unit. As it happens, we currently have a project that includes a couple of 22kW 400V VSDs and the input fuse switches on these are 63A units but fitted with 40A fuses.

The problem with making the input device higher rating is co-ordination with upstream protection. On a few occasions, as with the two units above, we've had to go for lower rating input fuses for that very reason.

I don't know how your supervisor got to 100A for the MCCB. It isn't what I would do and I've been in the VSD field for a very long time. But I don't know what rules apply in your geographical region (unless you happen to be in the UK).

As for conductor size, there are too many variables such as method of installation, type of conductor, length of run, ambient conditions to give a definitive answer.

 

[Rio]

Hi,

Thanks for your valuable input. I am based in India. Yes you are right, the input current is 40A and not 44A. He told me to follow B.S standards and I.E.E regulations. From the Danfoss VLT manual I have also found out the recommended Max Cable size as 35mm2 and Max pre-fuses as 63A. Sorry for asking again, is there anything I am missing out. I mean if you have any literature you could point me to. Is it possible they have just selected the MCCB as 100A to be on the safer side? If so than why did they selected the motor to be a 18.5KW and not a 22KW motor to give themselves some margin?

Any ideas??

Thank you once again

Kind regards

 

[petersonra]

here in the US the minimum size feeder conductors and maximum OCPD for a VFD is based on the manufacturer's ratings and nothing else.

 

[Besoeker]

Hi,

Thanks for your valuable input. I am based in India. Yes you are right, the input current is 40A and not 44A. He told me to follow B.S standards and I.E.E regulations. From the Danfoss VLT manual I have also found out the recommended Max Cable size as 35mm2 and Max pre-fuses as 63A. Sorry for asking again, is there anything I am missing out. I mean if you have any literature you could point me to. Is it possible they have just selected the MCCB as 100A to be on the safer side? If so than why did they selected the motor to be a 18.5KW and not a 22KW motor to give themselves some margin?

Any ideas??

Thank you once again

Kind regards

I've rechecked the calculations and I still make it 18.7kW. It might be that the hydraulic data, head and flow, has some margin built in so that the 18.5kW motor is OK. There is rather important piece of information that's missing - the speed at duty point head and flow. If this is lower than motor nameplate speed, a larger motor would be required.

I agree with the 63A fuses. But I see no justification at all for the 100A MCCB.
And 35mm² cable is certainly OTT unless you have very long runs and voltage drop could be an issue. You might also have a bit of a problem making off that size of conductor in the motor terminal box.

To correctly size the cable use BS7671, the IEE wiring regulations.

 

[Besoeker]

here in the US the minimum size feeder conductors and maximum OCPD for a VFD is based on the manufacturer's ratings and nothing else.

So conductors at minimum 40A rating and OCPD maximum 40A in this case?

 

[petersonra]

So conductors at minimum 40A rating and OCPD maximum 40A in this case?

hard to say. most times the max input current is slightly less than the max output current, and the minimum conductor ampacity is 125% of the rated input current.

the OCPD is generally rounded up from 250% of the input current rating as best i can tell. Might well up end being 100A CB.

However, not real long ago I used a VFD that limited the OCPD to about 125% of input amps.

The manufacturer is required to tell you what the max OCPD is.

X. Adjustable-Speed Drive Systems
430.122 Conductors — Minimum Size and Ampacity.
(A) Branch/Feeder Circuit Conductors. Circuit conductors
supplying power conversion equipment included as
part of an adjustable-speed drive system shall have an ampacity
not less than 125 percent of the rated input current to
the power conversion equipment.

UL now requires listed drives to have a maximum size OCPD specified in the instructions that come with drives, and since you have to follow that advice, that is where it comes from.

 

[Besoeker]

I mean if you have any literature you could point me to.

Not literature in the sense of published literature but here is actual data for the project I mentioned:

Tag No xxxxxxxxxxxx
Rating 22.0 kW
Ratet continuous current of VSD 46 A
Input fuse-switch OS63GB04N1P
Rating 63 A
Upstream fusing ≥ 40A
Peak load 30 seconds 51 A
Peak load 3 seconds 69 A
Speed variation Motor dependent
Efficienct of VSD >97%
PF of VSD Displacement PF is >.95.
Gland plate material Steel
Ingress protection
Cooling fan wattage 120W per fan
Earthing bar size Drive rating dependent
Panel heat load 1.1 kW
Max ambient temp 40C
Min 0C
Contactors TC1-D5011F7
AC3 rating 50 A

Harmonics per drive
3 0.0 A
5 19.0 A
7 11.6 A
9 0.1 A
11 2.1 A
13 2.1 A
15 0.1 A

The harmonics go up to 50th.
This is the specification which was submitted to a very demanding customer in the petrochemical sector and is now part of the contract documentation. Procurement for the project is now complete and build is about to commence. Changing anything at this stage would require something like an Act of Parliament....customer, consultants and end user would be all over it like a bad rash.....

 

[Sahib]

There may arise problems if one stops with simply seeing VFD hp (or kW)= Motor hp (or kW).

Here is why.

All VFD's are current rated; ......... power is a relative term.. The amps that a VFD can continuously source to its connected load..... is the guiding factor.

As an example, consider a 460 vac motor rated at 10 HP .... Would you specify a 10 HP VFD (rated at a nominal 14 amps continuous at 40 deg. C.)? Pretty Standard, right ?

So after shipping that VFD off to the customer and proper installation, you start getting phone calls saying the drive is tripping ....

How could that be? You supplied a standard 10 HP VFD for his 10 HP motor application, right ?

Well, when you dig into it a bit further .... turns out that 10 HP motor is an 8-pole machine in a 284T frame, drawing a rated FLA of 16 amps ...(Marathon Elect. Cat. No. 453) ....

So, it looks like that "standard" 10 HP, 14 Amp VFD wasn't a good choice. Probably should have supplied him a 15 HP VFD rated at a nominal 21 amps.

The point is ... that selecting a VFD on the basis of HP alone can... and will... get one into trouble. HP is a relative term. NFPA-70 table is not an absolute either.

ACTUAL MOTOR NAMEPLATE AMPS is the important parameter when selecting a VFD rating.

 

[Besoeker]

the OCPD is generally rounded up from 250% of the input current rating as best i can tell. Might well up end being 100A CB.

Seems a bit extreme. Why do you think it's that high? Is it perhaps a legacy of sizing them for direct on line starting?

 

[petersonra]

Seems a bit extreme. Why do you think it's that high? Is it perhaps a legacy of sizing them for direct on line starting?

Perhaps. OTOH, the drive is essentially a current limited device so basically it is the same thing as using a CB to provide the short circuit protection for a motor circuit and the overloads to provide the overload protection.

 

[Besoeker]

There may arise problems if one stops with simply seeing VFD hp (or kW)= Motor hp (or kW).

Here is why.

All VFD's are current rated; ......... power is a relative term..

I respectfully disagree.
Power is measurable, calculable and can be expressed as an actual value. It is not a relative value. 22kW is 22kW. No ifs, not buts.
Like current, it is also a limiting value for a VFD. Your example of the 8-pole machine taking more current is primarily owing to its poorer power factor. So, supposing you had a motor with a very much better power factor than standard then you could, by your logic, get more than the 10HP from it. But could you? Most likely not. The additional power would result in higher input current which could overload everything upstream from the bucket capacitor.

My point is that actual amps, important though it is, cannot be considered in isolation.

BTW. the bold font isn't necessary. It's sometimes considered, in internet etiquette, the equivalent of shouting. As is all allcaps.

 

[Besoeker]

Perhaps. OTOH, the drive is essentially a current limited device so basically it is the same thing as using a CB to provide the short circuit protection for a motor circuit and the overloads to provide the overload protection.

Sounds like a pretty good argument for not sizing the CB at 250% of drive input rating.

 

[Sahib]

Power is measurable, calculable and can be expressed as an actual value. It is not a relative value. 22kW is 22kW. No ifs, not buts.

I said power is a relative term and not a relative value.
Meaning that it is related to the current rating of the VFD

Your example of the 8-pole machine taking more current is primarily owing to its poorer power factor. So, supposing you had a motor with a very much better power factor than standard then you could, by your logic, get more than the 10HP from it.

If power factor improves, current drops and so power remains the same.

My point is that actual amps, important though it is, cannot be considered in isolation.

Please give a demo of what happens when current is considered in isolation just as I gave a demo of what happens when power is considered in isolation for VFD sizing.

 

[Besoeker]

I said power is a relative term and not a relative value.
Meaning that it is related to the current rating of the VFD



If power factor improves, current drops and so power remains the same.



Please give a demo of what happens when current is considered in isolation just as I gave a demo of what happens when power is considered in isolation for VFD sizing.

Stick to the topic please.

 

[Sahib]

Stick to the topic please.

Already.
The OP queried about his supervisor choosing 18.5kW motor for a 22kW VFD. I tried to justify it.

 

[Besoeker]

Already.
The OP queried about his supervisor choosing 18.5kW motor for a 22kW VFD. I tried to justify it.

The OP queried the MCCB rating and conductor size.

Stick to the topic.

 

[Jraef]

Perhaps. OTOH, the drive is essentially a current limited device so basically it is the same thing as using a CB to provide the short circuit protection for a motor circuit and the overloads to provide the overload protection.

From the upstream protection standpoint, VFDs are considered power conversion devices in North America, not the same as motor controllers. Some time ago, I'm not sure when, UL recognized the problem and started requiring that if you wanted to UL list your VFD as a motor controller, the VFD is required to be the SCPD (Short Circuit Protective Device) for the motor connected to it, because any upstream device is not going to see the true load current situation. So when that happened, UL then also made it a requirement for the VFD mfrs to start providing their MAXIMUM OCPD size for the line side protection of the VFD as part of the listing. That maximum size usually has to do with the ratings of the devices, so it can vary from one mfr to another. Therefore, there is no statement in the NEC that dictates the size of the OCPD device for a VFD, other than the general requirement to follow the manufacturers listing requirements. But the NEC section 430.122 was added a few years ago to dictate the MINIMUM size of the conductors feeding it at 125% of the VFDs maximum input current rating. So in effect, THAT then dictates the MINIMUM size of the OCPD, because you must then protect those conductors per the rules in section 210.

So for us here in NA, what you use as the OCPD to feed a VFD is something in between the size to protect the MINIMUM size of conductors, and the MAXIMUM size that the VFD mfr has listed the drive with. There is no other general statement that can be made. Many people just look at the maximum OCPD size required by the VFD mfr and go with that, not paying any attention to the term "maximum". The only thing you must be careful of is that someone may use the maximum OCPD per the VFD manual, then use the minimum conductor size per the NEC, and those two values may not match up! I have seen this happen more than once unfortunately...

But I can't help you with how this might relate to standards in India. I know that they used to follow British Standards for a long time, but I've heard that newer standards are now in place for India that although based on the older BS (no pun intended) are different in some ways. You would need to find a more relevant local website that addresses standards in India for that, this website was developed to assist people with issues related to our National Electric Code in the US and the Canadian Electric Code (which is almost identical). Good luck.

 

[Haji]

Is it possible they have just selected the MCCB as 100A to be on the safer side?

If the motor is to run by-passiing VFD, then 100A MCCB is required for over current protection of 22kW motor(also allowing the motor to start). For 18.5kW motor, 50A MCCB is enough.

why did they selected the motor to be a 18.5KW and not a 22KW motor ?

See post#9.

 

[Besoeker]

If the motor is to run by-passiing VFD, then 100A MCCB is required for over current protection of 22kW motor(also allowing the motor to start). For 18.5kW motor, 50A MCCB is enough.

See my comment in post #10.

See post#9.

Post #9 deals with the VFD size, not the motor.
If the original figures are used to calculate motor power, then 18.5kW isn't enough.