Standard VFD and transformer sizing practices?

Status
Not open for further replies.
I

Ingenieur

Senior Member
Location
Earth
Besoeker said:
The existing transformer survived unless I'm mistaken. And 125kVA would seem more than adequate.
The motor is 100hp/75kW. Again, it survived unless I'm mistaken.
So, I don't see any justification for upgrading either.

The drive worked for five years before it failed (from what cause we don't know) so one might assume it was fit for purpose.
So again, I don't see any need for an increase in rating.

Unless something has changed on the process side that we don't know about - then all bets are off.
:D
Click to expand...

You see no reason
I see a possible reason based on partial information
need drive mfg/pn

The moor is 108 HP unless a typo
afaik the drive has not failed?
drive is running at 140 A (at the limit for many 100 hp drives)
I'm surprised you did not ask how long it had been in operation?

As I said the xfmr may be fine if designed for this use
why do you like selectively chopping my posts up to distort their context?
so you can 'correct' me

load/xfmr = 140/150 = > 93% ignoring losses
not sure 'more' than adequate
if based on motor fla >95% 120 kva/125 kva
 
Last edited:
I

Ingenieur

Senior Member
Location
Earth
Besoeker said:
From the OP:
Click to expand...
I asked him that
surprised YOU did not

I believe in basics
i, v, load, operational methods, etc
data on the actual devices used
not esoteric treatise on xfmrs until we have that data

with the limited data
drive looks marginal at best
xfmr if designed fro vfd use ok but close to rating

need to know
Any life safety implications
revenue impact upon failure
in addition to tech data to make a vetted due dilligence opinion
otherwise idle speculation


Ingenieur said:
How long has it been operating in its current configuration?
Click to expand...
Ingenieur said:
has the xfmr temp ever been measured?
drive output i measured?
xfmr output i measured?
drive input i measured?
xfmr config: y:y, d:d, etc? pu Z?

for all we know this set-up may run to an expected service time without issues
Click to expand...

hmmmm
 
Besoeker

Besoeker

Senior Member
Location
UK
Ingenieur said:
I asked him that
surprised you did not

I believe in basics
i, v, load, operational methods, etc
data on the actual devices used
not esoteric treatise on xfmrs until we have that data

with the limited data
drive looks marginal at best
xfmr if designed fro vfd use ok but close to rating

need to know
Any life safety implications
revenue impact upon failure
in addition to tech data to make a vetted due dilligence opinion
otherwise idle speculation



[/B]
hmmmm
Click to expand...
OK.
 
P

paul_arc

Member
Location
CA
The motor is 108hp.
The drive failed from overheating, at least that's what the fault code said. Then it wouldn't run longer than 5 mins without over heating again. I cleaned it out and still couldn't get it to run any amount of time. An educated guess would be that it failed from running at the upper limit of the drive for so long. I had another 100 hp drive that I put in to get it going again but now looking to get a replacement of adequate size.
I'm looking at this 110kw drive, so about 150hp. My only concern is being too large for the 125kva xfmr and end up frying that.
There is another 90kw drive, so about 120hp. That matches the existing 125kva xfmr pretty good, but I feel it is still on the small size for the motor...
 
Besoeker

Besoeker

Senior Member
Location
UK
paul_arc said:
The motor is 108hp.
The drive failed from overheating, at least that's what the fault code said. Then it wouldn't run longer than 5 mins without over heating again. .
Click to expand...
Yet it ran without problems for five years?
Fans?
Filters?
Different operating regime?
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
As I see it you seem to think that there is a possibility that too large of a drive can somehow put the transformer at risk. Not so. The drive is just a source of power, the motor and pump
Is the load. The transformer simply sits in between. So long as the pump does not demand more load, nothing changes.

But that's where what I brought up earlier comes into play. If the VFD was automatically limiting the speed to 51Hz in order to maintain itself at the 140A Current Limit, using a drive with a higher current limit may allow the pump to run at the full 60Hz (worst case scenario). Again assuming a centrifugal pump, 51/60 = .85, so you are increasing the speed by 117.6% and that small change in speed on a centrifugal pump will demand roughly 163% more HP from the VFD. If allowed to do that, that then means 162% more power flowing through that transformer too.

Simple solution would be to simply buy the bigger VFD, but program the current limit to still be 140A, since you know the transformer has been ok with that all this time. So the only thing that changes really is the thermal capacity of the VFD. The new one would not be running at its upper limits.
 
P

paul_arc

Member
Location
CA
Jraef said:
As I see it you seem to think that there is a possibility that too large of a drive can somehow put the transformer at risk. Not so. The drive is just a source of power, the motor and pump
Is the load. The transformer simply sits in between. So long as the pump does not demand more load, nothing changes.

But that's where what I brought up earlier comes into play. If the VFD was automatically limiting the speed to 51Hz in order to maintain itself at the 140A Current Limit, using a drive with a higher current limit may allow the pump to run at the full 60Hz (worst case scenario). Again assuming a centrifugal pump, 51/60 = .85, so you are increasing the speed by 117.6% and that small change in speed on a centrifugal pump will demand roughly 163% more HP from the VFD. If allowed to do that, that then means 162% more power flowing through that transformer too.

Simple solution would be to simply buy the bigger VFD, but program the current limit to still be 140A, since you know the transformer has been ok with that all this time. So the only thing that changes really is the thermal capacity of the VFD. The new one would not be running at its upper limits.
Click to expand...


Yes, I could set the current limit so it doesn't exceed the transformer rating but a lot of times the operators like to get in there and pretend to be an electrician. Most of the time its just to adjust the speed for their needs. It's usually only a 1 or 2 hz change one way or the other. We get a lot of power flickers here and the equipment will drop out, when the operators try starting it back up without an electrician on site, they really get in the drives and screw them up some times.

I guess the part i'm concerned with is giving them the capability of really blowing something up. Because the pumps get stuck sometimes when it goes down which requires hard starting. Changing voltage boost, sync delay, current limit, accel time, and what ever else they get into while trying to get it started again.

Looking at the current limit of this drive, it is set at 185A@ 480v, which is 120% of the motor name plate. That exceeds the transformer already, but i'm not sure if the drive would even be able to hit that number since its only a 100hp.
I guess that's all i'm thinking about, limiting the power so that someone who doesn't know cant blow the transformer.
 
Besoeker

Besoeker

Senior Member
Location
UK
paul_arc said:
Yes, I could set the current limit so it doesn't exceed the transformer rating but a lot of times the operators like to get in there and pretend to be an electrician. Most of the time its just to adjust the speed for their needs. It's usually only a 1 or 2 hz change one way or the other. We get a lot of power flickers here and the equipment will drop out, when the operators try starting it back up without an electrician on site, they really get in the drives and screw them up some times.

I guess the part i'm concerned with is giving them the capability of really blowing something up. Because the pumps get stuck sometimes when it goes down which requires hard starting. Changing voltage boost, sync delay, current limit, accel time, and what ever else they get into while trying to get it started again.

Looking at the current limit of this drive, it is set at 185A@ 480v, which is 120% of the motor name plate. That exceeds the transformer already, but i'm not sure if the drive would even be able to hit that number since its only a 100hp.
I guess that's all i'm thinking about, limiting the power so that someone who doesn't know cant blow the transformer.
Click to expand...
Isn't the drive in an enclosure that can be locked?
 
O

Open Neutral

Senior Member
Location
Inside the Beltway
Occupation
Engineer
Jraef said:
As I see it you seem to think that there is a possibility that too large of a drive can somehow put the transformer at risk.
Click to expand...

Actually, I was thinking the transformer would be running at 51 HZ than its (presumably...) 60 Hz spec, and is handling a lot of non-linear input crapola; I'd think both would be sources of its running hot. I didn't mean to imply it was the source of the original failure; just that it may be what's next to die.
 
I

Ingenieur

Senior Member
Location
Earth
Current % full load motor ~ 90%
Current speed 85%
Current pump flow (assuming at 60 Hz) 85% of max

power increase to get 100% flow +63%
Flow increase at 100% fla +3.7%
new rpm 53 Hz
new drive 125 HP
Looks like max pump power > max motor power,
pump power at 100% speed/flow ~146, call it 150

125 HP drive
rla 39 A 108 Hp
53 Hz
flow increase ~4%
kva ~130 kva (xfmr is marginal)
if you need more the motor needs upsized

if you are ok with the 51 Hz of flow and need to replace the vfd 125 hp will do
xfmr should be ok at 116 kva
motor is ok 35 rla < 39 fla
 
P

paul_arc

Member
Location
CA
Ingenieur said:
Current % full load motor ~ 90%
Current speed 85%
Current pump flow (assuming at 60 Hz) 85% of max

power increase to get 100% flow +63%
Flow increase at 100% fla +3.7%
new rpm 53 Hz
new drive 125 HP
Looks like max pump power > max motor power,
pump power at 100% speed/flow ~146, call it 150

125 HP drive
rla 39 A 108 Hp
53 Hz
flow increase ~4%
kva ~130 kva (xfmr is marginal)
if you need more the motor needs upsized

if you are ok with the 51 Hz of flow and need to replace the vfd 125 hp will do
xfmr should be ok at 116 kva
motor is ok 35 rla < 39 fla
Click to expand...

I agree with the 125hp drive. Everything is sized marginal and feel that for how this is set up that 125hp will be a good fit.
It's always hard trying to size something that you know should be larger but the engineer didn't do that great with sizing the motor, transformer, and last drive.
 
Besoeker

Besoeker

Senior Member
Location
UK
Let me try this again.
The system ran OK for FIVE years.
Why is anyone trying to redesign it? Find the fault and fix it!!!
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Besoeker said:
Let me try this again.
The system ran OK for FIVE years.
Why is anyone trying to redesign it? Find the fault and fix it!!!
Click to expand...
Right. The "fault" was that the VFD eventually burned up, most likely from running at the extreme end of its capacity. Someone mentioned 140A as being "standard" for a 100HP VFD, it is not for most VFDs designed for North America; 100HP is typically 125A and although you can run the VFD at a higher value, you can't do that continuously. Some Asian drives that are designed for 75kW at 380V and SOLD as "100HP" will be rated for 140A, but only as "Variable Torque" drives. In truth, they are EXACTLY the same power transistors sold as 125A rated units here, they are "stretching" the capacity of the transistors knowing that most of the time on an HVAC load (which is what these are sold for), people almost never run them at the full rated current continuously, otherwise you would not bother having a VFD...

If he needs to run at 140A continuously, he needs to use a VFD rated for that and because the transformer represents additional FIXED losses in the system, that 140A should be the CONSTANT TORQUE rating of the VFD (in my humble opinion). Plain and simple; a 125HP constant torque rated VFD will be rated somewhere around 156A, perfect for this. And his question about the transformer was answered as well; a 125kVA transformer should be perfectly fine at 140A, as evidenced by the 5 years of operation.
 
Status
Not open for further replies.
Top