Sizing a transformer to a motor load

[cavejumper]

Hi everyone! Thank you all for your assistance, both now and in the past, and for being a great resource to myself and the entire electrical industry! I've certainly learned a lot reading through the forums, and if I have a question on anything, this is one of the first places I'll look for an answer.

As the title says, I'm doing calculations to size a step-up transformer and feeder for a dedicated motor load, and I want to make sure I'm doing it properly. My customer is on a budget, so I don't want to oversize it, but I want to make sure it'll work for him as well. I looked at other threads relating to this, and I think I got it, but I would like more knowledgeable people to look at this too and make corrections or suggestions as necessary. So here we go:

The equipment is an air compressor with a 75HP, 460V motor. The manufacturer says FLC is 105.5A, LRC is 479A. I have not see it, and I have no other info on it other than the manufacturer says there is no duty cycle listed. I'm under the impression that it doesn't have any variable speed drive or soft start or anything like that.

The building has a 240V delta service with enough capacity to handle this, but I would need to install a step-up transformer to do so. This transformer would be dedicated to this equipment (which does not need a neutral). Should I be looking for a 240V delta - 480V wye transformer, or would another configuration be better?

I've sized my motor feeder (480V) as follows:
Conductor ampacity (125% of FLC) - 131.9A
Overload protection rating (140% of FLC) - 140A
Short circuit protection rating (150% of FLC) - 175A

Here's where I'm uncertain - I'm not sure what number to use to size the transformer. If I go with 125% of FLC of the motor -> 105.5x1.25=131.9A -> 131.9Ax480Vx1.732=109.6KVA -> I would use a 112.5KVA transformer. Is this adequate?

Assuming it is, and I continue my calculations, my transformer rated current will be -> 112500VA/(240Vx1.732)=270.6A. Then my conductor ampacity will be -> 270.6Ax1.25=338.3A, and my overcurrent protection will be 350A.

I don't know how much voltage drop would be acceptable for this motor. I also don't know how much impendance there'll be in the system, but I can say that the compressor will be located close to the gear feeding it - I'll probably only have about 50-70' of conductor between the primary OCPD and the motor.

Please look over my figures and let me know if I need to change anything. Thanks for your help!

 

[petersonra]

A lot of 75 HP 3 phase motors can be wired either for 230 V or 460 V. Might be that the motor you have is like that. If so, I would just skip the transformer and wire it as 230 V.

If you end up using a transformer for whatever reason, I would point out that you may well end up with the transformer in saturation when the motor starts up. This can result in voltage sags and might well lead the motor to stall. At 75 HP, it might be appropriate to be looking at some kind of soft starter, but some compressors do not play well with soft starters.

 

[Besoeker3]

Maybe I'm just simplistic. Transformer output current rating should, at least, match that of the motor.

 

[cavejumper]

I am looking into whether the compressor can be converted to 230V. The short answer is that it can, it'll just be a matter of comparing the cost to do that vs. installing the transformer.

Help me understand transformer saturation. Does that happen when there is no load on the secondary for a period of time? What would correct that issue?

The transformer output current rating does roughly match the FLC of the motor, plus 125%.

 

[petersonra]

Transformers go into saturation when the current flow (actually the magnetic flux) through them exceeds their rating enough that no further current can flow. For instance, at startup, motor current flow may exceed normal FLA by 5-10 X. If the transformer cannot handle that amount of current the voltage will sag.

This is not always a problem. But, it can also cause the motor to stall and eventually trip the overload relay.

I have seen rules of thumb that are all over the place for transformer sizing for a single motor.

My personal opinion is to size the transformer FLC at perhaps 150-200% of motor FLC. Usually there is a transformer rating that makes sense in this range. My inclination is to error on the side of a larger transformer over starving the motor.

 

[powerpete69]

If you can make the motor run on 230V, then great.
If not, your numbers above look reasonable to me.
I simulated your situation in the PDF attached.
It shows your 75HP motor current, your 350 overcurrent device, 112.5 KVA transformer inrush, 112.5KVA transformer damage curves and maximum amps of your transformer.
It also lists the possible settings for your 350A breaker.
Guys above say your transformer may saturate, draw conclusions from simulation attached.
My two cents.

 

[mayanees]

The only way to remove the uncertainty is to do an analysis of the motor starting current and the Utility source capacity. If you're client is out in the boondocks, there may not be enough strength to avoid tripping breakers and dimming the neighborhood even when using a transformer that's two-three times the size of the motor (150-225kVA). But if he's close to a substation, you may get away with using a transformer sized at 125% (100kVA) and not see any dimming of the lights.
I suggest requesting Utility contribution information at the point of service, and also determine if the Utility has an across-the-line starting motor size limitation.

 

[cavejumper]

Powerpete - Thank you for making that simulation, that's very helpful!

Petersonra - Is there a max current rating or saturation level rating I can look for when shopping for transformers? Using Pete's simulation, it looks like I would be right at the limit of a 112.5KVA transformer, and it may be up to differences between manufacturers. Say, if I look at a 112.5KVA from Acme and it has a saturation level of x, then I'll be ok, but a 112.5KVA from Eaton has a saturation level of y, which is lower than x, I know I need to bump it up to a 150KVA? Is this a thing?

Mayanees - The entire building has a 1200A, 240/120V service that is no longer being using anywhere near that capacity. I would be feeding this compressor from an 800A subpanel coming off the main 1200A gear. It wouldn't hurt to ask the utility about any motor size limitations, but I doubt there would be any issues. Thanks for your input!

 

[retirede]

It’s likely that the compressor will only require a new motor starter and associated wiring.
That’s probably the most economical route.

 

[Cow]

Is this air compressor the type that comes self contained on a skid with the motor, pump, UL listed control cabinet, etc all in one?

If that's the case, it may be a surefire way to void the warranty and any listing that went with it. Since your components will essentially double in size, there is a good chance none of it will fit into the existing enclosure anyways.

If it's just a piecemeal air compressor made up of various components not packaged together, then I would absolutely look into just swapping the starter, controls, and associated wiring over to your 230v system.

 

[bwat]

I've sized my motor feeder (480V) as follows:
Conductor ampacity (125% of FLC) - 131.9A
Overload protection rating (140% of FLC) - 140A
Short circuit protection rating (150% of FLC) - 175A

Something to double check. Are you sure these are right? Specifically for your overload.

What I had in my head for overload was 115% or 125% depending on service factor according to article 430.

Short circuit protection of 150% may end up being on the lower side too, depending on fuse or breaker type.

 

[topgone]

Choose a circuit breaker and be done with it. What @bwat said, at 115%, your choice would be 125A TM breaker. The Instantaneous trip setting is fixed on some breakers and the only way to know is try to simulate the highest fault current you expect at your motor terminals and compare with the tripping characteristic of your chosen circuit breaker. Hope that helps.

 

[retirede]

Is this air compressor the type that comes self contained on a skid with the motor, pump, UL listed control cabinet, etc all in one?

If that's the case, it may be a surefire way to void the warranty and any listing that went with it. Since your components will essentially double in size, there is a good chance none of it will fit into the existing enclosure anyways.

If it's just a piecemeal air compressor made up of various components not packaged together, then I would absolutely look into just swapping the starter, controls, and associated wiring over to your 230v system.

The compressor manufacturer I worked for would have no problem with the conversion. We even sold conversion kits with a new starter and wiring harness!

YMMV!

 

[Besoeker3]

Transformers go into saturation when the current flow (actually the magnetic flux) through them exceeds their rating enough that no further current can flow.

Actually, voltage or V/f rating being too high saturates them.

 

[iceworm]

...Petersonra - Is there a max current rating or saturation level rating I can look for when shopping for transformers? ...

Following Bes post:
Saturation and the current load are not particularly related. Transformer saturation is an issue of V/F ratio. So, it is either over voltage, or under frequency. Neither of those should be an issue.

However, following other posts, the issue will be voltage drop caused by the transformer impedance and the conductor impedance..

https://electrical-engineering-port...e-voltage-drop-due-to-starting-of-large-motor
This guy has a pretty good model for the transformer. It is majorly brutal to follow. He works with KW, and inrush FLC multipliers, rather than Hp and KVA/HP (T430.7.B)

Using this model:
Neglecting the voltage drop on the primary and secondary conductors, for:
75Hp, code G

112.5KVA
480V
5.75 %IZ

%Vd at inrush = 23%
Voltage at motor terminals = 370V

Motor torque is proportional to V^2, so with the voltage dropping to 77%, available torque drops to 59%. No, I don't know is this is enough to start the compressor.

Changing to an energy efficient 112.5KVA, 4.4%IZ reduces VD to:
%Vd at inrush = 17.6%
Voltage at motor terminals = 396V
Motor torque = 68%

Changing to an energy efficient 150KVA, 3.8%IZ reduces VD to:
%Vd at inrush = 11.4%
Voltage at motor terminals = 425V
Motor torque = 79%

And there will be some voltage drop on the secondary conductors and the primary feeder (all the way back to the service transformer)
Inrush on 480V secondary ~ 600A
Inrush on 240V primary ~ 1200A

Just thinking, my inclination is to go with an energy efficient 150KVA

 

[iceworm]

A couple of other things to consider: (Most of this follows previous posts)
Changing the motor to 240V gets rid of the transformer, however:

75Hp, 480V takes a size 4 starter (~$3K) This assumes the transformer starter is on the transformer secondary. Which I likely would, just so that the system is not fighting the transformer inrush and the following motor inrush. Especially if the compressor is not continuous run.​

​

75Hp, 240V takes a Size 5 starter (~$6K)​

Motor CB:
For a TM, size at 225% of FLA by T430.250

480V 225A​

240V 450A​

Or consider a combination starter with a Mag only, sized 1100% - 1700%

Transformer Feeder CB, size at 125% Transformer FLA

112.5KVA 350A​

150KVA 450A​

Overloads:
For a 1.15SF motor, 140% overloads work well. I have set a lot of them there, and no issues. Except:
I have had several knowledgeable sorts carefully explain that the 140% overloads will burn up the motor. My response:

Nope. The motor load is not set by the overloads. It is set by design. If the motor is overloaded, setting down the overloads will not fix the overload. It is still overloaded. All setting down the overloads will do is cause more trips, resulting in more starts. And somewhere around 4 starts in an hour, the smoke comes out.​

​

For the case of an industrial grade compressor, the design should not have the motor running up in the service factor. Following bwat post, 125% (for a 1.15SF) is also a good choice. The lower overloads do marginally improve the chance of having a salvageable core if the bearing go out. Maybe the 125% overloads will shut down on dragging bearings before the rotor hits the stator. And maybe, no better than 140% overloads.​

Just some random thoughts from

the worm

 

[powerpete69]

Real interesting thoughts above, thanks. I can see my easypower simulator has a tab for "motor starting" and a tab for "powerflow" which I'm sure would graph voltage vs current over time. Unfortunately these tabs are not lit up which means we did not buy them in our yearly subscription. I will try to get them next time!!

 

[cavejumper]

Thank you all for your information and help! It does appear that the most economical route will be to convert it to 230V and avoid the transformer altogether. But at least this has been an interesting learning process for me, and I may be able to apply this to something else down the road. Be safe!

 

[GeorgeB]

As you look into the compressor system, determine if the motor is started "frequently", as in controlled by a pressure switch, or runs all the time and unloaded, or, running on an included variable drive to adjust provided volume to needed volume. If there is an inverter (or soft start) included at 460, and the motor terminations wired for 230, a new inverter (or soft start) would be required. If unloading is used, a soft starter is unlikely to give problems, and will help on a soft supply.

 

[topgone]

As you look into the compressor system, determine if the motor is started "frequently", as in controlled by a pressure switch, or runs all the time and unloaded, or, running on an included variable drive to adjust provided volume to needed volume. If there is an inverter (or soft start) included at 460, and the motor terminations wired for 230, a new inverter (or soft start) would be required. If unloading is used, a soft starter is unlikely to give problems, and will help on a soft supply.

That's another factor to look at. But most compressors I have dealt with do not put off the motor starter when the pressure limit is reached. There is an unloader/loader valve that cycles depending on the air pressure.