required kva to start motor

[mwuniseal]

I am looking for a calculation that I can size the required kva for a transformer to start a 460 volt 3 phase 200 hp motor across the line. I do not have any more specs on the motor nameplate yet. I have tried this: KVA = E x I x 1.732 x 600% (for inrush) and came up with needing 1147 kva to start the motor across the line. I have also been told that 1 kva per hp running, and 5 kva per hp starting can be used. Any help is much appreciated.

 

[Jraef]

I am looking for a calculation that I can size the required kva for a transformer to start a 460 volt 3 phase 200 hp motor across the line. I do not have any more specs on the motor nameplate yet. I have tried this: KVA = E x I x 1.732 x 600% (for inrush) and came up with needing 1147 kva to start the motor across the line. I have also been told that 1 kva per hp running, and 5 kva per hp starting can be used. Any help is much appreciated.

There is no specific calculation because there are way too many variables. You can use what is called Transient Motor Starting analysis tools like SKM or ETAP, but they are very expensive (and powerful) software programs, as in $5K+.

But transformers have an ability to handle BRIEF surges, so you do not need to size it to the LRC of the motor.

I have been using some "Rules of Thumb" that I was taught years ago, based on the starting method, and based on the need to avoid more than a 5% voltage drop on the primary side of the transformer. So far they have never failed me:

For Across-the-Line starting, XFMR kVA = 3x motor HP minimum. So for example if you have a 50HP motor, a 150kVA transformer is needed. You MIGHT be able to get away with less under certain circumstances, but this rule is for gross generalizations. For example if you are starting a remote pump station from a stiff line and don't care much about the voltage drop as long as the control power doesn't drop out, you might get away with a 112.5kVA transformer, maybe even a 75kVA. the problem is, you won't know if it worked until you already installed it, and at that point you have already spent the time and money.

For Y-Delta starting, XFMR kVA = 2.5x motor HP minimum

For RVSS starting, XFMR kVA = 2-2.5X motor HP (depending on starting profile / load)

For RVAT starting, XFMR kVA = 1.5-2x motor HP

For VFD, XFMR kVA = 1-1.5x motor HP

For multiple loads, use these values for the biggest motor on TOP of the running load kVA of the total load HP running when biggest motor starts. So for example a 150HP pump on a soft starter and 3 x 10HP fans that are already running when the pump starts, you have roughly 22kVA running load, plus 150 x 2 for the RVSS = 322kVA, next closest size is 500kVA. But if you can ALWAYS start the RVSS first, you can use a 300kVA.

This is not an exact science, just based on empirical data from the field.

 

[Besoeker]

There is no specific calculation because there are way too many variables.

But transformers have an ability to handle BRIEF surges, so you do not need to size it to the LRC of the motor.

I have been using some "Rules of Thumb" that I was taught years ago, based on the starting method, and based on the need to avoid more than a 5% voltage drop on the primary side of the transformer. So far they have never failed me:

I pretty much agree with that.
I would add though, that if other equipment is connected to the transformer secondary, you might want to ensure that the voltage drop on the secondary is not more than 5%.
I don't recall any circumstances where a 150kW motor was the only load on a low voltage 3-phase supply which is generally 400V (nominally) here.

 

[mayanees]

ditto..

ditto..

In full support of jraef's experience-based recommendations here as well.

And do take note of his watchout comment, that you're not certain if it'll start until you go to try it, and of course by then you're design is front and center, so error on the high side.

As a consultant, if I'm sizing a transformer for motor starting, I'll model the system in SKM and use the Transient Motor Starting program to see the voltage drop steady state, and perhaps more importantly, on startup. If I'm passing judgment that my design will work, I evaluate to NEC 695 Fire Pump starting criteria, which is 15% VD on startup, 5% maintained. You can tap the transformer up in the model and in the field if necessary to minimize the required xfmr capacity. But critical to the validity of the analysis is to get the source capacity information, and get the locked-rotor design rating off the motor nameplate if available.

And do note that jraef's recommended minimum for ATL starting is 300%. You're looking to start a 200HP motor, 300% of which is a 600 kVA xfmr, but 750s the next available, so that's probably a good choice.

John M

 

[Jraef]

I pretty much agree with that.
I would add though, that if other equipment is connected to the transformer secondary, you might want to ensure that the voltage drop on the secondary is not more than 5%.
I don't recall any circumstances where a 150kW motor was the only load on a low voltage 3-phase supply which is generally 400V (nominally) here.

LOL, I guess you don't see a lot of deep well irrigation pumps over there then. The biggest I have done at 480V was a 500HP submersible (consisting of 2 x 250HP in-line tandem* motors). It had 3 steel cables in the wire bundle to hold the weight, mostly of the wire, down the 4,000ft hole.

*I know the word "tandem" would typically mean side-by-side, not in line, but that's just how they are referred to in that industry.

 

[mayanees]

jraef,
What size transformer sourced the 500 hp submersible?
My experience with those pumps is that they need to be sized to run into service factor, but I expect they're not a large inrush on startup based on the application.
John M