Transformer Sizing

[rockman7892]

When sizing a transformer I usually take 125% of my load to arrive at a kVA value for the transformer. When sizing a transformer that will have multiple motors starting on it are there any inrush factors to take into consideration when sizing the transformer, or do the inrush currents not last long enough to have any effect on transformer?

In other words, If I'm sizing a transformer for which three motors will be started off of the transformer, can I stricly used the combined FLA of the motors to size the transformer, or do I have to take inrush/starting current of these motors into consideration?

 

[montoy]

The typical motor circuit consists of motor, overload protection, remote control (i.e. start/stop), disconnecting means (safety switch), OCPD (c/b). Should the question be, what size of circuit beaker or fuse to handle the inrush current of the motor load? What's really your motor circuit configuration?

 

[DRWeig]

Hi Rockman,

You'll need to consider the starting (inrush) current according to how the motor controls are configured. The inrush, if not accommodated, can drop the secondary voltage enough to cause trips or stalls. At worst, you'd like the voltage to stay above 90% of nominal during start conditions. You'll need to compute voltage drop with knowledge of the transformer impedance, line impedance, etc...

1) Will all three motors be allowed to start at the same time? If so, a good rule of thumb is to load the transformer no more than 60 percent with the combined FLA of the three motors.

2) If controls are configured so that only one motor can start at a time, you can creep up a bit -- consider one motor starting while the other two are running at full load.

Let's see if any of the other tipsters will weigh in with something more concrete...

Good on ya,

Goober Dave

 

[rockman7892]

I was thinking along the lines of a transformer feeding an MCC or some distribution panel which had various motor starters/contollers. Typically these motors would be started on at a time, but for a worst case scenaro of them all starting at once the 60% value is the sort of answer I was looking for.

Lets say for the sake of argument that I have (1) motor with a FLA of 100A. If I was only going to run this motor, then I would size a transformer capable of 125A on the secondary. Now if I had (3) motors at 100 FLA I would have 300A + (.25% * 100A)= 325A. So my question was more along the lines if I had these three motors, can I just size a transformer for a secondary of 325A or do I have to size a larger transformer to accomidate inrushes for possible multiple motor starting scenarios? I guess I'm looking for more or less "rules of thumb" for sizing transformers in this application if any exist.

Thanks for the help.

 

[jghrist]

You don't need to take motor starting into account when sizing the transformer for transformer loading. The thermal time constant of a transformer is high enough that no significant heating will occur.

You do need to take the voltage drop through the transformer into account, as indicated by Goober Dave. This indirectly affects transformer sizing because larger transformers have lower impedance (in ohms). You may need to use a larger transformer to lower motor starting voltage drops.

 

[dpc]

The best rule of thumb for sizing transformers in a plant - make it at least one size bigger than you think you need. You'll never get any credit for saving money on the transformer but you'll never be forgotten if the transformer ends up being a limiting factor in plant production.

The NEC actually says very little about sizing transformers. The main concern of the NEC is providing adequate transformer protection. The NEC load calcs are intended more for sizing feeders and branch circuits than for sizing transformers.

The NEC is a safety code, not a design guide, as you probably know.

In reality, you have to take a lot of factors in account, including future load growth.

Cheers,

Dave

 

[Morquea]

My usual rule of thumb is this :

Xmer KVA = 4(kVA/HP) * Pmotor(HP)

For your 100A motor (assuming it's a 575V), that rule would size a 400kVA.
I seldomly used that rule. I prefer do the following calculation :

1 - Evaluated the Locked Rotor Ampere - Usually a percentage of the FLA typicaly from 600% to 800% for On-line startup.

2 - Use the above result to calcuted the inrush power at rated voltage. LRA * V * sqrt(3) = Sinrush

3 - I multiply by 10. It's to take account that I don't want more than 10% voltage at starting. The results gives me the short-cirucit power capacity the transformer needed to start the motor without incurring a more than 10% voltage drop.

4 - Next, I multiply the above results by the Z%. I use 7% when I don't know wich transformer shall be choosed. The results should gives me the Rated kVA of the transformer if he is connect the an infinite source.

Example :
Sxmer = Scc*Zpu = 418 kVA
Z% = 7% => Zpu = 0.07 pu
Scc = 10*Sinrush = 5,98 MVA
Sinrush = LRA * V * 1.732 = 598 kVA
LRA = 600% * FLA = 600A
Rated Voltage = 575V
Motor FLA = 100A

It's still a rule of thumb since I use shortcuts assumption about the 10% voltage drop, the Z% and the RLA. But the essence is there... It think.








Danny Garant, ing.jr
Groupe Stavibel Inc.

http://www.stavibel.qc.ca