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Transformer Sizing for Motor Circuits

J

jthompson97

Member
Location
NJ
Occupation
PV Electrical Engineer
Hello all! This is my first post but I have used these forums many times.

I work in the PV industry and typically only work with continuous inverter generation, so my knowledge of non-continuous loads is limited.

We have a PV ground mount site with single axis tracker motors. The motors are 1.5kW, 480V 3ph, 3A continuous, 18A inrush. Only one motor has inrush at a time due to controls.

We designed the site for four motors, which has a 35A main OCPD at the 480V panelboard and a 25kVA transformer to step up to the system voltage of 600V. However, the racking vendor added a fifth.

I am confident on our OCPD of 35A and associated cable sizing.
(18A/motor * 1 motor * 1 factor) + (3A / motor * 4 motors * 1.25 factor) = 33A, less than 35A.

However, we had a 25kVA transformer. I am not entirely certain on how to size the transformer. Is it okay to size it only based on the continuous loads or should I consider this inrush as well? Which of the below is correct?

  • 33A (as determined above) * 480sqrt(3) = 27.4kVA - no good
  • I suppose the 1.25 factor is only for OCPD's. So if I did 18+ 3*4 = 30, 30*480sqrt(3) = 24.9kVA. Just makes it.
  • But if I only considered continuous, 3*5 = 15 -> 15*480sqrt(3) = 12.47kVA
 
jthompson97 said:
Hello all! This is my first post but I have used these forums many times.

I work in the PV industry and typically only work with continuous inverter generation, so my knowledge of non-continuous loads is limited.

We have a PV ground mount site with single axis tracker motors. The motors are 1.5kW, 480V 3ph, 3A continuous, 18A inrush. Only one motor has inrush at a time due to controls.

We designed the site for four motors, which has a 35A main OCPD at the 480V panelboard and a 25kVA transformer to step up to the system voltage of 600V. However, the racking vendor added a fifth.

I am confident on our OCPD of 35A and associated cable sizing.
(18A/motor * 1 motor * 1 factor) + (3A / motor * 4 motors * 1.25 factor) = 33A, less than 35A.

However, we had a 25kVA transformer. I am not entirely certain on how to size the transformer. Is it okay to size it only based on the continuous loads or should I consider this inrush as well? Which of the below is correct?

  • 33A (as determined above) * 480sqrt(3) = 27.4kVA - no good
  • I suppose the 1.25 factor is only for OCPD's. So if I did 18+ 3*4 = 30, 30*480sqrt(3) = 24.9kVA. Just makes it.
  • But if I only considered continuous, 3*5 = 15 -> 15*480sqrt(3) = 12.47kVA
Click to expand...
You are dealing with motors there. Our usual method is to factor in things when they are present. Motors have higher inrush currents if your controllers are not on VFD. That said, the success of the installation depends on a) controller sophistication, and b) whether all motors run at the same time or not. knowing you only have 35A.
Your problem is to know what coincidence factor to apply. (CF=system peak load/sum of the individual load peak loads) If the installation has been completed, I suggest you install a temporary breaker, bigger than the one "installed) and attach a recording ammeter/ demand meter so you can do a trial run -->to know what your "actual power peak" requirement is (don't mind the transformer, they can take some abuse for a short time). Take note of the peak load registered and divide it by the sum of the motor load peaks. Then you have a coincidence factor to use in sizing your CB.
Or, you can assume one of the two motors runs at a time (you could arrange a control logic to refrain the other motor from operating when the other is operating -> interlocking the two controls), CF=0.5. That simple interlocking halves your power requirement there. What you have there is more than enough. Don't overthink it.
 
As noted in post #4, inrush is dependent on the type of starter, which is an important consideration.

Transformer manufacturers publish documentation on how to select transformers for motor loads. I would turn to application data published by ACME in their transformer Q&A document for guidance and back-up. Note that what is published are rules-of-thumb that work for the majority of applications, but you can get crazy and model this in software to evaluate voltage drop upon motor starting, etc. (I wouldn't waste my time).

One additional item would be to perform all load calculations based on NEC table 430.250 FLCs and not your motor nameplate FLAs. I am assuming this is a 2HP motor per your listed kW and nameplate FLA so I would use 3.4A FLC per motor per the table.

Based on the above and assuming absolute worst case of >1 simultaneous DOL motor starts per hour, I get a minimum transformer size of 26kVA. I have incorporated the following parameters recommended by ACME:
  • 20% overcapacity for frequent starts
  • Motor running currents not to exceed 65% of transformer FLA rating for DOL motor starts
  • 5 * 3.4A * 480V * Sqrt(3) * 1.2 / 0.65 = 26kVA
Based on the above, a 25kVA is sufficient for your application as you note that the motors will not be starting simultaneously.
 
On a tracker, would you ever have all 5 motors operating simultaneously? Or really, worst case scenario is 4 motors already running and the 5th is started? Seems unlikely when there are a limited number of cardinal axes of movement. For example if you have North, South East and West drive motors, and add a "tilt" axis, that's 5, but you would NEVER run North and South or East and West at the same time. So 3 max simultaneously seems the thing to consider. I'm just guessing here though to point out that it may not be as it seems on the surface.
 
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