Transformer sizing

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I would like to correctly size a transformer for two 240VAC 3ph AC units.

Power available is 480VAC 3PH. The specs on the AC Unit only include the following:

Min. Amp Circuit 11.6A
Max Fuse Size 20A
RLA 8.4 (I assume Running Load Amps?)
No other details are avaialbe

I am assuming 11.6 is the FLA or full load amps.

11.6 X 1.25 X 2 = 29A (two compressor units)

3ph 15kw transformer at 240 provides 36A, 15000/240x1.73=apx 36A

I intend on using a 480/240 15KVA 3ph transformer. Is this correct?
Thanks in advance
 
control Issues said:
I would like to correctly size a transformer for two 240VAC 3ph AC units.

Power available is 480VAC 3PH. The specs on the AC Unit only include the following:

Min. Amp Circuit 11.6A
Max Fuse Size 20A
RLA 8.4 (I assume Running Load Amps?)
No other details are avaialbe

I am assuming 11.6 is the FLA or full load amps.

11.6 X 1.25 X 2 = 29A (two compressor units)

3ph 15kw transformer at 240 provides 36A, 15000/240x1.73=apx 36A

I intend on using a 480/240 15KVA 3ph transformer. Is this correct?
Thanks in advance
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11.6a is MCA (Minimum Circuit Ampacity) not RLA (Running Load Ampacity)

MCA is used to calculate the branch circuit. The manufacturer has already calculated the largest motor @125%(most likely the compressor) + the additional loads(condenser fan motor & controls) to arrive @ the MCA.

11.6x2=23.2amps
240v x 23.2a x 1.732 = 9643.776va/1000 = 9.64377kva

To limit the loading of the xfmr to 80%, multiply 23.2 amps x 1.25 = 29amps.
240v x 29a x 1.732 = 12054.72va/1000 = 12.05472kva
 
You better account for inrush on the units starting simultaneously unless you have controls that won't allow that to happen.

Transformers typically have to be oversized to start motor loads across the line due to ability to provide enough reactance during starting.
 
can you give an example of this?

can you give an example of this?

kingpb said:
You better account for inrush on the units starting simultaneously unless you have controls that won't allow that to happen.

Transformers typically have to be oversized to start motor loads across the line due to ability to provide enough reactance during starting.
Click to expand...


kingpb, can you give an example allowing (inrush current) on motor loads across the line starting, like his example? when sizing transformers?

what formula do you use when making allowances for inrush current?:D

thanks,
 
Going to be a lot of assumptions, but hopefully the method will be clear. I must preface this with saying, the calculation is only an approximation, and does not account for cable voltage drop, nor actual voltage on system prior to starting. This method also assumes that the utility system, or in this case the 480V system, can be viewed as an infinite source.

Assume the motor on the unit is 3Hp with a FLA = 9.6A, and LRC = 600%
15KVA XFMR has a FLA of 36A @ 240V, 3ph with an impedance of 3%.

Therefore the LRC = 9.6 x 6 = 57.6A
% of LRC to XFMR FLA = 75.6/36 = 1.6 or 160%

An estimation of VD during starting is then 1.6 x 3% (XFMR Z) = 4.8%

Starting two motors simultaneously will mean 2 x 4.8% = 9.6% drop on the LV terminals of the transformer

Motor starters can usually handle a 15% drop on starting, but you need to check, because some can only handle 10%.

Also, conductor voltage drop will contribute to initial bus voltage, meaning you may be starting with a 2-3% drop already, especially combined with not having a full 480V on the primary side of transformer. Taps on the transformer can be used to help accomodate for this.

For large(r) motors a more detailed analysis may be required, that may include dynamic motor starting especially in applications requiring high starting torques.
 
kingpb said:
Going to be a lot of assumptions, but hopefully the method will be clear. I must preface this with saying, the calculation is only an approximation, and does not account for cable voltage drop, nor actual voltage on system prior to starting. This method also assumes that the utility system, or in this case the 480V system, can be viewed as an infinite source.

Assume the motor on the unit is 3Hp with a FLA = 9.6A, and LRC = 600%
15KVA XFMR has a FLA of 36A @ 240V, 3ph with an impedance of 3%.

Therefore the LRC = 9.6 x 6 = 57.6A
% of LRC to XFMR FLA = 75.6/36 = 1.6 or 160%

An estimation of VD during starting is then 1.6 x 3% (XFMR Z) = 4.8%

Starting two motors simultaneously will mean 2 x 4.8% = 9.6% drop on the LV terminals of the transformer

Motor starters can usually handle a 15% drop on starting, but you need to check, because some can only handle 10%.

Also, conductor voltage drop will contribute to initial bus voltage, meaning you may be starting with a 2-3% drop already, especially combined with not having a full 480V on the primary side of transformer. Taps on the transformer can be used to help accomodate for this.

For large(r) motors a more detailed analysis may be required, that may include dynamic motor starting especially in applications requiring high starting torques.
Click to expand...

Is voltage drop across the transformer the only thing that you have to worry about when starting multiple motors on a transformer as explained? Do you have to take avaliable kVA of the transformer into consideration in this scenario?

Do you also have to take these combined starting currents into consideration to assure that they dont damage the transformer which should be protected by and overload device?
 
15kva 3phase 60 hz 480 volts > 240 is ok, but i normally prefer the next available size. ( 17.5 kva if it is available).

This takes care of any future additions.

cheers.
 
The outcome of temporarily overloading transformers during motor starting presents itself initially as poor voltage regulation. There are limits to the temporary overload. Certainly, the protection needs to be coordinated, as the temporary overload may be high enough, and sustained long enough to trip the transformer and feeder protection devices.

Use of transformer damage curves that show inrush requirements, will need to be plotted with the motor starting curves and motor protective devices to be certain proper selective coordination takes place. With the maximum setting possible, while still protecting the transformer from damage, it may not be high enough, and therefore a larger transformer may be necessary.

In looking at protection curves, it seems that as long as the motors starting simultaneously achieve full speed within about 30 seconds, you are going to be OK (assuming they can start on a low voltage condition) if using a protection device on the primary, not set higher than 125%.

To truly understand this issue and see how all the pieces need to come together you really need to look at the motor starting curves, and load speed torque curves. That may seem overkill for what appears to be a simple set-up, but then, that's why equipment fails prematurely and no one understands why, or it won't start properly and the service tech or engineer gets called.
 
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kingpb said:
In looking at protection curves, it seems that as long as the motors starting simultaneously achieve full speed within about 30 seconds, you are going to be OK (assuming they can start on a low voltage condition) if using a protection device on the primary, not set higher than 125%.
Click to expand...


How did you calculate the fact that if they are up to speed within 30s then everything should be fine? Do you simply add the two starting currents of the motor to determine the combined starting currents of the motors when comparing this current value to transformer damage curves and OCPD's?
 
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