Step up transformer load calculation.

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iceworm said:
Size 4 on the 480V side, size 5 on the 208V side.

3000A to 12,000A inrush (depending on the xfm) every time the xfm is energized

1100A LRC when the motor is energized.

Hummm .... I suspect I would lean toward putting the starter on the 480V side.

ice
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The motor is 460 volt, and the compressor comes as a complete package, and would be hooked up to 460 Volt side. The xfmr will be on 24/7, as the compressor cycle would vary quite a bit. This is at a foundry.
 
A little more info.. The load side would be fused at 150 amps, manufacture specs, the line side would have a 400 amp breaker, 65000 AIC. Thanks for all the responses, it's really been helpful.
 
I haven't seen anyone mention impedance of the transformer. What you need to be sure of is that the system can provide the reactance to the motor when started; i.e. VARs. The impedance of the transformer plays a huge role in this.

Starting pf of the motor is what you need to look at, then determine what size the transformer and transformer need to be such that you can meet this requirement. Once it is running should not be an issue.
 
kingpb said:
I haven't seen anyone mention impedance of the transformer. What you need to be sure of is that the system can provide the reactance to the motor when started; i.e. VARs. The impedance of the transformer plays a huge role in this.

Starting pf of the motor is what you need to look at, then determine what size the transformer and transformer need to be such that you can meet this requirement. Once it is running should not be an issue.
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Sounds like you are discussing the voltage drop on start up. See post 7. The VD at locked rotor was the reason I suggested a 150kva xfm.

I'm always up for a new modeling technique. How does one calculate the required tranformer impedance? I likely would not order a custom wound xfm, so one would have to take what ever the mfg are offering. Additionally, where does one get the motor startup pf? I've not seen that on the mfg spec list

Here is an example.

Say the motor pf at strtup after the first few cycles is .3. And the 112.5kva xfm %Z is 3.5%. Is that okay?

(clarify request is for the model)
ice
 
kingpb said:
... Starting pf of the motor is what you need to look at, ....
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Physics Question:

Q: Does the motor locked rotor pf change with the applied voltage? Yes, the voltage is down, reducing the vars, but so is the i^2 losses.

Here is the model I am using:
Consider at locked rotor, the motor is not turning, so power out = 0. The circuit can only deliver real power to the winding copper resistance. All the rest of the E x I is reactive.

Consider the motor is on a long feeder or small xfm - either case there is significant VD at LRC.

Q: Does the locked rotor PF change with applied voltage?

I've never given it much thought. If I was concerned about the VD on startup, the model I use is: motor pf at .3, conductors per Table 9, and xfm impedance at factory nominal (all reactance). I have not gotten in trouble with this yet - but tomorrow is a new day.

ice
 
At starting, the pf of the motor is very low, i.e. mostly all reactance. Hence why the steady state concept of voltage goes down when current goes up, does not apply. During starting as voltage goes down, so does current and is the basic principle of reduced voltage starters, albeit with torque limitations/consequences and is why V/Hz (VFD) is much better suited for high torque applications.

A motor with starting pf around 0.3, FLA = 78A and inrush of 6x FLA, and transformer (112.5KVA, Z=3.5%); the VD through the transformer (not including cable) would be:

(Mtrinrush)/XfmrFLA X Z% = VD%

468/135 x 3.5% = 12.2% VD at transformer terminals. If you add another 3% in cable from transformer to motor, your looking at around a 15% VD on start-up.

Now say you have a transformer with Z = 5.75%, the VD goes to 19.9% without cable. The problem can be compounded by the fact that as the voltage goes lower, you may not be able to overcome the starting torque of the mtr+load and it will never reach full speed and protection will trip.

Increasing the size of the transformer helps too, because that lowers the ratio of motor inrush to xfmr FLA. As you say, your not going to order a custom transformer with the impedance you want (not at this size anyway) so its easier to go up in KVA rating instead.

For this example; using a 150KVA instead of 112.5KVA, the new FLA is 180A, which translates to a VD of around 9% instead of 12.2%; again not including cable.

Hope that helps
 
I guess it depends on how you look at it; because the theory behind the voltage drop is related to starting pf (vars) and capability of transformer to allow the needed vars from the system for starting (Z%). They are just disguised in the short cut calc.

For smaller motors, the estimated calc may suffice, but certainly for a larger motor, or for high torque applications, or where the starting pf of motor is not in range of 30-40%; these assumptions might not work, and it is best to get the speed torque curves, motor data, including reactance; and model it using a program with transient motor starting analysis.
 
king - I'm just polking fun here. This is not meant to be mean
kingpb said:
... because the theory behind the voltage drop is related to starting pf (vars) and capability of transformer to allow the needed vars from the system for starting (Z%). They are just disguised in the short cut calc. ...
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Yes, the motor var requirements are well disguised. VD is out in the open - in plain sight.

kingpb said:
... For smaller motors, the estimated calc may suffice, but certainly for a larger motor, or for high torque applications, or where the starting pf of motor is not in range of 30-40%; these assumptions might not work, and it is best to get the speed torque curves, motor data, including reactance; and model it using a program with transient motor starting analysis.
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Butch, I think that is enough dynamite
(Credit to someone else on the forum - not original with me. And I appologize for not remembering who it was)

ice
 
I would think a soft starter or VFD would have some impact on transformer selection when it comes to whether additional capacity is necessary for starting, or even just for allowing lower overcurrent device settings.
 
iceworm said:
John - Wack her with this

ice
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I read the paper, interesting, I'm going to read it again. The engineer talked to I.R., and they are now saying that no warranty due to the high leg effect on the electronics. I guess some of the sensors are resistive loads. Back to square 1, at least I've learned something.
 
John1T said:
I read the paper, interesting, I'm going to read it again. The engineer talked to I.R., and they are now saying that no warranty due to the high leg effect on the electronics. I guess some of the sensors are resistive loads. Back to square 1, at least I've learned something.
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Well, that takes care of that one, although, as with some VFDs and other electronics the more likely effect would be on surge protection devices installed from line to either ground or "neutral" rather than the sensors or other parts of the electronics.
They probably pick a single phase from which to derive their control power, and as long as that does not include the high leg they would be OK. But I guess it might be too much of a risk of miswiring and blowing everything up.
 
Well the client wants to continue trying to use reverse wiring the step down transformer. I'm going to meet with the engineer tomorrow, he feels that we should get the proper step up transformer, which would solve any high leg problems, and help with the inrush. Maybe they will listen to him, but we will see. This has been a real learning experience for me. John
 
Met with the engineer yesterday, he went through the schematic and did not feel that high leg could be properly isolated from the control circuit. He also could not get anyone at IR to put in writing that that transformer would be acceptable, so he will not give us a stamped set of drawings using that transformer. The client has agreed to the proper step up transformer. Thanks to everyone for their input, I've learned quit a bit on this. John
 
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