need help finding the Locked rotor amps on this cut sheet

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cptviggi

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We need to run power to a jib crane where the hoist motor is 480v. We only have 208v in the building. We are trying to giving the GC a quote for a step up transformer. The crane suppler is telling us to size everything for 30amps at 480v. The electrical supplier is telling us that we need to supply a transformer good enough to cover the Lock Rotor Amps on the hoist motor or we will have a severe voltage drop upon start up. The FLA is 12 amps. How do I find the LRA?
Oh, and the cut sheet is for a European made motor. I can't read it! lol

Thanks
 

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cptviggi said:
We need to run power to a jib crane where the hoist motor is 480v. We only have 208v in the building. We are trying to giving the GC a quote for a step up transformer. The crane suppler is telling us to size everything for 30amps at 480v. The electrical supplier is telling us that we need to supply a transformer good enough to cover the Lock Rotor Amps on the hoist motor or we will have a severe voltage drop upon start up. The FLA is 12 amps. How do I find the LRA?
Oh, and the cut sheet is for a European made motor. I can't read it! lol

Thanks
Click to expand...
430.231 table,, I hope.:)
 
cptviggi said:
We need to run power to a jib crane where the hoist motor is 480v. We only have 208v in the building. We are trying to giving the GC a quote for a step up transformer. The crane suppler is telling us to size everything for 30amps at 480v. The electrical supplier is telling us that we need to supply a transformer good enough to cover the Lock Rotor Amps on the hoist motor or we will have a severe voltage drop upon start up. The FLA is 12 amps. How do I find the LRA?
Oh, and the cut sheet is for a European made motor. I can't read it! lol

Thanks
Click to expand...
Fortunately the cut sheet is in English even if the format is not what you are used to. Translate "," to "." in numbers though. :)
SWAGs:
1. The specified fuse (time delay) for 480V operation is 20Amps. That indicates to me that although the operating current (FLA, also close to IA on this sheet) will be less than 20A the starting current will be much higher than 20A.
2. The starting current and the LRA will be in the same ballpark, although with a different duration depending on how heavily loaded the motor is at startup.
3 In the table row for DCPro-25 (two pole) at 480 volts, the "normal" current, IN, is shown as 8.7 Amps. The heading of the last table tells us that the starting current is symbolized by IA. Then going back to the first table, we find that the ratio of IA to IN is 4.95.
Therefore, IA is equal to 8.7 times 4.95, or 43A. Enough higher than 30 to cause me mild concern.
 
LEO2854 said:
430.231 table,, I hope.:)
Click to expand...
The devil is in figuring out which table to apply!

Table 430.251(B) [2011] suggests close to 81 amps LRA for a 10 HP motor, but it is very hard for me to tell whether or not the motor construction matches any of the Design classes called out for use of that table.
The fact that Table 430.250 shows an FLA of 14A while the motor cut sheet appears to show 8.7A makes me question the applicability of the tables to this particular motor. Not that the NEC would not require us to use those figures anyway IF the various requirements for using the tables are met. :)
 
How did you size the motor at 10 HP?

How did you size the motor at 10 HP?

Where did you come up with that? Everything else makes sense.
 
SO check this out

SO check this out

430.250 has a 5 hp at 7.6amps 430.251 has a 5hp at 46amps our motor is 8.4amps and the bottom of the cut shows 53amps for the starting current. I say 53 amps is plenty big enough for starting and LRA. do you agree?
 
cptviggi said:
430.250 has a 5 hp at 7.6amps 430.251 has a 5hp at 46amps our motor is 8.4amps and the bottom of the cut shows 53amps for the starting current. I say 53 amps is plenty big enough for starting and LRA. do you agree?
Click to expand...
Read the bottom table again please!
That 53 is not Amps it is Meters. What that table shows is the conductor size which will result in an acceptable voltage drop during starting as long as it is shorter than the length shown. In this case conductors of 2.5 mm2, no longer than 53 meters.
It refers to the conditions under which it is determined as the starting current, IA, but nowhere does it tell you what that value is for each motor.
 
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cptviggi said:
We need to run power to a jib crane where the hoist motor is 480v. We only have 208v in the building. We are trying to giving the GC a quote for a step up transformer. The crane suppler is telling us to size everything for 30amps at 480v. The electrical supplier is telling us that we need to supply a transformer good enough to cover the Lock Rotor Amps on the hoist motor or we will have a severe voltage drop upon start up. The FLA is 12 amps. How do I find the LRA?
Oh, and the cut sheet is for a European made motor. I can't read it! lol

Thanks
Click to expand...

If the transformer is not large enough to deliver maximum locked rotor current it does drop voltage. So does a soft starter should you decide to use one.

Unless severely undersized or if motor is started pretty frequently, I don't see too much problem with it, in fact it may still be better to have this natural "choke" in the circuit if there is frequent starting as the higher starting current would probably result in excess heating in the windings. JMO.
 
kwired said:
If the transformer is not large enough to deliver maximum locked rotor current it does drop voltage. So does a soft starter should you decide to use one.

Unless severely undersized or if motor is started pretty frequently, I don't see too much problem with it, in fact it may still be better to have this natural "choke" in the circuit if there is frequent starting as the higher starting current would probably result in excess heating in the windings. JMO.
Click to expand...
I can see quite a few starts per hour in this application (jib crane). The cut sheet for the motor rates it in terms of up to 120 starts per hour. I would think that would be based on across the line starts, and the motor would be correspondingly beefy. How would that relate to your disclaimer?
The motor would be designed to handle it, but the transformer and conductors would have to be be up to that service also.
 
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GoldDigger said:
I can see quite a few starts per hour in this application (jib crane). The cut sheet for the motor rates it in terms of up to 120 starts per hour. I would think that would be based on across the line starts, and the motor would be correspondingly beefy. How would that relate to your disclaimer?
The motor would be designed to handle it, but the transformer and conductors would have to be be up to that service also.
Click to expand...

True, the thing that will suffer the most would be the transformer. Like you said, the motor is probably already derated to be able to handle this kind of conditions, the transformer is what will likely see excess heating if it is not derated.

I'm not quite as concerned with motor conductors, they seem to be able to handle this kind of thing better than windings do.
 
Being a European motor, it will be rated in kW, not Hp. Consequently, the LRA is not going to be listed it will be a multiplier (as shown 4.95) , and there is no standard LRA for KW motors. I have seen them as high as 12x and as low as 2.5x, just depends on the design.

As far as transformer, you would probably best be served by contacting your supplier, explain what you you need, and let them tell you the best option for transformer. I do know that as a rule of thumb 2.5x the base KVA is what you would need for normal motor starting through a transformer, but this application may need more consideration.
 
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