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Sizing a generator for inrush current
- Thread starter mbrg
- Start date
I wonder if CE's tech support could give you some idea... They are pretty darn helpful folks.
This will be determined by looking at the max primary current of all the transformers that will be connected upon power-up.
I run into this when a new press is installed and the question comes up of what size breaker to feed the transformers with.
The core of the transformers need to saturate on the first waveform and that will draw max current, even if the secondary and all load is off.
If everything in question is on one circuit, look to the breaker feeding them for a good idea of the inrush current.
Unless of course that breaker is subject to tripping on power up, then you'd need to calculate 10-20% more current for the backup
switchover to be reliable.
I think your electrican's calculation of 17kw on power-up is pretty good with a fair safety margin.
As far as the generator capacity, it will depend on its current limiting, or ability to deliver that first sine wave without stumbling.
You may need a 15kw generator, or one that will deliver that much current for at least 1/60 of a second.
I run into this when a new press is installed and the question comes up of what size breaker to feed the transformers with.
The core of the transformers need to saturate on the first waveform and that will draw max current, even if the secondary and all load is off.
If everything in question is on one circuit, look to the breaker feeding them for a good idea of the inrush current.
Unless of course that breaker is subject to tripping on power up, then you'd need to calculate 10-20% more current for the backup
switchover to be reliable.
I think your electrican's calculation of 17kw on power-up is pretty good with a fair safety margin.
As far as the generator capacity, it will depend on its current limiting, or ability to deliver that first sine wave without stumbling.
You may need a 15kw generator, or one that will deliver that much current for at least 1/60 of a second.
Some of the manufacturers have some sort of a load calculator on line. Even answers to what the locked rotor current of an air conditioner would do to a genset. The two sites with A/C I'm associated with have greatly oversized gensets running about 50% average load. A 175 kVA and a 750 kVA set. One good thing is the A/C units all seem to have a long delay before restarting after a power outage.
T
thebeaver32
Guest
Very few online calculators know anything about a broadcast transmitter. Here are a few "Rules of Thumb" from some other Engineers, and myself as well:
Tube-type transmitters: Figure 3 to 4 times the TPO output of the transmitter. This will cover inrush surge, tube filaments, blowers, and driver stages.
Is your tower lighted? Be sure to figure tower lights into the equation. A beacon bulb is up to 700 watts per bulb, and there are two bulbs in a beacon. Each side light (marker light) is about 120 watts. And these bulbs ALSO have an inrush current. Figure the total wattage of the lights and double it.
Is your site air conditioned? Find the power consumption of the system, and multiply that by 1.5.
If you have resistance element heating, just add its power draw. There's no inrush on electric. And you only need to add EITHER the air conditioning load, OR the heating load--whichever is bigger, since you shouldn't be using both at the same time.
Building lighting should be negligible. Add about 750 watts for each equipment rack.
If you have any other tower users who will use generator power, include them.
And finally, if there is any additional load anticipated down the road, now's the time to include it!
This should give you a pretty good estimate of the generator size you need. If the total calculated load is between rated generator sizes, go UP to the next level. Electrical loads tend to go up, not down.
Tube-type transmitters: Figure 3 to 4 times the TPO output of the transmitter. This will cover inrush surge, tube filaments, blowers, and driver stages.
Is your tower lighted? Be sure to figure tower lights into the equation. A beacon bulb is up to 700 watts per bulb, and there are two bulbs in a beacon. Each side light (marker light) is about 120 watts. And these bulbs ALSO have an inrush current. Figure the total wattage of the lights and double it.
Is your site air conditioned? Find the power consumption of the system, and multiply that by 1.5.
If you have resistance element heating, just add its power draw. There's no inrush on electric. And you only need to add EITHER the air conditioning load, OR the heating load--whichever is bigger, since you shouldn't be using both at the same time.
Building lighting should be negligible. Add about 750 watts for each equipment rack.
If you have any other tower users who will use generator power, include them.
And finally, if there is any additional load anticipated down the road, now's the time to include it!
This should give you a pretty good estimate of the generator size you need. If the total calculated load is between rated generator sizes, go UP to the next level. Electrical loads tend to go up, not down.
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