Quantar in-rush current for stand-by generator calculations
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Quantar in-rush current for stand-by generator calculations
My Quantar book is in the radio on a mountain top not easy to get to and I have been asked what the initial inrush current would be so the site owner can calculate what size generator is needed for all customers there. It is a single quantar, UHF, 100 watt transmitter and is about 5 years old. Single user P-25 radio. Not trunked, no additional receivers, no extra options in cabinet. Any help would be appreciated
My 300w Nucleus 2 900 MHz paging transmitter manual doesn't specify anything about inrush current.
I do know that the two power supplies have some kind of soft-start circuit in them. They also automatically adjust for any input voltage from 100 to 260. When I turn them on, there's no noticeable dimming of anything else on the same circuit. I'd guess it's pretty low, just from the way they act at power-up.
I could go to my site with a clamp-on AC current meter, set it to measure inrush current, and turn just one power supply on. The Nuke is on its own outlet. This would give you a true measurement. The site is less than a mile away and quite easy to get to. Let me know via PM if that would be any help.
Bob M.
I do know that the two power supplies have some kind of soft-start circuit in them. They also automatically adjust for any input voltage from 100 to 260. When I turn them on, there's no noticeable dimming of anything else on the same circuit. I'd guess it's pretty low, just from the way they act at power-up.
I could go to my site with a clamp-on AC current meter, set it to measure inrush current, and turn just one power supply on. The Nuke is on its own outlet. This would give you a true measurement. The site is less than a mile away and quite easy to get to. Let me know via PM if that would be any help.
Bob M.
Inrush Current for Generator Starting
I'm guessing that the generator is many times the size of your Quantar load in which case the inrush current should not enter into the equation for alternator sizing. Usually when we deal with generator sizing at telecom sites, we are concerned about the worst case load: Battery charging, tower lights on and air conditioner running. Like your Quantar, most rectifiers will not exceed their input CB rating much beyond listed maximums. They use various methods to limit inrush current to reasonable values. Then the DC output gets enabled a short time later. Any proper air conditioner control will wait a couple of minutes after restoral of power before it starts up. Many electronic telecom controllers will also go through a boot up routine before keying up a transmitter. And battery systems should not be too demanding after waiting < 1 minute for the generator to come on-line. So there should be plenty of capacity available when everything transfers to generator to charge up the input caps.
The inrush is very short, so smaller engine-alternator combinations can flinch, but won't choke on inrush currents. You can also purchase an alternator with a Permanent Magnet Exciter, which will assist in keeping voltage up during an inrush. Some companies, like Kohler for instance, offer the PMG as standard on many models.
I've seen many telecom sites with way oversized generators. They take an absolute worst case scenario and then add some extra fudge factor. Compounding this is that the difference in installed price between a 20 kW generator and a 50 kW generator is about 4%.
One extra tidbit, be wary of power supplies/rectifiers that are not power factor corrected. They create harmonics and eat up generator kVA. All my Quantars are 24V DC input , so I don't have experience with the AC versions.
RF Dude.
The inrush is very short, so smaller engine-alternator combinations can flinch, but won't choke on inrush currents. You can also purchase an alternator with a Permanent Magnet Exciter, which will assist in keeping voltage up during an inrush. Some companies, like Kohler for instance, offer the PMG as standard on many models.
I've seen many telecom sites with way oversized generators. They take an absolute worst case scenario and then add some extra fudge factor. Compounding this is that the difference in installed price between a 20 kW generator and a 50 kW generator is about 4%.
One extra tidbit, be wary of power supplies/rectifiers that are not power factor corrected. They create harmonics and eat up generator kVA. All my Quantars are 24V DC input , so I don't have experience with the AC versions.
RF Dude.
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MSS-Dave
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- What radios do you own?: Harris XL200M. XPR7550E, NX300
A few years ago, had a Quantro that kept futzing up for no apparant reason. It would require a recondition of the codeplug then a reload of the data to operate again. After a few extended site visits, we determined the problem was the transfer switch for the backup genset. It would break shore a/c then remake on the genset before everything stabilized and scramble the codeplug. The customer had a big UPS there for other things but this repeater had been neglected and not hooked into it. Since the UPS had the capacity, we rewired to it and the problem "went away". RF Dude has the right idea in using 24 VDC. Use inverters and a big battery stack and lots of AC related problems will go away. You can even program the radio to drop power level on AC fail to save battery life.One extra tidbit, be wary of power supplies/rectifiers that are not power factor corrected. They create harmonics and eat up generator kVA. All my Quantars are 24V DC input , so I don't have experience with the AC versions.
BTW, from the Q-Tar installation section in the training manual, max draw is 766 watts, nominal 8.5 amps at 110 volts. If there is instantanous inrush on TX keyup, it shouldn't exceed the 8.5 amp max.
Dave
I went to my site this morning and put my Fluke 337 current meter on it. I unplugged the two AC fans; this is for the Nucleus 2 transmitter only.
At 130 watts RF output, it draws 6.9 amps with a 7.0 amp peak.
At idle, it draws 0.8 amps.
I turned it off, waited 10 minutes, and turned it back on (both power supplies are plugged into a switched power strip - that's the switch I used). With the meter set for inrush current, it read 3.5 amps.
I did this again, but set the meter for regular current. It stored a maximum value of 7.7 amps at power on.
I think the Fluke 337 only measures inrush current during the first 250 mSec of applied power, and the Nucleus takes longer than that to really start doing anything, due to the soft-start power supplies. So I tend to doubt the inrush value of 3.5 amps. I think it's higher than that.
Before I put the Nuke on the air, I measured the current when it was sitting on the bench at home: 10 amps (1200 VA) when putting out 300 watts RF. Based on that, the inrush current is less than the maximum operating current, and that seems reasonable.
Bob M.
At 130 watts RF output, it draws 6.9 amps with a 7.0 amp peak.
At idle, it draws 0.8 amps.
I turned it off, waited 10 minutes, and turned it back on (both power supplies are plugged into a switched power strip - that's the switch I used). With the meter set for inrush current, it read 3.5 amps.
I did this again, but set the meter for regular current. It stored a maximum value of 7.7 amps at power on.
I think the Fluke 337 only measures inrush current during the first 250 mSec of applied power, and the Nucleus takes longer than that to really start doing anything, due to the soft-start power supplies. So I tend to doubt the inrush value of 3.5 amps. I think it's higher than that.
Before I put the Nuke on the air, I measured the current when it was sitting on the bench at home: 10 amps (1200 VA) when putting out 300 watts RF. Based on that, the inrush current is less than the maximum operating current, and that seems reasonable.
Bob M.