Motor Inrush Currents

[dereckbc]

Hey guys need some feedback on standard motor start up current vs. soft start motors. Since Motors are not my strong point thought I could get a few good replies.

I know a standard motor will draw say 7 to 10 times FLA. Is that an accurate general statement?

What is the average start up current on a soft start motor?

The reason I am asking is because I am getting specs to run cell tower sites with off-grid solar PV systems with air conditioning. My first thought is it is not practical because it would take a huge amount of battery capacity and solar panel wattage rating. But one manufacture makes a cabinet type unit with built-in refrigeration of 1-ton cooling.

On a related note to inverters I am initially thinking the inverter would have to be rated 7 times KVA to handle the start up current of the cooling, but then soft start popped into my head.

None of this is real application yet, I just have to come up with some design and cost proposals to show the executive management so they can determine to proceed or not.

 

[coulter]

Look at (2005) T430.7.B. I've seen: one code S (LR>17 X FLA), several "energy efficient" where the LR was in the 10 - 11 XFLA, and the majority were code G (LR ~ 6 X FLA). I'm not sure that any of this applies to a hermetically sealed compressor.

Next to look at is Art 440. There are abunch of strange issues with hermetically sealed compressors. I think it's because the motors are running/immersed in coolant and the MFGs overload the snot out of them (that would be an engineering term:smile: )

Hermetically sealed compressor, soft start, inverter fed - hummm ... I don't have any experience with combining the three.

Just a suggestion (which you already likely know) Unless one of our whizs have specific experience, I'd start with a typical cooler mfg and ask them: Can the compressor be started on a soft start?

Then ask a softstart mfg; What am I looking at for harmonics and pf during the soft start time?

Then ask the inverter mfg: What do I need for an inverter to do this?

The reason I suggest this is I have never seen this type of data in equipment technical data sheet. Again, maybe one of our whizes knows more.

You have an interesting project. Keep us informed on how it comes out. I guarrentee I'll make use of the knowledge.

carl

 

[quogueelectric]

Hey guys need some feedback on standard motor start up current vs. soft start motors. Since Motors are not my strong point thought I could get a few good replies.

I know a standard motor will draw say 7 to 10 times FLA. Is that an accurate general statement?

What is the average start up current on a soft start motor?

The reason I am asking is because I am getting specs to run cell tower sites with off-grid solar PV systems with air conditioning. My first thought is it is not practical because it would take a huge amount of battery capacity and solar panel wattage rating. But one manufacture makes a cabinet type unit with built-in refrigeration of 1-ton cooling.

On a related note to inverters I am initially thinking the inverter would have to be rated 7 times KVA to handle the start up current of the cooling, but then soft start popped into my head.

None of this is real application yet, I just have to come up with some design and cost proposals to show the executive management so they can determine to proceed or not.

When searching for an answer to another thread I ran across the pdf for a fujitsu mini split and it already has a soft start built in to its unit I think this may help your situation both practicality high quality and cost efficient. Check out page 5 for the freq drive current info. http://www.fujitsugeneral.com/PDF_06/halcyon06_brochure.pdf Really cool unit I am thinking of one in my house I have to check out price though

 

[tom baker]

Quoted inrush is 6 x LRA. But depends on how the motor starts. The faster the motor comes up to speed the lower the LRA.
Softstarts have no inrush. But they are limited in the time for starting and stopping.

 

[dereckbc]

Thanks so far for the comments. What I am being asked to do to my knowledge has not been done yet. I don?t know of any equipment vendors that make such a hybrid set up.

For the past few years I have designed a number of cell tower sites where no POCO was available like mountain tops or rural areas, and we had to design from the ground up a Solar PV system. It was not to difficult because all the equipment uses 24 VDC. The radio equipment is all designed for outdoor cabinetized and as such has mil spec heat grade electronics. The radios burn about 600 watts 24 x 7 so it give off a good deal of heat.

Well now a vendor has come out with a much less expensive outdoor cabinet, but instead of using very expensive mil spec electronics, they use refrigeration and the electrical interface is single phase 240, and then uses standard rectifiers and batteries to power the radio equipment. However the AC uses AC power.

So now I have to determine if the equipment savings can be justified. My gut says no way because the additional PV panel wattage, an oversized inverter, and a lot of extra batteries will add up to more then the equipment cost savings. My dilemma is I don?t know how much to over size the inverter if at all.

 

[quogueelectric]

Thanks so far for the comments. What I am being asked to do to my knowledge has not been done yet. I don?t know of any equipment vendors that make such a hybrid set up.

For the past few years I have designed a number of cell tower sites where no POCO was available like mountain tops or rural areas, and we had to design from the ground up a Solar PV system. It was not to difficult because all the equipment uses 24 VDC. The radio equipment is all designed for outdoor cabinetized and as such has mil spec heat grade electronics. The radios burn about 600 watts 24 x 7 so it give off a good deal of heat.

Well now a vendor has come out with a much less expensive outdoor cabinet, but instead of using very expensive mil spec electronics, they use refrigeration and the electrical interface is single phase 240, and then uses standard rectifiers and batteries to power the radio equipment. However the AC uses AC power.

So now I have to determine if the equipment savings can be justified. My gut says no way because the additional PV panel wattage, an oversized inverter, and a lot of extra batteries will add up to more then the equipment cost savings. My dilemma is I don?t know how much to over size the inverter if at all.

I would call fujitsu and ask thier engineers as a potential customer I am sure they would bend over backwards to try to help you meet your goals.
I have done some tower jobs and there is lots of stuff you cant talk about in them so I wont. Nice work though.
Everything has to go together smoothly and I mean smoothly or your (edit to say buttocks) is newmown grass

 

[winnie]

IMHO DC -> Inverter -> 60Hz AC -> soft start -> motor is a wasteful approach.

Consider DC -> VFD -> motor

A search for "24V air conditioner" will bring up several manufacturers of low voltage DC air conditioning systems. Some of these use inverter fed 'brushless DC' compressors; some might even be variable speed (and thus variable cooling, rather than cycling on and off for temperature control).

-Jon