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#193859 - 04/23/10 06:52 PM Compensation of reactive power single phase motor  
Wolfgang  Offline
Joined: Sep 2005
Posts: 153
the very West of Germany
I'm in discussion with a US colleague about the equipment of an installation in Maine for which we supply the hardware and he supplies the electric part.

This installation will be equipped with 2 5hp blowers that draw 25 A on 240V single phase each. My colleague is worrying about the over all current of his equipment.

Here in Europe we sometimes use capacitors more or less directly on the motor terminals to compensate reactive power drawn from the grid and to reduce the amperage of the installation. Is this a suitable way in a US installation?

Work Gear for Electricians and the Trades

#193870 - 04/24/10 06:31 PM Re: Compensation of reactive power single phase motor [Re: Wolfgang]  
Tesla  Offline
Joined: Jun 2004
Posts: 1,273
Sacramento, CA
At this power level 'run caps' ( ie dedicated kVAR directly attached to the motor ) provide enough power correction such that the utility is happy.

5-hp 1-phase is just about the upper limit for most Pocos...

After that you might get complaints.

BTW, in today's America, 1-phase motors above 5-hp are custom orders. Last time I tried to get some 10-hp units I was told to get a converter and a 3-phase: a custom build would cost so much more.


#193874 - 04/25/10 05:32 AM Re: Compensation of reactive power single phase motor [Re: Tesla]  
Wolfgang  Offline
Joined: Sep 2005
Posts: 153
the very West of Germany
Many thanks Tesla,

In this North America job I'm just responsible for compliance with our general design guidelines. The limitations on site are an existing 1 phase 37,5 kVA tranny and some equipment of a third party. So our power supply has been limited to 70A @ 240V. I just wanted to make some proposals how to use these 70A in the most effective way. With regard to 2 VSDs I would have head pain because of extra cooling requirements as the whole installation is exposed to sun and all other weather influences.

I will ask my colleague to check that way.

Btw. this is my first single phase installation after 20 years in that business. And the client is a PoCo.

#193876 - 04/25/10 06:07 AM Re: Compensation of reactive power single phase motor [Re: Wolfgang]  
frenchelectrican  Offline
Joined: Feb 2003
Posts: 939
Wi/ Paris France { France for ...
Typically in USA I know single phase motor I can get them in larger size however many POCO's will throw a fits due they have issue with their transfomers unless you get a written pole motour { this useally found in 30 CV or larger } but for common size the 7.5 CV is pretty much top of most single phase useage { very few POCO can allow larger single phase motour useage but again it is very limited}

as far for 5 CV motours they are listed between 28 to 32 amps @ 240 volt 1

so you are pretty close to the limit on the supply requiement for your customer.


Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)

#193883 - 04/25/10 04:02 PM Re: Compensation of reactive power single phase motor [Re: frenchelectrican]  
Alan Belson  Offline
Joined: Mar 2005
Posts: 1,803
Mayenne N. France
Wolfgang, with a 1.15 service factor, [ ie 5,75hp ], often built in by makers, the blowers will still not draw more than your allocated 70A @ 240V. I make it 27x 1,15 = 31,0 AMPS, running each motor = 62,0 AMPS. Starting amps could be reduced by sequential starting of course.
With a 37,5kVA transformer, you will need 13.44 kVA or 36% of its capacity, with 20% more [16.2 kVA = 43%] if starting more than once per hour. Unless reactive load or power factor is mentioned in your contract I think adding capacitance, to bring pf closer to one, is just added expense, for the figures seem to indicate that your equipment will operate well within the specification. I'd guess the poco allocated the 70A limit from motor amperes tables anyway. If I were writing your specification of the supplied equipment, I'd qualify starts per hour, voltage sag limitations, start amps and transformer share limits to protect your position.

Wood work but can't!

#193885 - 04/25/10 04:44 PM Re: Compensation of reactive power single phase motor [Re: Alan Belson]  
Wolfgang  Offline
Joined: Sep 2005
Posts: 153
the very West of Germany
Hi Alan, thanks for your comment!

My colleague (from Wyoming) who is responsible for the electric design makes up a different calculation:

The 70 A breaker may be charged with 80% maximum. So he says he is limited to 56A. Each motor draws 25A at full load according to the data table, service factor is 1 (I'm still trying to figure what that term stands for). So he calculates that there are 6A left for anything else which is not sufficient.

Without any idea of the "why" for those rules I just have to accept those figures. That's why I wanted to propose to reduce apparent power.

#193887 - 04/25/10 06:04 PM Re: Compensation of reactive power single phase motor [Re: Wolfgang]  
Alan Belson  Offline
Joined: Mar 2005
Posts: 1,803
Mayenne N. France
Good evening Wolfgang,

Reducing the apparent watts to that of the actual motor power amps can be had by getting the power factor pf to an ideal of 1.0 , with an Active Power Factor Corrector, without the experiments/calculation needed for a capacitor bank. Whether that will be enough to drop the amps to an acceptable figure- dunno? If the pf of your motors were 0,90 then you might gain perhaps 10% or 5 Amps. If that's not enough, then the only other option is to fit smaller blowers, for nothing else will do it! Here's one site on Active PFC:

Not to be confused with 'service factor', which just means the maker is slightly overloading the motor to improve the performance [ for the marketing department perhaps! cool ]
If Wyoming says sf is 1.0, you will be drawing about 10hp/7.5kW.

Wood work but can't!

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