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Paul_NJ
03-05-2009, 10:39 PM
I know this is OT, but I've learned great answers come from this group no matter the topic.

I'm going on a volunteer trip to Africa in a couple months, where available power is 220v/50Hz, and was asked to spec a portable generator for a remote medical facility. Among a number of devices they need to power, they've been donated a couple U.S. medical centrifuges. I haven't seen them but assume they are 60 Hz. From what I recall, if you power a 60 Hz motor with 50 Hz electricity, it will turn at 5/6 of the speed, and can overheat due to the lower fan speed. Am I correct on that? Is there such a thing as a frequency converter? I also seem to recall that European 220v has 220 volts on one hot lead, a neutral, and ground, compared to U.S. 220v which we know has two hots, a neutral, and a ground. So it's not a simple plug in either. Can someone check me on all this? Thanks.

rdfeil
03-06-2009, 12:01 AM
Paul,

You are correct in the things you have said. Now for the real world... You will need to check with the manufacturer of the centrifuges as to the effect of 50Hz power. They will run at 5/6 of normal speed which may be a problem for that type of equipment, fan speed normally is not a problem. As for the hot / neutral issue... Power in the UK is indeed 220 volt hot in respect to a neutral. This is not a problem for equipment that is rated for 220 volt operation only. The equipment rated for 220 only does not use the neutral, only a safety ground which is NOT the neutral wire. If it is rated 120/240 then there is a problem and it will not work. The generator is a simple issue. If you are intending it to be used as a backup to the land power just specify that it MUST be 220/240 volt 50Hz, and if you can find out specify the neutral/ground point to match the power where it is going to be installed. All of the major generator manufacturers will custom build to specification, often times for no extra charge. I hope this helps.

Robin

Paul_NJ
03-06-2009, 12:16 AM
Robin

Thanks for the response. These will be portable generators that will be taken into remote areas that have no electricity.

My question on the 220v issue is what will happen if you plug a U.S. designed device that is designed for two hot 120v leads, (across which you get 220volts because they are 90 degrees out of phase with each other), into a UK type circuit that has a single 220 volt wire and a neutral? Will the device care?

J Tiers
03-06-2009, 12:59 AM
No it will not, in most cases.

Only if it uses the neutral, and is therefore expecting 120V from a hot to the neutral will it be an issue. Most equipment won't be a problem that way.

You can tell, because any equipment with a 3 prong plug, where one is ground, and two are power, is ONLY using 220/240.

To need a neutral, it will, unless it's an old stove, have 4 prongs on the plug..... one neutral and 2 hots.

rdfeil
03-06-2009, 01:18 AM
Paul,

The phase relationship is 180 out :p Now that I have picked on you the answer to your question is ........ It will work fine:D To explain how our power works... Our 120/240 single phase (think house service) is simply a transformer with a 220 volt output which is center tapped. This gives 120 volts on each side of the tap and 220 across the whole thing, because of the center tap the phase relationship between the two ends and the center tap is 180 degrees out of phase. In the US the center tap is the ground AND the neutral. This is done at the transformer by the power company and is the only place that the electrical code allows this connection. This allows us to use equipment at both 120 and 240 from the same service and still maintain a safety ground. In other countries the configuration is different, but I suspect that the neutral side is actually grounded like our center tap is. A transformer does not care where it is grounded or for that matter if it is grounded. The ground is only for safety.
As for the portable generators for use where there is no power, if the equipment can run on 60Hz then any generator with a 220 volt output will work. Again, the only issue is the grounded leg. One thing you might want to do is try to figure out if the generator has the neutral connected to the frame ground, if it is then you may want to either get a isolation transformer or try to have the neutral bond disconnected. This can be done by any competent motor shop. Be forewarned, the motor shop might balk at disconnecting the bond so be sure to explain that it will be going out of the country and be willing to sign a release form to relieve the motor shop of the liability. Also, CLEARLY mark the generator as having a isolated neutral so you know the ground is not normal.
Also, remember that you may need some adapter plugs made up if you intend to use both US and Non-US equipment on the same power system. Most countries plugs and outlets are different.

I hope this helps.

Robin

Edit: I just read J Tiers response after I submitted this and he is right about identifying the needed power. If the device is 120/240 it will have a 4 prong plug, 2 hot, 1 neutral and 1 ground.

Paul_NJ
03-07-2009, 08:45 AM
Robin and J Tiers

I appreciate for your responses and help. I had surmised that the element of a resistive device such as a heater, or lamp, couldn't tell the difference whether it was connected between a single 220v wire and a neutral, versus two 120v wires 180 degrees(thank you) apart . But with the internal world of an electric motor being a black box to me, I didn't know if the same could be said, and if motor connections had to be changed or handled differently. I suppose this comes up when folks consider bringing 220 v machines from most anywhere to North America.

Thanks again.

J Tiers
03-07-2009, 11:36 AM
As a rule, most equipment sees no difference between 220 with hot and neutral vs 220 with 2 hots center-tapped.

The safety aspect is not an issue with any modern equipment. very old equipment MIGHT be counting on that grounded neutral, and in such a case it could be unsafe with 2 hots. But no such equipment has been allowable for 30 years or so, and probably long before that.