208 three pase to 220 two phase convert

[Craighell]

Please excuse my ignorance, but I need to convert a three phase 208 building to operate a system that requires 220vac.


Back ground:
I understand that 220 vac in the USA operates two 110 leads 180 degrees out of phase and 208 has its out puts at 120 degrees of phase shift. This was why I believed the equipment we were installing was having problems. at first when using only two leads of the 208 power.

The equipment is from Europe and expects a 230 phase and a neutal. The machinerey does not have any neutral connections going to ground in any way, therefore typical installs in the USA work fine with standard US 220vac building wiring. The equipment has always been quite happy with the voltage potential supplied by the two 110vac phases 180 degrees apart.

The install team installed a Buck-boost transformer to get 230 (228 actual) when measured with a multimeter. They used two phases of 208 and the two leads out of the buck-boost. The equipment however, still failed to work poeperly.

The next attempt was to put one 208 phase and the neutral as an input and the equipment worked (I think this only provided 150+ vac across the out puts). Unfortunately it draws a shite load of current.

Questions:
What's wrong?

Is the 120 degree phasing of 208 making the 230 output of the buck boost unusable with my equipment? (It's an eddy current retarder) The rep at ACME says no, the output is 180 deg.

Is the rep wrong?

Thanks for any and all suggestions!

 

[alehman]

If the equipment has no neutral connection and only two wires from the three-phase system, it doesn't know any thing about the relative phase angles (120 vs. 180).

When the equipment failed to work on the boost transformer, did you measure the actual voltage?

Is your transformer 1-phase or 3-phase? Take a close look at the boost transformer connections. That can be tricky to get right.

One phase and a neutral from the 208V system should give you 120V. If the equipment has motors, it is understandable that the current would be much higher. This would probably burn out the motor(s) in short order.

 

[stevenal]

Sounds like single phase to me, see faq238-777

 

[jraef]

I saw your error just as alehman did. In connecting directly to the 208V system without the buck-boost xfmr, you should have used 2 line connections from the 208V, not 1 plus neutral. That gave you only 120V potential. If they made that basic mistake, it's also likely they connected the buck-boost transformer wrong as well. The phase angle difference should not have made any difference.

It might help if you describe your equipment better if that doesn't solve your problem.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

The normal single phase voltage in the US is 240 V not 220 V or 230 V.
Often 50 Hz equipment requires a higher voltage on 60 Hz but not always. It depends on what type of equipment. You may still have a low voltage problem at 230 volts.
If you care to give us a little more information on what the equipment is and what it is doing when it fails to operate properly maybe we can help you.
yours

 

[Craighell]

Thanks for the suggestions guys,

What happened is that two legs of the 208 failed operate correctly.

A buck boost was installed an used two phases of the 208 feeding the buck boost. Still failed to operate correctly.

When used with a single leg of the 208 and neutral to the input of the buck boost it made about 150vac output and worked OK.

The unit is an eddy current brake (telma retarder). Failure here equals surging rather than smooth holding. With low voltage, i.e. the 150vac from the single leg through buck boost, the unit is smooth, just not capable of full holding capacity.

I'm trying to get more info from the customer site. I just heard the customer has sourced a transformer that can boost the single leg to 240. Never heard of this before. I only heard of 55% max boost....then again I'm out of my element here.

I have measured 220vac in typical garage environments (like house hold dryer outlests) and the units work just fine. I've never had a unit hooked to a single leg of 240 so I have no idea how that would work, but as I said it's euro designed so it's very happy with the 230 single leg power over there so single 240 I suppose would work just fine.

I've also never had a customer with a 208 three pahse building before.

Why would 228 vac when measured at the buck boost cause surging when 220 household does not?

 

[jghrist]

If your application is two-wire and does not require a grounded conductor, then you should be able to just wire from two phases of the 120/208Y service. The voltage will be 5% lower than the 220 volt, but may be OK without the buck-boost transformer. As Alehman said, the equipment doesn't know or care what the phase relationship is; it is just a single phase source.

The surging would have nothing to do with the configuration. Either it is a function of the buck-boost transformer or the voltage is not stable regardless of the configuration. Maybe there was a bad connection to one of the phases or to the buck-boost transformer when you connected to two phases.

 

[waross]

If you have had good luck using single phase in other locations, try using 240 volts instaead of 230.
I would also try the equipment on a single phase 240 volt circuit that other similar equipment works on. You may have a faulty piece of equipment.
The main rectifier circuits may not react the same to a change in frequency as do the control circuits.
It sounds like it was hooked up "every-which-way" including loose. It may have been damaged by one of the "Let's try this" connections.
It sounds like your control circuitry or you power supply may have a problem that may not be related to the voltage.
If you want to let us know what the make and model of the equipment is, you will be surprised by the amount and accuracy of the information and advice you will receive.
yours

 

[rbulsara]

It seems the equipment is missing some kind of a choke or reactor/resistor to limit the inrush (surge?/).

If 150V works, and you have problems with 208V (surging?) then making it 220 or 230 is only worsen the "surge".

I bet the particular device (the same unit) will have the same problem with a 220V supply. As mentioned in earlier posts phase angle betweent the two leads is irrelavant, all a single phase device cares about is rated voltage between the two leads feeding it. Phase angle matters for 3 phaseloads such as motors.

 

[dwftx34]

If the Buck/Boost transformer was wired correctly are you sure it was sized for the load. This might explain why it worked but not correctly the transformer needs to be sized porperly for the connected load I do not remember the ratio but I bet someone will respond with it.