Do I need the transformer? your opinoin please

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lquadros

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We have a 480V system. We often use a transformer 20KVA-50KVA as needed to step it down to 208Volts.
to power up Servo panels and such and then we install a 3-5KVA transformer to further step it down to 120V. I really do not think we need his 120V transformer and can use the "Y" node.Is it something to do with fault currents or voltage regulation? Or it can be done without a transformer? Have they chosen 208V servos for the same purpose? Is this just a designers choice that we are following?
Thanks very much for you replies.
 
Re: Do I need the transformer? your opinoin please

If you have transformer the voltage from 480 delta to 208/120Y, then I don't see a need to further transform the voltage for small loads as you describe. Use the 120V off of the 480-208/120V transformer.
 
Re: Do I need the transformer? your opinoin please

Depending on the nature of the 120V load, some designers will install the second smaller transformer to provide isolation from the 208V part of the system. Noise and transients are typically the concern.

I have see cases where there were operational problems with PLC's that required a small isolation transformer to cure.
 
Re: Do I need the transformer? your opinoin please

There is something that I do not understand about the way you described your system. It is this part:
. . . then we install a 3-5KVA transformer to further step it down to 120V.
Click to expand...
What is the primary voltage of this transformer (i.e., 480, 208, or 120), and how is it connected (i.e., single phase line-to-line. single phase line-to-neutral, 3-phase Delta-Delta, or 3-phase Delta-Wye)?
 
Re: Do I need the transformer? your opinoin please

Wouldn't an on line UPS do the same for the PLC and then you don't have to have grounding electrodes for the transformer?
 
Re: Do I need the transformer? your opinoin please

Isolation transformer's don't fix problems with PLC's they just sweep the problem under the rug sorta speak. Most logic problems in PLC's occur because of multiple bonding at different location's on the machinery or at different locations in a plant. Now if there is any current flowing between these grounding paths then it will also flow on the shielding of data cables running from these remote sensors back to the PLC. And there are many other reasons that can come into play that can cause problems like this too but to just install an isolating transformer and call it fixed is not the solution just a quick fix. The grounding problems should still be looked into as some could be dangerous.

Think of this way it is kind of like a sound man for a PA system that cut's off a ground prong on a cord to remove a ground loop hum. But at least with an isolating transformer it can be grounded for a SDS to keep the grounding continuous

[ March 31, 2004, 10:50 PM: Message edited by: hurk27 ]
 
Re: Do I need the transformer? your opinoin please

CLARIFYING MY QUESTION for DO I NEED A TRANSFORMER
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What we have is a 3phase delta/wye transformer
480 pri. and 208-120 secondary. The 208 seconday is used to power up the servo and also to power up a 1 phase transformer that steps it further down to 120V. This voltage is used to power up PLC, regulated power supply for 24VDC, receptacle for laptop, panel lighting and such. The PLC runs I/O modules, positioning or motion control modules and analog modules if needed.My other question is What would be the need for using 208V servo system instead of 480V?
Thanks in advance for your replies
 
Re: Do I need the transformer? your opinoin please

Let me further explain why your situation has me confused. You say you have a 1-phase transformer that ?further steps it down to 120V.? But you already have 120 volts! Your first transformer gives you 208 volts phase to phase, and 120 volts phase to ground.

So back to the primary connections of the second transformer. If the primary is connected line-to-ground, then the transformer has a 120 volt primary and a 120 volt secondary. It is not stepping it further down to 120. This would be an isolation transformer, not a step-down. Sam and Wayne have given their thoughts on this concept.

If the primary is connected line-to-line, then it is stepping 208 volts down to 120 volts. Here again, you do achieve a degree of isolation between the 208 volt system and the new 120 volt system. But I can?t see why it would be done this way. Perhaps by sharing the load over two phases, you can get some balancing of loads. But with a transformer so small (10% of the overall 480 volt load), this would seem to have a minor impact.

I can?t help with the servo question. It?s a design area in which I have no experience.
 
Re: Do I need the transformer? your opinoin please

a lot has to do with the configuration of the system itself. i installed a system years ago and asked the same question. the distributor told us that we did not have to use the 120 volt transformer but cautioned us that the transformer was meant to isolate the 120 v system from the servo unit to prevent surge currents and other noise from getting into the system, as well as help prevent the 120 volt /24 volt devices from getting unwanted surges that would burn up the system. the transformer was only $150 so we put it in versus the $1000+
it would take to replace the destroyed components.

please get with the people you purchase the system from as well as the local distributor for more specific details.
 
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