how to choose the right transformer

[Guest sam_koz2003]

hello can any help me Can any one help with this problem, im trying to chose the right transformer for the system im using , its 240v and the secondry is 120 v How would I determine the right part And what is kVA ? I know its power but how can I determine it from the motors I have Thank you

[Chris Elston 79]

Try this: http://forums.mrplc.com/index.php?act=Downl...&CODE=02&id=132 Once you have a total AMPERAGE, then you'll know how big of a transfer you need. Use the download above to help you along.

[Guest sam_koz2003]

i couldnt downdload the excel program , the screen froz when i try to open it can you show me the way to calculate the kva for the transformer, i guess the version for that excel sheet is older than the version i have thats why i cannot open the excell file thanks for you help

[JimRowell 1]

Sam... VA is simply the voltage X the amperage. If you wish the secondary to be at 120 volts and you will be drawing 2 amps then your load will be 2 X 120 = 240 VA. Adding the K in front makes it one thousand times the VA (1,000VA is the same as 1 KVA). So, you need to have a rough idea of your total amperage requirements and then multiply by the voltage to get the VA. Divide that by 1000 to get KVA. You should then buy a transformer that is much bigger than that. The absolute limit for loading of transformers is 80% of their rating but 50% or less is better. The higher you load the transformer, the lower the output voltage will be. If you are talking about voltage sensitive loads such as motors, relays and solenoids, then it can be a real issue. Also, there are many different types of transformers. For example, "control" rated transformers have a low impedence to reduce the voltage drop on high load. In most cases you will need an "isolation" transformer as opposed to an "autotransformer" for both safety and performance issues. More to the point though.... what is it that you are doing? If you are modifying or designing a machine's control system, there are many, many pitfalls that await you. Sizing a transformer is a very basic skill. I don't mind explaining it in great detail for you. Always glad to help out, teach people, etc. But if you have to ask such a question, it makes me nervous about what you are taking onto your shoulders! Do you understand the basics of E-Stops, master contactors, mixing voltages, control isolation, proper supply grounding, etc, etc? I don't mean to offend. But I AM hoping to scare you a bit. Anyway, feel free to provide more detail. Jim Rowell

[Guest Guest_sam_koz2003]

thanks Jim

[darrenj 0]

Jim...your are correct in your transformer calculation...However i believe you forgot about 3phase loads...This throws a little bit of a curve in there!! Also if i am reading correct it is for motor loads..so you will also have the inrush to worry about..All in all as jim said its fairly easy if you know what to look for...

[JimRowell 1]

Well, the original poster stated that the primary was 240 volt and the secondary was to be 120 volts. That would generally indicate that we are talking about single phase. As for inrush, you don't need to consider that for most setups. Exceptions to this would be if you are doing a lot of continual motor starting or if you have motors that are exceptionally slow to start but neither is all that common. You should oversize your transformers such that they run very cool for long life anyway. Just like breakers and fuses, when properly sized for the running load, the normal inrush encountered on startups can be completely ignored. Control transformers are slightly different in that they often feed high inrush loads as well as loads that are very sensitive to voltage drops. That's why one would typically use a separate transformer for control that is rated as a "control" transformer. These have very low impedance values so the output voltage varies less as the load changes. Anyway, you are right that 3 phase is different. For 3 phase, VA is the voltage times the amperage times the square root of 3 (or 1.73). KVA is VA divided by 1000. Jim Rowell

[JimRowell 1]

I should probably add a point or 2 to my previous post before someone gets in trouble. Typically, transformers supply a mix of motors but that's not always the case (especially in an automation application). If you have a transformer where the majority of the load is a single motor (or multiple motors that are started together) and you are using full voltage, across-the-line starting then you may want to consider the voltage drop caused by the starting current when sizing the transformer. Voltage drop will affect the motor's starting time. Excessive drop can make the starts very slow or even prevent the motor from reaching speed. It's not likely to be a problem if you size the transformer at say 70% or less of its rating. The secret is knowing the starting current and the impedance of the transformer. The "percent impedance" rating of a transformer as stamped on the nameplate is defined as the percent of the total circuit impedance that exists inside the transformer when at 100% full load. In other words, in a 5% transformer, at full load, 5% of the total circuit impedance will be inside the transformer while the other 95% will be in the load and the wiring. This also translates to a 5% voltage drop at the transformer's terminals when at full load. If you size this transformer at 100% of a motor's running current and that motor has a 500% starting current, you would see a 25% voltage drop during the startup (500% or 5 times the normal 5%). If you sized it at 50%, you'd only see a 12.5% drop on startup (much better). I'm not sure how reliable such a linear assumption always is but it seems to pretty much hold true in my experience. Bear in mind that these drops are in addition to any created by wiring. And of course, as the motor comes up to speed, the current falls and the voltage drop decreases. Motors generally have a startup current of about 4 times the running current. The typically stated range is 4 to 6 times with 5 times being the accepted value for use in calculations. In real life, most motors are closer to 4 times. For transformers, 5% impedance is very common with a few being 3%. Since you normally would want to size a transformer at 30 to 70% of its full rating anyway, you would not go too far wrong even with a single large motor as the load. But as you can see from the above, staying closer to 50% or less would be way better for single motor transformers. It's an important point for those (such as me) who might otherwise load transformers quite high and then rely on external cooling to prolong their life. I like external cooling via fans. A force-cooled transformer will run at a very low temperature even when loaded above 80%. Unfortunately, this cooling does not help in the voltage drop department. Jim Rowell