POWER TRANSFORMER - Page 1
Forum section: electronics / beginners

author: WIRENUT
author_rank: Contributor
timestamp: August 10, 2014, 11:51:55 pm
content: Hi,I have a Duel Voltage transformer. The Primary side is 220V AC or 120V AC, depending on parallel configuration vs series. I have to use 220V AC to supply power to the transformer for my application. The secondary side of the transformer is outputting 6.3VAC which is being rectified to a DC voltage for other uses.I need to run a motor that runs off of 120VAC. I want to take the 220VAC feed and use a voltage divider to cut in half to obtain the 120V AC. I am not sure how to calculate the wattage so I can select the proper rated resistors to use for the divider. Anybody have any suggestions??The 220V is a typical feed from either a house or commerical building. It can be assumed that the current is rated at 15A breakers, which usually runs at 80% making the current feed approximately 12A.Thank youWIRENUT

author: WIRENUT
author_rank: Contributor
timestamp: August 11, 2014, 12:01:21 am
content: Does any one need more info?

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 11, 2014, 12:18:53 am
content: If you are using a resistive divider (or simply a voltage-dropping series resistor) to provide 120V for your motor, then none of that informataion about your transformer seems remotely relevant. What was the purpose of even mentioning the transformer?Using resistors to drop (by whatever method) mains voltage for a motor (unless it is the size of a walnut) is an ABSOLUTELY HORRIBLE method. Don't even THINK about trying to do this.Of course, since you did not mention the current/power/rating of the motor, we have to assume it is something more than trivial.

author: WIRENUT
author_rank: Contributor
timestamp: August 11, 2014, 12:44:20 am
content: Sorry....should have named it "voltage divider". Any suggestion of a better method to get 120V from a 220V supply? because of the application I am trying to not use a step down transformer.

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 11, 2014, 01:05:01 am
content: Unless you can disclose what the load is, nobody can answer this question. Not enough information.What is your reason to not use a step-down transformer?Do you have somereally good reasonto reject the only logical solution?And you did not answer the question about why you provided all that info for the (apparently irrelevant) transformer?

author: G7PSK
author_rank: Super Contributor
timestamp: August 11, 2014, 08:25:00 am
content: With a very small motor you might be able to tap off one of the 120 V coils on the transformer but this would of course stress the other coil more so the motor current draw would have to be far smaller than the transformers.

author: German_EE
author_rank: Super Contributor
timestamp: August 11, 2014, 03:57:26 pm
content: Again this depends on the current draw of the motor but why not use a capacitor to drop the voltage? There are plenty of motor start capacitors around in various values and they are all rated for 220V operation. I can't believe I'm recommending this guy but here's a demonstration of the power involved when you use resistive dividers at 220V:

author: David Hess
author_rank: Super Contributor
timestamp: August 11, 2014, 05:30:15 pm
content: I agree with G7PSK. If the transformer is large enough, then I would use the primary side as an autotransfomer to provide 110 VAC to the motor.

author: SeanB
author_rank: Super Contributor
timestamp: August 11, 2014, 06:05:11 pm
content: That method was used for decades with tape decks and record players where they used a 110V motor and used the transformer to run it. You need to have the power use of the motor, and have a transformer with a higher power rating than that for it to work well. EG you have a 120V 10VA motor, and a transformer with a 12V 4A secondary ( 48VA) so you can safely use the motor with the transformer, provided you remember the secondary now no longer can deliver the 4A, but is now limited to delivering 48-10=36VA, so now you can only draw 3A from it. Scale accordingly for your motor and transformer and see if it works.

author: David Hess
author_rank: Super Contributor
timestamp: August 11, 2014, 07:54:26 pm
content: I built a 240 VAC phase splitter so I could power 240 VAC loads with my 120 VAC backup generator but it works in either direction. The secondaries are not connected to anything and I used three big 1000+ watt transformers in parallel because I decided that was large enough to handle 3 kilowatts and I could not lift four of them when attached to the same 2x6. In operation they run barely warm.