Transformer theory

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Skiroy

Well-known member
Joined
Apr 13, 2010
Messages
233
Location
Panama City Florida
Okay Im a noobie. My understanding was that the primary side of the transfomer was what you feed to the transformer and the secondary is what the transformer outputs.

Primary equals input of 240V and secondary equals output on 120V.

Well my boss today, which I dont have the most confidence in looked at a transfomer that its label siad the primary was 480V and the Secondary was 208V.

He told me that the transformer converted 208V to 480V,but I thought it was vice versa.

1.So is he wrong or can you take a transformer and supply voltage to the secondary and step up voltage and use the primary as the output?

2. And a unrelated question, They have a motor rated for 220V running on 240V voltage. I know the voltage range is +- 10% for whats on the motors namplate. So that means the max upper range for the motors is 242V. So is this cutting it too close?
 
1. In most cases, transformer work both ways, step up, step down, 1:1, whatever... Say, for a tube preamp PSU you can step down 20:1 (240:12) for 12v filaments, and then attach another identical transformer "backward" to step back up (12:240) for plate supply. Now whether that 480v transformer of your was design to step up, or step down, I don't know, but the 'primary' is meant for your input.


2. Large industrial motors are usually built to withstand a lot of abuse... I wouldn't worry about it, but I'm no power equipment expert.
 
> use the primary as the output?

1) Yes. Just be SURE you are putting a nearly-right voltage on the winding. 480V on a "208V" winding will overload the core, blow fuses, smoke.

2) Maybe. There's always some non-ideal loss. When you use it the "right" way, they wind-up a few percent to cover the losses. Say you had a "100V-200V" part. It is really 100V to 220V sagging to 200V at full load. Put it backward, 200V to the "200V" side. Now the wind-up allowance works against you. You will get maybe 90V dropping to 80V full load. On very large iron (480-208 is usually large) the sag allowance may be only 2% and negligible compared to other system slop. Little wall-warts used backward (120V:12V into 12V:120V) can show awful sag.

Other concerns. The nominal primary is connected to "infinite" power, a utility company. If there is a short, over 20,000 Amperes may flow before the breaker can shut it down. This can cause major damage. OTOH the secondary power is limited by the core and windings, probably not more than 10 times the rated power ever. Heavy transformers may have more protection on the "primary" so they don't throw molten metal.

> motor rated for 220V running on 240V voltage. ...is this cutting it too close?

Forget about it. You ain't the Boss or the Boss's Brain. You drag the stuff in, and drag it out when it smokes, not your job to wonder "Why?"

If an AC motor is not being worked RIGHT UP TO its capacity, the voltage is not very important. And when it IS worked to its limits, line-loss in the typically skimpy wire from the fusebox will reduce the terminal voltage, probably down to 230V.

Also there hasn't been 220V power in the US for a long time. All 234V, 240V, or higher. The motor companies know that, and also the typical line-loss. If they sold "220V or die" motors in today's world, they'd be killed by an avalanche of warranty returns. They know where the motors are going and do NOT want them to come back. They will take standard voltages. And 240V is ultra-standard today.
 

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