Anderson Video - Transformer Example

Professor Anderson
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Okay, so in this problem it says we have a toy train transformer, and it is a- it has a turn ratio of 8 to 1 and so it looks like this. Here's our iron core, we've got a whole bunch of turns on this side and then we just have a few on this side. Okay this is our primary and this is our secondary. So first off, the way we drew this, is this a step-up transformer or a step-down transformer? What do you guys think? Down, right? We're going down in voltage. Why? Because it's for the toy train and you're gonna hand it to your kid and then you don't want them running around with 120 volts you want them running around with you know, like 12 volts. Okay, something similar to that. So it's a step down transformer, there's lots of coils on this side, there's very few coils on that side. And let's give you the following. We're gonna say that the current running through the secondary is 1.6 amps and we need to find Ip. Alright that's probably not too bad, right? What we said was the voltages step up or step down, but voltage is linear with current thanks to Ohm's law and so in fact current is also going to step up and step down. Is over Ip is Np over Ns. Okay, and we're looking for Ip so let's just solve this equation for Ip. We've got Ip is equal to Is times Ns over NP. And now we have all these numbers. Is is 1.6 amps, Ns is 1, Np is 8. We said it was an 8 to 1 turn ratio and so we just get 1.6 over 8, this is of course 0.2 amps. Okay, so that's the first part and let's think about the second part. And the second part is a question, how much power does the train use? Alright. We know that this is coming from the wall, right? So the voltage over here, Vp, is 120 volts. This voltage over here is going to be Vs, And we know what that is now, right? There's a step down by a factor of 8 so it's 120 over 8, 120 over 8 is 15 volts. So how much power does the train use? Well, the train is on the secondary side and so Ps is just secondary current, which we knew already, times secondary voltage. Okay, and so you can plug this in, you've got 1.6 amps times 15 volts, and if you do 1.6 times 15 what do you get? Hopefully... I'm gonna guess 24 is that right? It's got to be right because this is the same at 16 times 1.5 that's 1 and a half of 16 which is 24. Okay, so that's how much current is in the secondary, that's how much power is used by the secondary. Okay, questions about transformers?
Okay, so in this problem it says we have a toy train transformer, and it is a- it has a turn ratio of 8 to 1 and so it looks like this. Here's our iron core, we've got a whole bunch of turns on this side and then we just have a few on this side. Okay this is our primary and this is our secondary. So first off, the way we drew this, is this a step-up transformer or a step-down transformer? What do you guys think? Down, right? We're going down in voltage. Why? Because it's for the toy train and you're gonna hand it to your kid and then you don't want them running around with 120 volts you want them running around with you know, like 12 volts. Okay, something similar to that. So it's a step down transformer, there's lots of coils on this side, there's very few coils on that side. And let's give you the following. We're gonna say that the current running through the secondary is 1.6 amps and we need to find Ip. Alright that's probably not too bad, right? What we said was the voltages step up or step down, but voltage is linear with current thanks to Ohm's law and so in fact current is also going to step up and step down. Is over Ip is Np over Ns. Okay, and we're looking for Ip so let's just solve this equation for Ip. We've got Ip is equal to Is times Ns over NP. And now we have all these numbers. Is is 1.6 amps, Ns is 1, Np is 8. We said it was an 8 to 1 turn ratio and so we just get 1.6 over 8, this is of course 0.2 amps. Okay, so that's the first part and let's think about the second part. And the second part is a question, how much power does the train use? Alright. We know that this is coming from the wall, right? So the voltage over here, Vp, is 120 volts. This voltage over here is going to be Vs, And we know what that is now, right? There's a step down by a factor of 8 so it's 120 over 8, 120 over 8 is 15 volts. So how much power does the train use? Well, the train is on the secondary side and so Ps is just secondary current, which we knew already, times secondary voltage. Okay, and so you can plug this in, you've got 1.6 amps times 15 volts, and if you do 1.6 times 15 what do you get? Hopefully... I'm gonna guess 24 is that right? It's got to be right because this is the same at 16 times 1.5 that's 1 and a half of 16 which is 24. Okay, so that's how much current is in the secondary, that's how much power is used by the secondary. Okay, questions about transformers?