# ⚡ 2 New Autotransformer Videos For The Weekend



## Zach Stone P.E. (Jan 29, 2021)

*How do you feel about autotransformers for the PE exam?*

I took a break from interviewing engineers that have passed the new Power CBT PE Exam to post two new auto-transformer videos for the weekend.

I've been receiving a lot of emails lately asking for help with autotransformers such as how to:

Solve for the common winding current
Determine which direction the common current is flowing
Solve for the autotransformer power rating
Apply the transformer ratio formula to the autotransformer
Solve step up vs step down autotransformers
and much more
If you (or someone else you know) is studying this weekend for the new *Power CBT PE Exam*, here are two brand new videos I just uploaded to our YouTube channel to help show you how easy these problems can be if you learn how to apply circuit analysis techniques that you are already familiar with to these circuits.

You'll never need to memorize another formula or even flip through the Reference Handbook again for autotransformers once you know how to apply these simple circuit analysis techniques.

By the way, have you noticed that the Reference Handbook only includes the circuit and formulas for the *step-up* autotransformer and leaves out the *step-down* autotransformer?

If you get a *step-down* autotransformer problem on the PE exam, it will be up to you to know how to re-arrange the terms and formulas to solve it correctly.

Here is the first video that reviews these* circuit analysis techniques for the step-up autotransformer:*



Much easier this way, isn't it?

Once you're comfortable with the step-up autotransformer, (since it appears in the Reference Handbook), here is how you apply these same *circuit analysis techniques for the step-down autotransformer:*




Now that you've got a better grasp on autotransformers, try applying these techniques to practice problems you've recently got wrong and watch how easy these problems become.

By the way, this week we covered autotransformers during day 1 and day 2 of Live Class #3 - Transformers.

This coming week we will be teaching similar techniques for *Induction and Synchronous Machines* (Motors and Generators) during Live Class #4 - Rotating Machines :

Tuesday, February 2nd, 2021 (Day 1)
Wednesday, February 3rd, 2021 (Day 2)

If you're taking the exam this year, it's not too late to join our live class for the Power CBT PE Exam.

Once you enroll, you'll have access to the recordings of live classes that you've already missed (like transformers) so you can catch up with the rest of the class.

If you have any questions about autotransformers, comment down below and I'll do my best to answer you.

I hope you have a great weekend, and if you're studying for the new CBT PE exam, keep up the hard work!


----------



## akyip (Jan 29, 2021)

Zach, you're informative as always.

IMO, the most helpful video from you about autotransformers is the one that shows how to create an autotransformer from a standard transformer:



Also, one thing I haven't really seen much in many of the practice exams and materials is autotransformers with opposing winding polarities (e.g. a 480 V / 120 V standard transformer being converted to a 360 V / 120 V autotransformer).

As far as I can recall, the only practice problems that I saw regarding autotransformers with opposing winding polarities were from the Wildi book.


----------



## Zach Stone P.E. (Jan 29, 2021)

Thanks @akyip. That video is an old but good one.

Additive vs subtractive polarity tends to be more confusing than it really should. As long as you add up voltage correctly with KVL, and current currently with KCL, you'll always end up with the correct secondary voltage or current.


----------

