Install a Control Transformer

In this guided applications exercise we'll examine, install, and test a control transformer. The purpose of the control transformer is to establish pilot level voltage. Pilot level voltage is utilized by control elements in a ladder logic diagram, like push buttons, relays, and coils, to govern the operation of primary or power level devices, like electric motors and hydraulic systems.
For those interested in replicating this application exercise here's the parts list from Automation Direct:
FG2 (COVER UNFUSED ALL 75VA & 100VA MLI & MGJ 50VA PRIMARY OR SECONDARY SIDE)
FGF2 (COVER FUSED ALL 75VA & 100VA & MLI/ MGJ 50VA PRIMARY OR SECONDARY SIDE)
PH100MGJ (TRANSFORMER CONTROL 100VA 380x277x208VAC TO 240x120VAC)
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For more FREE online technical training check out the following playlists available at the bigbadtech channel:
DC Circuit Analysis • DC Circuit Analysis
Single Phase AC Circuit Analysis • Single Phase AC Circui...
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Hydraulics • Hydraulics and Electri...
Pneumatics • Pneumatics
Motor Control • Motor Control
Motors and Generators • Motors and Generators
Motor Drives/Variable Frequency Drives (VFD) • Motor Drives
Programmable Logic Controllers (PLCs) • Programmable Logic Con...

[@michaelcostello6991]

Thanks for such a detailed and understandable video for us novices

[@krzysztofkapcia4521]

Jim, you connect the primary input of the transformer between L1 an L2. You use terminals 1 and 2 of the transformer reserved for 208V. But line to line voltage is 400V. Is this just a mistake on your draw or maybe I dont understand something? Thank you very much for creating such an oustanding learning materials. It helps me a lot. Thank you.

[@melvinalvarez6009]

great information!

[@krzysztofkapcia4521]

I've just answered my previous question. You've got 208 V line to line voltage in your country.

[@HungryGreeny]

I often see control voltage as large as 220 V left floating in electrical diagrams. What can be a rational behind leaving control voltage floating? Sometimes it explained as a safety feature, but in every national or international code it explicitly said that floating systems must have some kind of isolation resistance control device, otherwise it will be safe only untill first ground fault.

[@scottwestlake8407]

I was wondering why you connect one side of the step down transformer to ground. I think I understand that when measuring voltage across the coil, it gives you a reference of ground to give you a more accurate voltage reading. But wouldn’t that just allow the current to flow straight to ground and not through the control circuit as that would be the easiest path for the electrons to flow?

[@bigbadtech]

The load (ie: control circuit) is between the hot and grounded neutral. Any current must travel through the circuit to reach the ground referenced neutral. Any errant low resistance path from the hot to neutral would have substantial current traveling through it and blow the fuse. Here's more on center tapped transformers and transformer connection diagrams: kzbin.info/www/bejne/pGq8l5-IjN6Lgrs kzbin.info/www/bejne/iZe3lpeMbZ2CgZI

[@james77011]

Hahaha! 😂😂 Giving permission to beat ur lad partner... lol

[@johnc4352]

Hi Jim. How is the secondary current limited on a step down transformer such as your control transformer, it says 100 VA, or say a battery charger for an iPad or digital camera etc? If we step down the voltage say by aprox. half, 208v to 120 volts, or from 120v to 5 volts, what keeps our available secondary current from doubling or more?

[@bigbadtech]

Excellent question, I've got a whole series of lectures on transformers in the works. Short answer, the load on the secondary determines the current in the secondary and current in the primary is inversely proportional to the turns ratio.

[@unicko1985]

The VA of the transformer will dictate how much Amp you are able to draw from it either secondary or primary. I.E.: I= S(Kva) x 1000/ E, so lets say you have a 150VA 1phased transformer that does 120 to 24 amp. I= 0.15 x 1000/120=1.25Amp on the primary I= 0.15 x 1000/24=6.25 Amp on the secondary. For the 3 phased transformer you must do a little more complicated equation: I= S(KvA)x1000/E (volts) x 1.732 (square 3), So if you have a 1500 kVa transformer: I= 1500x1000/4160*1.732= 208.19 Amp at the primary I= 1500x1000/600*1.732= 1443.42 Amp at the secondary. Hope that helps.

[@mikeadler434]

👍👍

[@raedslekh133]

Hi Jim, can i reverse primary and secondary of transformer?

[@bigbadtech]

Check out the transformers lecture at: m.kzbin.info/www/bejne/i2PZm5xmbciZpNU

[@johnc4352]

If I wind a step down transformer, for 120v primary and 60v secondary, my turns ratio is 2:1. So if I figure 15 amps max into the primary (limited by the breaker size), I will get 30 amps out of my secondary and have to size my wire accordingly. Now I look at my cell phone charger, it has 120v input and steps voltage down to 5v, a 24:1 ratio. If I was winding a transformer for this ratio, and assumed 15 amps max. on primary I would figure my secondary could produce 15 X 24 amps, or 360 amps. But on my phone charger it states output current is only .55 amps, and even if I short the secondary out it will only produce .55 amps, no where near the theoretical 360 amps. My question is, what's in that little charger that limits the output current? Maybe this question is better left till you do a transformer series, but I always wondered about it.

[@johnc4352]

.55 amps, 550 milli amps

[@bigbadtech]

Long story short, there's more than a transformer in your power supply and some of the questions will be answered in the upcoming power electronics lectures as well. It's what's known as a regulated power supply with predictable maximum outputs.