# Controlling the Inrush Current Required by Large Transformers

Large transformers have a huge current demand when they are initially turned on. This is because, until the magnetic field and inductive resistance builds, they are essentially short circuits.   For example, you may have turned on some large tool or appliance and heard an initial large "HUMMMMMMM".  That is the transformer say "Ow".  The circuit breaker for that outlet might also go "Whoa, what are you doing!"

The transformer above (Avel Y236907 800VA 45V+45V Toroidal Transformer), for example, will try to draw over 100 Amps on the first cycle of 60 Hz Power.

To keep a large transformer from being damaged at turn-on (and to keep it from saying "ow"), or to keep a breaker from popping, you put in an inrush current limiter circuit.  This Instructable will detail how to do that.

## Step 1:

The thing we are after is a way to limit the current initially at turn on, and then to not limit current afterwards at all.

The circuit I use contains a thermistor, a relay, and some resistors, a capacitor and a couple transistors.

spark light3 years ago
This is a really cool and ingenious instructable, but does it have to use a resistive load + relay?
could it not be that you just wait till the zero crossing of the AC line, then switch it full on, considering that because it's an AC wave, it will ramp up the voltage in a sine wave pattern from 0? Or does the ramp have to be relatively slow?
-Thanks. Most transformers i work with are air core resonant.
3 years ago
I think the problem there is it may take the big coil longer than the cycle time to build up to its full magnetic flux. So for that first fraction of a second you're good, but what about the next wave? Inductance is reluctance. A coil this size has big feet to drag.

Now I'm wondering if one couldn't build a circuit of cascading chokes to start a big coil. Kind of like fighting fire with fire? Or would that just aggravate the problem? Inductance is one of my weaker points of understanding. I just never seem to get the stuff.
2 years ago
Maybe a motor starter
3 years ago
If you switch a transformer on at the zero crossing the current is theoretically infinite! Switching at top or bottom and current is zero I learned at school. So do not use SSRs or SCRs with zero-crossing circuitry on transformers ever!
saratogadude (author)  spark light3 years ago
As I understand it, if you switch at the first zero crossing, that is the time that it will draw the 100+ amps - on that first cycle.

Most of the books I have read said to give the transformer limited inrush current for 1-2 seconds. I may have gone overboard with 2-5.
rimar20003 years ago
This seems very useful. Can this method be used with motors? Forgive my ignorance.
Forgive me as I must plead ignorance as well - I do not know much about motors.

I googled "motor inrush current" and got many sites that seemed to have thoughts on it though.
3 years ago
Haha! What a coincidence!

I ask that because some time ago I bought a generator, and the vendor said that for using it to feed the fridge and/or the freezer, I need 7 times more power than the nominal intake of the motors, due to initial pulse. I think the case is like that you show, of tranformers.
3 years ago
I think the motors need that current just to overcome their own inertia. Inductance motors have start windings in them that draw more current just to get them starting to spin. I think there is also a phase shift involved with the start windings that aids in motors beginning to revolve, but I could be wrong about that.

Really, it is amazing electric motors run at all. Motors and transformers have coils in common, but after that the similarities end.

In order to reduce starting current with electric motors I think they have to be designed differently before they're built. I've heard of large industrial motors with special coil arrangements to bring them up to speed and limit the current they need in order to start.

To sum up, your vendor was right, and I do not know about anything you can do to reduce the current draw of an electric motor from what it is. Well, maybe if you hooked up a pull starter off a lawnmower ... it is the initial pulse that is the problem, once motors get moving they go within their rating.
3 years ago
Thanks, Fred!

I asked advice from a manufacturer of generators, and they said that maybe I could use a small generator if I change my refrigerator and my freezer for a new ones, because currently there are not made with the motor fixed to the compressor but through a centrifugal or electric clutch to prevent the inrush pulse is so high. Clever, isn't?
"Ow", you confuse thyristor and thermistor.
You´re using a thermistor or NTC.