If I am making a torroidal inductor for a DC circuit with a maximum 20 A current at 120V DC, what size magnet wire would be required...and if the magnet wire must be rated for the full 20A, is it permissable to double smaller gauge wire? I know there's a correction factor for doubling conductors (80%, I think?), but I'm not sure about the ampacity requirement in an inductor.

Thanks

 

Show us how it is to be used in a circuit.

If you were to put 120V DC across the coil it would increase in current until the voltage drop across its internal resistance is 120V and it will produce 2400 W of heat.

 

Can you explain why you need an inductor for a DC circuit? They don't really do anything in a DC circuit.

 

It could easily be for a boost or buck converter.

 

For 20A you need 3mm^ wire, so how much can you get on the toroid?

 

It could easily be for a boost or buck converter.

It is true that the inputs and outputs are DC for those devices, but the in between parts hardly fit that description. If that is the case a more precise description of what the TS is contemplating would be handy. An SMPS at those power levels is not a trivial design and we have no way of knowing how this inductor will be used.

The wire gauge you need for handling 20A depends on the expected temperature rise of the wire, so there are multiple possible answers.

 

Use the inductor design tool on Micrometals' website
https://micrometals.com/design-and-applications/design-tools/inductor-designer/
Usually, you can ignore AC effects such as skin effect, and just work on cross sectional area. However, if the inductor is too small, enough AC could remain for skin effect to become important. If the inductor is too small, then the peak to peak flux excursion will be larger and core losses will become important.

 

Normally for an SMPS application you would be looking at the minimal inductance to ensure CCM (Continuous Conduction Mode) operation. This would lead to the selection of a core material and a calculation for the number of turns. The TS does not seem to be concerned with inductance, which to me reduces the chances of this being an SMPS application. Still, I could be wrong.

 

Can you explain why you need an inductor for a DC circuit? They don't really do anything in a DC circuit.

Its a motor choke...to reduce ripple after the rectifier in a triac based speed controller that I cobbled together.
The chokes can be purchased, and I have 2 factory made chokes on 2 other motors using KBIC-225 SCR speed controllers and they make a huge difference in motor noise and smoothness...but I made this controller and I want to make the choke as well...because I like to make stuff if I can.

 

Its a motor choke...to reduce ripple after the rectifier in a triac based speed controller that I cobbled together.
The chokes can be purchased, and I have 2 factory made chokes on 2 other motors using KBIC-225 SCR speed controllers and they make a huge difference in motor noise and smoothness...but I made this controller and I want to make the choke as well...because I like to make stuff if I can.

So how is a triac based speed controller classified as a DC circuit? A schematic of your controller to see the relation of the inductor to other components would be helpful -- at least to me.

 

Hi,
as i understand, the motor controller KBIC-225 SCR outputs DC voltage, so a DC coil can be used to improve motor performance. And yes, wire can be doubled . Take in account, an air gap to be provided in DC coils, in most cases.
For 20A current you should use a 2.5mm dia wire. Or two wires 1.8 mm each.
Wire diameter in mm can be calculated:
d=1.13*sqrt(I/delta).
I is current in amps, delta is current density, it's about 4A/mm2 or lower

 

Hi,
as i understand, the motor controller KBIC-225 SCR outputs DC voltage, so a DC coil can be used to improve motor performance. And yes, wire can be doubled . Take in account, an air gap to be provided in DC coils, in most cases.
For 20A current you should use a 2.5mm dia wire. Or two wires 1.8 mm each.
Wire diameter in mm can be calculated:
d=1.13*sqrt(I/delta).
I is current in amps, delta is current density, it's about 4A/mm2 or lower

Thank you! My question arose from an observation that a couple of EI core / wound DC chokes that I do have (and use) are wound with AWG 16 magnet wire or metric equivalent, and the motors they are connected to are rated in excess of the ampacity of the magnet wire. I haven't measured the current under load, so I can't say that they (the chokes) are actually subjected to the nameplate current. They all work great and do not get hot, so I figured either the current passing through them is much less than the motor info suggests, or there is some sort of "magic" that occurs in the winding that reduces the wire's resistance.

Anyway, thank you again. I appreciate the answer and have learned something.

 

So how is a triac based speed controller classified as a DC circuit? A schematic of your controller to see the relation of the inductor to other components would be helpful -- at least to me.

I can't draw the entire circuit, but it starts with a cheap AC controller like this one, but with a 10KV rating...which is doubtful.

The output of this would go through a bridge rectifier to get the DC for the motor. I'm just trying to smooth the DC output. It works without the choke, but it can be quite noisy.

Thanks

 

In fact, when i worked at a papermill, we did have some DC motors , 1-10kW. And DC chokes were used to improve motor performance. Performed on a E- I laminated steel cores with air gaps.
In other hand, the controllers were specially made for DC motor, two/three diodes and two thyristors, feedback circuits to improve speed stability, trimmers to limit motor current,trimmers to set min/max speed, external speed setting pot, etc

 

So how is a triac based speed controller classified as a DC circuit? A schematic of your controller to see the relation of the inductor to other components would be helpful -- at least to me.

The OP quoted using a KBIC controller, these do not use Triacs but SCR's in a typical dual diode, dual SCR bridge controller. Output is phase controlled/pulsed DC.
Sounds like the OP is creating a common mode toroidal choke, often bi-filar wound..

 

I see now the idea is to use a cheap Triac controller followed by a bridge.
With this method, a Pi filter arrangement could be used, e.g. 2 large electrolytic's with an inductor between them.

 

I see now the idea is to use a cheap Triac controller followed by a bridge.
With this method, a Pi filter arrangement could be used, e.g. 2 large electrolytic's with an inductor between them.

View attachment 303418

The Pi filter will burn down SCR's due to a huge uncontrolled current peaks. Capacitor C1 to be removed

 

Certainly multiple strands of thinner wire can work very well in a filter inductor. That is often done to make winding a lot easier, because the thinner wire is much more flexible, and also it has more surface area to transfer heat from. So there is no derating required to use multiple strands. There are wire size tables for transformer applications that should be adequate for a filter inductor as well. You will need to read the application notes for the different tables, as operating conditions do matter.

 

The OP quoted using a KBIC controller, these do not use Triacs but SCR's in a typical dual diode, dual SCR bridge controller. Output is phase controlled/pulsed DC.
Sounds like the OP is creating a common mode toroidal choke, often bi-filar wound..

View attachment 303415
View attachment 303416

I have three machines using the KBIC-225 controllers running PMDC motors, 2 of which have EI laminated type chokes that I've added, while the other has the torridal type shown in the illustrations you've posted (the dual lead one, specifically...not sure if that's considered a common mode choke or not). What I'm putting together now is a very cheap Triac based controller feeding the AC output through a bridge rectifier. Since my original post I have wound another torroidal choke and placed it on the AC input of the rectifier and I am very happy with the result. I've measured both the AC and DC current under full load and it's well below what I expected, and well under the conductors' ampacity. No measurable heat issues with the heat sinks I've used on the Triac and rectifier as well.

I have no way to measure ripple other than what I hear in the way of audible brush noise and heat, unfortunately, but through trial and error using various value capacitors and "home made" chokes including a re-purposed ceter tap transformer core, I have managed to get them running pretty well for my purposes.

 

...not sure if that's considered a common mode choke or not). .

A common mode choke is one that is contra wound, the laminated is for LF suppression, the HF for High freq.