# How to model a current transformer in SPICE.

#### Ian0

Joined Aug 7, 2020
8,396
I am trying to model a current transformer in SPICE. It's one of those with a hole through the middle where a current-carrying wire goes to make the primary.
The secondary measures as 6H in series with 100Ω.
The usual method of measuring leakage inductance doesn't seem too successful: with a shorted turn where the primary goes, the secondary inductance only reduces to 5H.
The CT in question is rated 100A:50mA.
Obviously, I could just model it as L1=1.5uH, L2=6H, K=1, but I'm pretty sure that K≠1

#### crutschow

Joined Mar 14, 2008
32,910
Why are you concerned about leakage inductance?
Are you trying to measure a high frequency current?

If using K=1 gives the correct output current versus the input current, than I see no problem with that.

#### Ian0

Joined Aug 7, 2020
8,396
Why are you concerned about leakage inductance?
Are you trying to measure a high frequency current?

If using K=1 gives the correct output current versus the input current, than I see no problem with that.
It's 50Hz, but it's inside a feedback loop.

#### crutschow

Joined Mar 14, 2008
32,910
It's 50Hz, but it's inside a feedback loop.
So are you looking for phase-shift or other non-idealities that may affect the loop response.
At 50Hz, I doubt there any any in the transformer that would do that.

#### ronsimpson

Joined Oct 7, 2019
2,748
Obviously, I could just model it as L1=1.5uH, L2=6H, K=1, but I'm pretty sure that K≠1
Measure the resistance of the secondary. (wire resistance) and add that to your SPICE model.

#### Ian0

Joined Aug 7, 2020
8,396
I found a transformer with a 100A secondary (not everyone has one of those lying about), and measured a few current transformers.
IME 100A:1A has L_secondary = 35mH, L_leakage = 3mH, R=0.4Ω.
micro-transformer.com ZMCT123 (100A:50mA) has Ls>20H L_leak=200mH, R=120Ω
yhdc.com (100A:50mA - the split-core one that's all over Ebay) has L=6H L leak =4H (perhaps) and R=100Ω
The phase shift on the yhdc part is about 5.5°, the other two are as close to being in phase as I could measure (tested with a burden resistor of 1Ω for the IME and 20Ω for the other two)
I don't know how good the leakage inductance measurements are, as they vary with the size of wire used to make the primary short.