# Why does my variac have only 60 mA of magnetization current?

I bought a 3 kVA variac (230 V in, 0-300 V out, 10 A max). I made some tests, including a load test (it seems to withstand the advertised 10 A without overheating), energy efficiency test (over 97% at full load, when the voltage is modified by only a small amount) and a magnetization current test which indicated that the magnetization current is only 60 mA.

I was totally surprised by this 60 mA magnetization current. I was expecting an autotransformer rated for 10 amperes to have far more magnetizing current. I know that a transformer/autotransformer has two currents: the load current and the magnetizing current. If you have no load, then the current is only the magnetizing current in the primary winding. This is purely reactive current, out-of-phase with the voltage, and does not consume power except by resistive and magnetic losses. If you connect a load, then the load causes a current in the secondary winding, which encourages more current to flow in the primary winding. The primary and secondary winding currents in this case cancel each other's magnetic effects, so neither is creating any magnetization in the core.

To reduce the magnetizing current, you can have more turns. But if you have more turns, the resistive losses in those turns cause so much power loss that it's probably better to accept a higher magnetizing current. So based on this, I was expecting the variac to have maybe few amperes of magnetizing current. But clearly, it does not. It has only 60 mA!

Is it really typical for 10 A variacs to have a magnetizing current of only 60 mA? Or is it likely that my variac has a capacitor inside that's cancelling the reactive impedance if the input has the correct frequency?

• Please edit question to include link to the variac datasheet. Commented Aug 11 at 11:21
• Lm should be 12 H without a capacitor. With a capacitor of 10 uF in parallel, Lm should be ~ 1 H. Commented Aug 11 at 11:28
• Unfortunately, this Chinese variac probably does not have a datasheet, and it's in a case I haven't opened, but from weight (10 kg) and external dimensions, it probably has 7 cm radius, 26 cm2 area toroid transformer, which would give 400 turns for a µr=10000 core. Maybe it could be that the magnetizing current is that small. Commented Aug 11 at 14:42
• Another measurement: winding resistance 2 ohms. A 5.1 cm x 5.1 cm square "toroid" core would require 82 meters of wire for 400 turns. This would put the wire at 19AWG, which should handle 10 amperes, but in a toroid the wires are close to each other, but on the other hand, the insulation of transformer wire should handle higher temperature than the insulation of the power cord. Commented Aug 11 at 14:47
• More calculations: at 400 turns and 0.06 A RMS (0.085 A peak) current, the magnetic field reaches around 1 Tesla for a 7 cm toroid made of µr = 10000 material. I understand this is near the limits of most transformer cores, so indeed, it could be that the magnetizing current is naturally that small. Commented Aug 11 at 14:54