Different AC voltage in multicore cable

Moonwalker · Mar 31, 2022

I wanna control (reduce) the speed on my AC duct fan by inserting a variac between the mains and the duct fan.

The duct fan is currently connected to the mains (230v) in a junction box close to where I wanna install the variac (3 meters).

The variac has 4 connection-pins:

L - mains
N - mains
L - fan
N - fan

The easiest solution would be to pull a 5 wire multicore cable (300/500v) between the variac and the junction box.

But will the higher voltage on two of the wires (to the mains) impact (EMI?) the lower voltage on the other two wires (to the fan)?

Avo Mk8 · Mar 31, 2022

Moonwalker said:
But will the higher voltage on two of the wires (to the mains) impact (EMI?) the lower voltage on the other two wires (to the fan)?

No.

But depending on the type of motor you have driving the A/C fan, altering the voltage may not control the speed very effectively, being mainly governed by the mains frequency and the number of poles. Also having reduced the speed, if you turn it off, and then back on at reduced voltage, the motor may not start.

Moonwalker · Mar 31, 2022

Thx for the clarification.

It's a PSC motor.

The inefficiency shouldn't be a problem - I have already made a test comparing the temperature of the fan when using step 5 (full voltage) and step 2 running for two hours. On step 5 the fan was nicely warm, around body temperature, and on step 2 it wasn't warm at all. I actually expected the opposite.

The temperature was felt, on the top, in the center, on the plastic part of the fan after being removed from the duct.

The fan can start up from zero on step 2.

Avo Mk8 · Mar 31, 2022

Moonwalker said:
The inefficiency shouldn't be a problem - I have already made a test comparing the temperature of the fan when using step 5 (full voltage) and step 2 running for two hours. On step 5 the fan was nicely warm (felt on the plastic part on the top of the fan after being removed from the duct), around body temperature, and on step 2 it wasn't warm at all (on the same spot as before). I actually expected the opposite.

The fan can start up from zero on step 2.

Cool. Sounds like you've investigated the heating and starting issues I was concerned about - all good!
Well done.