Different AC voltage in multicore cable

[Moonwalker20]

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)?

 

[BrianPetersen]

The individual wires are insulated, right?

The application is one that is not sensitive to electromagnetic interference, right?

 

[badservo]

Most AC motors will not like having the voltage changed while the frequency remains the same.

 

[BrianPetersen]

Reducing the voltage fed to an AC induction motor won't appreciably change its speed until the voltage drops so low that the motor can't develop sufficient torque near synchronous RPM to stay near that speed, at which point current and slip increase drastically and the motor heats up and trips its thermal overload (assuming it has one) if it doesn't trip the breaker first.

But that isn't what the original poster asked...

Original poster needs a VFD, not a variac. Or maybe change pulleys, if so equipped.

Unless the motor is a universal motor, which is unlikely in that application.

 

[FreddyNurk]

It's a duct fan, chances are there's a suitable dimmer or similar device (or a variac) that would do the job, subject to OEM recommendations.
I suspect the application is domestic thus usual discussions about 3 phase induction motors may not apply.

This is at the level an electrician should be able to cover.

 

[Moonwalker20]

Thx for the inputs!

The individual wires inside the multicore cable are insulated.

"The application is one that is not sensitive to electromagnetic interference, right?" That's why I started this thread, so I was hoping you guys could answer that.

It's domestic and it's a one phase PSC motor.

The variac has already been tested and the speed reduction works (frequency not changed - only voltage). Power consumption (watt) is lowered when speed is reduced. Current is not rising as well (there's a 0,5 amp fuse inside the variac).

The temperature shouldn't be a problem. I already made a test where I compared 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, so no start up issues with reduced speed.