I have a remote 7.5kW motor (400V 3 Phase) with local power that I need to control (off and on) from about 1,000 m away.

There is an available telephone copper pair (22 gauge) between the control location and the motor.

What would be my best option?

I'm thinking a 50v supply at the control end driving a small relay at the motor that operates a 220v heavier contactor - but I have no idea of the relay specifications.

An electronic or radio solution would also be considered, but there is a lot of radio noise and power spikes in the area.

 

You can use a 24V ac coil contactor to switch the motor on/off remotely, just send 24v down the telephone cable to the contactor.

like one of these...
https://inverterdrive.com/group/Contactors/WEG-CWM18-11-30D02-Contactor/

 

Thanks Dave.

If I'm reading the contactor spec correctly, it has a transient closing current of about 3A and a hold current of about 300mA.

I guess that is okay over 22 gauge telephone pair?

 

Thanks Dave.

If I'm reading the contactor spec correctly, it has a transient closing current of about 3A and a hold current of about 300mA.

I guess that is okay over 22 gauge telephone pair?

22 gauge wire has a resistance of 53 ohms per 1000 meters. Your 22 gauge pair is 2000 feet round trip so about 100 ohms. To switch 3 amps down that wire, you'll need a control voltage of 300 volts and about 100 volts for the holding current.

 

Certainly have a local contactor to switch the motor.
But 1000m of telephone wire has too high resistance to power the contactor coil directly as explained above.
So have a second sensitive relay to switch the power to the contactor coil. For example you can get a relay with a 24VDC coil with 1000ohm resistance and contacts that will switch 3A.
Don't forget to suppress both coils with diodes (if DC) or VDRs (if AC).
Also, telephone wire is not specified for high voltages so for safety keep the voltage 24V or below and isolate from the mains supply.

 

Thanks guys.

Seems to be a bit of disagreement.

For safety I'll go with a relay controlling the contactor to keep the current down in the copper pair.

48v should be fine. Analog telephone circuits have been using it for years.

Or maybe I'll put a board on the roof with "off" or "on" written on it, read it with a telephoto video camera from the other end, do the OCR, and have a Raspberry Pi run the contactor