changing a motor from 1 to 3 phase: do i need to change the cable?

sme

in about 1 month we are getting 3 phase installed and are going to have a 2hp motor replaced with a 2hp 3 phase motor.
the motor is approx 120m from the electrical installation point. there is currently 6square cable running the motor.
will this cable need to be replaced or is it adequate?

Eurorunner

Can't help with your query, but just curious as to what it cost you to go for 3 phase?

Michael Collins

Not sure what you mean by 6 square cable. You will need a cable with 4 seperate conductors in it going to the motor. In general for a given HP 3 phase is more efficient, and the current through the cables is less (higher voltage => lower current). So if there are 4 cables available they should* be capable or running the 3 phase motor, given that they had no problem with the single phase one.

*No responsibility is implied on my part!

Borzoi

sme wrote:

in about 1 month we are getting 3 phase installed and are going to have a 2hp motor replaced with a 2hp 3 phase motor.
the motor is approx 120m from the electrical installation point. there is currently 6square cable running the motor.
will this cable need to be replaced or is it adequate?

Size wise yes, but as others have pointed out, you don't have enough cores.

3-phase generally needs 3 lives, nuetral, earth - 5 core wiring
1-phase needs 1 live, nuetral, earth - 3 core wire

WHy are you getting 3-phase, it seems like a lot of effort for a single 2HP motor

Michael Collins

Yes, of course an earth would be required as well.

cast_iron

The 6-square refers to the thickness of the cores (6mm squared).

Like other said, you will need 3 cores for the phases, 1 for the neutral and 1 for the earth.

The 6-square may well have 5 cores, if it does - you'll be fine, if not - it will have to be replaced.

sme

Eurorunner wrote:

Can't help with your query, but just curious as to what it cost you to go for 3 phase?

the installation costs won't cost us anything as we are getting it through a government initiative called clar. we will just have to pay for an electrician to wire it. but i think had to pay if it, it is somewhere in the region of €1800.



currently the only large electrical appliance we have is the 2hp motor which is running a cold store, so really it would not make much sense to change over for this alone. but we are planning to build 2 more coldstores later this year and our irrigation system needs to be overhauled and 3phase motors are much cheaper and won't burbout as quickly.


thanks for ye're help. the cable is only 3 core so it will have to be changed.

fishdog

It would be normal for a 3 phase motor NOT to require a neutral. This means 4 cores is enough, 3 phase and an earth. A 3 phase motor is more efficient, quieter running, cheaper to maintain, more reliable and has a lower starting current than its equivelent in single phase.

However 2 hp is only about 1.5 kW, this would draw a full load current of less than 3 amps per phase, which is not a big a load. If the motor was close to the distribution board a 4 x 1.5 cable would be sufficient. Due to the fact that the cable run is so long (120 metres)the cable will have to be alot larger, otherwise the a significant volt drop would occur. That would cause problems. I would have to do a few sums to tell you what size cable, but at a guess a 4 core 4 or 6 sq. would do.

You could ring Eurosales in Sandyford ind. est. they have a computer programme that will tell you instantly and it is free!!! I do it all the time, and then I buy the cable from them.

Michael Collins

fishdog wrote:

It would be normal for a 3 phase motor NOT to require a neutral...

Yep, was thinking about this again myself. Since the impedance of the coils would be equal, the 3-phase system would be balanced. Hence the return current would be zero...

cast_iron

Yes, but would there be any control electroncis on board. Even a small control panel (though I'm not sure if one would be on the motor) would need a neutral to function?

As a general rule, I'd say it's best to run the wiring for a neutral in any case, as who knows what will be required when it's replaced next time.

fishdog

Control wiring can require a neutral, we need more information. You may find that whatever is switching on the motor at present (presure switch, on/off start/ stop station, time clock, etc....) would be able to operate a 3 phase contactor to switch the motor on and off. A contactor with a 415 volt coil can be obtained exactly for this reason (ie does not requie a neutral).

Red Alert

The 3 phase motors are quieter as well as they deliver constant torque as opposed to pulsating torque.