3 Phase Machinery

[JimM]

Hi guys

I'm in the market for a milling machine and will probably be going for an old 3 phase machine (30+ years old) Any ideas if something of this age will have a a dual voltage or single voltage motor ?

Reason I ask is that although I haven't yet found a suitable machine I have sourced a 3HP inverter but this will only work with a dual voltage machine - I don't want to dive in a buy it though if it's likely it won't be any good to me

Thanks

Jim

 

[Wedg1e]

My '77 Boxford lathe has a dual-voltage motor on it (if you mean 220/440). However my '80 Bridgeport mill had a single-voltage (440) fitted! So age would appear to be no guide...

 

[eddie49]

Most 3-phase motors contain three separate 240-volt windings. Any two windings in series would need 2 x 240 = 480v, but in fact on 3-phase it is square root of 3 ( which is 1.73 ) x 240 = 415v. To get the three windings into series pairs, they are wired in the "Star" or "Y" mode.
To use a 3-phase motor on an inverter that is running from a single-phase 240v AC mains supply and producing a 3-phase output at 240 volts, the three windings have to be individually across the three legs of the supply. Thus the motor has to be set up in "Delta" mode ( indicated by a triangle ).
A Dual-voltage motor will be marked 240/415. But the real issue is whether you can get to the start and end of each winding, to convert it from Star to Delta. The junction box of a modern motor will have 6 terminals. For Star, the lower three of the six terminals will be linked horizontally across the bottom, by copper bars. For Delta, these are removed, and instead three vertical links are installed. The 3-wire output of the inverter goes directly to these three links.
A very old Star-connected motor may have just three terminals, but even they can usually be converted to Delta by opening the motor to get at the common point of the three windings.
There is another factor with using old motors on inverters: weak insulation may be broken down by the multiple fast square pulses that are used to simulate the 3-phase sine waves. So-called "Inverter Duty" motors have windings that are resistant to voltage spikes.

 

[piman]

Hello Jim,

there are single phase to 415v three phase inverters available. I bought a 2 KW one recently, but they are not cheap.

Alec