A milling machine problem

But one would have thought that his industrial electrician would have known about that

The inverter should current limit if set up with the correct motor parameters and it is 'modern' enough.

Neil

My own inverter reduces motor amps when it calculates the motor is likely to be overheating.

Neil

Gentlemen (and Ladies)

Hello again from the still frozen wastes of Northern Ontario, Canada.

The diagnosis of the problem has been made!

The essential problem is that the Ex-Cell-O 602 mill was made in 1971 and as such has a 1971 vintage motor on it. Recall that when connected to the inverter the motor ran for a few minutes but seemed to get hot then the inverter would trip off. The inverter however would run a 2009 vintage motor of the same hp and specifications with no problems at all on my lathe!

At a motor shop today, a new 1 1/2 hp 230 V 3 phase motor ran off the inverter with no problems and an amp draw of about 1/3 of the motor's maximum rated amp draw (whereas the old motor would draw 8-12 amps despite the motor plate saying 5.1 amps max and this with the motor not even connected to the mill spindle!)

Some research has shown that a number of companies that supply motors and inverters for industry all state that older (pre 1990 in North America) motors are often not inverter rated and as such will not run well off such a unit. This appears to be due to various things related to type of wire used, insulation class, and other things beyond my simple brain.

I thus feel that the first motor shop found no problems with the old motor when they benched tested it because they had it hooked up to a "pure" 3 phase supply and as such it ran drawing only around 1/3 of it's rated amperage as expected rather than almost twice the rated amperage that it was drawing when running on my inverter at home.

So I must advise fellow enthusiasts to consider this possible cause if they find their "older" 3 phase motor does not run well off their lovely, shiny new inverter.

Thanks for all the help and suggestions.

A grateful member.

Paul

More information about inverters and older motors in this document.