Problem- mill motor issues. help please

[Deano7/11]

howdy all.

have been lurking around here for a while now just reading and learning.

i recently brourght myself a Ostmac (king rich KRV2000) knee mill and a chinese lathe. both second hand and 3 phase. the plan was to use 2 vacon vp10 single phase to 3 phase invertors to run these machines on 1 phase power,

so far all going great, i have the lathe converted and running great but the mill is proving to be a little harder. i have sorted all the electrics out have have the motor running off the invertor but that is were the problem starts.

both the mill and lathe have 3hp motors on them. the mill motor is a modified king rich motor. it was a star winding only motor so had to have the delta winding dug out and brought to the terminal box.

the lathe motor is a brand new 3hp 3phase motor that i wired in delta. when running the lathe at full speed (1800rpm) the motor draws 5.5odd amps runs a treat. when i run the mill it draws 3odd amps and when i wind up the variable speed pullies the motor drops alot in rpm so much so it would stall if i keep winding the handle. i have watched the amp meter on the invertor and it stays at the 3 amps only even when loaded. (the motor is rated at 8 amps and the drive at 9.6 amps)

keep in mind the invertor is the exact same invertor i only have 1 invertor at this stage so it does work well with the lathe motor but doesn;t work on the mill motor.

i have even hooked the motor wires from the mill motor to the lathe so i am using the mill electrics but powering the lathe motor and all seems great.

i have spoken to a guy that has done this exact conversion and has had great sucess except he has used a different brand invertor but other than that the conversions are identical.

i spoke to the guy that did my mill motor mods and he said the motor is fine. i spoke to the invertor people and they say the motor is at fault.

i am now banging my head agaist a wall trying to nut this one out. for some reason the motor doesn't want to run on the invertor. if it was an easy job to install a new universal fit motor i would but it will mean making adapter plates and shafts and i want to avoid this if i can.

anyone got any ideas? before i throw a match to the lot.

cheers guys
dean

[Algirdas]

It's normal. Motors are not identical. You can find some difference easy.
1. check resistance of insulation at (800 ... 1500)V
2. check inductance of windings
and once again. Are winding connections the same? Both star?

[Deano7/11]

both motor windings seem the same connections. the motor guy connected the windings on the motor i am having trouble with in 3 pairs. so there is 2 winding connections coming off each terminal.

the lathe motor seems the same. i had to change the 'bridges' from joining only 3 terminals together to joining 6 terminals in 3 pairs.

how do i measure the resistance of the insulation of the inductance of the windings.

is there something in the above results that i should be looking for.

thanks again

[RCaffin]

Before coming to any decisions, measure the output of the inverter when under load. I suspect that it just can't handle the current, and the voltage drops.

Best solution: see what it would cost to get 3-phase brought in to your shop.

Cheers

[Deano7/11]

Originally Posted by RCaffin Before coming to any decisions, measure the output of the inverter when under load. I suspect that it just can't handle the current, and the voltage drops. Best solution: see what it would cost to get 3-phase brought in to your shop. Cheers

3 phase is not an option as i am only in this place for a year or so.

the thing is this invertor works great on my other 3hp motor (lathe) so i don't see why there is such a huge difference in the performance between the 2 different 3hp motors.

with no load on the motor it draws 3 amps with the any type of load it still draws 3 amps. i would understand if the amps sky rocketed breifly then dropped down to 3amps but this does not happen.

i will try to measure the current when i get my new invertor. the invertor i have is now on the mill

thanks
dean

[RCaffin]

Originally Posted by Deano7/11 the thing is this invertor works great on my other 3hp motor (lathe) so i don't see why there is such a huge difference in the performance between the 2 different 3hp motors.

Well, there are motors ... and motors.

Originally Posted by Deano7/11 with no load on the motor it draws 3 amps with the any type of load it still draws 3 amps. i would understand if the amps sky rocketed breifly then dropped down to 3amps but this does not happen.

There may be an internal electronic limit in the inverter, set at 3 A. If so, and it would surprise me if it were NOT so, you won't see any surge.

Without measurements we know very little.

Cheers

[Deano7/11]

Originally Posted by RCaffin Well, there are motors ... and motors. There may be an internal electronic limit in the inverter, set at 3 A. If so, and it would surprise me if it were NOT so, you won't see any surge. Without measurements we know very little. Cheers

the current limit in the invertor is set to 12.5a

the motor current is set to 8 amps.

i am happy to measure whatever you think i should but i need to no how to measure these things. i have a multimeter if that helps

[RCaffin]

Originally Posted by Deano7/11 the current limit in the invertor is set to 12.5a the motor current is set to 8 amps. i am happy to measure whatever you think i should but i need to no how to measure these things. i have a multimeter if that helps

Hum, yeah, well, that should be enough current!
I suggest measuring the AC voltage (with care) on all three phases when running both motors. If the voltages are all the same, then I am puzzled.
Hard to debug things over the internet!

Cheers

[Al_The_Man]

It appears to me you have something mis-connected in the motor, see if you can get it tested on straight three phase and put a load on it.
Did you bring all 6 ends out and connect in delta?
If you have below normal current on load then with your VFD settings then I would say it is something motor related.
What is the original star connected voltage rating?
Al.

[mactec54]

RCaffin

You can't measure the output of the inverter unless you have an oscilloscope, because you do not get true AC out of a VFD, so it will not read on a multimeter correctly

I think you have a wiring problem, or a bad motor, most likely just the wiring of the motor

Deano7/11 can you do some photos of the inverter wiring & the motor wiring

[Deano7/11]

thanks for the replies guys.

i will get some pics of the connections in the next day or so. my camera is at my gf house.

the invertor is a brand new unit. it is currently fitted to the lathe an is working a treat on a new 3hp motor draws 5.6a when the lathe is in high gear (1800rpm).

i guess i could get the motor tested by someone else as the guy that did the mods to the windings for me wasn't really intrested in testing it cause he is certin it is not the problem. i can send it across town to another guy that has done this mod with great sucess to other king rich motors

[Algirdas]

i need to no how to measure these things
simple. first You take all ends of windings separate. 1.1 ... 1.2, 2.1 ... 2.2, 3.1 ... 3.2 like this. Next you measure resistance of each winding @(1 ... 15)kHz. Note - DC operated measurement doesn't works. You need to feed AC.
Third, You connect two windings in series like 1.1 winding 1.2 connection 2.1 winding 2.2 and take measurement between 1.1 and 2.2. If the resistance is not double, change connection like 1.1 winding 1.2 connection 2.2 winding 2.1. and check the resistance (between 1.1 and 2.1) now. Correct connection is when Your resistance is rising. Mark the ends of windings and then You will have possibility of correct wiring.
If this doesn't helps, then You need special high voltage device to measure resistance of insulation.