Three phase motor checks

[Peter Simpson 1Participant @petersimpson1]

Could anybody out there give me some advice on an electrical problem I have in my workshop. I have a Meddings HS4 Donkey saw that has developed a fault. The saw runs off an inverter as its a dual voltage 3 Phase motor. Yesterday whilst sawing a piece of bar stock it tripped the workshop RCD, I have disconnected the three outgoing load wires from the inverter to the motor. When the saw is switched on the inverter runs to 50 HZ on the display all ok. When the motor is reconnected to the inverter the RCD trips as soon as the saw is started. I have checked the three motor connections to earth with my multi meter there are no shorts to earth in these. Not sure what to check next. The motor is a Newman B56D/ FB214 cannot find any info about this frame size on the net.

[Peter Simpson 1Participant @petersimpson1]

[EmgeeParticipant @emgee]

Peter, you need to check for earth faults on the motor and associated cables with an insulation resistance tester at 500volts, Megger or similar.
Disconnect outgoing motor cables from the VFD before testing.

Emgee

Edited By Emgee on 26/07/2019 18:14:29

[Peter Simpson 1Participant @petersimpson1]

As I do not have a megger so it looks like I'm stuffed. If there was an insulation fault it would probably cost more than the saw is worth. As the motor is flanged to the body of the saw I cannot see the possibilities of changing the motor for a single phase motor.

[Martin CargillParticipant @martincargill50290]

I would think that the fault probably lies with the inverter. Is there anything else connected to the circuit that feeds it? The RCD will trip at 30ma but some of the 30ma could be coming from something else. The inverter (if its a modern one) should give a fault code and switch off, if it detects something being wrong with the motor

Martin

[Peter Simpson 1Participant @petersimpson1]

Hi Martin

The inverter is a Jaguar Cub the fault code on tripping is LU. The RCD is being tripped by the saw. I Will do some investigating into the fault code. I think it's an under voltage fault. Thanks Peter.

[Robert Atkinson 2Participant @robertatkinson2]

Hi Peter,
If you don't have test equipment like a megger there re other checks you can do to determine if it's the motor or the inverter. While it's likely to be the motor, there are inverter faults that will only trip an RCD with a load connected. A quick check would be to connect 3 incandescent lamps wired in "star" in place of of the motor. initialy just connect the 3 phases an leave tthe center star point unconnected. The lamps should come on when the drive output is on. The brightness hould vary with speed, lower speed, dimmer and higher speed brighter. A difference in brightness between the lamps indicates a inverter fault. If all looks OK conect the center point to the drive neutral. If its still OK it likely the motor is at fault.

Were are you located? Maybe some one near you can help.

Robert G8RPI.

[Peter Simpson 1Participant @petersimpson1]

This is not the first time this has happened but all has been well for ages, could it be the motor / inverter pairing. The motor is a 1/2 HP the inveter output is 0.4kWatt is the inverter on it's limit.

[Martin CargillParticipant @martincargill50290]

LU is probably (from memory) undervoltage, it would show this if the power supply to it has tripped i.e. the RCD has shut off the supply.

Martin

[EmgeeParticipant @emgee]

Just within the VFD max rating.

Emgee

[Peter Simpson 1Participant @petersimpson1]

This morning I wired up a spare motor to the inverter . The motor ran OK, sad to say the spare motor is not suitable for the saw. I have stripped down the original "Newman" motor, other than a bit of dirt on the stator winding every thing looks ok. This afternoon I will give the internals a clean and the rebuild it. Fingers crossed that it works.

[Les Jones 1Participant @lesjones1]

Don't you have any friends with a megger that could check the insulation between the windings and case for you ?

Les.

[Peter Simpson 1Participant @petersimpson1]

As stated earlier, this afternoon I stripped the motor down, cleaned the internals with a degreaser, blew it dry with the air compressor then rebuilt it, Rewired it to the inveter on the bench. All went well. Now to refit all the bits back onto the saw…..Fingers Crossed.

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