DOL starter switch Trips out

[Lucien Nunes]

So, to recap:
Problem with conveyor motor tripping overload relay after a minute.
Nameplate FLC 5.1A
Starter overload was found set at 6.5A
Measured line currents 12.5 / 7.6 / 1.5A at 218 / 240 / 230V L-N after O/L
Winding resistances L-star point all 2.99 ohms

The disparity between the line currents is much greater than that of the voltages at the starter, so I think the different voltages are more a symptom of different drop in the circuit conductors carrying different currents, and less likely to be the cause. As yet we don't have confirmation of the voltages at the motor terminals themselves so there is a possibility of a high-resistance connection between the two.

Another possibility is that the motor has a faulty winding with a few shorted turns that is too small a fraction of the winding resistance to register on the resistance test. Nonetheless, when energised with AC, the transformer action with the shorted turns causes all sorts of problems within the motor, not least lots of heat, although it doesn't run for long enough for that to become apparent.

One simple but effective test is to change the leads around in the motor terminal box to put each winding on a different supply phase. I.e. move L1 to L2, L2 to L3 and L3 to L1 to preserve the sequence. Then re-measure the line currents. If the cable with the 12.5A overcurrent is now L2, the problem is in the motor. If it's still L1, the problem is with the wiring or starter.

 

[James]

above is a stock picture of roughly what you have.
Imagine that the contact between 4 and C has burned away
with the contactor energised.
If it has completely gone and there is no connection at all then you will get the following slightly misleading readings (approximate)
N to L1 230v
N to L2 230v
N to L3 230v

N to 2,3,4 all 230v assuming the fuse's have not failed

N to A 230v
N to B 230v
now here's where it can get a bit confusing
N to C will be less than 230v but not a lot less. It will not be Zero volts as you might expect.
this is because the voltages from L1 and L2 pass through the motor windings and return to point C to be measured by your meter in reference to N

the following readings are what i would expect in the picture above
2 to A 0v
3 to B 0v
4 to C 400v

now in the real world, the contacts will probably not have burned away so there is absolutely no connection, just a very high resistance so the figures above are all approximate. but you get the idea? this type of fault finding is a quick and easy way to determine where you are dropping voltage on one phase that is causing overcurrent on the other phases.
You can use the same technique on the overload, any fuses or isolator in line etc.

 

[Lucien Nunes]

Your point about the regeneration of a mssing phase is good and valid and this is a trap for the unwary. However, one thing pointing away from the intermittent / burnt-out contact scenario is that the OP has not mentioned any issues with starting. The motor appears to start from a standstill every time, which a 3-phase motor won't generally do if one line is missing entirely. My experience is that it is more likely on a star-delta starter where it starts OK in star, then loses a phase when it transitions to delta but carries on running as a single-phase motor.

 

[marconi]

To have a true sense of the 3 phase voltages please measure L1-L2, L1-L3 and L2-L3 ie the line voltages instead of the phase voltages with respect to E. Depending on the earthing system the E you touch with your meter could be at some arbitrary potential with respect to the supply N.

Do some L-L (line) and L-N(phase) voltage measurements at the incomer and also the distribution board feeding the supply to the conveyor motor.

Study the distribution switchgear to discover if there are any large unbalanced single phase loads and on which phase they are connected.

 

[davesparks]

Amps at out going after overload
L1 12.5A
L2 7.6A
L3 1.5A

Resistance between star point and incoming phases at motor all at 2.99 ohms

So what is the insulation resistance of the motor?

 

[Pip]

So, to recap:
Problem with conveyor motor tripping overload relay after a minute.
Nameplate FLC 5.1A
Starter overload was found set at 6.5A
Measured line currents 12.5 / 7.6 / 1.5A at 218 / 240 / 230V L-N after O/L
Winding resistances L-star point all 2.99 ohms

The disparity between the line currents is much greater than that of the voltages at the starter, so I think the different voltages are more a symptom of different drop in the circuit conductors carrying different currents, and less likely to be the cause. As yet we don't have confirmation of the voltages at the motor terminals themselves so there is a possibility of a high-resistance connection between the two.

Another possibility is that the motor has a faulty winding with a few shorted turns that is too small a fraction of the winding resistance to register on the resistance test. Nonetheless, when energised with AC, the transformer action with the shorted turns causes all sorts of problems within the motor, not least lots of heat, although it doesn't run for long enough for that to become apparent.

One simple but effective test is to change the leads around in the motor terminal box to put each winding on a different supply phase. I.e. move L1 to L2, L2 to L3 and L3 to L1 to preserve the sequence. Then re-measure the line currents. If the cable with the 12.5A overcurrent is now L2, the problem is in the motor. If it's still L1, the problem is with the wiring or starter.

Hi Lucian,

Did as you suggested, but at the out going at the switch not at the motor, and yes the higher current moves to the next phase so I'm going with the motor being at fault.

Thanks for your input

Pip
[automerge]1579598814[/automerge]

To have a true sense of the 3 phase voltages please measure L1-L2, L1-L3 and L2-L3 ie the line voltages instead of the phase voltages with respect to E. Depending on the earthing system the E you touch with your meter could be at some arbitrary potential with respect to the supply N.

Do some L-L (line) and L-N(phase) voltage measurements at the incomer and also the distribution board feeding the supply to the conveyor motor.

Study the distribution switchgear to discover if there are any large unbalanced single phase loads and on which phase they are connected.

Hi,

There is no Neutral at the switch or motor.

Pip

 

[marconi]

Pip; May I express some minor frustration with you? A number of folk have asked if you measured the Insulation Resistance of the motor windings - have you? I asked you to measure the line voltages which you don't need an N or E to do - did you?

That said - well done Sir for finding a defective motor.

 

[Pete999]

Hi Lucian,

Did as you suggested, but at the out going at the switch not at the motor, and yes the higher current moves to the next phase so I'm going with the motor being at fault.

Thanks for your input

Pip
[automerge]1579598814[/automerge]

Hi,

There is no Neutral at the switch or motor.

Pip

Marconi said Measure L-N voltage at the incomer Pip

 

[Chris1234]

Some very good advice on here. I often swop the phases round twice at the DOL so that I can see if a fault is incoming or on a motor.

One thing I want to add is removing the motor connection bars completely and checking the bolts underneath are tight, I´ve found them loose on old and brand new motors!
Be wary on old motors as the plastic terminal holder can become brittle and snap!