DOL starter switch Trips out

[Pip]

Advise Please.

Called out to my local farm, who deal in fruit and veg, to a DOL starter switch which cuts out after a minute or so. This switch controls a 415V 5 amp 3 phase motor running a conveyor belt, and has done so for a number of years.

I replaced the switch with a second hand one I had just see if that was the issue, but to no avail.

Due to low IR readings I rewired from the switch to the motor via the original Cee Form plug and socket, and the on/off switch on the conveyor belt unit, re installed the original switch, but still have the same problem.

So before I advise replacement of the motor, what else could be causing this DOL switch to trip out?

I have not changed any settings on the overload part of the switch.
When it trips out, after a few minutes the switch will work and the motor will start again, (the Bimetal strip cooling down and resetting?)
So would this be the motor drawing more current than stated so overload occurs? or just old switch / overload?

Thanks in advance

 

[Pete999]

Advise Please.

Called out to my local farm, who deal in fruit and veg, to a DOL starter switch which cuts out after a minute or so. This switch controls a 415V 5 amp 3 phase motor running a conveyor belt, and has done so for a number of years.

I replaced the switch with a second hand one I had just see if that was the issue, but to no avail.

Due to low IR readings I rewired from the switch to the motor via the original Cee Form plug and socket, and the on/off switch on the conveyor belt unit, re installed the original switch, but still have the same problem.

So before I advise replacement of the motor, what else could be causing this DOL switch to trip out?

I have not changed any settings on the overload part of the switch.
When it trips out, after a few minutes the switch will work and the motor will start again, (the Bimetal strip cooling down and resetting?)
So would this be the motor drawing more current than stated so overload occurs? or just old switch / overload?

Thanks in advance

Is the DOL starter compatible with the motor, does the Motor need a better way of starting eg Star Delta, is the conveyor blocked in any way causing the Rotor to lock?

 

[James]

check for volt drop across each phase of the contactor
check you have equal current on each of the phase lines
check voltages, have you dropped a phase?

 

[Pete999]

Have you IR tested the windings?

 

[James]

Have you IR tested the windings?

or the sparky's nose test?

power off, open terminal box on top of motor, stick nose close to motor and inhale.
95% effective and saves getting the expensive meter out of the van!!

 

[Dabbers]

What you might find is the bearings in the motor at starting to seize or the gear box that the motor is driving has become warn out, you could do with a clamp meter on it and if you adjust the overload up you can see the current rise you might find due to its age that another amp makes the motor continue to run pointing towards increased labouring on the motor. "seen this before"

 

[Lucien Nunes]

check you have equal current on each of the phase lines

And compare it with the motor FLA on the nameplate and the current setting of the starter. Surely if the starter keeps tripping on overload, the first thing you would do is see if it is overloaded?

 

[Pip]

Is the DOL starter compatible with the motor, does the Motor need a better way of starting eg Star Delta, is the conveyor blocked in any way causing the Rotor to lock?

Hi,

As I say it has been working this way for a number of years without a problem, the starter and motor have been a pair for the same period.

The conveyor is not blocked, how freely it travels I can't tell as I have no knowledge of it before the issue, also not sure of the maintenance regime regarding greasing etc but will advise it.

Thanks
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check for volt drop across each phase of the contactor
check you have equal current on each of the phase lines
check voltages, have you dropped a phase?

Hi,

Have not lost a phase, Only briefly checked voltage on each phase before it tripped out but nothing untoward from first impressions, will check current and voltage again. Thanks
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Have you IR tested the windings?

Hi Pete,

No not gone that far, IR tested the feed cables, and changed as I mentioned due to low readings, (and all the cuts and nicks and routing) which is why I'm suspicious of the motor.

Thanks
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or the sparky's nose test?

power off, open terminal box on top of motor, stick nose close to motor and inhale.
95% effective and saves getting the expensive meter out of the van!!

Ha Ha. Have done that, but no strange smells or burning, but might try and get closer and give it another go
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What you might find is the bearings in the motor at starting to seize or the gear box that the motor is driving has become warn out, you could do with a clamp meter on it and if you adjust the overload up you can see the current rise you might find due to its age that another amp makes the motor continue to run pointing towards increased labouring on the motor. "seen this before"

I like the way you're thinking, the motor, and gear casing attached, do move when the shaft of the belt rotates, which does suggest something is not aligned

Will check current as per other reply

Thanks
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And compare it with the motor FLA on the nameplate and the current setting of the starter. Surely if the starter keeps tripping on overload, the first thing you would do is see if it is overloaded?

As I say this Switch and Motor have been working together for a number of years without a problem. See pic regarding the motor and switch. The current setting on the overload is at 6.5A, and as I say I have not changed this, but following another reply I will check with a clamp meter and change settings.

Saying that when I swapped the switch out for a second hand one I had I did change the load setting on that one and it still tripped out

 

[James]

ok,

*****
assuming the following is true.

motor is not physically overloaded
if the motor draws 5A at max rated current and the overload is set at the same,
******

if you measure the current on each phase and it is roughly equal on each phase and less than 5
it is a faulty overload relay

if the currents are unequal then
you have a failure in the circuit on one phase, find it by measuring where the volt drop is.

i.e. measure the voltage between the incoming L1 and the outgoing L1 on the contactor, repeat for other two phases
same again for the overload relay.

note that you can get a phantom voltage on a cable coming back from the motor if you are measuring in reference to earth.

example,
if the contactor has failed on L2 only and not passing any current,
you will still measure 230V from the outgoing terminal to earth or N
the voltage is actually a combination of L1 and L3 going through the motor and returning.

for testing, in the past I have linked out the N.C contacts on the overload relay to allow the contactor to stay energised for long enough to test.
be careful as by doing this for a long period of time, (1 minute or more) you could overheat the motor.

there are a few possible outcomes.

1. contactor has failed on one phase
2. overload relay is faulty
3. cable from overload relay to motor is broken (including any junctions)
4. somewhere upstream of the contactor you are loosing voltage ( isolator, cable, plug/socket, circuit breaker)
5. faulty motor

if you believe it is a faulty motor,
test resistance between star point (terminal in motor casing where 3 are joined) and each of the incoming phase connections.

should be roughly equal
if one is significantly different to the other two then send motor to the rewind shop for investigation.

 

[davesparks]

As I say this Switch and Motor have been working together for a number of years without a problem. See pic regarding the motor and switch. The current setting on the overload is at 6.5A, and as I say I have not changed this, but following another reply I will check with a clamp meter and change settings.

6.5A is too high, the overload should be set at the FLC for the motor, so acvoridng to your pictures the overload should be set at 5.1A.

How many times have you tried to run the motor like this? It sounds like you've tried it, and it tripped, many times. This is not advisable and could be degrading the overloads.

Test the motor, both continuity and IR, I'm surprised this wasn't your first port of call after a visual check.

 

[David Prosser]

Start with a visual inspection of conveyor system. Nearly every time I'm called out to "it's an electrical fault because it keeps tripping" !! It will be a mechanical fault. the fact that the overload has been turned up previously points to this even more so.

If your still can't find the fault then DM as I'm often down Chipping/Blockley way.

 

[Pip]

ok,

*****
assuming the following is true.

motor is not physically overloaded
if the motor draws 5A at max rated current and the overload is set at the same,
******

if you measure the current on each phase and it is roughly equal on each phase and less than 5
it is a faulty overload relay

if the currents are unequal then
you have a failure in the circuit on one phase, find it by measuring where the volt drop is.

i.e. measure the voltage between the incoming L1 and the outgoing L1 on the contactor, repeat for other two phases
same again for the overload relay.

note that you can get a phantom voltage on a cable coming back from the motor if you are measuring in reference to earth.

example,
if the contactor has failed on L2 only and not passing any current,
you will still measure 230V from the outgoing terminal to earth or N
the voltage is actually a combination of L1 and L3 going through the motor and returning.

for testing, in the past I have linked out the N.C contacts on the overload relay to allow the contactor to stay energised for long enough to test.
be careful as by doing this for a long period of time, (1 minute or more) you could overheat the motor.

there are a few possible outcomes.

1. contactor has failed on one phase
2. overload relay is faulty
3. cable from overload relay to motor is broken (including any junctions)
4. somewhere upstream of the contactor you are loosing voltage ( isolator, cable, plug/socket, circuit breaker)
5. faulty motor

if you believe it is a faulty motor,
test resistance between star point (terminal in motor casing where 3 are joined) and each of the incoming phase connections.

should be roughly equal
if one is significantly different to the other two then send motor to the rewind shop for investigation.

Thank you James for a clear and precise evaluation of the situation. I will en devour to follow these instructions and report back.

Again thank you very much

 

[Pip]

Thank you James for a clear and precise evaluation of the situation. I will en devour to follow these instructions and report back.

Again thank you very much

Hi James

Voltage on incoming side of contactor No load
L1 230
L2 238
L3 230
Contactor side with load
L1 218
L2 240
L3 230
Overload side with load
L1 218
L2 240
L3 230

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

 

[James]

I presume that you are measuring between a live pin and an earth or N when taking the measurements.

when measuring voltage across the contactor, measure like this.
p.s. it works better when the contactor is under load and use a volt meter instead of a pair of biro's

EDIT,

you should see 0v between incoming and outgoing contacts as in picture.
if you find a voltage between in and out contacts then that is where your fault lies.
after all, a contactor is just a set of mechanical switches, the in and out for each phase should be at the same potential.

 

[Dabbers]

Hi James

Voltage on incoming side of contactor No load
L1 230
L2 238
L3 230
Contactor side with load
L1 218
L2 240
L3 230
Overload side with load
L1 218
L2 240
L3 230

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

Definitely a fault on L1 I would check what James has listed It may well be an isolator or supply cable upstream feeding the DOL that is failing causing a high resistive joint looking at the current readings

 

[Pip]

I presume that you are measuring between a live pin and an earth or N when taking the measurements.

when measuring voltage across the contactor, measure like this.
p.s. it works better when the contactor is under load and use a volt meter instead of a pair of biro's

Yes Live and earth on contactor, will find some biro's and do it again

 

[James]

Yes Live and earth on contactor, will find some biro's and do it again

I have edited that post to make it a bit more informative.

 

[Pip]

Definitely a fault on L1 I would check what James has listed It may well be an isolator or supply cable upstream feeding the DOL that is failing causing a high resistive joint looking at the current readings

Upstream from the DOL is another CeeForm connector plug, will check that. and there is then a choice of sockets as there are various other feeds going to various other Motors on the spud grader. Which were not powered at time of test by the way.
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And compare it with the motor FLA on the nameplate and the current setting of the starter. Surely if the starter keeps tripping on overload, the first thing you would do is see if it is overloaded?

Hi Lucian

See pic of the overload, I'm reading it as 6.5A, what do you think?

Either way turning it up or down makes no difference to the problem, its now set at 5.5

Thanks for your input

 

[Dabbers]

Good Luck! won't be back on till Monday i'll see how you get on then! just another thought is that its worth checking the voltages on the other equipment upstream from the contactor as the voltage would still be low on L1 further up stream

 

[marconi]

The forum at its very best - a brain trust:

Brain trust - Wikipedia - https://en.wikipedia.org/wiki/Brain_trust

 

[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
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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
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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!