DOL starting 3 Ph Motor and an RCD

[netblindpaul]

darkwood,
In post #12 OP says the machine has an inverter, this may have leakage, if this powers up the same time as the main DOL motor it could be this taking the trip out, even if the inverter driven motor is not running the inverter may power up to ready state when the DOL motor is energised.

 

[netblindpaul]

Re-reading the OP in the light of the post by darkwood above, I would say that a soft start will probably be worse for tripping an RCD then DOL.

 

[silva.foxx]

I got a machine from another branch of our company and they told me it didn't work so i am testing it by plugging it into a 3 phase socket protected by the 30mA RCD. It's a dry clean environment - a bakery to be precise. The machine is plugged into a socket in a factory and that socket is protected with an RCD. The reason for the RCD is because the machine is portable and can be plugged in at a few locations around the factory.

The machine has one DOL 3-phase motor (about 1kW I think) started through a contactor and thermal overload, and a 1ph - 3ph inverter to drive a drum type motor. Starting the DOL motor causes the RCD to trip instantly.

I checked the ohm readings across the motor windings as I dont have a megger. The motor is in sound condition.


Is the starting of the motor causing this and would soft-starting the motor solve the problem?

If so, could I just take the RCD out of the circuit?

So you think there is no need for an RCD for this machine?

Just had to rehash the tale to read it properly.

1. The motor needs an IR test to prove its integrity, as your interpretation of a 'sound' motor catches out a few skilled men!

2. I'm not too well up on 3-ph RCDs in motor circuits. I only recall RCDs working on an imbalance of currents! Have you checked for any control faults? What voltage is the control circuit?

3. Can you bench test the motor by setting up your own, non-RCD protected rig?

4. Can you try to disconnect the 1ph-3ph VFD and try the DOL?

5. A machine needs a better understanding than just plugging-in-and-hope-it-plays!

I think you'll need to offer better info, pictures, diagrams, currency to get a better ROI.

Good luck... I'm sure you'll sort it.

regs// s.f

 

[gerard]

hi all and thanks for your help. I have a 3ph socket with a 100mA RCD and it is running fine with that. So, as someone pointed out, the 30mA RCD doesn't get on too well with the VFD. Now that this has happened, I do remember this happening before(a long time ago) and probably should have known better. thanks again.

 

[Darkwood]

darkwood,
In post #12 OP says the machine has an inverter, this may have leakage, if this powers up the same time as the main DOL motor it could be this taking the trip out, even if the inverter driven motor is not running the inverter may power up to ready state when the DOL motor is energised.

Apologies there paul missed #12 and yes all makes sense now, depending on size of inverter it would be a big suspect as 1ph to 3ph inverters create more noise than a straight 3ph to 3ph of the same size inverter, had this situe before with portable motor control rigs been plugged into 13amp sockets on different sites, im looking into a few systems to get around this fitting an earthing system monitor that cuts power if it detects a break in the earthing to reduce risk of shock as well as having adjustable rcd which will be set 30mA above natural leakage of the drive.

 

[silva.foxx]

hi all and thanks for your help. I have a 3ph socket with a 100mA RCD and it is running fine with that. So, as someone pointed out, the 30mA RCD doesn't get on too well with the VFD. Now that this has happened, I do remember this happening before(a long time ago) and probably should have known better. thanks again.

Does this 100mA RCD satisfy personal safety? If the lead of this portable equipment become damaged and exposes a live conductor, does this 100mA RCD offer correct protection to the poor sod that picks up this cable and receives a new hair-do? .

 

[Darkwood]

Does this 100mA RCD satisfy personal safety? If the lead of this portable equipment become damaged and exposes a live conductor, does this 100mA RCD offer correct protection to the poor sod that picks up this cable and receives a new hair-do? .

This is a common problem with having inverters in portable equipment and really a good all round solution dosnt exist, because leakage is expected to cause nuisance tripping you have to depart from the reg's on this occasion and make it as safe as practically can be done, this includes using cable with earthed braiding and having earth circuit monitors on, the regs are not always practical in every situe but as long as counter measures are taken then youve done you bit.

 

[gerard]

I am going to check the regs but is a 100mA RCD not better than none?! If I make everything as safe as possible then what else can I do?

 

[victory12]

agree , paul. the correct terminology is " shove a megger up it"

Have you taken any IR readings? I can't understand why you've got a RCD on the drive, but never the less it shouldn't trip it. Makes you wonder why your other department gave it away?
________________
virtual assistant

 

[Robo1401]

If the motor is not faulty then why not just take out the RCD and install a 10A triple pole type C or D mcb? This would allow for the higher starting current and provide the necessary protection needed for this type of circuit?....

 

[gerard]

I want to protect against leakage current and an MCB won't do this. At the moment it is protected with an MCB - still waiting for supplier to get an Si type RCD and this will solve problem - hopefully!

 

[sparkyjames]

What make is the VSD as some are able to disconnect the filters that cause the tripping by a simple jumper inside the terminal cover. This is a risk based procedure based on the sites requirments for 30mA RCD prtection vs EMC issues. New Boy Ducks head and waits for flack:tounge_smile:

 

[gerard]

It's a WEG CS10 - two of them.

 

[silva.foxx]

OK. Lets clarify this for the recent new posters...

The RCD is at the socket that the portable machine is plugged in to.

Gerard has the job in hand by replacing said RCD with a special type.

When he has fitted the new RCD I'm sure he'll post back here with his results.

Clear?

 

[gerard]

OK. Lets clarify this for the recent new posters...

The RCD is at the socket that the portable machine is plugged in to.

Gerard has the job in hand by replacing said RCD with a special type.

When he has fitted the new RCD I'm sure he'll post back here with his results.

Clear?

I sure will. As I said -I'm just waiting for part to come in, there is a delay for some reason.