Discuss DOL starting 3 Ph Motor and an RCD in the Commercial Electrical Advice area at ElectriciansForums.net

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G

gerard

Hi,

I am having trouble with a 3 phase motor (about 1kW I think) and a tripping RCD. The motor is DOL started through a contactor and thermal overload.
Starting the motor causes the RCD to trip instantly. 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?

Cheers guys
 
It's a 30mA RCD. I got the 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. Would you recommend putting it on circuit with no RCD or up sizing the RCD to 60mA?
 
It's a 30mA RCD. I got the 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. Would you recommend putting it on circuit with no RCD or up sizing the RCD to 60mA?

Er, ... i think so!!!
 
Doubtful you will get a 60mA RCD.
Why do you want an RCD in the first place.
What machine, what motor, guessing it has a motor, what motor starter/drive, what else on the machine etc.?
 
First thing you should do is PAT test the motor is seems clear you have a earth leakage somewhere, there shouldnt be a reason for a leakage on a good sound motor, what does the motor do?
 
seems like all the motor does at the moment is trip the RCD.
 
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?
 
Darkwood,
Sorry mate but I wholly disagree with your use of the term PAT test the motor in the post above.
LOL yes well recently got my PAT cert and a free Martindale Micropat so just zapping everything with it but yes megger will do but just done 20 motors with 3phase socket kits on so just went on my own tangent.:D
 
The machine has one DOL motor and a 1ph - 3ph inverter to drive a drum type motor. The machine is plugged into a socket in a factory and that socket is protected with an RCD. So you think there is no need for an RCD for this machine?
 
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?
I checked the ohm readings across the motor windings as I dont have a megger. The motor is in sound condition. The reason for the RCD is because the machine is portable and can be plugged in at a few locations around the factory.
 
What inverter are you on about Paul?, O/P says its DOL with contactor and overload, makes sense now about you PAT comment which defo' wouldnt do through an inverter, hes only enquiring about whether soft starting would cure the problem.:whatchutalkingabout
 
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.
 
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
 
Last edited by a moderator:
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,
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.
 

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