Current Reading Between VFD Line Side and Loadside

First: What is tripping? The inverter drive? If so, what's the fault message? There aren't many inverter drives on the market any longer that don't have a fault record.

Second: Unless you have a VERY sophisticated meter, reading anything related to a VFD is pointless, you are not going to get accurate readings.

That said, the 0.7A reading on L1 is still troubling if there is really voltage on the terminals. I would suspect that you are reading by touching the wires, not the drive terminals, and you have a loose input terminal on L1 or a broken conductor inside of the drive between the L1 terminal and the diode bridge rectifier. That would cause all of the current to go through L2 and L3, which would increase by the sq. rt. of 3 on those two terminals and would be over heating the diode bridge.

Hi Jraef, thanks for a quick response, it was the magnetic contactor. as when found tripped they just push the button to be ON, there was a time it was from the breaker.

The breaker is undersized, which means the wiring is also. Have a qualified Electrician sort it out.

If he has O/Ls on the contactor it will trip on imbalance. Shoving a bigger breaker in isn't the answer.

Correct.

<...tripping...opinion...>

Could the forum please have the original poster's opinion on the error code that the inverter is displaying, then?

PWslack,

Thanks, the inverter doesn't show any code. I forget to include on my original post. The history was as per my electrician the original motor was 7.5kW only but when the production add more carding unit the air velocity must be increase to increase the cotton material supply to the units. however few months ago when they replaced the motor from 7.5 kW to 12.6 kW they didn't replace the breaker to 50A.. I considered it was wrong and will be replaced once we replace the inverter. I am wondering why the current reading of line side L1 was only .7amps on that can I conclude that the inverter was the fault triggering the breaker to tripped of or there something wrong besides this?

"The trip is doing its job of maintaining the facility's popularity with its fire insurance company."

Very well said, I might like to borrow that phrase sometime, if I may.

There is no copyright on it. Feel free to do so.

A 12.5kW motor at 440V ought to have a FLC of around 23A, and the CB should be around 50A. Something doesn't compute here.

If the drive is tripping, there ought be a fault message or code. Or is it the CB that is tripping?

<...If the drive is tripping, there ought be a fault message or code. Or is it the CB that is tripping?...>

The Original Poster doesn't say.

<...the CB should be around 50A...>

In that case it is the Original Poster's installation that is wrong, and a local qualified Electrician needs to sort it out using local wiring codes.

Very correct Tornado, anyway we will replace it soon same time we replace the inverter and the wire size will do as well. No fault message displayed on inverter.

Then it is useless, and may as well be isolated until it is corrected.

0.7A line surely show unbalanced supply.First isolate the problem at line side. what type of drive are using? Quite cheap type, if it does not features error/fault log? Almost all drives, current, is equipped with diagnostics now.

What is the output voltage?

Power in is approx power out.

I agree with Raef - look for single phasing input.

By the way, if your contactor is between the power supply (substation) and the VFD, you have two other problems.

First, the VFD manufacturer is probably not happy with you turning it on and off this way. The problem comes from all of the energy rush to charge up the DC power section from which the output transistors draw energy for your motor. This rapid build up of energy into the DC section will exceed your contactor ratings, but because it is over quickly enough, the thermal overload section (a separate module) may not heat up enough to trip. Initially this may work, but the relay points (contacts) will degrade rapidly.

Second, each time you switch the power on and off, you are placing a large capacitor bank in parallel with the substation windings. So, you have a nice big L-C tank circuit which can bounce very large currents through the contacts causing them to pit and vaporize little by little until you start blowing fuses or breakers.

Generally, it is best to leave the power to the VFD most of the time. It should not be turned off and on more than twice per day. Otherwise, the inrush currents are going to make you spend some serious money.

If you already have a burnt contact (as in the low current for L1) then the arcing that is going on is what is tripping your safety device. The more it arcs, the more it heats up with surge currents.