Help with megger

[zog]

One up-man-ship....I used it for a bit more than 40 years and never had a problem. I've seen new cable megger less than 100 megs, and nothing was wrong with it (no damage). Zog would have you buying new motors and replacing the cable! Not necessary.

You are missing the whole point. I y ou own the equipment you feel free to make up any procedure you want. I am a certified level 4 NETA tech, that means I am required to follow the ANSI/NETA standards because I am hire to. Some companies are required by thier insurance carriers to have thier testing done by a certified tech, some just require the equipment to be tested to recognize standards such as IEEE, ANSI, or NETA. If the result of one of those tests is below the minimu spec I recommend repair or replacement, thats my job, the owner of the equipment can do whatever they please. But dont try telling me I am wrong because you have used some thumbrule for 40 years and I folow real specs .

For an electrician wiring houses or taco bells I think any megger test is better than nothing. And I wouldnt care if it was <1000M either.

 

[VinceS]

I use NEMA MG1 2007

I use NEMA MG1 2007

The MG-1 standard is on NEMA web site www.nema.org/std. You want to use chapter 9 on testing. It costs, but if your boss requires information, I'm sure he will pay. There are illegal copys found on the web, if you trust them...

 

[Besoeker]

there was a pop in the vdf and the circuit breaker tripped, upon examining the vdf unit, one of the contactors is welded shut probably from arching.

Is the contactor inside the VFD module itself or is it external to the VFD but mounted inside the same panel?

 

[sii]

Here is the scenario a VFD controlling a 3PH 480 V motor, the vfd stops working, is replaced with a new one, a few minutes after being energized it fails also, the boss orders the vfd to be bypassed, wires are checked for voltage and connected without the VFD, motor runs perfectly.

Thanks

I'm curious how you bypassed the VFD. Did you replace the VFD in the circuit with a motor starter? Otherwise, how did you start the motor since it mentions being fused and not run off a 3 pole breaker? I guess my question is: is there a contactor or starter between the motor and VFD? If you open the circuit between the motor and VFD, you will almost certainly fry the drive. If not the first time, it won't be long.

 

[dmanda24]

Is the contactor inside the VFD module itself or is it external to the VFD but mounted inside the same panel?

The contactor is inside the VFD, is the first of 3 bypass contactors, it powers up the converter.

Today one of the electricians replaced the contactor and reconnected the unit, the VFD was powered up without any load, and worked perfectly on bypassed mode but as soon as the converter was engaged the contactor popped and the breaker tripped? supply wires were megged @ 1000V and obtained the meter limit of 2.2 Gohm. We are at a loss as why the drive is doing this, we will call eaton to see if they could shed some light on the situation.

 

[chaterpilar]

My first reaction is phase reversal at the output during changeover.

Can you check again the phase sequence at the contactors .

I never prefer contactors either at the output or input of VSD because of these hassles.

I would not worry about the cable as the system is running in bypass mode hence cable fault is ruled out.

I would prioritise to see the voltage/amps rating of the motor and check whether the VSd current rating is suitable for the motor.

Photos of the contactor arrangement can be of great help.

Cheers.

 

[RHJohnson]

You are missing the whole point. I y ou own the equipment you feel free to make up any procedure you want. I am a certified level 4 NETA tech, that means I am required to follow the ANSI/NETA standards because I am hire to. Some companies are required by thier insurance carriers to have thier testing done by a certified tech, some just require the equipment to be tested to recognize standards such as IEEE, ANSI, or NETA. If the result of one of those tests is below the minimu spec I recommend repair or replacement, thats my job, the owner of the equipment can do whatever they please. But dont try telling me I am wrong because you have used some thumbrule for 40 years and I folow real specs .

For an electrician wiring houses or taco bells I think any megger test is better than nothing. And I wouldnt care if it was <1000M either.

I don't believe I am missing the whole point. I believe you are looking at those megger reading with an idealistic point of view.
Testing companies such as yours do a good job. I have used their services in the past, and definitely appreciate what they do. And as you say the owner of the equipment can do as he pleases with the information received.
I can't say I always agree with their decision either, but it is their choice, you and I are only able to tell them what they have.

I am sure you would have an awakening if you were to go to an industrial plant and megger all the small motors. You would find some very low megger readings that have been low for years. If they are not overloaded, and do get greased, and are ventilated, they continue to run for years. It is not unusual for the plant to have new spares or rewinds on hand in their warehouse.

I make recommendations, and tell the clients what I think, based on over 40 years of experience. Part of those years was running an electrical repair shop for a large plant where we made extensive repairs to equipment used in a very adverse environment. We dismantled motors from 13.8 kv to 120vac, and rewound our motors below 4166v (due to size limitations of our oven).

Zog, I am not trying to be disagreeable, I am trying to explain what it is like in the real world. The OP will not go far if he runs to management screaming wolf!

 

[Besoeker]

The contactor is inside the VFD, is the first of 3 bypass contactors, it powers up the converter.

Three bypass contactors and inside the VFD?
If it powers up the VFD surely it is not a bypass contactor?
I'm with Chaterpilar on this - a photograph or, even better, a single-line diagram of the contactor arrangement would be of great help.