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Looking for transients in 600V 3 phase system, need opinions
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Sterno:
Old picture, line reactor has been added, its located  between breaker and VFD.  I dont have current pic and machine is 3hrs away.   The MOV in question is located between L1 and L2 on input of VFD.
I am not braking the motor, letting it coast.

I have kicked this up the command chain to get someone with the proper gear / knowledge to solve this as I'm just a Electronics Engineering Technologist, not an Electrician.  Nonetheless, my boss wants answers from me, so here I am.
jmelson:

--- Quote from: Circlotron on July 04, 2019, 01:23:28 am ---
--- Quote from: Sterno on July 03, 2019, 11:22:42 pm ---Power comes from electrical room through a disconnect, circuit breaker, and line reactor to the VFD.

--- End quote ---
If the VFD suddenly stops pulling current, could the line reactor be giving the VFD input an inductive kick?

--- End quote ---
YES, this is very possible.  But, it is not just when it stops drawing power, it is EVERY CYCLE of the line, it draws current for a moment on each phase, then stops as the sine wave begins to decrease.  As soon as the rectifier stops conducting, there will be a spike from the inductor.  You need some kind of circuit to absorb that spike to protect the rectifiers.

Is this model supposed to be run with a line reactor?  Does the manufacturer specify the inductance for a line reactor for this size of VFD?  Is the line reactor within that spec?

Presumably, for a 600 VAC VFD, they must be using 1200 V (or higher) rectifiers, so this must be a pretty bad spike to take out the rectifier section.

Jon
Sterno:

--- Quote from: jmelson on July 05, 2019, 07:21:29 pm ---
--- Quote from: Circlotron on July 04, 2019, 01:23:28 am ---
--- Quote from: Sterno on July 03, 2019, 11:22:42 pm ---Power comes from electrical room through a disconnect, circuit breaker, and line reactor to the VFD.

--- End quote ---
If the VFD suddenly stops pulling current, could the line reactor be giving the VFD input an inductive kick?

--- End quote ---
YES, this is very possible.  But, it is not just when it stops drawing power, it is EVERY CYCLE of the line, it draws current for a moment on each phase, then stops as the sine wave begins to decrease.  As soon as the rectifier stops conducting, there will be a spike from the inductor.  You need some kind of circuit to absorb that spike to protect the rectifiers.

Is this model supposed to be run with a line reactor?  Does the manufacturer specify the inductance for a line reactor for this size of VFD?  Is the line reactor within that spec?

Presumably, for a 600 VAC VFD, they must be using 1200 V (or higher) rectifiers, so this must be a pretty bad spike to take out the rectifier section.

Jon

--- End quote ---

--- Quote from: Sterno on July 04, 2019, 11:16:13 pm ---Line reactor is 6 inches above VFD.   Rated for 3.9A, 3%, 600V, 6.74mH.  Unit specified by VFD manufacturer

--- End quote ---
Line Reactor was added after first failure in an attempt to solve the problem.
jmelson:

--- Quote from: AlfBaz on July 05, 2019, 01:47:36 am ---Can the VFD handle the motors regen? You said it's a conveyor motor, does it have a contactor on the vfd's output, input or both? Perhaps your rundown params are too aggressive or the motor's model parameters are incorrect

--- End quote ---
Good point!  Conveyors are a special problem, as they have VAST inertia.  If asked to stop, the load on the conveyors can keep it moving for a long time.  If the VFD is set for dynamic braking, then it needs an EXTERNAL braking resistor to handle the dissipated energy.  It may also need to have the braking time adjusted so that the regenerated energy is kept in check.

What you can do is connect a DVM rated for the voltage (going to be close to 1000 V!!) to the braking resistor and then stand well clear while starting the conveyor and then stopping it.  You will see the DC link voltage in bursts across the braking resistor during the deceleration.  (Some VFDs also allow you to program the panel display to show DC link voltage - you can use this instead.)  You want to keep the DC link voltage well below the maximum for the VFD, to protect the semiconductors.  Actually, the VFD should fault out with an OV fault code if this happens, but maybe the micorcontroller is not programmed to be conservative.

Jon
jmelson:

--- Quote from: Sterno on July 05, 2019, 07:26:17 pm ---Line Reactor was added after first failure in an attempt to solve the problem.

--- End quote ---
OK, has the unit blown again since the reactor was added?
I'd still try to do the DC link voltage reading while the conveyor is decelerating.  This is easy if available on VFD display.
Otherwise, it is tricky and a bit dangerous to get to this with a DVM, but many drives do have these available on the screw terminals inside. 

I'm still a bit surprised to see the VFD with NO EXTERNAL braking resistor in a conveyor application.

Jon
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