Contactor voltage rating for VFD DC bus 2

[Kani77]

Hi,

I have two VFD 45kW, 600V, 3Phase, 60Hz. The DC bus of two VFDs are required to connect trough a contactor. What is the voltage rating of the contactor I should consider? Will 690V rated contactor suitable for this? Your comments is much appreciated.

Thanks,
Kani

 

[waross]

Extinguishing the arc associated with breaking the current is much more difficult on DC than on AC.
There are different methods of interrupting the arc. The effectiveness of the arc suppression will often not follow the same ratio of safe voltage between AC and DC.
The ratio between peak AC voltage and DC voltage is definitely not safe.
AC arc suppression depends much on the zero crossing to extinguish the arc.
DC of course, has no zero crossing hence the unsuitability of using that ratio.
So what do you do?
Use a contactor rated for the DC working voltage.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[wcaseyharman]

I would assume you can find the working voltage of the DC bus to determine your DC contractor rating. At a minimum if the VFD had a standard 3 phase rectifier on the front end the DC voltage is 1.35*AC_RMS (phase to phase) voltage. At 600VAC thats 810VDC - I probably would pick a 1000VDC contactor or thereabouts to handle some potential overvoltage.

 

[waross]

At 600VAC that's 810VAC - I probably would pick a 1000VDC contactor or thereabouts to handle some potential overvoltage.

Forget the AC rating completely.
I had an adventure with a 600 VAC 10 Amp relay.
600 VAC rated, double break.
Two contact sets were used in series.
That should be good for 1200 VAC, right.
Certainly more than 600 VAC.
The relay was used to control a 110 VDC brake coil on a motor.
Just a couple of Amps at 110 VDC.
The relay arced over all four break points.
With just 2 or 3 Amps, there was no major damage, but the arc persisted.
The arc melted the silver alloy contact faces, which then flowed together and shorted out the circuit.
Once the arc was replaced by molten metal, there was little heat generated and I found the brake energized and four little silver alloy hourglasses bridging the four contact sets.
DC interrupting is much harder than AC interrupting.
Use a DC contactor rated at somewhat above the peak AC voltage, as Wcaseyharman recommends.
Don't even think about trying to convert an AC rating to DC.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

Kani77 (Electrical)(OP)2 Jul 24 03:40
"...I have two VFD 45kW, 600V, 3Phase, 60Hz. The DC bus of two VFDs are required to connect trough a contactor. What is the voltage rating of the contactor I should consider? Will 690V rated contactor suitable for this? "
I have the following opinion for your consideration.
1. In the IEC world, major contactor OEM have the following in their program.
a) Utilization category DC-1, L/R <1ms up to 20A dc with 4-poles in series for 440V dc.
b) Utilization category DC-3, L/R <2ms up to 8A dc with 4-poles in series for 440V dc.
1.1 Proposal: Connect 2 coils in parallel, but with two each 4-poles in Series. To form 8-pole, for higher voltage rating. BTW: for say <10A dc, 1000V dc contactors are not readily available on the IEC market.
2. Refrain the use of 1000V ac rated contactors.
Che Kuan Yau (Singapore)

 

[wcaseyharman]

Ooops, that was supposed to be 810VDC- I corrected it.

I learned early on in my career that DC was much harder to break. On one of my first PLC jobs, (Micrologix 1100 I think, but it was the first PLC installed at a hydro generating powerhouse originally built in 1904) I found out a 10 A 250V relay did not mean 10A DC - after letting the smoke out of one of them I read the fine print - a 10A 250VAC relay had a .2A 125VDC rating, and I was picking up a 44 Watt coil…

 

[Kani77]

Thank you very much your reply. I will go with 1000VDC contactor.

 

[jraef]

I hope you fully understand all of the ramifications of switching the DC bus in a set if VFDs, including any pre-charge considerations and what happens s if you open that contactor under load. This is NOT something that I have ever heard of doing, and I have been involved in engineering or building literally thousands of VFD systems over the past 40 years. I cannot for the life of me think of why this would be necessary. Common DC bus systems are quite common, but the need to SWITCH the DC bus is not. If you have a common DC bus and don’t need to use one of the inverters connected to it, simply don’t modulate it. If it’s a maintenance issue, use a manual disconnect. Contactors add the risk of inadvertent operation, or lack thereof.

Be that as it may, as the others have said, you cannot infer a DC capability based solely on an AC rating of a contactor. You must find a TRUE DC rating, and on a 600VAC input VFD, it would need to be rated for 1000VDC. That will be difficult to find, very expensive, and probably very large.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

jraef has been sharing good VFD advice with us on this forum for over 20 years.
A lot of smart engineers have learned more about VFDs from reading Jeff's posts over the years.
Jeff's post is all that you really need to know.
Thanks Jeff.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[RedSnake]

Well I agree with you jraef a strange and dangerous solution for a "double" VDF solution.

NAFO Sergeant Anna Gr 69th Sniffing Brigade

 

[iop95]

May I ask what was the logic behind the requirement to connect the DCbus of the two VFDs through a contactor?

 

[Kani77]

Thank you for your feedback!

On a bucket wheel reclaimer, there are two Variable Frequency Drives (VFDs) for the long travel drive motors: one for the left side and another for the right side. The client wishes to synchronize both sides using a coupled DC bus while operating the machine remotely. Consequently, the client requires a contactor controlled by a PLC to connect both VFD DC buses as needed. Under normal operation, the two VFDs operate independently. For a visual reference of a similar machine, please refer to the link provided below.

https://www.youtube.com/watch?v=TdpNxXKqvHY

 

[RedSnake]

Some basics about VFDs the DC bus is not what determines the speed of the motors connected to each VDF its the servo part of the VDF that does that, with some kind of feedback on the motor taco or impulse generator if you want a exact speed.
The DC bus only converts AC to DC that is then converted back in the VDF to a certain frequency that decides what rmp the motor will run at.

In short the DC bus can't synchronize anything.

NAFO Sergeant Anna Gr 69th Sniffing Brigade

 

[iop95]

VFD synchro is related to speed synchonization of attached motors in a stable ratio (1:1 or other) but can't be made by DC bus.
Common DC bus is used mainly in multi-axis motors machines to obtain some energy recovery as at a time some motors may be in braking (regenerative mode) same in ramp-up.
In such machine you need speed synchronization and as no need for high precision I think standard encoders is enough with VFDs that has encoder input/output.
May use or not common DC bus but this is related to VFD / power supply power size, VFDs distance, VFD-motor distance, input cables size limitations and maybe others.

 

[jraef]

The only possible reason to connect the two DC busses is if one of the conveyors might be overhauling while the other is motoring, so they could share the energy between them. But from the looks of that setup, would either of them ever be overhauling? If not, there is zero reason to connect the DC busses.

By the way, I want that 2 minutes of my life back for all that silly lead-in graphics in the beginning of that video... someone was a bit too full of themselves with that.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[RedSnake]

If not, there is zero reason to connect the DC busses.

Not quite true we have setups with one power supply and two or more frequens converters on the same DC bus without running regenerative mode.
This is to save space in the cabinets and you need less power supplies in this setup, we still run totally separate movements on the motors.

NAFO Sergeant Anna Gr 69th Sniffing Brigade

 

[Heaviside1925]

OP,
I have gone back and read some of your previous posts. You seem willing to take on projects that are out of your area of expertise and figure things out and educate yourself as you go along. This can be a way to learn a new subject area but only if the expectations of the client are realistic.

The client wishes to synchronize both sides using a coupled DC bus while operating the machine remotely

Yes, this can be done but not by using a coupled DC bus. I suspect your client has heard descriptions of the motor control for a machine like the one in the video. They might have heard "common DC bus", "synchronized outputs", "A contactor to couple and decouple", "PLC controlled" and put them together, in their direction to you, in way that made sense to them. I hope that by doing some research on your own as well as the answers provided on the forum will direct you to go back to your client, to determine, if the direction you were provided was truly what was intended, or, as I suspect, the client only wishes their machine to function in a "like" manner. If the latter is true, then there are paths forward.

 

[Kani77]

I really appreciate your professional feedback. It appears that I made an error in saying synchronization of both sides using a coupled DC bus. Under normal operation, the DC outputs of the VFDs are interconnected through the contactor. In the event of a VFD failure, the operator will deactivate the contactor and utilize the other VFD to drive all motors. The DC connection pertains to braking and the brake resistor. Please find attached the power wiring diagram for your reference. I selected 2P 1000VDC contactor form Eaton. As jraef said, the contacor is expensive and large.