Order of Line Reactor, MMC/MCB, and Contactor

[Shackled Designer]

Salutations.

My present project involves planning a motion control system where the servo power supply receives 3-phase AC line power through an MCB or MMC, a contactor, and a line reactor. The contactor exists in this circuit to ensure that the servos do not move if the safety and motion control systems do not give a "ready" signal. The servo power supply is similar in some respects to a VFD, but it supplies power on a DC bus to several servo drives.

In servo/VFD power converters (or inverters) that I have used in the past, the circuit protection occurs where the feeder meets the branch (which may or may not be close to the power converter), the contactor comes usually shortly thereafter, and then the line reactor is installed in close proximity to the power converter's infeed. In the vendor's support documentation for this system, though, the device order is shown in the sample diagram as follows:

3-Phase Line --> MCB/MMC --> Line Reactor --> Contactor --> Servo Power Device

A couple questions arise out of this:

1) Is following this order likely to have significantly different operational results than reversing the positions of the contactor and line reactor?
2) If I follow the vendor's example and separate an MMC/Contactor UL508 Type E combination to put the line reactor between them, does this negate the UL combination rating?

The only change I see in 1) is where the voltage spike is directed from the reactor when the contactor opens under load {vL = -L*(di/dt)}, that is, whether the spike is absorbed by the power converter or by the line. This consideration leads me to one more question:

3) So long as I keep the line reactor close to the power converter, would it be OK to put it before the current protector and contactor, i.e. the line reactor comes between the 3-phase line and the MCB or MMC?

Is there anything else that I should consider?

Best regards,
The Shackled Designer

 

[Besoeker]

Salutations.

My present project involves planning a motion control system where the servo power supply receives 3-phase AC line power through an MCB or MMC, a contactor, and a line reactor. The contactor exists in this circuit to ensure that the servos do not move if the safety and motion control systems do not give a "ready" signal. The servo power supply is similar in some respects to a VFD, but it supplies power on a DC bus to several servo drives.

In servo/VFD power converters (or inverters) that I have used in the past, the circuit protection occurs where the feeder meets the branch (which may or may not be close to the power converter), the contactor comes usually shortly thereafter, and then the line reactor is installed in close proximity to the power converter's infeed. In the vendor's support documentation for this system, though, the device order is shown in the sample diagram as follows:

3-Phase Line --> MCB/MMC --> Line Reactor --> Contactor --> Servo Power Device

A couple questions arise out of this:

1) Is following this order likely to have significantly different operational results than reversing the positions of the contactor and line reactor?
2) If I follow the vendor's example and separate an MMC/Contactor UL508 Type E combination to put the line reactor between them, does this negate the UL combination rating?

We manufacture variable speed drive systems and a few servos too.
A few points in no particular order.
We always have the MCB or fused isolator first, the reactor next, the the drive then the contactor.
If you put the contactor ahead of the drive then you need an auxiliary supply to keep the controls powered.
Also, if the DC link capacitor gets discharged, which it likely will, then a re-start requires them to go through the pre-change sequence unless the aux. supply can do that.
Then there is the safety consideration. Most customers we deal with want to see a physical disconnect between drive and motor. With the contactor on the input side and an aux. supply you don't get that electrical disconnect.

The only change I see in 1) is where the voltage spike is directed from the reactor when the contactor opens under load {vL = -L*(di/dt)}, that is, whether the spike is absorbed by the power converter or by the line.

The design should not allow it to open under load. An early break auxiliary contact should shut down the drive before the main contacts open. There are a few ways of ensuring this - it's just a bit of logic.

 

[petersonra]

we rarely if ever put a contactor in, and almost never a line reactor. both serve little or no purpose in modern servo systems.

 

[Shackled Designer]

Thank you both, Bob & Besoeker.

Bob, the inline contactor is also outside of my experience, but this is a recently developed servo controller (in the last 2 years), and the line power contactor is a requirement of the vendor relating to functional safety. As far as the line reactor is concerned, I would imagine that, apart from a small amount of current limiting, the reactor is there to protect the line from the converter rather than the reverse.

Besoeker, my converter does have a separate low-voltage DC line for controls power. Also, the power converter and the servo integrated motor/drive combinations are separated from each other by a connection module which is also part of the functional safety system, so I suspect that effective motor power removal occurs there. With these facts in mind, your point about logic controlling when the contactor opens is well taken. Much of that determination will rest on the motion control and safety programmer(s), most likely myself in this case.

From both responses so far, it seems that the order of the supply components is not a significant issue.

Perhaps interestingly, I now recall seeing a relatively recent VFD system that includes hardware for mounting a UL508 Type E motor protector directly on the drive's infeed for space savings. However, reading the fine print for that VFD, it only achieves a 5kA SCCR with the motor protector if a line reactor is not used. (With the line reactor, the SCCR jumps to 65kA using the motor protector.) So in the case where the integral protector mounting is used, the line reactor would have to come before the motor protector infeed.

Best regards,
The Shackled Designer

 

[petersonra]

Thank you both, Bob & Besoeker.
Perhaps interestingly, I now recall seeing a relatively recent VFD system that includes hardware for mounting a UL508 Type E motor protector directly on the drive's infeed for space savings. However, reading the fine print for that VFD, it only achieves a 5kA SCCR with the motor protector if a line reactor is not used. (With the line reactor, the SCCR jumps to 65kA using the motor protector.) So in the case where the integral protector mounting is used, the line reactor would have to come before the motor protector infeed.

Best regards,
The Shackled Designer

why would it matter any?

 

[Shackled Designer]

In response to my comment,

So in the case where the integral protector mounting is used, the line reactor would have to come before the motor protector infeed.

thus spake Bob:

why would it matter any?

Sorry for my not being clear enough. There is no space for the line reactor between the motor protector and the drive when the MCB integral mounting is used, so the reactor would have to be placed before the protector's infeed. The motor protector mounts directly to the infeed of the drive with neither space nor terminals for connecting an intervening reactor.

Respectfully submitted,
Shak