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Old March 23rd, 2011, 08:54 PM   #1
DamianInRochester
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Contactors on Output or Input

I am not a big fan of placing contactors on the ouput of VFDs.

We are working on a machine that was brought in from China. It utilizes Lenze 9300 drives. It is a web handling machine so the risks are getting pulled into pull roll nips and winders, etc. Interestingly, the Chinese did not see fit to install any type of disconnecting means for any of the drives.

Normally, I like to just put a disconnect on the line side and also give the drive a "quick stop" signal simultaneously with the Contactor dropping out. This way the drive uses the rest of the stored energy to help bring the motor to a stop quicker.

What complicates this arrangement is that all drives (six in total) all have their busses tied together. These busses all go to a global DB brake that dumps the buss to a resistor.

Therefore, if I drop the contactor out on say the winder section of the machine, it will still be getting power from all the other drives.

So .... I am thinking of putting the contactors instead on the output of each drive. I would put a saftey PLC in. When an estop occurs i would first give the drives a "quick stop" signal. A short time later I would drop the enable input to the drive to turn off the output semiconductors. Then finally I would drop the contactors on the output.

Any thoughts? Anyone else have experience in a case where the drives are all buss connected?
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Old March 23rd, 2011, 10:41 PM   #2
leitmotif
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Paralleling the DC buss is basically paralleling power supplies. This should have no effect on the VFD operation ie from DC buss thru IGBT and to motor. The VFD must stay energized to do dynamic braking. IF you are relying on VFD to quickly brake the motor on an E stop you should be OK even with paralleled DC bus. The efficacy of this is more determined by the programming in the VFD controls and how you set the parameters.

With six of these you can pull an E stop and power from dynamic braking will go to the paralleled DC buses. In this case you should not need braking resistors. HOWEVER if there is an Estop option for all six motors simultaneous then you will need a dynamic brake resistor to ensure all six DC buses do not reach HI DC bus voltage.

I would not have the contactors between VFD and motor - again - the VFD must stay on and motor must remain connected for VFD to dynamic brake the motor.

Look in the manual and see if you can wire the Estop into the VFD on a separate control circuit with parameters to set the slope (deceleration rate) for dynamic braking.

I hope Dick DV will answer here and give you some better ideas on how to handle.

Dan Bentler
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Old March 24th, 2011, 04:54 AM   #3
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There are caveats to both methods

Putting contactors on the line side of the drive is best for the health of the drive but when a estop condition occurs it takes a while for the drives to power back up and re initialize themselves. This is not a problem unless the estop is ativated quite a bit e.g. light curtain accross a point that a operator has to access from time to time.

Putting contactors on the load side of the drive gets rid of the wait time for power up issue that you have with line side contactors but opening a contactor between the drive and motor while the drives IGBT units are still conducting "motor turning" and "drive still running" will spike the IGBT units and can cause damage to the drive. You may do this for 15 years and never have a problem and you may do it the first time and kill the drive. There is just no way to measure it.

A decent solution is to put load side contactors on but not open them until the motor is completly stopped and the drive is stopped and not enabled.

Pneumatic delay contactors are sometimes good for this. Set the delay long enough to ensure the load has stopped and the motor is no longer turning.
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Old March 24th, 2011, 05:16 AM   #4
DamianInRochester
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Quote:
Originally Posted by leitmotif View Post
Paralleling the DC buss is basically paralleling power supplies. This should have no effect on the VFD operation ie from DC buss thru IGBT and to motor. The VFD must stay energized to do dynamic braking. IF you are relying on VFD to quickly brake the motor on an E stop you should be OK even with paralleled DC bus. The efficacy of this is more determined by the programming in the VFD controls and how you set the parameters.

With six of these you can pull an E stop and power from dynamic braking will go to the paralleled DC buses. In this case you should not need braking resistors. HOWEVER if there is an Estop option for all six motors simultaneous then you will need a dynamic brake resistor to ensure all six DC buses do not reach HI DC bus voltage.

I would not have the contactors between VFD and motor - again - the VFD must stay on and motor must remain connected for VFD to dynamic brake the motor.

Look in the manual and see if you can wire the Estop into the VFD on a separate control circuit with parameters to set the slope (deceleration rate) for dynamic braking.

I hope Dick DV will answer here and give you some better ideas on how to handle.

Dan Bentler
Hi Dan,
When you said "Paralleling the DC buss is basically paralleling power supplies", this is the very reason I started the thread. It means that putting contactors on the line side is useless to me.

Let me explain an example. This thing has a winder that will likely be in it's own fence circuit such that the we aren't killing the whole machine just for a roll change. If I kill the input contactor just to the winder, well, I really haven't done anything. That is my dilema.

Yes, the DC bus are all tied together and share a big DB resistor. It is pretty clear the Chinese only were only doing a quick stop and that the DB resistor was only to keep the buss voltage normal during that short burst of regen when the all decel to a stop. We can't get away with what the Chinese were doing.

I do want to have at least a short lived powered stop, which Is why I mentioned using the safety PLC and sending a quick stop first, and then dropping the enable once the motors have had a chance to decel.

Thanks,
Damian
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Old March 24th, 2011, 05:52 AM   #5
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Damian, I like the plan you lay out in the second to last paragraph of your OP. The way I read it, you would do the same thing separately for each of the six sections.

Your proposed system would take the best braking possible with the snubber resistors and, once stopped or nearly stopped, cut off the motor.

I like it. Good job! E-stop is always messy and your solution is well thought out.

As to the concern over opening contactors in the VFD output leads and damaging IGBT's, first, some manufacturers are now providing IGBT's and protection diode networks so, they claim, you can open the output under load without damage. I would suggest a disconnect with a really good flash rating, however. Second, back in the old days, if we needed an output disconnect, we would get one with a pre-action switch on the handle. Tie this switch back to the Run Enable on the drive. That way, the drive cuts off its output just before the contactor opens.
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Old March 24th, 2011, 06:36 AM   #6
BITS N BYTES
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Quote:
Originally Posted by DamianInRochester View Post
I am not a big fan of placing contactors on the output of VFDs.
That's why many drive manufacturers are incorporating Safe rated E-stops into their drives!

The Lenze units have the following option you could use:-


The “safe torque off” function (safe standstill) enables the implementation of applications up to control category 3 according to EN 954-1. All shut-down devices required have been integrated into the drive to save space. The complicated process of disconnecting the motor cable for fitting a motor contactor has been dispensed with, which results in shorter installation times and improved EMC performance.
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Old March 24th, 2011, 07:12 AM   #7
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Quote:
Originally Posted by BITS N BYTES View Post
That's why many drive manufacturers are incorporating Safe rated E-stops into their drives!

The Lenze units have the following option you could use:-


The “safe torque off” function (safe standstill) enables the implementation of applications up to control category 3 according to EN 954-1. All shut-down devices required have been integrated into the drive to save space. The complicated process of disconnecting the motor cable for fitting a motor contactor has been dispensed with, which results in shorter installation times and improved EMC performance.
Agreed, however I have to work with what I have been given. These drives are about 6-7 years old and were not ordered with the STO function on them.
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Old March 24th, 2011, 07:12 AM   #8
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Quote:
Originally Posted by DamianInRochester View Post
I am not a big fan of placing contactors on the ouput of VFDs.

We are working on a machine that was brought in from China. It utilizes Lenze 9300 drives. It is a web handling machine so the risks are getting pulled into pull roll nips and winders, etc. Interestingly, the Chinese did not see fit to install any type of disconnecting means for any of the drives.

Normally, I like to just put a disconnect on the line side and also give the drive a "quick stop" signal simultaneously with the Contactor dropping out. This way the drive uses the rest of the stored energy to help bring the motor to a stop quicker.

What complicates this arrangement is that all drives (six in total) all have their busses tied together. These busses all go to a global DB brake that dumps the buss to a resistor.

Therefore, if I drop the contactor out on say the winder section of the machine, it will still be getting power from all the other drives.

So .... I am thinking of putting the contactors instead on the output of each drive. I would put a saftey PLC in. When an estop occurs i would first give the drives a "quick stop" signal. A short time later I would drop the enable input to the drive to turn off the output semiconductors. Then finally I would drop the contactors on the output.

Any thoughts? Anyone else have experience in a case where the drives are all buss connected?
We do this often with servos if the risk assessment calls out for it. A stop signal to the drive then a couple of second delay before opening the contactor. google category 1 stops. A category 0 stop is instantaneous and has priority but is hard on drive output circuits.
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Old March 24th, 2011, 07:15 AM   #9
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Just found this:
Stop function to DIN EN 60204 (VDE 0113)
The Stop function is defined as follows:
There are three categories of Stop functions:
 Category 0: Shut-down by immediate switching-off of the energy supply to the
drive machinery (i.e. an uncontrolled shut-down);
 Category 1: A controlled shut-down , whereby the energy supply to the drive
machinery is maintained to perform the shut-down, and the energy
supply is only interrupted when the shut-down has been completed;
 Category 2: A controlled shut-down, whereby the energy supply to the drive
machinery is maintained.
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Old March 24th, 2011, 07:17 AM   #10
DamianInRochester
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Quote:
Originally Posted by DickDV View Post
Damian, I like the plan you lay out in the second to last paragraph of your OP. The way I read it, you would do the same thing separately for each of the six sections.

Your proposed system would take the best braking possible with the snubber resistors and, once stopped or nearly stopped, cut off the motor.

I like it. Good job! E-stop is always messy and your solution is well thought out.

As to the concern over opening contactors in the VFD output leads and damaging IGBT's, first, some manufacturers are now providing IGBT's and protection diode networks so, they claim, you can open the output under load without damage. I would suggest a disconnect with a really good flash rating, however. Second, back in the old days, if we needed an output disconnect, we would get one with a pre-action switch on the handle. Tie this switch back to the Run Enable on the drive. That way, the drive cuts off its output just before the contactor opens.
Dick,
Thanks for the advise. It is always nice to get a sanity check on something like this. Some of the electrical work I have seen done on this system makes me wish we could just start from scratch. They actually used two normally open buttons in parallel going to a regular PLC input and labeled them E-stop. They have another enclousre with qty 8 10hp drives and the only thing protecting them is one large main 200amp breaker. They burried the main CB for the enclosure behind a fixed panel. The lists goes on and on ..........

Damian
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Old March 24th, 2011, 07:27 AM   #11
Jeff23spl
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Like Dickdv, we safely use an early break contact on the cuttout device between VFD and motor(Switch or contactor) this signal is connected to the enable input of the drive so in any case, if the plc program or anything else goes wrong or someones open the cuttout whille motor is at full speed, the igbt won't blow....A switch is often needed when the motor isn't on the same floor or far away and it is cheaper to cut the output because of wires lenght.
Then you need braking ....i never dealed with parrale dc buss but it seem to be already ok at this point if you make a fast coasting down with the e-stop and open the contactor when the speed read 0...

By the way if it is a real safety integrated e-stop, you either need a safety drive logic (very rare) or you have no other choice to cut the power to the motor by 1 or sometimes 2 serial contactor depending on the safety category needed...Just make sure your estop wiring and logic also respect the safety design.

I will look if the VFD i use are protected from voltage peak like mentionned...Maybe sometimes i do thing a bit too much conservative...and lose some quotes
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