Use of VFD output side contacter for E-stop 1

[fuzzydiode]

I am looking at a system that consists of multiple VFD's each driving a single 1HP (or less) motor. I would like informed opinions regarding the need/tradeoff of providing a separate contactor in the output side for each VFD (between motor and VFD) versus a single MC on the input side (supply)for all VFDs. The purpose of the contactors would not be to start/stop the system in normal operation, only to stop the system as quickly as possible in a emergency/abnormal situation.

Thanks
RP

 

[ScottyUK]

I guess it depends on your application:

You might damage a drive by having the contactor after the drive, and this practice is usually not recommended by the drive manufacturers for this reason.

You might think that risk is acceptable if you have a continuous process that would cost more to shut down than it would to replace a drive.

I assume as you are considering the two options, it is safe to either shut down parts of the process in isolation and leave other running, or to simply kill the whole system.

Can you fit the contactors upstream of the individual drives? The drive manufacturers don't like this either as it causes inrush onto the front end rectifier and shortens the life of the drive, but it is preferable to the downstream contactor option from this point of view.

 

[fuzzydiode]

My personnel preference would be to use a single contactor upstream to supply the all inverters and simply kill the whole system in the event of emergency. Other parts do not have to continue to operate. However, I am being told that the motors may continue to move due to energy stored in DC link portion of VFD. I know the VFD output supposed to shut down on loss of input power within 15ms to 100ms so it seems this would be insignificant. Anyway, this is the reasoning for having seperate contactors in each output.

RP

 

[CB2]

If you remove power from a vfd without removing the start/run command the vfd will see the removal of power as an under voltage condition, the energy stored in the DC bus and motor/load inertia will allow the vfd to continue running untill the dc buss voltage decays to a shutdown condition.
Please define what each motor is to do in an E-stop condition. Coast to stop, ramp to stop?
I have found that there is not much in the way of "E-stop" standards in North America, however there are some new standards in Europe that you may want to check out. Based on what is going on in the "normalization" of standards I think you will see something in the near future for North America.

 

[jraef]

Another thing to consider is that it may be possible for the VFD to stop the motor FASTER than if you just disconnected it. Is it Stopping Time that you are concerned with, or electrical safety?

If you proceed with output contactors, look at the DC motor ratings of the contactors. The PWM output of the VFD will act like DC when you open under load, striking a longer arc and potentially welding an under rated contactor, defeating your purpose. The same holds true by the way for safety disconnect switches on the load side of the VFD.



Quando Omni Flunkus Moritati

 

[jbartos]

Suggestion: Visit

http://www.meau.com/eprise/main/Web...on_Guide/VFD-SG-Section/P-VFD-Packaged-Drives

where the Mitsubishi offers "output contractor," which should probably be "output contactor."

 

[jbartos]

Suggestion: Visit

http://www.electricalline.com/images/mag_archive/14.pdf

etc. for more info

 

[jOmega]

I think you'll find that Article 430 of NEC requires separate contactor or disconnect device and overload relay for each motor; regardless of whether or not you intend to start/stop via contactor. Protection provided in VFD will not protect individual motors.

Contactor should be rated for interrupting an AC Motor.

DC motor rated contactors typically have blow-out coils or magnets... and cannot be be used in an AC circuit...even though that circuit is bi-polar dc.

Any contactor that is rated my its manufacturer for ac motor duty, is ok. Suggest NEMA rated as opposed to IEC as an IEC contactor will likely shorten motor life.

You could also use a HP (kW) rated disconnect switch.

Some people use HP rated circuit breakers (i.e. motor protector class).

Approval to interrupt power flow from VFD output to motor must come from VFD manufacturer. Some cannot tolerate the interruption (arcing etc. occurs... poles don't open simultaneously etc...)

Some manufactures, such as Danfoss, specify that their products tolerate output contactors.

Some manufacturers add nomnal 100 uHy inductors between output terminals of VFD and line side of contactor to tolerate disruption (reduces di/dt). You could probably install 1-1/2% or 3% gapped-iron core inductors between output terminals of VFD and line side of interrupting device (contactor or disc. sw. or c/b).

Three types of emergencies:

1) danger to personnel
2) danger to machinery/equipment/process
3) danger to both

In the first case, it is ok to sacrafice anything, including VFD, to preven/limit harm to personnel

In second case, companies have different policies; many companies will allow a $5k VFd to be sacraficed as opposed to damaging a $300k machine

In third case, because of danger to personnel, VFD is sacraficable.

How much is a finger, limb, life worth .... ? How much is lost production and replacement of machine worth ?

 

[afterhrs]

I strongly suggest you review "NFPA 79,Electrical
Standard for Industrial Machinery 2002 edition ."

Regards,
AH

 

[jOmega]

afterhrs:

Good point... for Industrial Applications.

NFPA 79 not applicable to HVAC and Water/Wastewater and commercial applications.

jO

 

[GGOSS]

Hello fuzzydiode,

If your intent is not to switch the contactors for start stop control then I would suggest you look at the control options available to you from the drive.

Many drives provide a 'free run stop' faciility which when utilised will cause the motors to 'coast to rest' rather than be decelerated by the drive.

If your drive offers such a facility it may be worth considering opening the input contactor shortly after the 'free run stop' command is given, thereby also providing electrical isolation.

Having said all of the above, jraef has quite rightly advised that it may be possible to stop the motors more quickly with the VFD than without.

Regards,
GGOSS

 

[TheBull]

Well, here is my $.02 worth...

As I recall, years ago the drive manufacturer's did not want anything on the output side of the drive due to damage that would be caused by opening it under load. However, most of them have solved the problem and now actually recommend that you put them on the output side.

If you put the contactor on either side, keep in mind that the motors will "free-wheel" when the contact is open. In some cases this may be acceptable. If this is not acceptable, you need to look at a time-delay relay off of your main e-stop contact that will allow the drive the opportunity to stop the motor faster than if it were to free-wheel (regen/dynamic braking). In the case of an AC drive, you will need to look at braking resistors for the regen capabilities.

One thing that I have always had a problem with in putting the contact on the input side of the drive is that once the e-stop is reset, you have to wait for the drive to do its POST before you are ready to run again. Operators don't like having to wait once they reset the e-stop.

 

[ScottyUK]

JOmega,

I'm genuinely interested in why a NEMA contactor is kinder to a motor than an IEC contactor. What's different (better?)about the NEMA design?


Scotty.

 

[jOmega]

Hi Scotty.

The IEC contactors have a shorter throw length and studies have shown this to cause motor insulation failures.

jOmega

 

[afterhrs]

What studies....please cite one.....maybe this is a topic for a new thread.

regards
A.H.

 

[GGOSS]

jOmega,

I too would be very interested to see the results of these studies. Can you point me to a paper?

Regards,
GGOSS

 

[jOmega]

New Thread Started.
jO

 

[steamboat444]

Suggest you check up on the OSHA required gear and circuitry for operator safety, if op safety is a consideration.

 

[jbartos]

Suggestion: If NEMA contactor dimensions are compared to IEC contactor dimensions, the IEC contactors tend to have much smaller cube dimensions. Therefore, the reduction of the arcs during contact parting has to be accomplished by the contactor contact parting time.
Consider:
E=Ldi/dt..........................................Eq.1
i(t)= (1/L)integral from 0 to t of (E dt) + Constant...Eq.2
v=ds/dt............................................Eq.3
s=distance
v=speed
t=time
If the IEC contactors have reduced the distance s in Eq.3 because of smaller cube dimensions, then under an assumption that v = constant for NEMA contactors as well as for IEC contactors, and it is about the same. Then, the time t has to be reduced in:
s = v x t..........................................Eq.4
This implies that the IEC contactors open faster since t has to be smaller in Eq.4. Now, if I or i(t) in Eq.2 is supposed to be about the same for NEMA and IEC contactors, then the voltage E in Eq.2 has to increase to have Eq. 2 valid. Therefore, this is where the problem with the motor insulation starts. The higher E has a negative impact on the motor insulation. Then, come to think of the contactor selection, the choice is clearly seen.

 

[jOmega]

JBartos:

You said: "Suggestion: If NEMA contactor dimensions are compared to IEC contactor dimensions, the IEC contactors tend to have much smaller cube dimensions. Therefore, the reduction of the arcs during contact parting has to be accomplished by the contactor contact parting time."

Suggest you go back and review CORONA theory.


jO