Estop wiring to AC drives

[TheApprentice]

Hello:
Can anyone shed any light on how to properly wire estop circuit to AB 600VAC 3phase AC drive? To be more specific, if you have a field Estop push button, pushing that will deenergize a contactor, and disconnect the output from the drive to the motor. The motor also have brakes that works off the drive running contact output and hitting the estop will de-energize the brake relay as well. This means the motor will coast to stop. What I am not sure is from a few reading that talks about safety relays with drives, that the estop output contactor should not be open until the drive is output voltage/current has decreased to acceptable level to prevent damage to the drive. Some wiring setup I've seen have the estop contactor at the input of the drive as well, but the principle is still the same - don't open the contact at estop until the drive output is low. Is this a comment practice? And does this practice only apply to safety type relays or should it also be done to any drives estop wiring in general.
Thanks In advance

 

[Skogsgurra]

Very relevant question!

I have had drives in safety critical applications where the E-stop just disconnected the primary blower (boiler application) mains connection which made the whole inverter section blow up. On the other hand, I have seen installations where the same stopping method seemed to be OK.

Considering the energies involved (rather large blowers, 600 - 800 kW) in the DC link, in the magnetics and the inertia of the fan itself, I think that a controlled shut-down and, after that, a disconnect is the only way to go.

The safety guys then probably will tell you that they do not trust the electronics in a critical situation. My answer to that is that you should do the controlled shut-down and then (after ten or twenty seconds) a brute-force disconnect if you haven't reached still-stand or if the motor hasn't de-energised.

Using the latter method guarantees that the drive will stop under all circumstances. And if the electronic shut-down fails (which it never does, really) the brute-force disconnect will be your ultimate safety.

 

[TheApprentice]

Hi skogsgurra
Thanks very much!
Let me get this straight, so you are saying,
at the activation of estop button, ramp the drive output to zero and then open up the output contactors? And let's say if a delayed opening of the output contacts are what I need, is there a rule to determine the time delay requirements?
Now another related question is, some wiring have the contactor at the estop input side, some at the output side.
Which one is better? In terms of the wiring I have in mind, removing the input power will kill the HMI/fault display which is not necessarily what I want, and I know there are way to wire the drive so that it works off the external power. But what is the reason to have the contactor at the input vs the output?
Thanks again

 

[kh6723]

Apprentice,

Dependant on where in the world you intend to apply this installation, specific regulations apply to the use of E-stops. In the EU (Europe) these requirements are laid down in the machines directive which specifically excludes the use of non-hard-wired E-stops for safety critical applications. As E-stops are, by definition,safety critical, these directives apply. There are exceptions to this. These are by using SIL rated equipment which is proven by probability criteria to have a low risk-of-failure level.

OK, so what are the relative merits of input .v. output connection of contactors......

U are quite correct. Positioning the contactor upstream of the VSD without an auxiliary supply (which is the usual case) will kill the HMI. I personally don't go for this arrangement.

I tend to prefer the contactor positioned on the output of the VSD (i.e. between the VSD and the motor). My reasons for this is that most codes of practice do not allow semi-conductor devices to be used for isolation purposes. As the DC link of a VSD will (at the moment of switch-off via an upstream contactor) still contain a finite amount of energy, then any failure of the semi-conductors between the DC link and the VSD output could result in the stored energy being transferred to the load. This would result in a momentary inching of the motor. Not very clever if anyones hand is close to the shaft (for whatever reason) or if the drive operates some form of device like an elevator.....

these are just my views by the way.... but they're probably a good starting point for discussion !!!!!

 

[ScottyUK]

I agree with some of kh6723's comments, other than I prefer to see the ESD contactor upstream of the drive. The stored energy in the majority of drives is not sufficient to be concerned about in most instances. The contactor upstream is preferred by many drive manufacturers because it improves the chances of the drive surviving the opening of the contactor. In a true emergency situation blowing up the drive is secondary to protecting personnel, so it may be a moot point. A drive can always be replaced; people can't.

Skogsgurra's comments are worthy of consideration, but you must closely look at the application. Operation of the ESD must not cause a bad situation to get worse, but in the majority of cases a delay of 20 seconds would be totally unacceptable for a ESD function. Exceptions might be include servo systems where sudden loss of power may cause violent uncontrolled motion of a load, for example, and operation of the ESD should ideally avoid this.

Some drive manufacturers, of which Allen Bradley are one, have a module which is approved for use as an ESD function. In principle it appears to open the DC link using a semiconductor switch and remove the gate drive from the output bridge. I have not used one of these functions, preferring the old-fashioned way with a contactor. The ESD function must be virtually infallible which I equate to simple design and robust construction.



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[Skogsgurra]

Yes, Scotty. Twenty seconds is a long time. It worked in the boiler primary air fan situation. For other cases, the delay can and shall be made much shorter.

 

[jraef]

TheApprentice,
This is an oft repeated query in this forum, and the debate goes on. As you can see, there is no "unified theory" as to how to deal with this issue, mainly because it is application and jurisdiction dependant. You might be well served to do a keyword search on this to read the many other threads, they are all lively and informative.

An issue I question from your original query however is your description of the brake operation. If as you say there is a brake module attached to the motor, the typical application of that type of device is that it is spring loaded to ENGAGE the brake when power is lost, so the wiring of it to the VFD output contact is likely a N.O. contact that closes immediately upon a Start command. This would ENERGIZE the brake coil and RELEASE the brake. Therefore when you de-energize the brake coil, it will BRAKE, not coast as you descibed above. It would be highly unusual for a brake module to operate as you have described. You should check it out very carefully to be sure before proceeding with any further strategy.

If it is a typical motor brake system as I have described, and you end up with an output contactor connected with your Estop, you could simply wire the brake coil circuit in series with a N.O. auxiliary contact off of that contactor. When the contactor opens, the brake coil would de-energize and apply the brake.

"Venditori de oleum-vipera non vigere excordis populi"

 

[rbulsara]

I am just curious, what is the risk to the drive if we trip (shunt or UV) the upstream breaker feeding a VFD drive?

If the drive is going to be damaged then, how diffreent it is from losing a power source (utiltiy outage)?

 

[jraef]

Good point rbulsara.
I for one do not see the inherent risk in opening an upstream device of any sort as an ESD. The problem usually associated with upstream devices (and the basis of the warnings about this in their manuals) has to do with using an upstream device as the normal way of starting and stopping the VFD. This leads to some incremental thermal stress of the precharge resistors and can lead to eventual failure. By definition however, ESD is not something you do on a regular basis (hopefully). I generally fall into the upstream side of this discussion, WHEN it has been otherwise decided that braking using the VFD is not the best choice.



"Venditori de oleum-vipera non vigere excordis populi"