Connecting electronic VSDs (slightly OT)

G'day

Once again I have been thinking how I do my designs and how to improve.

Generally when designing VSD (Variable Speed Drives - electronic type) circuits I will take a feed from the 3 phase bus, run it through a motor circuit breaker (thermal and magnetic type - this way I can avoid a thermal overload relay), then through a contactor and then to the VSD. Post VSD I would run it straight to the motor, often via a field isolator depending on other design factors.

Due to safety requirements, the 3 phase bus is normally supplied via double contactors wired to the safety circuitry.

Recently, I have seen some designs that do away with the motor contactor supplying the VSD, and connect straight to the circuit breaker. This seems a good idea to me, since under most conditions the extra contactor seems to be redundant. If the safty circuit is tripped, the double contactors up-stream of the bus will take care of it.

What are your thoughts?
Would this be a better design method?
How do you guys normally design these circuits?

Thanks in advance

Doug

U need to be careful just cutting off the power of a VSD.
U need to consider the inertia of the load, ie In an application with a huge band saw at a timber mill if u were to just cut the power to the VSD the saw would free wheel to a stop. This could take half am hour due to the inertia of the load. This is totally unacceptable ina safety situation. The VSD can pull up the load much quicker through DC injection etc.......Hence you get saftey relays with a timing funciton. Upon an Emergency stop situation arising this allows the VSD to stop the drive quickly, the power is then removed from the VSD. Therefore the contactor would still be required (for cat 3/4 two of them...)

Remember, when deisgning and constructing the machine, for saftey purposes for operators and to cover your own arse in court (in case god forbid that some should get injured), u must document document document the Risk Assesment, Identified Risks, What and whay did u choose the equipment saftey equipment etc....

If an accident happened on a machine u deisgned and workcover got involved, the posistion of Workcover cover is that u are guilty of malpractice/negligence in the first instance...its up to u after that to then prove you innocence. Thats just the work cover side. If a death was involved then the Civil Court Follows.

In Victoria (Australia) there is a bill deemed the industrial manslaughter bill. In NSW the unions are trying to get this passed as well. Basically the Owners of the company down to the direct superviors are held responsible, and the fine vary accordingly from $100,000's to jail sentences.

All i am trying to emphaise is Don't sacrifice safety for just a few bucks in savings....!! Ultimately it your decision.

Have u done a risk assesment ?
Sometimes its good to have some else look at the mechanical design / dangers who doesn't know the machine. The might help u see the forest thru the trees so to speak... Drop me an email if u need some local support and i'll see what i can arrange....

I normally use a main circuit breaker, wire directly to the VFD (VSD) and then to the motor. This provides branch circuit protection, and allows disconnecting power to service the VFD.

I don't see what function having a contactor between the VFD and the breaker serves. It certainly won't eliminate the coasting to a stop if the VFD's dynamic breaking isn't working, and I fail to see how cutting off power with a contactor is better in that regard than opening the breaker.

It should come as no surprise that safety issues are not simple and require a thinking person's analysis before doing a project. For example, protection against shock hazard in a drive motor system is often at odds with the need to quickly stop a high-inertia machine.

In general, in my experience, VSD's are connected to the power supply thru a fused or circuit breaker disconnect and the motor leads are continuous between the drive and the motor. This is usually considered acceptable with e-stop and safety functions in the drive control circuits thru either Run Enable or External Fault loops. The benefit with this is that the inverter can be used to quickly stop the machine. The disadvantage is that, with the inverter stopped, there can be enough leakage current thru the output transistors to present a high voltage on the motor leads even tho the motor shows no sign of being energized. This can represent a serious hazard to anyone coming into contact with the motor leads. You have a similar situation with drive-motor systems that are programmed to operate the motor at zero speed. The motor appears to be "off" but, in fact, is energized.

The US auto industry has addressed this issue by requiring a break contactor in the motor leads so leakage current cannot flow. This has been their standard until recently when they have moved to permit Run Enable loops in the drive without a contactor as long as the terminal assignment for the Run Enable loop is not defeatable in the drive software. Some drive manufacturers have moved to modify their control inputs to accommodate this new standard.

In my view, there is no useful purpose in placing overload blocks or safety contactors in the supply side of a drive. The drive overload protections plus the input fusing offer better, faster protection that any overload block. An input safety contactor seems like a good idea until you consider that the drive likely has 2-3 minutes of lethal energy stored up in its DC bus which continues to exist after the input contactor has supposedly brought the sytem to a de-energized condition.

My point here is that the system hazards in each application need to be identified and addressed individually. Across-the-board solutions, as usual, are convenient and often ineffective. At worst, they create an illusion of safety which is worse than an obvious hazard, in my view.

Thanks for your replys.

To Sleepy Wambat:
Normally I design the system to Cat 3 or 4, and I will usually provide a group safety circuit with large double contactors feeding the rest of the circuit. This will then feed multiple VSDs and DOLs. Often I will use a timing safety relay with a 1 second timer to give the VSDs time to stop before the contactors are released.

What I (used to) do was to add an additional contactor after this safety circuit and before the VSD. The reason I did this was because the engineer at the first company I used to work for did it this way. I continue to do this because all my colleuges also design these VSDs this way.
Periodically though, I question whether I am doing things the best way, hence my question.
(I am also following the goings on in the Australian liability scene, scary isn't it.)

Tom Jenking:
I see you have been thinking along the same lines as me, only you got there first.

DickDV:
Thanks, you actually answered another question I had ages ago on a design that we modified. It was some SEW servo drives with the safety contactor wired between the motor and VSD. What would happen is the safety circuit would trip, the contactor would open and go "BANG", and the motor would stop. The contactor would have to be replaced before we could re-start the circuit. A timer allowing the motor to stop before opening the contactor would have worked but, since we already had a workable method, we weren't going to experiment.
You also indirectly brought up another problem. We are mandated by our local official government industrial safety organisation (Worksafe WA) to have padlockable safety isolators on all equipment. These generally interrupt the power flow just befor the motor. All our people (and contractors) have personal padlocks to lock these isolators. If any work is required on a motor, it is pad locked in the 'OFF' position. There then becomes no way the motor can start.
The new problem we may get is that changes in the rules now mean that we can isolate from the board using the circuit breaker. This would be done by mounting the CB on the door of the cabinet with the 'switch' part external to the board. This circuit breaker must also be pad lockable. This is OK for a DOL starter, but as you have pointed out, a VSD can store a fair amount of energy in it's DC bus. We will have to take this into consideration when planning our new designs.

Thanks

Doug