Motor Contactors and Unbalanced Loads

<...will an inverter fried (spoil) if the motor contactor connected at the drive output is found unbalance between phase? I recently found some of my motor contactor has poor contact and the voltage unbalance ia more than 5% between phase....>

View the incoming power as raw stuff that the inverter processes and sends to the motor. The inverter will only respond to power imbalances if the load current exceeds its overload setting, which should be the same as the full load current stated on the motor label.

"I recently found some of my motor contactor has poor contact and the voltage unbalance is more than 5% between phase ..."

I don't think it's very likely that your 5% difference is down to bad connections at the contactor. With that kind of voltage drop, your contactor would fry.

Quite.

The imbalance is possibly to be the result of a heavy single-phase load elsewhere on the network. Habitually, electricians start connecting single phase loads on the dis board at the top-left, and keep going. Urban myth has it that the red phase is more heavily loaded in many cases, because in nine dis boards out of ten, the red phase is at the top of the fuse stack.

The fact that you are using the contactor on the output of the inverter and suffering problems with the contacts and inverter IGBT failure seems to show that this is incompatible in your design.

Either the contacts are not rated for the load disconnect and contact deterioration leads to bad contacts which then cause IGBT problems seems most likely, or the contactors / materials are not good enough and you need to use better quality.

What frequency are you opening the contactor at, if its near zero you may have a seup pushing current and at low Hz, not a good combination.

Since inverters form the PWM from fixed DC voltage in the link, I cannot see why you should have any imbalance in voltage, is this measured at inverter output terminals?

A quick solution is to use the inverter inhibit input (freewheel stop) to switch off all IGBT before opening your contactor, this would be easy using a slave relay to get your timing. The IGBTs should all be OFF well before the contactor breaks, particularly if its an AC coil (slower).

If the contactor is correctly sized then this removes all problems and should give 100% reliability.

The contactor is only breaking the motor current and I've never experienced much difficulty with this over many years, however, remember it's rated to break AC and at low Hz the current is nearly DC. Still, if you sequence this correctly as above, you should have no problems.

Good points - if this is what OP is doing (breaking motor circuit after VFD). But, if this is the case (and it reads like it from the question), I'd go a step further, and say "don't do it!" - put the contactor on the VFD input. With a decent design, I don't believe there should never be any need to break the motor circuit directly (but I'm ready to be corrected, given a good reason).

This is where we all hang around to see whether OP is going to give us any more, or maybe even acknowledge our replies.

Quite often you use an output contactor in mechanical handling and safety applications for motor isolation purposes since the frequency of using this is too high for the input circuit causing tripping and failure of the inrush circuit. This tends not to be so important on process applications (fans, pumps, mixers etc) as when they're stopped (OFF) they stay in this state for a while.

If you re-establish power before the inverter has even powered down when using input contactor, you can defeat the bypass causing higher input currents (stressing the rectifier) or overload the inrush resistor thermally (its only low duty rated) causing restor burnout (open circuit at switch on)

Safety applications are aided by use of a dedicated and approved safety enable input normally replacing one contactor but this is only available on certain inverters.

Also, different makes of inverter have differing robustness depending on the problems caused by output switching.

In general, frequent switching leads to using an output contactor but much depends on the timing rather than the nr/hour for instance.

Operating a motor contactor connected between the variable speed drive (or variable frequency drive, depending on what country your in) output and motor while the variable speed drive is running can cause serious damage to the variable speed drive. As such I wouldn't ever recommend it (and nor would any variable speed drive manufacturer I am aware of).

The variable speed drives are fitted with start and stop inputs and also software-programmable ramp or braking functions (meaning you almost never need contactors to switch the variable speed drive on and off). HOWEVER, if you need to use series-connected contactors for additional safety (as dictated by some requirement or standard), or automatic or remotely-controlled isolaton (for motor maintenance purposes), then place them at the variable speed drive power INPUT side. The documentation supplied with the drive (or available on the drive manufacturers website) should clearly indicate this.

As for your damaged IGBTs, it is possible that the motor contactor has caused this, although it could also be a power quality issue (in which case you should be using reactors to protect the variable speed drive) or even an environmental issue (such as vibration). Without more information it's easy to guess but hard to tell...... BUT, if you are running the variable speed drive continuously and using the motor contactor to start and stop the motor (which I have seen people do, and which you are NOT supposed to do, read the manual people that's what the start and stop inputs are for), then that is almost certainly what is killing your variable speed drives.

Can you supply us some more information on how you are operating and controlling your variable speed drives please?

Thanks for all the good info.. to further elaborate on the VSD operation and control mode:

In general, this installation is originally from OEM. An MCB is protecting the VSD and the VSD output is connected to a contactor with overload to the motor.

At time for operation, contactor will activate when the motor is selected via DCS. then VSD will activate when is motor put to run mode.

I have found this contactor has a poor contact point (measuring contactor's input and output resistance, i got some read almost in Mega ohm on some particular phase) Will this cause VSD damage? or it just gave effact to motor?

Any intermittent connection between the VFD and the motor will almost certainly have a detrimental effect on the VFD. This would be the case if your poor contact was arcing, allowing momentary current flow. It will also affect motor performance.

I cannot agree with your view re output switching as since the late 80s when IGBT replaced bipolar I have run thousands of applications with motor contactor on the output as use on the input WILL definitely damage inverters if used other than infrequently. There is normally a limit put upon the number of times you can power up varying from one to ten times per hour, typically. This, however, is only a rough guide as the timing and nr in a small time is more important. Some larger inverters may take up to 30s to power down which makes it hopeless in a guarding application, for example.

I would be happy to advise how to achieve this reliably if you have poor experience of this arangement in your applications to give a good result with the asociated benefits of doing so.

As you say, some manufacturers don't recommend this as their design is not robust enough to withstand the arcing but many also accept this and I have many years of experience to support this. I have applications where output contactors are operated on a regular basis in the car, drinks, construction and machinery industries with no problems but I would also advise to design in avoiding contactor wear and associated problems anyway where possible.

However, in the context of this design it causes problems so it should definitely be sequenced with the electronics and there will be no issues if done correctly.

From post #7 I think you misread what I said as I clearly indicated that the start and stop inputs built into the variable speed drive are there for that purpose (ie-regular starting and stopping of the drive). I also indicated the instances where you would want to isolate the drive (infrequent operation as apposed to normal regular drive control) using a contactor or isolator, and regular start/stop control was not one of them (for the reasons you have stated).

The variable speed drives are fitted with start and stop inputs and also software-programmable ramp or braking functions (meaning you almost never need contactors to switch the variable speed drive on and off). HOWEVER, if you need to use series-connected contactors for additional safety (as dictated by some requirement or standard), or automatic or remotely-controlled isolaton (for motor maintenance purposes), then place them at the variable speed drive power INPUT side

From your post #10 you say

Some larger inverters may take up to 30s to power down which makes it hopeless in a guarding application

As I said above, the variable speed drive is fitted with ramp and brake functions for quick stopping. The output and control circuitry of variable speed drives can still stay powered from their internal capacitors for a prolonged period of time after the input supply is removed if the motor is stopped, BUT if the motor is running the capacitors are discharged very, very quickly stopping the drive.

I stand by my statement which is supported by all of the variable speed drive manufacturer literature I have seen (both in America and Europe) HOWEVER, your application experience sounds different so maby you are working with different site standards and practices (such as to the safety standard EN 60204-1 which I have not had much experience in) which may require the variable speed drive output to be switched directly to comply with a Cat3 or Cat4 safety level.

So, two sides which differ but neither is necessarily wrong (depending on the application).

I think I understood your main point and can appreciate your concerns as this is often voiced to me from various designers, however, it is really a concern of the quite distant past and motor isolation of a running output is actually mentioned in some common inverter instruction manuals but often hitorically 'ignored'.

Apart from experience (I work for a supplier of inverters and have done so for 25 years+), I have had all such information confirmed at a senior engineering level at a major Japanese manufacturer. I have, of course, large experience in many factories (especially bottling canning and the auto industry) of this application that support this.

However, this doesn't cover all eventualities and sequencing avoids situations which may cause IGBT problems or associated high noise getting back to ICs causing aberrant firing and so failures, for example, arcing from bad connections etc.

Still, frequent isolation requirements are better done at the output than the input and repeated failures normally means a bad control design or defective components, not normally the central principal.

I believe that the requirements of EN 60204-1 are met by removing the primary power source - I don't think there's a distinction between input and output of a motor drive (provided that it can be shown that the drive system is not capable of initiating or sustaining motion when primary power is disconnected).

The tricky bit comes in with VFDs - how can anyone guarantee that a contactor on the output side is not switching (near)DC currents? Getting a contactor rated to break DC at any substantial current is a big problem.

May I know where can I find the detail elabotation of standard EN-60204-1 ?

(I've tried to do web search but to no avail to get a satisfied understanding of the standard)

You can get it from BSI - but unfortunately, you have to pay. In the UK, standards can be viewed in public libraries.

Thank you for the info