Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.


Variable Frequency Drives- Wiring diagrams

Variable Frequency Drives- Wiring diagrams

I have a problem with  selecting between available variable speed dive wiring diagrams especially the location of the main contactor, as indicated below,

1) No contactor- drive is between the feeder MCCB and the motor. This is the simplest.

2) After the feeder MCCB before the VFD- Has the advantage of diver being isolated when not in use but the diver capacitors are subjected to frequent charging discharging cycles. How serious is this issue is not clear.

3) After the VFD before the motor. Do not see any advantage other than isolation of the motro when the motor is at rest which can also be achieved  using the feeder MCCB.

I come across all these configurations, but unable to decide which is the best. My application is pumping of water and the start stop cycle is  about  8 hours. I appriciate if you can comment on this based on your experence/ knowledge.

RE: Variable Frequency Drives- Wiring diagrams

Capacitors are charging discharging six times a cycle when the drive is energized.
The contactor ahead of the drive is the safest.
Our drive gurus will report any possible issues with the contactor between the drive and the motor.

"Why not the best?"
Jimmy Carter

RE: Variable Frequency Drives- Wiring diagrams

Some VFDs do not like driving into an open circuit.  You need to talk to specific vendors if their drive is suitable or not.
My preference is to put the contactor at the input to the VFD and an interlocked isolator adjacent to the motor.

RE: Variable Frequency Drives- Wiring diagrams

Why do you feel the need for a contactor at all? If it has to be there, how would you intend on using it?


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Forum Policies  

RE: Variable Frequency Drives- Wiring diagrams

If the contactor is there to provide an emergency stop then use a no-volt release or a shunt trip into the MCCB. Some drives have a 'secure disable' function (can't remember whose trademark that is...) which is a high inegrity means of killing the drive output in response to a safety input. Some are qualified to the EN954 Cat 4 machine safety standard which is not easily met. Personally I'd choose the NVR in the breaker to kill the drive in an emergency because it will fail safe and will quickly clear a downstream fault regardless of what it is, but it would be foolish to disregard the effort the drives guys have put into meeting some tough requirements.

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

RE: Variable Frequency Drives- Wiring diagrams

If you put the contactor between the VFD and Motor then you will potentially have two issues:
1) If the VFD is running and you open the contactor then most VFD's will register this as an error. An error may indicate that you have 'lost the motor' or it is indicating 'no-load' or something. The VFD may alarm or even trip; either way it is not a good thing to happen.
2) If the VFD is already running and the contactor is open (assuming it hasn't tripped), then closing a contactor into a VFD that 'thinks' it is running a motor will either cause the VFD to go into current limit (the motor will in effect be looking to start DOL or it will trip the drive on overcurrent). Worst case scenario is that there could be severe damage to the VFD due to the nature of high voltage transients when a contactor is closing. This could destroy certain VFD's.

All in all, not recommended for a contactor between the VFD and Motor unless you have some aux contact into a 'coast stop' on the VFD or other means to bring the VFD to an immediate stop if the contactor is opened and (preferably) a timer if bringing the run command after closing.

Some VFD's have built in isolators in the drive itself. They will isolate power to the VFd and can be locked off if security of maintenance is required.


RE: Variable Frequency Drives- Wiring diagrams

As for the line side, you have to make sure the VFD powers down before re-applying power. It's not a good thing to de-energize the line power input and then re-energize it while the VFD still has the pre-charge circuit closed. This will create a large current surge that can damage parts.

If you "must" do it on the line side then use it as an emergency stop only or use a timer to ensure the drive has been powered off for a suitable period of time before re-closing.


RE: Variable Frequency Drives- Wiring diagrams

In respone to LoinelHutz, we have had this same exact problem he is describing. We were using contactors on the line side of our drives as part of a machine guard type safety circuit. Basically there were safety gates installed on our new line, through normal operations the operators would open the gate, dropping out the contactors on numberous drives. The operator could open and close the gates in a second or two if he wanted. Our drives were dropping like flies on our new line. We had the drive rep come out and scope everything.He determined that we need to put a timer on the contactors for say 10seconds or so, to eliminate this inrush current on the percharge circuit. We also installed line reactors between the drive and the contactor. We haven't had a failure since.

RE: Variable Frequency Drives- Wiring diagrams

What you probably should do is properly design your safety circuit to require a re-start if anyone opens the door! The fact that they can open and close it so fast and have an automatic restart is inherently dangerous to my mind. It's an accident waiting to happen.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Forum Policies  

RE: Variable Frequency Drives- Wiring diagrams

The best way I have seen is the contactor on the output of the drive that is emergency stop enabled.  The same enable signal is given to the drive for enable signal.  

So during emergency stop the enable is taken from the drive first, then at same time the contactor is opened.

During normal stop this same  contactor is opened after a off delay.  The off delay signal comes from the PLC estop enable.  The off delay time is typical of the ramp down of the drive and a little more.

When the start button is pushed the contactor is pulled in and the vfd drive is turned on.


RE: Variable Frequency Drives- Wiring diagrams

I left out alot of details to make the point about the electrical problem. These gates are designed with a solenoid that prohibits you to open the gate if all of the interlocks aren't in order. You then have to press a pushbutton to unlock and to relock the gates, again providing all of the interlocks are in order. These are specially designed 2 channel safety switches wired into safety relays.

RE: Variable Frequency Drives- Wiring diagrams

I would put the contactor between the VFD and the motor with an auxilliary contact which starts the VFD when the contactor closes.
If it was a gate safety circuit I would have a key operated safety lock on the gate which would do two operations: a)open the VFD contactor instantly and b) initiate a delay timer wich would time out before the gate could be opened. This would allow sufficient time for the machine to run down to a stop before entry.
To take it a stage further I would install two contactors between the VFD and the motor (contacts in series, coils in parallel). The contactors could then be monitored by a safety relay and if one contactor should stick or "weld" closed, the machine could not be restarted after a stop command because the safety relay would not reset.    

RE: Variable Frequency Drives- Wiring diagrams

Geeze, I don't see the point in nit-picking the details of cjhut's example. Basically, he posted that he ran into a case where using a contactor to switch the input power to the VFD on and off too quickly caused a field issue, re-enforcing the possible issue I described. Isn't it enough just to know that allowing a circuit the ability to quickly switch the power on and off isn't a good idea?

The OP has provided enough detail to know why he's looking to install this conactor so going into details about how to do a safety circuit is pointless as it doesn't address the posted question at all.

RE: Variable Frequency Drives- Wiring diagrams


Thank you for all the voluble contributions made to this thread.

"Why do you feel the need for a contactor at all?" – I really do not have an answer other than that I have seen it as a practice.

Deep discharging of the capacitors seems to be the issue, as high inrush current is generated every time the drive is switched on.

If the contactor between line and the drive is for the emergency stop, better way would be to use "use a no-volt release or a shunt trip" as suggested by ScottyUK. This will save space, cost and additional heat dissipated in the contactor coil.

If the contactor between the drive and the motor for emergency stop, there may be an advantage as complete isolation can be achieved immediately without waiting for the drive capacitors to discharge. But the drive rep claims there is a logic input to achieve the same.

However for my application (pumping water) I do not see any advantage in having a contactor, if I can use shunt trip to disconnect power under fault.


Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close