Variable Frequency Drives- Wiring diagrams
Variable Frequency Drives- Wiring diagrams
(OP)
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.
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
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.
Bill
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"Why not the best?"
Jimmy Carter
RE: Variable Frequency Drives- Wiring diagrams
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
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Variable Frequency Drives- Wiring diagrams
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If we learn from our mistakes I'm getting a great education!
RE: Variable Frequency Drives- Wiring diagrams
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
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
RE: Variable Frequency Drives- Wiring diagrams
"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: Eng-Tips.com Forum Policies
RE: Variable Frequency Drives- Wiring diagrams
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
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
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.