ptonsparky
Tom
- Occupation
- EC - retired
The overload protection should open the Run command to the drive.
Single VFD controlling two motors
- Thread starter ChefTurnedSparky
- Start date
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
Because the aux contact from the ol is wired into the VFD enable. It results in both motors shutting off if either motor overloads, which is one of many reasons I do not like it.
If you use a mpcb, conceivably you could not wire the aux contact to the drive as the mpcb itself will open
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Not all VFDs have protection for output phase loss, so there is no guarantee that the drive would protect the motor from single phasing. But also, the act of blowing a fuse involves drawing an arc across the fuse element parts until that arc is quenched. Arcing on the output of a VFD runs a risk of damaging the transistors from dV/dt (rapid increase in voltage). While this is not as critical in modern drives as it once was, there's also no guarantee that any particular drive has what it takes to prevent this damage, making it a bad practice. It's kind of like saying it's OK to run your car engine with the oil a quart low for a while; some cars will be OK with that, some will not, so it's a good idea not to do it..
The overload are usually wired in series with the stop circuit so overload trip turns off the electronics. Shutting off a motor disconnect while a VFD is running can damage the drive. Had a contractor use the outdoor 480 volt non fused safety switches to turn four 75 HP cooling tower fans on & off rather then walk down to the drives in third basement level ( over 75 steps climb ) while performing maintenance. First visit he did no damage but next trip he blow up a board on one drive and something else on another drive. The Danfoss drive tech said some times you can disconnect a running motor maybe 8 or even ten times before luck runs out. Should be a NEC article to demand that all disconnects on output side of drives have a Normally Closed switch that opens before the motor leads. Years ago I came across them on a lot of drive safety switches but not one in last ten years or so.
I always ordered VFD'S with a built in disconnect and made sure they had fuses on input side. Had an input fuse blow while drive was running but never did any damage to the drive. At my hospital had the B phase blow on an Critical AHU return fan. They used the A & C phase to power control transformer to control board. Fan was only running maybe at 60% of full speed. They would not allow me to turn power off for a minute to replace the fuse until around 4 AM the next morning. I went into the parameters and reset the maximum speed to think 60% until I replaced the fuse. The two supply fans provided enough air to not set off any alarms. A three phase 480 volt drive normally produce around 660 volts DC . Of course a VFD with a blown fuse would never be able to run any where near full speed.
alan_505
Member
- Location
- Melbourne Australia
- Occupation
- Electrician
I did a Commissioning job once where there were 23 motors for long travel on a ship loader with 1 VSD (500A), they had it configured that if more than 2 O/L's tripped they would stop the VSD any less and it would keep going, They also had 2 motors for the boom conveyor and they each had a single 500A VSD controlling each motor, so I think rather than having 23 VSD's for the long travel motors they went with the same size VSD that they were using on the boom conveyor motors to save on spare parts and panel size. Each long travel motor had their own O/L and the fault bits must have gone to the PLC inputs to be able to make the decision when to stop.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
We are talking about fuses on the output, not input. Fuses on the input are not a problem.
Sora Komi
Member
- Location
- Charlotte, N.C.
- Occupation
- Facilities Controls Technician
After reading all of this, I have a question about wire protection. We have one drive with an output current rated at 55 amps. This drive feeds two identical motors, each with an 18.1 FLA rating. If I were to run wire rated for 55 amps from the drive to a power distribution block, and then continue this 55 amp rated wire to the line side of two separate motor overload devices, I think that would be ok. BUT, after I leave the overload devices, I have to drop the wire size down to #10 to the motors. Now, if I installed fuse blocks with 25 amp fuses, that would protect the wiring, but I think that most people believe that installing fuses anywhere on a VFD output is not a good idea. So, can I just set the overload protection on the device to protect the motor, and also use that setting to provide over current protection for the wiring to the motors as well?
MTW
Senior Member
- Location
- SE Michigan
From my understanding fuses are not a good choice
a better choice would be, "MPCB (Motor Protection Circuit Breaker, also known as an IEC Manual Motor Starter) and that is a good choice for use on multi-motor VFD applications, because it ALSO provides the necessary Short Circuit protection for the motor, as well as a way to disconnect one motor without shutting them all down (if necessary). I don’t know if anyone else is offering that as of yet."
You can read more at a recent thread here. The VFD should be set to V/H mode and the overload NC contacts wired back to the drive enable circuit.
https://forums.mikeholt.com/threads...approved-for-the-use-on-a-vfd-output.2585996/
a better choice would be, "MPCB (Motor Protection Circuit Breaker, also known as an IEC Manual Motor Starter) and that is a good choice for use on multi-motor VFD applications, because it ALSO provides the necessary Short Circuit protection for the motor, as well as a way to disconnect one motor without shutting them all down (if necessary). I don’t know if anyone else is offering that as of yet."
You can read more at a recent thread here. The VFD should be set to V/H mode and the overload NC contacts wired back to the drive enable circuit.
https://forums.mikeholt.com/threads...approved-for-the-use-on-a-vfd-output.2585996/
Last edited:
Macbeth
Member
- Location
- Livonia NY
- Occupation
- Automation
Here is an example of a newer VFD, with disconnect detection.
The ABB 580 series, along with the ACS380 and ACH180, has a motor disconnect detection feature that enables the drive to detect when an output disconnect is opened. The 580 series is made up of the ACH580, ACQ580 and ACS580. The use of this feature eliminates the need for a disconnect to have an auxiliary contact wired back to the drive.
The motor disconnect detection configuration is found in Group 95. Parameter 95.26 is only one step to configure this feature in the drive.
Set parameter 95.26 to be [1] Enable.
This obviously only works for singe motor or when all motor disconnects are open..
The ABB 580 series, along with the ACS380 and ACH180, has a motor disconnect detection feature that enables the drive to detect when an output disconnect is opened. The 580 series is made up of the ACH580, ACQ580 and ACS580. The use of this feature eliminates the need for a disconnect to have an auxiliary contact wired back to the drive.
The motor disconnect detection configuration is found in Group 95. Parameter 95.26 is only one step to configure this feature in the drive.
Set parameter 95.26 to be [1] Enable.
This obviously only works for singe motor or when all motor disconnects are open..
Thanks . Only worked with older model ABB drives ( maybe ACH 500 'S ) in 6 & 18 pulse styles. All drives 40 HP & larger were on 18 pulse drives. While troubleshooting drives I have disconnected the motor leads. If the parameter 95.26 was on enable would you have to temporarily disable it while having motor leads disconnected. While troubleshooting a drive that tripped out on over current if the motor was directly bolted to a pump and had no external fan to turn to see if motor or pump was seezed up after reading motor winding resistance and megger ding would disconnect the motor leads and run the drive in hand from 25% to full speed.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Drives still don't play well with load (even partial load) being switched on or off on the output side of the drive. Best practice AFAIK is melting alloy type overload units and have them interrupt a control input to the drive if they trip. This will of course shut down all drive output. If the process can not afford to have one motor end up taking down all the output then you really need multiple drives or at least smaller groupings of common drives.