Encoder to restart a VSD after a fault

[SA07]

Hi We have a VSD 800 kW on a boiler fan 700 kW motor speed 742 rpm voltage 690 Vac. Sometimes when there is a network disturbance, the VSD tripped. We have program reset fault and restart for 3 times. We have put a delay in the PLC program to give the VSD enough time to restart. Most of the time the restart after a fault works. Will the installation of an encoder on the motor shaft improve the VSD restart after a fault? If after some time of operation, the encoder fails during normal operation, any idea how the VSD will react? Will it trip? VSD is Siemens Sinamics S150

 

[thebard3]

I gather that the drive is configured to stop on a loss of network communication. If your start/stop signals to the VFD are via network then that's pretty much how I think it should always be done unless there are specific differences on how things need to work on your system. Assuming that's the cause of the stoppage, why does the motor fail to restart? Are you attempting to issue a start command before communication is restored? Is there a drive fault and if so what is it? I do not understand the discussion of an encoder. Is an encoder in use or are you asking if one can be employed to improve performance?

Brad Waybright

The more you know, the more you know you don't know.

 

[waross]

We have put a delay in the PLC program to give the VSD enough time to restart.

Possibly not enough time for the residual voltage to decay.
Are you letting the fan stop or are you trying to restart "on the fly"?
Is there a natural draft that keeps the fan running?

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[thebard3]

Bill,
I was wondering about that too. Not sure what the encoder would contribute. AB drives (I'm most familiar with those) have a 'flying start' option to allow starting a motor that is already turning. Otherwise, bad things happen when the drive starts trying to ramp up from 0 hz. Waiting on more input from the OP

Brad Waybright

The more you know, the more you know you don't know.

 

[SA07]

Most of the time the VSD does not trip and sometimes restart after fault. However sometimes, it trips by mini speed.

Boiler trip by minimum speed on ID fan

The setting is less than 19 % for 2 min. (19% =9.5 Hz)
This is set in the boiler PLC which gets a feedback of the VSD ID fan.

We do not lose the communication which is done by profibus and is on UPS. We want the VSD to restart on the fly as soon as possible because everything is operating the turbine, alternator, boiler. Sometimes after a disturbance/fault on the network, we island, the VSD ID fan trips. Normally it should reset the fault and restart and make 3 attempts. But after few minutes during island mode, the VSD does not restart and the boiler, thus the whole plant trip.

We want to investigate other means to ensure that the VSD restart after a fault during normal operation.

 

[SA07]

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/raw/upload/v1611289431/tips/Eng_Tips_trip_VSD_ID_fan_66_kV_undervoltage_mug355.docx[/url]

Plz see attached part of fault recording for undervoltage on 66 kV network which has caused the VSD ID fan to trip.

 

[thebard3]

I'm still trying to understand the nature of your shutdown condition. From your enclosed graph, it appears that you have a significant voltage drop on the 'Y' phase. I do not know what unit is represented in the 'x' axis so can't determine the duration. How is the blower running speed (fault status) determined? By this, I mean how does your system determine that a fault has occurred? Is the encoder feedback used by the VFD for closed loop speed control or is it direct to the PLC (separate from VFD circuitry), or is it VFD run status, output frequency, load %, or similar via the network communication?
Is the shutdown the result of a VFD fault loss of phase, supply undervoltage, voltage unbalance, or something like that?
Following a fault condition, when the PLC attempts to restart the drive-

Is the line voltage restored to normal before the restart sequence is initiated?​

Does the PLC or VFD automatically reset a drive fault?​

Can the PLC attempt to restart the drive if it is in a fault condition or if the line voltage is still abnormal?​

Brad Waybright

The more you know, the more you know you don't know.

 

[SA07]

ID fan speed is determined by boiler operation and the PLC sends the set-point to the VSD.
x-axis is time/ s . The duration of voltage drop is about 0.1 s

Now we do not have an encoder on the motor & VSD. We are enquiring if an encoder will improve the restart on the fly on the VSD or are there other means to improve this restart after fault.

When there is an under voltage, the VSD may trip by DC bus under voltage.

The PLC does not restart the VSD. The VSD has an inbuilt function which enables it to reset itself after a fault and restart to operate the fan.

The network disturbance is usually short duration 100 ms, then the voltage becomes normal. Or often we island from the network and our turbo-alternator is supplying our plant so the voltage becomes normal.

 

[thebard3]

Now it's making more sense.
You state the VFD will attempt to restart the motor 3 times. What condition causes the restart sequence to fail? Is it a drive fault?

Brad Waybright

The more you know, the more you know you don't know.

 

[jraef]

1) If the drive is still seeing the conditions that made it trip, it will not restart. You need a deeper understanding of the faults that the drive is seeing, it records each fault in a "fault buffer" (LIFO stack), up to I think the last 8 faults, along with a time stamp and conditions at the moment of the fault. If it is tripping 3 times in a row and stops attempting, all of those fault codes will be recorded.

2) Siemens drives have Flying Restart, do you have that enabled? If not, why not? It's been too many years since I have seen a Siemens drive to know what it is called or how to access it, but it's there.

3) Adding an encoder would do nothing to help anything you have described here.

Call Siemens, get some help. Their drives are very capable, but very complicated.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[SA07]

In 2008, someone from Siemens came on site. He replaced a card, changed some settings to enable the restart after a fault. As I said, most of the time we island properly, the VSD restarts after a trip. But there are a few cases, that it does not restart.
Flying restart has been enabled.

If during normal operation, there is a disturbance and the VSD tripped. Then it will reset itself and restart. It will search for the speed of the motor and then reenergize the stator. Will an encoder not help it to search quicker the speed and restart on the fly quicker?

We have tripped several times due to mini speed. This means the motor speed has decreased to 9.5 Hz for 2 mins while the VSD was trying to restart.

If the VSD can restart before the motor speed decrease to 9.5 Hz, it will be able to restart.

 

[itsmoked]

No an encoder will not help. What the drive is doing is trying to detect the voltages in the motor stator and line up the rotor position so it can re-apply excitation. It gets hard to do this after all excitation has faded. I don't know of any field drives that will hand that over to an encoder. I have seen it only in machine tools that actually use the encoder for all commutation.

Keith Cress
kcress -

http://www.flaminsystems.com