Siemens Midimaster 1500/4 drive starting problem 1

[sawal]

We have a Siemens Midimaster Vector 1500/4 freq. drive, 15kw, 575 vac, 3 ph. that drives (6) sets of (4) blower motors for a Tamglass furnace. the drive starts the first set of motors, and after reaching the set speed the power is transferred to the 575 vac mains. Then the drive starts the next four motors and again after reaching the set speed the 575 vac mains takes over. This sequence of starting continues up to the 6th set of motors where the drive stays on controlling these 4 motors.
Problem: Since the beginning (three years now), we have operated the furnace 24/7. Now that we only operate the furnace during the day and shut it down at night, we are noticing the starting problem. In the early morning (bet. 4 & 5 am ), as soon as we try to start the blowers through the InTouch HMI operator's panel, the F002 (overcurrent) alarm indication would appear immediately on the drive. To clear the alarm we have to reset the breaker to the drive and then the drive would start normally again. My question is what is causing the drive/motors to not to start after sitting for sometime. The motors are in good conditions.
If anyone could help, we would be very appreciative.

 

[jraef]

Arthritis?

No seriously it is not that uncommon for motors and their associated power train components to require more starting torque (which translates to current) when cold as opposed to warmed up. Bearing races are full of stiff grease, tolerances are different, motor windings may have absorbed some moisture etc. etc. etc. When new, the delta from cold to hot was probably not enough to create a problem in the VFD. As the system has aged it is entirely likely that an OC can occurr. And when you have multiple motors on one drive, the problems can be exacerbated.

I don't know anything about that particular drive, but a little torque boost on the low end might help.

"Venditori de oleum-vipera non vigere excordis populi"

 

[sawal]

Hi jraef, thank you for the reply, I appreciate it.

Do you think increasing the ACCEL time would help? It is at 11 sec. right now. I will try to give some starting boost, as you recommended.

Thanks,
sawal

 

[jraef]

ACCEL time increase may help, because acceleration requires torque, which requires energy in the form of current and time. The total amount of energy required to accelerate a load is constant. So the lower your current is, the longer your acceleration time is going to be. Conversely if you stretch out your ACCEL parameter you will lower the peak current requirement. However this is load dependant, i.e. some loads are best accelerated as quickly as possible, so be careful. Blowers are usually not a problem however, unless they need to be at speed quickly to facilitate lubrication.

"Venditori de oleum-vipera non vigere excordis populi"

 

[DickDV]

Starting multiple motors on a single VFD is a tricky business due to the unusually low impedance the drive sees when initially energizing the motors.

Changing accel time rarely helps because that implies that the motors actually got started. Usually, the trip occurs before the motors ever get a chance to move.

Increasing the starting torque boost will likely make the problem worse since that is normally accomplished by increasing the starting voltage to the motors which will cause even higher initial currents. I have solved the starting fault in a few cases by reducing the starting torque boost to just below the trip point on the drive.

Since this is a vector drive, I would think that the drive processor is going to control the starting surge current based on its own algorithms rather than on a parameter setting, but, I'm not too familiar with the Siemens drive so it may or may not.

For me, the most likely solution is to reduce the drive max current parameter to a level that represents normal full load current of the four motors at full speed. Since these are on fans, that should not present an operational problem. This way, the drive automatically limits its output to whatever setting is entered, pretty much without regard to the impedance it sees on the motor leads. An S curve accel ramp could possible help in getting the rotors to break into motion before the ramp starts up.

The underlying problem here is that, most likely, the drive is undersized for the application. For multiple motors, I use the rule of adding up the nameplate full load amps and adding 10% for each additional motor over one. So, in this case, the total of the motor nameplate amps plus 30% should be the drive continuous current rating. I have never had a starting fault problem using this rule.

Unfortunately, simply adding up motor hp or kw and selecting a drive that equals that hp or kw will lead to these kinds of problems in many cases. It looks like, in this case, the drive was just large enough for new motors but, as age set in, it had nothing extra to cover the tougher starts for the reasons mentioned in an earlier post.

If all else fails, I have heard of applications where a 5% motor lead reactor was added to increase the initial impedance the drive sees. I have never done this but it seems like a more economical solution that replacing the drive.

Good luck!

 

[Barry1961]

I will agree with DickDV about the low voltage boost probably causing the problem to get worse. Low voltage boost does give you more starting torque but uses more current. I think that the boost you get is very inefficient due to the V/F ratio being so skewed. They may have improved the low voltage boost since I messed with it.

I have been involved with some multiple blower/fan applications where due to a induced draft the fans will "free wheel" in the direction opposite of the normal rotation. This of course causes a major start up problem.

Barry1961

 

[ozmosis]

sawal
I would check parameter P077. This is the 'Control Mode' and the default is set up for a control method called FCC (Flux Current Control), a sort of basic 'vector control' strategy. FCC is not suitable, nor is setting P077=3 (vector Control) when running multi-motors. The drive will automatically check the stator resistance of the connected motor but in the case of multi-motors this can cause a wrong reading as you have various stators connected and will lead to an incorrect resistance reading seen in P089. I would advise set P077=0, set the Autocalibration P088=0 (off). Doing this just provides a linear volt/freq to the motor rather than thinking it is being clever and adjusting the volts/hz to what it thinks is the optimum performance of the 'motor' when in reality it is 4 motors all with different characteristics.
This may or may not help. If they've been ok for the past three years then it could be something else, maybe an insulation issue with the motors, especially as the supply volts is 575, this can cause premature problems with certain motors that initially might not be detected by normal motor testing but could be picked up by the VSD due to their sensitive monitoring.

 

[sawal]

Thank you for all the excellent advice. You all deserve a star.
Apparently, after coming to the plant for several nights to check on the situation, I thought I have narrowed down the cause of this “mysterious” problem. After shutting down the furnace cooling fans, the frequency drive indicator would still show the running frequency and I could measure 384 vac at the drive’s output terminals, which I thought should be both “0.0”, null. I replaced the drive with a new unit but for some reasons I could not download the parameters that I uploaded from the original drive. The error message says that the versions of the device configured and the device identified do not match. Together with Siemens Tech Support, we tried to match the versions to no avail.
Finally, they suggested to enter the (100 +) parameters manually and get their latest DRIVEMON v5.3 upload/download software. We have v5.2 which according to Siemens would not matter much. I installed back the original drive with the same problem until I have a definite solution to the problem because we need the furnace tonight.

If there is anyone out there who is familiar with Siemens drives and softwares, please we need your inputs. Thought that uploading and downloading of parameters should be a very simple task.

Thank you so much!

 

[Marke]

Hello sawal

I have experienced a similar problem that was not related to the number of motors etc, but was an issue of condensation. Try meggering the motor circuits befor you apply a start signal and you may find a low reading.
What I found some time ago, was that over night there was condensation in an isolator and when we first applied power, there was enough energy to vapourize the moisture and becuase the energy was limited by the drive, no permanet damage occured. On the second start attempt, the moisture had gone and there was no problem. A megger test after a successful start showed no problem as did a megger test after the trip occured, however a megger test befor the initial start showed up the leakage.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[sawal]

This particular problem has been solved. The PLC program has been modified to include an OFF2 command which is basically giving a 100 ms time delay before disconnecting the motors from the drive. Everyting was OK, except for this additional time delay which reduced or eliminated the interference caused by the disconnection of the motors from the drive.

Thanks to all of you who replied!!!