We are using a VFD for our fan control application . We are presently using Abb make samistar drive in our fume extraction system. The drive is used for driving an induction motor of the capacity 1200kw . The motor is of 660 V rating and vvvf is used to soft start it and then raise it to a frequency of 40 Hz.
Though the application doesnt need a variable frequency drive it is used more as asoft starter.

We are facing lot of problems with this drive like frequent blowing off of GTO's and the card failures . So we are thinking of a option of converting this sytem to a dol starter type.

We are thinking in terms of the following changeover scheme.

The motor is for fan application the strating current required will be very high so we will give it a soft start using The VVVF drive. Then we will raise the frequency up to 50 Hz and will run the fan till it achieves its full speed.

Once the full speed is achieved we will switch off the VVVF drive and start the DOL breaker.( the breaker will be of 3000 A 1000 V rating with all the features of a normal Breaker.)
This breker will start as soon as the VVVf drive is switched off and the outgoing breaker is switched off.

Problems

What all problems are anticipated in this change over scheme ?

Please post your comments.

I will be very thankfull to all those who will respond.

Regards

Parish

 

Responding to Parish's Sun, Jan 19, 1:52pm query:

It may be possible to do away with switchover and use just one breaker as the DOL starter. Several of the more important questions for a qualitative analysis are:

1) What is capacity of upstream supply transformer? If unknown, then utility short-circuit MVA?

2) What are motor parameters?

3) What is driven-load in percent of motor rating?

4) What is ratio of driven-load inertia to motor-inertia?

5) What is NEMA (or equivalent metric) torque-speed curve type (eg, B, C)?

6) What is insulation type and rated temperature-rise?

7) Can fan damper, vane, or louvre, remain closed during motor runup?

Regards,
Phil Corso, PE
(Boca Raton, FL)
[[email protected]] {[email protected]}

 

Further to my earlier response listing (7) questions needed for a qualitative analysis:

8) The VVVF transfer to DOL CB method is not recommended. A hazard exists any time reduced voltage starting is used for large motors, and the starting sequence includes a period when the motor is momentarily without power. This period is referred to as open-transition. Numerous cases can be found in the A-List archive. Following is a synopsis:

During the transition time between disconnection from VVVF (with a another breaker I might add) and DOL CB closure, the motor's residual voltage could be substantially out-of-phase with the supply. Thus, the motor inrush could be twice the normal value resulting in damage to the motor.

Regards,
Phil Corso, PE
(Boca Raton, FL)
[[email protected]] {[email protected]}

 

Hi Parish,

I would assume that the motor you have is a standard squirrel cage motor and not a wound rotor. I my self would be throwing the VFD out and replaceing it with a DANFOSS VFD or Soft Starter.

But if your hands are tied and you have to keep your exisitng equipment the idea of bypassing the VFD when it is up to speed will work ok !! ..

VERY IMPORTANT to watch for is PHASE ROTATION if the phase rotation out of your VFD is differant to the incoming supply well when you go into bypass mode the motor will not be very happy with you!!!!!

I would all so think about incorperating another contactor in front of the VFD to disconnect it from the supply when the bypass contactor kicks in

I wish you luck

Matthew
[email protected]

 

It seems likely that your VVVF gets the most stress during motor starting. Using a DOL breaker to bypass the VVVF after starting may not help your situation.

I think you should find out why your VVVF is failing. These days VVVF drives should give long life. If necessary get an ABB engineer to your site to help you out.

If you still want to go with the DOL breaker idea, there are several major difficulties.
1. Using a breaker to disconnect the motor from the VVVF while the VVVF is running is not recommended and will likely damage the breaker or the VVVF. You will need to be very sure that the VVVF firing circuits have stopped before openning the breaker.
2. You must provide robust interlocking between the VVVF breaker and the DOL breaker. Applying line voltage to the VVVF output will destroy the VVVF.
3. You must make sure that the DOL breaker will not operate until the motor is up to speed.

 

Dear Parish,

a) Recently I came across article on advancement in drives which states that present drives have come up with facility of "inbuit bypass" which bypasses the VFD and runs like DOL. I will send you a text if u mail me on below address.

b) Apparently, changing from VFD to DOL shouldnot give you problem. Case would have been different if you have to take DOL to VFD (In this case VFD should have 'fly to catch' option enabled) but you should take care of following points:
1: Ensure direction when starting from DOL.
2: Disconnect ckt to output of VFD otherwise DOL output will directly go to VFD output and damage drive as your teminations for both is same i.e. motor terminals.

c) Talking about your present failures of VFD plz send me details of VFD.

Regards,
[email protected]

 

Dear Parish
I will suggest rather in different way. Use a H.T motor and H.T switchgear in combination with a vaccume contactor panel for frequent on-off. This will solve the problem parmanently.
Afterwords sell 1200KW LT motor with drive.
We are also having fume extraction system having all H.T motors ranging from 180KW to 450 KW which runs on H.T DOL system without any problem.
Ofcourse you have to see fault current/capacity of transformer for this.

Regards,
G.N. Muley

 

Dear Parish

I believe the best is to try to know the reason behind the GTO blown out which is not usual with such high rate failure devices, Sami Star also requires a maintenance after five to seven years of service depending on the operational conditions, for instance you might have to replace the pulse amplifier cards, power supplies and the tricky ribbon cables as their faults are not easy to discover, further you may need to replace the dc bus capacitors, SAMI STAR is not a super drive but it is a heavy duty one when properly maintained.

ABB can provide with such service.

Good luck
M.A.Aleem
Electric manager
Suez Cement Co.
Egypt