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Bumpless transition
2

Bumpless transition

Bumpless transition

(OP)
I start a high inertia load (rotary saw- SC AC motor)) using a
VFD and after acceleration time I switch to the grid in an open transition.
(My VFD is not capable to run in parallel with the grid)
The transition frequently is not smooth.
What can I do to overcome this problem?
Thanks


RE: Bumpless transition

Use a synchronizing relay. Run the VFD up a little above grid frequency, disconnect from the VFD and let the sync-check relay control the grid tie contactor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Bumpless transition

(OP)
Thanks Waross
Sync-check relay I had used to connect busbars acting over theirs
respective generators in order to synchronize them.
The relay compared voltage levels, phase angles, frequencies, I think. Is it the same relay?
In such a case, why I need this sync-check relay?
How can I change the voltage or angle if needed?

RE: Bumpless transition

Because the instant you disconnect the VFD the motor will go out of sync with the supply.  When the relay closes you have a crapshoot of out-of-sync possibilities, that's going to happen - like death and taxes.

That's why you want to over speed slightly and have the sync check pull in the relay just as the speed falls through the power waveform.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Bumpless transition

To add to itsmoked's information.-
The motor generates a back EMF. The relay checks the phase angle between the grid and the back EMF from the motor.
Motor shafts have been broken by re-energizing a spinning motor. (And a lot have not been broken.) The torque transient may be worse if there are capacitors on the motor terminals.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Bumpless transition

(OP)
Waross and Itsmoked
You are really great !
Now it is clear why old saw design (belt driven) "accepted"
the re-energizing of a spinning motor and new (shaft driven) dont.
The safety pin breaks.
I will install CS Relay (25 ANSI)using a PT 690/115V at motor terminals and another equal PT at incoming busbar.
Is the phase angle checking enough or the voltage level comparision is also important?
Thanks again.

RE: Bumpless transition

Phase angle is most important.
The bumps come from out of phase closing. The back EMF may be lower than the line anyway. Voltage matching is important when paralleling generators to avoid a reactive current bump.
If you match your phase angles you should get a smooth transition.
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Bumpless transition

(OP)
Thanks again waross
The last question.
Why open transitions like wye/delta starters or even the
old system of helper motor for starting of high inertia loads
do not cause significative bumps?

RE: Bumpless transition

But a wye-delta does.

If you mean starting with a pony motor then starting the main motor there actually is no transition occurring. The main motor will create an inrush just like starting at 0 speed with the inrush time being much shorter.
 

RE: Bumpless transition

When a running motor is disconnected the back EMF is quite high. When a motor is spun up mechanically, the back EMF may be much lower. (Capacitors on the motor terminals will often increase the back EMF.)
There may be an energization surge or transient as Lionel mentions. This surge may be from one to several cycles rather than the starting time of several seconds. An oscillograph will show a very high current peak during the first quarter cycle of  a motor starting graph. This transient may be several time the value that is normally considered the starting surge.
The transient is so short as to be ignored by an analog ammeter, but the early electronic clamp meters would indicate this value when the peak lock was used. The value of the transient would also vary from start to start by a factor of 2 to 3 times. That is, the peak current from one start may be three times the value shown for another start. Those early meters were almost useless for motor trouble shooting.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Bumpless transition

wye/delta starters cause nasty bumps all the time. They cause lots of "I wonder why the breaker tripped?" issues.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Bumpless transition

(OP)
Waross, Itsmoked, Lionel
It is not clear for me that disconnected motor(Squirrel Cage-
500kW/690V)back EFM will provide enough power/time to magnetize
the step down measuring transformer and to load the Check Sync
Relay voltage input.
Thank You very much indeed  

RE: Bumpless transition

If the transition is not smooth as you stated in your first post, there is probably enough back EMF. You may be able to boost the back EMF with a capacitor across the relay/motor terminals.  

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Bumpless transition

(OP)
Waross
The bump was so strong that gear box was broken.
We are awaiting for the repairing be finished to continue the electrical test.
The idea is to measure the phase angle, wait for the synchronizing and close the grid CB.
As we need power to feed the Check Sync Relay (motor side step down transformer + voltage input) and the grid is still disconnected, the rotor  remanent magnetism is the key point.
Am I right? This was the reason of the question.
Thanks

RE: Bumpless transition

Yes.
How much torque do you think that it takes to break a gear box? The torque is proportional to the current squared. The back EMF may not be 100%, but if there is enough back EMF to break the gearbox there is probably enough back EMF to get a usable sync check signal.
Power the sync check relay from the grid.

Quote:

the rotor  remanent magnetism is the key point.
Yes.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Bumpless transition

Bill;  I can imagine electrolitic's concerns based on terms like "probably" verses a blown multi-thousand dollar gear box.

Can his setup be arranged to show that it works correctly before actually committing to a 'crunch'?

For example could it all work correctly with the exception of the power actually being hooked to the motor on the second connection?

I haven't thought this out really. just surmising.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Bumpless transition

Thanks for the clarification question, Keith.
My use of the word probable:
It depends! grin
I have seen apparently different rates of decay of back EMF in the field. I have seen fan applications with 30 HP motors where the shutdown procedure was to trip the circuit breakers. This left the contactor coils connected to the motor terminals and the back EMF provided enough voltage to hold in the contactors for several seconds.
On the other hand, I used to demonstrate induction generation to students with a DC motor coupled to an AC induction motor with a resistor bank connected across the motor terminals. I would overdrive the induction motor and then cut the AC to the motor and the motor would continue to generate into the load bank. I moved to another location and I was not able to duplicate the induction generation with the available equipment.
It depends. However, as I stated, if bad connection timing broke a gear box, that would indicate a lot of back EMF.
Proof of concept? I'm throwing this out for discussion and comments.
How about a lamp check? Similar to the old synchronizing lamps for paralleling generators. You will probably need transformers and the lamps should be rated for twice line voltage.
One lamp (lamp A) will be connected from L1 to T1 across the run contactor. The second lamp (lamp B) will be connected from L2 to T3 across the run contactor.
The lights should cycle on and off at the error frequency. If you have driven the motor up to 1830 RPM (61 Hz) the lamps should cycle on and off once a second.
You are in sync and good to close the breaker when lamp A is dark and lamp B is at maximum brightness.
Another check may be to set a sync check relay to light a lamp rather than closing the breaker and look at the waveforms from the utility and the motor on a dual trace scope. The wave forms should be within about 15 degrees when the sync light comes on.
Comments Keith?
Next possibility;
Resistors in series with the motor to limit the current while pulling the motor into step and then shorting the resistors.
Again for discussion.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Bumpless transition

I like your two-light method. After re-reading the OP I think you could run that test multiple times without actually engaging the direct connection.

Setup the entire system with the sync check relay etc., add the lamps and study the results, before ever allowing the direct connection.

Thanks Bill.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Bumpless transition

(OP)
Sirs
Thank you all.
When the gear box be ready ( I need the high inertia load for slow decceleration) I will check the synchronizing (disconneted
motor slightly over speeded) using a step-down transformers and a
Check Sync Relay, looking mainly for phase angle matching, in order to close the CB.(Safety simulations before the real closing.)
I hope that back EMF of disconnected motor be enough to feed my synchronizing circuit(of course, the sync relay auxiliary voltage supply will come from the grid).
I will inform you the results.
Thanks again.

RE: Bumpless transition

Thank you for the update electrolitic. Let us know how the testing goes.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Bumpless transition

You should be able to test it fine without the gearbox connected. You don't actually synchronize when the frequencies exactly match. At a slight frequency difference the phases go in and out of alignment relatively slowly and the relay should be able to catch it the first time the phases come close to lining up.

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