VFD and motor contribution to lineside faults 6

[veritas]

Hi

Not dealt with VFD's very often so need some advice please. I have read that a 4-quadrant VFD allows operation of the motor in all 4 quadrants, i.e. rotation and torque in the same direction as well as rotation and torque in opposite directions. When torque and rotation are opposite to each other then motor is breaking and acts as a generator feeding current back to the VFD. Hope this is correct.

Does this apply when there is a line side fault? Does the motor see the dip due to the fault as a "brake" and thus pump current back to the system? Is this how a motor controlled by a VFD drive can provide motor contribution to lineside fault cuurent? For a typical induction motor, how much contribution (as seen on the lineside of the VFD) can one typically expect from the motor via the VFD for a lineside fault?

Thanks.

 

[davidbeach]

Pre-VFD the elevators were almost always DC driven from an MG set. If the overall elevator system was regenerating the MG set became a GM set. The need to dump power somewhere when running elevators on standby systems has been around for a very long time. Buildings large enough to need to run elevators on standby power are also large enough to have significant amounts of air handling equipment, perhaps including elevator shaft pressurization fans. Not that hard to come up with the ballasting load to keep the elevators from motoring the generators. One of the few ways that hydraulic elevators are far superior to traction elevators, but hydraulic don't often go more than 5-6 floors.

 

[Skogsgurra]

Huge mining trucks have the same problem, "squared". They don't know where to put their excess energy when going downhill with hundreds of tons of ore on them, some up to 500 tons, plus dead weight about the same. The ones I've seen used braking resistors (with high temperature alarms) if electric. God knows where the energy went in all-mechanical trucks. Oversized and water cooled disk brakes?
Some dynos used "evaporation brakes" with brake drum filled with water that produced lots of steam when testing truck engines.
Why not regeneration? Because some of the test facilities outgrew the supplying grid that couldn't absorb the peaks. It simply wasn't worth the money to increase capacity for the few hours of testing at peak loads. Base load in some of these places were only a few percent of peak braking power.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[LionelHutz]

Slow the lifts down. Decelerate them slower. Make sure they are sequenced so that there is not multiple units hitting peak regen at once.

 

[waross]

I would Keep It Simple.
I would monitor the load on the generators.
Start stepping in resistor banks when the load drops below a set amount.
Load resistors based on the generator load avoids having to change any parameters of the elevator drives or programming.
Changing drive or elevator parameters may lead to warranty and safety issues.
A subset of Murphy's law is the law of unintended consequences.
Breaking the law of unintended consequences in regards to elevator control and programming can land you in jail.


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

 

[waross]

I just saw Lionel's post. Good solution IF you can get the elevator manufacturer to add that as an approved feature of the elevator drive.

Skogs;
The all mechanical trucks use Jacobs^TM or compression release brakes. The exhaust valves are opened at the top of the compression stroke and the energy is dissipated as hot air.
Clessie Cummins of Cummins Diesel Engines had retired from Cummins Engines when he invented the compression release brake but Cummins management was not interested in the idea.
Clessie partnered with the Jacobs company (Who remembers the adds for the Jacobs Three Jaw Chuck?) to build and market the "Jake Brake".
Jake Brakes have been around since the 60s and are in wide spread use in heavy trucks in North America.

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

 

[Skogsgurra]

Thanks a lot, Bill. Obviously, not only God knows

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

Bill; generator-side load banks... Brilliant.
The radiator fan might even take care of the heat removal..

Keith Cress
kcress -

http://www.flaminsystems.com

 

[FreddyNurk]

I am aware of multiple installations of what was a boiler integrated into the radiator cooling loop as a dump load for generator installs, it worked quite successfully at both keeping the engine at the optimum temperature whilst alleviating the potential reverse power issues. However, that was for integration with wind turbines, rather than elevators, and I'd expect that the regeneration from the elevators would be too much for a radiator cooling loop on an engine to handle for longer periods of operation.

The boiler in that case was driven by a regenerative inverter, but the boiler was DC connected, not AC.

EDMS Australia

 

[waross]

Hi Freddy. The boiler could be connected to the DC bus, with suitable switching.
That would be a good solution for a downhill conveyor or other overhauling load.
I am very nervous about any modifications to the drive of something critical such as an elevator.
And with the load bank at the generator we are looking at one large stepped load bank, rather than 24 individual drives.
It may help to cycle some elevators out of service when on the generator.

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

 

[sibeen]

Bill, one of the first suggestions made was for a load bank to be installed in the generator room and switched in whenever the regen exceeds the load. Haven't heard from the client so they are still mulling over the initial report.

 

[FreddyNurk]

waross, I agree, the elevator drive shouldn't really be touched.

What was easy in my previously mentioned case was that all the generators where the prime source of generation, the activation of the dump load was managed on a per generator basis, and was triggered by power measurement (reverse power condition) at each set. All completely independent of the wind turbine system.

The case for the elevators is a bit different, although not impossible. A dump load based on total generation, as suggested, would work quite well. That might be more difficult if the generators are not all in the same location.

EDMS Australia

 

[itsmoked]

I can see the generator in a large building having a load bank for testing already installed and it's heat removal in place. Dumping the elevator heat into it would be a controls issue more than an infrastructure job.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Hi sibeen; The only tweak I would suggest is that it may be safer to switch in the load bank steps whenever the regen gets close to the load rather than switching in when the regen exceeds the load.
Try to avoid going into net regen at all.
You would be trading a little regen savings for added dependability.

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