Elevator Motor in-rush current on renewable backup power 2

[PrimalPete]

Hi all,

I am designing a system that will provide backup power (batteries) to an elevator in the event of a grid outage. I have 150KW of total power at my disposal to power the load. The renewable inverters cannot handle in-rush that will go beyond 150KW without faulting out.

The elevator motor has a Locked Roter Amperage rating of 400A RMS (3 phase 208Vac). I am assuming this is that maximum draw that our backup system will need to provide to manage a soft start of the motor. If my calculations are correct, the max power would be about 200KW. It looks to me that more backup power needs to be added to cover the gap here.

I was hoping to get suggesting on how one can limit the in-rush current so that we do not have to increase our power capacity. Any hardware suggestions or tips would be helpful.

Thanks.

 

[BrianPetersen]

Disclaimer, I'm not an elevator technician so this might be a dumb question, but ... Is the motor driven through a VFD?

If it is, it can (and should) be programmed to ramp up to speed. Starting at 0 Hz (or close to it, i.e. slip frequency) and ramping up avoids the in-rush jolt.

If it isn't (and there isn't any means of smooth starting at all, just direct across the line), the elevator in its current installation will not only have high inrush current but will also start with an enormous jolt, because huge inrush current also means huge motor output torque!

It's possible that the motor has some other means of starting smoothly, i.e. reduced voltage or some such thing.

 

[PrimalPete]

Thanks Brian,

This is helpful. This is early stage of design and so I'm just trying to find out any potential show stoppers now. (I also know nothing about elevator control systems) This is a new construction building so it looks like I need to sync up with the elevator installers and team up with them to solve the issue.

Thanks.

 

[ScottyUK]

Why batteries?

A generator is usually a better solution for longer-term power interruptions because fuel stores more energy per unit volume than batteries, and is cheaper per kW of output. A generator is inherently more able to handle loads like induction motors than a static inverter and provides better fault-clearing capability.

Not saying you're wrong - it's just a less common solution. What limitations and constraints are governing this design?

 

[itsmoked]

less common solution

As in never ever heard of before.

Yes PrimalPete, you do need to get with the elevator vendor on this.

Elevators are highly regulated and you could do all this battery back-up only to find the resulting system to be unapproved. Scotty is also correct in that typically if the power fails the elevator comes to a smooth stop. The generator starts, comes up to speed, stabilizes, and takes on the loads it's selected to support (including the elevator). The elevator controller reboots and if the elevator is not at a floor is moved to the closest floor and the doors opened and it's back in service and capable of remaining in service until the building management allows the generator to run out of fuel.

With batteries the elevator may continue to operate uninterrupted for a few minutes post power outage before the batteries go flat leaving the elevator in the same state it would've been in without any batteries at all! Hardly worth the expense, hassle, and maintenance, of managing the batteries.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[PrimalPete]

@ScottyUK

Thanks for the input. I'm sort of not in the loop on the business decisions for not having a diesel genset. My job is to design the renewable backup system and make sure it works! But I agree, a diesel would make more sense given the emergency load panel consists mainly of electric motors.

Limitation would be on the battery side. We simply cannot add more batteries to this design. The batteries can handle up to 300KW of instantaneous power, but we have only spec'd 150KW worth of inverter power. So we can double up on the inverters if necessary to get up to 300KW of power. But it seems like having some sort of soft starter mechanism would be a cheaper alternative.

Thanks.

 

[ScottyUK]

Think about an induction motor starting: the current is very high, but it's at a low power factor. It's almost all reactive power, and reactive power is an AC-only quantity: it simply doesn't exist in a DC system. That's of huge significance here because it means those high currents during starting are confined to the inverter output stages which generate the synthesised AC. The DC link and the battery don't see the high currents at all. Doubling up the inverters could be a very smart move.

I still think you should install a generator though.

 

[EdStainless]

I have seen systems with battery back up.
When they come on it would go to the ground floor and then shut down.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[sibeen]

You had also best think of what you are going to do with the energy the elevator is going to regen and try to feed back into your inverter. You're going to have to dump that excess somehow.

 

[PrimalPete]

Well let me clear a few things up. The idea (to use batteries) has already been decided by the main contractor, and we have been hired to design the backup system.


@itsmoked
These are fairly large batteries. 240KWH of capacity, enough to keep the elevator on for 3 to 4+ hours on a full charge. The batteries will be grid-tied and be doing demand response on a daily basis. The battery will never go below a threshold of reserve so that it will always have enough juice to power the elevators for at least 2 hours in standalone mode.

@ScottyUK
Yep I agree, which is exactly why I want to limit that in-rush. If our inverters simply fault out then there won't be any backup system at all.

I understand Diesel Gen is king, but my concern was mostly about getting help on how to deal with in-rush currents. That is going to be our largest hurdle at the moment.

Thanks.

 

[Skogsgurra]

PP, you do not get any inrush or even very high starting current. The LRA occurs only in DOL and the VFD makes your motor produce a torque that can, depending on rotor type, be way above the DOL starting torque. That is how all PWM VFD:s work.
So, 200 kW is ample if your motor is 150 kW. This picture explains that.

https://upload.wikimedia.org/wikipe...ple-uf.svg/2000px-Variation-couple-uf.svg.png

The lowest frequency shown is 10 Hz. But the elevator drive starts just above zero, "catches" the motor, makes sure that it is held properly before opening the brakes. The current drawn will not be any higher than normal operating current.

I do not see any problems. Except that of approval by authorities and committees. Most drive suppliers have models where all this and other elevator-related functions are built into the firmware. It is wise to consult a skilled seller of such drives. Do not talk to VFD peddlers. They know less than you about this.

And, of course, you shall be aware that the "easy way" with an empty or less than half-laden cage is up. Not down. But you need to handle the fully laden cage as well. So four-quadrant is a must.

Gunnar Englund

http://www.gke.org

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

 

[ScottyUK]

Have we got a VFD-equipped elevator, or a fixed-frequency static inverter trying to handle a standard motor starting direct-on-line? Very different sets of conditions, and right now I see two diverging sets of answers to two different sets of problems.

 

[waross]

Rather than doubling up on fixed frequency inverters, I suggest using a frequency agile inverter. It's more commonly called Variable Frequency Drive (VFD).
If you are going to spend money on an inverter, buy an appropriate inverter ie: a VFD.
Personally I would consider first, not a diesel generator but a natural gas fueled generator but I guess that that boat has sailed.
On the other hand I remember being asked to select and install a standby generator for a building hemmed in by other buildings.
How do we get fresh air in?How do we get rid of the exhaust?
How do we handle the rejected heat?
How do we handle transfers for multiple, metered occupancies?
The owner put so many restrictions on my design that I ended up by saying;
"I'm sorry. I'm afraid that you need a better engineer. I can't handle this."
They found someone else and proceeded from disaster to disaster.
I was very glad that I had walked.
Just saying, sometimes you have to think outside that box. (And then move outside the box.)

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

 

[Skogsgurra]

I couldn't even imagine a DOL for an elevator. But Scotty is right. A VFD is not mentioned anywhere. It should be. If it isn't a slow elevator for goods only. All elevators I have seen lately (the last 15-20 yrs) were VFD types. Easy, safe and simple standard solutions with lots of support.

Gunnar Englund

http://www.gke.org

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

 

[ScottyUK]

skogs,

It could be an old Ward-Leonard set for all we know!

 

[jraef]

VFDs qualified to be used on passenger elevators are not as simple as you might think, as mentioned earlier everything having to do with passenger elevators, at least here in North America, is highly regulated and requires special certifications.

You need to start with the information from the elevator supplier as to how they are starting their motors, this is critical with your concept. If they are starting Across-the-Line (DOL), that is VERY different from being started with Wye-Delta (very common in the elevator industry) or with a Solid State Soft Starter, or with a VFD. For example Wye-Delta starting may be the sort of thing that damages your backup inverter systems, so size may become moot.

If it is a VFD, you can then consider connecting a battery backup directly to the DC bus of the VFD(s), the extra inverters may be altogether unnecessary.

So again, you really cannot do justice to a backup system design in the dark, you MUST first know the details of what you are dealing with.


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