Operation of asynchronous engine in escalators moving down 4

[Sempronio1960]

I am interesting about operation of asynchronous engine in escalators moving down. Is the engine working as generator? If so, can an overload beyond maximum couple cause an escalator overspeed? Moreover, in generator operation, is the energy lead into grid or dissipated in a rhesostat? Thank for attention

 

[Skogsgurra]

The Asynchronous motor tries to run close to synchronous speed. A four-pole machine (on a 50 Hz grid) that is 1500 RPM.

But it doesn't run exactly at that speed because it needs some "slip" (difference between synch and actual speed) to have enough rotor current (induced in the rotor bars) to balance the load torque. So a motor that is "motoring" as opposed to braking, will always run slightly below synch speed. And the slip increases as load increases, there is a drop in speed when load increases.

In the braking mode (which seamlessly and without change in anything but the load torque being opposite) the motor speed is just above the synch speed. The slip is negative, so to speak, and instead of drawing power from the grid the motor delivers power to it.

The torque/speed curve is symmetrical around 1500 RPM and it is all very simple, actually.

Study the picture below (found on the internet, thanks to unknown)

Gunnar Englund

http://www.gke.org

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

 

[itsmoked]

Motor not engine.

Usually it would be sent back into the grid when large loads are going down. Buildings big enough to require escalators can easily re-consume the escalator power. There has to be some sort of backup in case the grid becomes unavailable. The escalator VFD will remain powered as long as the load is descending but will still need somewhere to dump a lot of energy if the grid becomes unavailable. Likely this would be solved by an electrically held-off mechanical brake that simply friction stops the escalator into expensive stairs.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[crshears]

The "asynchronous engine motor" is most commonly an induction motor, and exactly the same phenomenon observed in the descending loaded escalator mode is harnessed for induction generators; these have become commonplace ubiquitous in jurisdictions where distributed generators have been granted access to electricity markets.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Sempronio1960]

@Skogsgurra
Ok, my question is: if |T|>|T|[sub]Run away poin[/sub], is the motor accelerating, also if is it in generating mode?

@itsmoked Motor not engine....
Of course

 

[xnuke]

Yes, the machine will accelerate when the load torque magnitude exceeds the runaway torque point magnitude, which is the maximum torque the machine can put out when operating as a generator.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[waross]

I have a bit of a problem with the picture.
The curve is a typical curve for a DOL induction motor.
However the normal operation of a VFD is in the region between the full load operating point and the full load braking point.
To start a motor with a VFD frequency set so that the motor is at the point shown as the starting torque is wasteful and does not take advantage of the reduced starting current of the motor VFD combination.
To say this another way, that picture/curve and the operating points shown are valid for an induction motor started DOL.
Normally the limits of VFD operation will be between the breakdown point and the runaway point or between the current limit points if set, for short periods of time.
Steady state operation or load RMS* operation will be between the full load operating point and the full load braking point.
*Link See .pdf pages 49, 50, 51. RMS HORSEPOWER LOADING
Comments?

A question for Gunnar and Jeff:
It is my understanding that that curve may be used for VFD operation if the 0 point is the VFD output frequency and the slip is the motor normal slip.
eg: 1800 RPM - 1760 RPM = 40 RPM slip for a 1760 RPM motor
1500 RPM - 1460 RPM = 40 RPM slip for a 1460 RPM motor.
Is this correct or close to correct?
Bill
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Most escalators do not use VFDs, there is no value in varying the speed. Most use Across-the-Line (DOL) starters or at most a form of Soft Starter that reduces the voltage when the escalator is unloaded (Nola energy saver circuit), but only IF the escalator must be running all the time regardless of use, as in airports and 24/7 facilities like casinos. In places like large retail stores where the escalators are not used when the store is closed, they don't even bother with that, they just use ATL / DOL starters. As far as over speeding, the motors are connected to right angle worm gears with a very high gear ratio, so they are not really subjected to a lot of regen force anyway. They are basically motoring regardless of going up or down, there is just a lot less motoring needed going down.

If a down escalator's load rating is exceeded so that the motor overloads or if the line power is cut off, a mechanical safety clutch/brake engages and stops the escalator. It does so slowly so as to avoid making any passengers fall forward and end up in a pile at the bottom. So no matter what, the escalator cannot over speed. But the motor size / gearing is such that you could have 300 pounds on each and every stair tread and it would not be enough to over load that motor.


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

 

[itsmoked]

Thanks for that eye-opener Jeff!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

Jeff

" It does so slowly so as to avoid making any passengers fall forward and end up in a pile at the bottom. So no matter what, the escalator cannot over speed."

CSKA Moscow fans injured in escalator accident in Rome.

An escalator collapse in Rome's La Repubblica metro station on Tuesday night, has left 24 CSKA Moscow fans injured, with one person in a serious condition, city administrators have confirmed.

The cause of the collapse is not yet clear with a video of the incident shows the escalator moving at an excessive speed, with people being thrown into a crush at the very bottom.

Photographs of the escalator show a concertina effect of the steps at the very bottom of the escalator and the local authorities are now investigating the cause.

http://www.espn.in/soccer/uefa-cham...ow-fans-injured-in-escalator-accident-in-rome

Muthu

http://www.edison.co.in

 

[jraef]

I’m not an expert, but I’d hazard a guess that it was a mechanical failure, such as the stair chain broke on both sides and without that, the brakes have nothing to stop. It will likely turn out to be some violation of standards, and/or lack of enforcement. There are lots of accidents that take place around the world, most of which can be attributed to people cutting corners to get around expensive requirements and regulations.

My brief delving into the elevator /escalator world opened my eyes to some of the strictest standards I’ve ever run into. The cost of testing and design changes was significant and restrictive to entry, a situation ripe with temptations for unscrupulous players.


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

 

[benta]

My main question about escalators is: why are the handrails always a bit faster that the escalator itself? I've never found a reason for this phenomenon, which should be easy to solve.

Sorry, totally OT.

 

[Compositepro]

I have never noticed that. Perhaps you are referring to what you feel with your hand as the escalator changes slope from up/down to flat at the landings. That is actually caused by the lean of your body changing with respect to the angle of the escalator.

 

[cswilson]

I'll second benta's question. And Compositepro, I usually notice this on the constant-slope portion in the middle of the escalator.

 

[davidbeach]

Yep, handrail moves at a slightly different speed on most escalators I'm aware of. No idea why though.

 

[itsmoked]

They are elastomeric as compared to the steel treads. Driven by pulleys they run at a slightly different rate because they can slip and because the rubber drive wheels physically wear, changing the ratio.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

In the last few days, I watched carefully the escalator hand rails in a shopping mall, airport and underground train station. I can definitely say that that handrails move at exactly the same speed as the treads.

Muthu

http://www.edison.co.in

 

[itsmoked]

Suuuuuure they do Muthu, sure they do.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

Keith

Suuure they do move at same speed or suuure they do move at a slightly different speed?

Muthu

http://www.edison.co.in

 

[waross]

I have seen some that will gain a foot or so over the length of travel.
Others do not.
To add to the confusion, some escalator hand rails appear to gain or lose a little due to the geometry as the travel transitions from near horizontal to the working angle, but do not gain or lose on the straight portion of the travel.

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