Regenerating voltage on elevator

[kwired]

Even if the loads were perfectly balanced, their mass means they have inertia, so the motor must overcome it to start and to stop the motion. Add friction to this. Even a perfect balance can only exist at one point in the elevator's travel...as the cab descends the cable on its side of the sheave increases, adding considerable weight...and there is less cable on the counterweight side, reducing weight on its side.

As I mentioned earlier, the common counterweight is set to match no more than the cab plus 40% of its max capacity. The regeneration system can and should be set up to work regardless of which way the imbalance exists. It can be utilized to retard lift speed if the cab is empty or otherwise weighs less than the counterweight, to retard descent speed when the cab weighs more than the counterweight. Either way, no scenario exists where adding weight to the cab should reduce dynamic load while descending. At least not on Earth....

Yes you must overcome inertia regardless.

If balanced motor only has to overcome inertia when starting and stopping.

If not balanced then the motor has to either work harder to raise the heavier side or to maintain steady speed when lowering the heavy side, that maintaining steady speed when gravity is trying to speed it up is where the "running" regeneration is going to come from.

 

[Craigv]

Yes you must overcome inertia regardless.

If balanced motor only has to overcome inertia when starting and stopping.

If not balanced then the motor has to either work harder to raise the heavier side or to maintain steady speed when lowering the heavy side, that maintaining steady speed when gravity is trying to speed it up is where the "running" regeneration is going to come from.

Exactly, which is why any increase in the falling mass should increase the force required to slow it, not increase it, as the OP seemed to be inferring. Adding cab weight should have increased the regeneration power while descending.

 

[winnie]

Exactly, which is why any increase in the falling mass should increase the force required to slow it, not increase it, as the OP seemed to be inferring. Adding cab weight should have increased the regeneration power while descending.

Which suggests that the counterweight was too heavy, meaning that there was too much regeneration when the cab was going up. The OP never said which direction was having the regeneration problem, simply that the cab was traveling.

As you have noted, adding cab weight will not remove regeneration. At most it could shift the regeneration to a different part of the duty cycle. In other words, if the system is regenerating when the cab is going up, adding cab weight will reduce that but could cause regeneration when the cab is going down or when there are different numbers of people in the cab.

Additionally, part of the regeneration is being absorbed by the losses in the system, so if the system were severely out of balance, adding cab weight might have reduced the amount of regeneration until it was less than system losses, thus reducing the apparent regeneration. You still have mass moving in a gravitational field and still have to deal with falling mass releasing energy, but if that goes into heating your gearbox and motor and not burning up your resistors, then the problem is 'fixed'.

-Jon

 

[GoldDigger]

If the counterweight is indeed too heavy (sheer speculation at this point), then the proper solution (but one requiring some engineering instead of stacking up weights) is to reduce the counterweight.

Sent from my XT1585 using Tapatalk

 

[RumRunner]

Gentlemen,

We have an elevator that is regenerating voltage during travel and not just on braking. Our power regeneration is connected to a resistor bank and is not fed to the building. It is regenerating so much power it caused a small fire. The drive has been replaced. The temporary fix was to load 1500 pounds of weight in the cab which causes the voltage to go away. The control cabinet is MCE with a Hollister motor.
Any ideas what would cause runaway regeneration?

I see a disjunct.

Regeneration happens when the drive (motor) is acting as generator and when it is not doing the driving.

The only time it is connected to the resistor bank is when a command is given to stop and thus applying the brake. . . doesn't matter whether it is regenerative or dynamic braking. This is happening when the motor is acting as a generator.

The resistor bank is connected, so that a tremendous load on the generator is applied in order to force it to slow down before it goes to a total stop. . . and before the (mechanical) friction brake takes over.

OP did not say whether the motor is DC or an AC motor with VFD. The braking scheme would be different for each particular type motor.

(Not familiar with Hollister motor)

 

[winnie]

The only time it is connected to the resistor bank is when a command is given to stop and thus applying the brake. . . doesn't matter whether it is regenerative or dynamic braking. This is happening when the motor is acting as a generator.

The resistor bank is connected, so that a tremendous load on the generator is applied in order to force it to slow down before it goes to a total stop. . . and before the (mechanical) friction brake takes over.

I am going to disagree partially, but this is probably dependent on the situation.

With a VFD controlled AC motor, the common situation is to connect the resistor bank when the DC bus voltage exceeds a threshold. So _if_ the motor enters a 'regeneration' condition for whatever reason (either braking to a stop or an 'overhauling' load), and if that regeneration is sufficient to raise the bus voltage, then the resistors get connected to protect the bus from excessive voltage.

-Jon

 

[Jraef]

I am going to disagree partially, but this is probably dependent on the situation.

With a VFD controlled AC motor, the common situation is to connect the resistor bank when the DC bus voltage exceeds a threshold. So _if_ the motor enters a 'regeneration' condition for whatever reason (either braking to a stop or an 'overhauling' load), and if that regeneration is sufficient to raise the bus voltage, then the resistors get connected to protect the bus from excessive voltage.

-Jon

This is correct. There does not need to be a "stop" command for the dynamic braking transistor to be engaged, it is simply going to respond to the DC bus level. In fact it doesn't even need to be a overhauling load. That's why I said earlier that if the LINE voltage is high, it will conduct into the brake continuously and burn it up. Happens more than you might think because many drives do not have an Over Voltage sensing circuit or if they do, all it does is shut down the drive. But on some drives, the DC chopper (brake transistor) is an EXTERNAL device so although the VFD may be shut off, the rectifier is still connected and the DC bus is still being charged, so the chopper still fires into the resistor.

Bottom line, there are numerous details left out of this scenario and there is really no way to diagnose the situation without more feedback from the OP, who has never posted again since his first post. At this point, we don't even KNOW if there is a VFD involved.