Regenerating voltage on elevator

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alanconga

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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?
 
180924-1300 EDT

alanconga:

I don't think regenerated voltage is your problem.

You indicate that under heavy load conditions, 1500 #, the problem goes away.

Put a DC voltmeter across the DC bus. With the elevator not moving monitor this voltage.

Then with very little load in the elevator raise the elevator and monitor the bus voltage.

Same while lowering the elevator.

Look in the manual and see at what bus voltage the the regen resistor is connected.

Report back.

.

 
alanconga said:
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?
Click to expand...
Seems the bulk of regen should be when the cab is descending, with maybe only a small amount when decelerating while ascending. The more mass you have in the cab the more energy it should take to decelerate it while descending. Seems to me that 1500 pounds extra weight would cause it to regen even more energy not less:huh:

Has to be more we haven't been informed about if adding 1500 pounds causes less regen??
 
kwired said:
Seems the bulk of regen should be when the cab is descending, with maybe only a small amount when decelerating while ascending. The more mass you have in the cab the more energy it should take to decelerate it while descending. Seems to me that 1500 pounds extra weight would cause it to regen even more energy not less:huh:

Has to be more we haven't been informed about if adding 1500 pounds causes less regen??
Click to expand...
A traction elevator always includes a counterweight which is sized approximately to the average weight of full can and empty cab.
If the counterweight is oversized there will be large regen when the cab is ascending, because the counterweight is descending.
Adding 1500# to the cab would then bring the system more into balance.

Sent from my XT1585 using Tapatalk
 
LarryFine said:
Wouldn't that be dynamic braking rather than regenerative braking?
Click to expand...
Yes and no. If a motor is used as a generator during deceleration, then it's a globally referred as a dynamic braking system. If the power is dissipated through resistance grids it's additionally described as rheostatic, and if power is fed back to the grid (or other original power source) it's regenerative. So a diesel locomotive (and the OP's elevator) would have rheostatic dynamic braking. A Prius would have regenerative dynamic braking.
 
Craigv said:
Yes and no. If a motor is used as a generator during deceleration, then it's a globally referred as a dynamic braking system. If the power is dissipated through resistance grids it's additionally described as rheostatic, and if power is fed back to the grid (or other original power source) it's regenerative. So a diesel locomotive (and the OP's elevator) would have rheostatic dynamic braking. A Prius would have regenerative dynamic braking.
Click to expand...
And in the OP's case, since there definitely is a braking resistor and it is getting hot, the braking is dynamic rather than regenerative.
Possibly the intention was to use a VFD that offered regeneration and to reduce the size of the braking resistor accordingly, for use just as a back up to regeneration. But if the VFD was not programmed properly for regeneration it might overload the braking resistor.
I think it is more likely that the design was for dynamic braking and something went wrong in the details of the mechanical and electrical design. In that case the OP simply misused the term regenerative. (The motor was, of course, acting like a generator after all. :))
 
GoldDigger said:
A traction elevator always includes a counterweight which is sized approximately to the average weight of full can and empty cab.
If the counterweight is oversized there will be large regen when the cab is ascending, because the counterweight is descending.
Adding 1500# to the cab would then bring the system more into balance.

Sent from my XT1585 using Tapatalk
Click to expand...
Thanks, I know very little about passenger elevators. Sounds like maybe there is too large of a counterweight then.
 
180924-1951 EDT

I like GoldDigger's suggestion that overvoltage occurs on a light load going up from a counterbalance.

This may mean a bad design in terms of the dissipation resistance power rating, and/or the standby DC bus voltage is too high, or not enough bus capacitance.

.
 
Is there a VFD involved? One aspect of a VFD that uses Dynamic Braking (which by the way COMES from a regenerating motor) is that the braking resistor is tied to a "chopper" transistor that in turn is triggered not by a specific "brake" command, but rather by simply monitoring the DC bus voltage level and firing when it reaches a predetermined threshold voltage. The regenerating motor raises the DC bus level and THAT's what automatically triggers the chopper to fire into the resistor. But if the LINE INPUT voltage is too high, the result on the DC bus voltage level is the same and the chopper fires.
 
kwired said:
Thanks, I know very little about passenger elevators. Sounds like maybe there is too large of a counterweight then.
Click to expand...

The OP's description sounds backwards. Adding weight to the cab increases the mass to be slowed, which physics tells us should generate more, not less power for a given rate of deceleration.
 
Craigv said:
The OP's description sounds backwards. Adding weight to the cab increases the mass to be slowed, which physics tells us should generate more, not less power for a given rate of deceleration.
Click to expand...
That is what I thought at first, but if there is a counterweight then what he describes makes sense when the cab is empty.
 
alanconga said:
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?
Click to expand...
Insufficient braking capacity.
 
kwired said:
That is what I thought at first, but if there is a counterweight then what he describes makes sense when the cab is empty.
Click to expand...

The presence of a counterweight doesn't change the physics...adding weight to the cab increases the downward acceleration regardless of the initial balance of the assembly. Adding cab weight should still increase the braking force needed to reduce the cab's downward momentum, not decrease the braking force.

Consider a pair of weights hanging by cable on a pulley. Weight A is the cab. Weight B is the counterweight. If Weight B is more than Weight A, very little force is needed to stop the downward motion of Weight A as it is lowered. If we add weight to Weight A to any degree...even if the total is still less than that of Weight B, the force needed to stop the downward motion of Weight A is increased.
 
All well and good, but the OP specifically mentioned current in the resistor during "travel" not just braking. That is what I was trying to address.
In theory there will be no deliberate motor resistance to motion during acceleration and constant speed for both upward and downward cab motion unless the cab always rises empty and descends fully loaded. If the actual symptom is that the OP is specifically concerned with braking resistor current during upward cab motion, then the counterweight factor is relevant.
If that is not the case, then Jraef's over voltage scenario makes the most sense IF you can explain why the chopper is not turning on when the elevator is stationary.

Sent from my XT1585 using Tapatalk
 
Craigv said:
The presence of a counterweight doesn't change the physics...adding weight to the cab increases the downward acceleration regardless of the initial balance of the assembly. Adding cab weight should still increase the braking force needed to reduce the cab's downward momentum, not decrease the braking force.

Consider a pair of weights hanging by cable on a pulley. Weight A is the cab. Weight B is the counterweight. If Weight B is more than Weight A, very little force is needed to stop the downward motion of Weight A as it is lowered. If we add weight to Weight A to any degree...even if the total is still less than that of Weight B, the force needed to stop the downward motion of Weight A is increased.
Click to expand...

But if weight B is greater than A, the braking will need to occur when A is traveling up, not down. Increasing weight A will then lessen required braking.

This would be the case where the elevator counterweight is grossly oversized.
 
retirede said:
But if weight B is greater than A, the braking will need to occur when A is traveling up, not down. Increasing weight A will then lessen required braking.

This would be the case where the elevator counterweight is grossly oversized.
Click to expand...

Which isn't the case here. Max counterweight is 40% of the maximum cab weight.
 
There _should_ sometimes be regeneration on travel. The elevator has a counterweight, but the counterweight doesn't get adjusted to match the passenger load; it is just an average value.

Depending on which side is heavier (counterweight or cab) and the direction of travel, and the amount of friction in the system, either the motor is doing work lifting a load, or the motor will be regenerating.

The added weight did not eliminate the problem, but simply moved the problem to a different operating regime; if the elevator starts at the top of the building and lots of people get on to go down, the problem will show up again.

The gravitational potential energy stored by the elevator moving loads up must go somewhere when it goes down again.

-Jon
 
retirede said:
But if weight B is greater than A, the braking will need to occur when A is traveling up, not down. Increasing weight A will then lessen required braking.

This would be the case where the elevator counterweight is grossly oversized.
Click to expand...
Doesn't really matter which is heavier, the cab or the counter weight. One way the motor is loaded more to raise the heavier object, the other way it is tasked with constantly decelerating the heavier object.

Seems the counter weight is not sized per the most common load on the cab. How much use the elevator sees will have an impact on what size weight would be optimal. Take an office building - if most people are going up early morning - the cab is loaded when going up, but then if they are returning empty to ground level for the next group, you are loading motor with raising of the counter weight. At end of day load conditions are likely reversed. Optimal conditions for the motor/drive is when cab and counter weight are same weight regardless which way it is traveling. User conditions aren't always going to be that way though.
 
kwired said:
Doesn't really matter which is heavier, the cab or the counter weight. One way the motor is loaded more to raise the heavier object, the other way it is tasked with constantly decelerating the heavier object.

Seems the counter weight is not sized per the most common load on the cab. How much use the elevator sees will have an impact on what size weight would be optimal. Take an office building - if most people are going up early morning - the cab is loaded when going up, but then if they are returning empty to ground level for the next group, you are loading motor with raising of the counter weight. At end of day load conditions are likely reversed. Optimal conditions for the motor/drive is when cab and counter weight are same weight regardless which way it is traveling. User conditions aren't always going to be that way though.
Click to expand...

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....;)
 
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