There are a number of factors which can prevent an elevator from restarting after the power is restored post power failure. Most of these factors can also be a factor in preventing an elevator for coming back on-line when an emergency power generator kicks in. We will explore some of the causes of these power related elevator shut downs.
Power failures are not really power failures.
Typically elevators restart very well if the power is suddenly shut off, stays off for a length of time (about 20 seconds or more) and then the power suddenly comes back on with full power. This scenario, however, is not the typical pattern for most “power failures”. Often the power flickers on and off a time or two either as it is failing, or as it is being restored. The power also often drops to a lower voltage as it is failing or being restored. This situation is called a “brown out”. Even a momentary brown out condition can cause circuit protection fuses, circuit breakers or software to trip out in an attempt to protect the expensive portions of the elevator equipment from permanent damage. As the voltage drops, or flickers, the current can rise quite rapidly. If this situation is not addressed quickly enough, it can burn out electronic components, circuit boards, relays, motors, or other electrical parts of the elevator equipment. The fuses and other protection devices in an elevator controller are designed to protect these components as much as possible, but as in any electronic system design, there is a balance between excessive nuisance tripping and the level of protection. Most elevator electrical protection features are intentionally designed not to be self-resetting because it is prudent to have a knowledgeable elevator technician inspect the equipment prior to restarting the elevator. The technician can then ensure that there is not some kind of permanent problem with the elevator power or power circuit that will cause greater damage to the elevator equipment from multiple resets.
What about power surges?
Power surges occur when the line voltage rises above the regular voltage. Momentary or prolonged surges also often occur in conjunction with power outages. Power surges typically have an even greater tendency to trip out protection features and damage the elevator equipment than brown outs or power fluctuations. The results are typically similar to the discussion above.
Are new elevators more susceptible to shut downs related to power failures?
Yes and no. The older elevator systems (the ones with few electronic components) typically have components which are less sensitive to power variations and therefore usually don’t have as many power protection features. The lack of protection on these more robust components usually results in fewer nuisance trips due to power problems. It often takes, a more severe, or more prolonged power variation to shut the system down. The reduced protection and nature of the many relays in these systems, however, usually result in numerous shut-downs as the relays fail due to age and the cumulative extra stress that the power issues put on them.
The newer elevator systems often have some adjustments that can be made to fine-tune the level at which some of the protection features trip out. For example, many of the newer motor drives have minimum and maximum input voltage detection. At a certain percent variance, the motor drive will trip out to protect itself and the motor. The acceptable percent variation can be increased, but increasing this value always puts greater risk on the protected components. The technician tries to find an acceptable balance between these two factors.
What can I do if my elevator does not restart after a power failure?
You can have a qualified electrician or building maintenance personnel verify that you have full power on all of the phases leaving the elevator main disconnect switch in the elevator machine room. If there is a power feed problem upstream of the elevator controller, this check will usually find it and often save the expense of having an elevator technician come to your site just to tell you that he can’t help you until full power is restored to the elevator controller.
If the power leaving the elevator main disconnect switch checks out fine, the next step should be to call your elevator service provider.