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Elevator and Escalator Consulting Engineers

Elevator Emergency Power Operation

The operation of elevators on emergency power systems has special problems that should be recognized by the building owner and the electrical consultant.

Elevators during their normal operation cycles will at times be drawing energy from the electrical supply system and at other times will be supplying energy to the electrical supply system.
 
It is necessary that the electrical supply system be able both to provide energy to the elevator but also, under negative elevator loads (e.g. empty car in the up direction) to absorb the energy fed back to the system by the elevator. Failure to absorb this energy that the elevator feeds back will lead to overspeeding, mis-operations and shutdowns.
 
The normal electrical supply system, since it is connected to an entire electrical network extending across the continent, generally has no problem in handling these requirements.
 
An emergency power generator can normally supply energy to the elevator as required but it may have trouble dealing with the energy that the elevator feeds back.
 
To avoid problems it is best to follow a couple of simple rules:
 
1. The elevator load on the generator should be assumed to be at least twice the lift motor nameplate rating;
2. The elevator load should be less than 20 per cent of the total load on the generator;
3. Of the total load on the generator, at least 40 per cent should be resistive.
 
The idea is that there will be a large resistive load connected to the generator at all times which will absorb any energy that the elevator feeds back to the generator.
 
These rules are particularly important when the elevator uses a solid state drive.
 
Some smaller variable voltage variable frequency drives use resistors in the machine room to disperse the energy developed by the elevator under negative loads. These drives are less of a problem for an emergency supply system since the emergency generator does not have to deal with energy fed back to the system.
 
However, the larger solid state drives which incorporate invertors will require the appropriate emergency power generator arrangements to handle the power they feed back to the electrical system.
 
It is also important to note that the emergency power requirements for any solid state drive can be somewhat more stringent than for the old motor-generator drives. This results from the distortion of the current wave form caused by the solid state drives.
 
One important rule to follow: Do not try to make any emergency power tests unless there is adequate resistive load connected to the emergency power generator. This applies to any elevator drive system. The elevator needs this resistive load in the power circuit.
 
If the elevators are being modernized and there is an existing emergency power generator it may be necessary to update the generator since the size of the generator and the control system for the generator may not be adequate to supply power to the modernized elevators with their solid state drives. Some manufacturers of emergency power generators recommend that the generator be increased in size by an additional 20% to handle this type of load. In some cases, simply updating the generator controls will work. In many cases nothing has to be done; this would typically be the case if the generator were originally generously sized.