How to use bidirectional DCDC converter in elevator automatic rescue device to improve efficiency and reduce costs

1 Introduction

Since elevators transport millions of people every day, operational safety is critical. Have you ever wondered what happens when the main power to an elevator is turned off? Will the lift descend onto the lift rails, or will it get stuck somewhere between the landing doors? To prevent the first consequence, a fail-safe braking mechanism ensures that the elevator car stops immediately when the main power supply is switched off. After a stop, in order to prevent people in the elevator from being trapped in the elevator until power is restored, the automatic rescue device ( ARD), also known as the elevator emergency power supply, comes into play.

2. Introduction to automatic rescue device ( ARD)

ARD is a backup power device that continuously monitors the main elevator power supply. Unexpected shutdowns can occur due to grid failure or faulty wiring that interrupts the input phases of the elevator drive. The ARD detects such a fault condition, immediately starts supplying power to the elevator drive and sends a fault signal to the elevator controller. The controller then releases the motor-driven brake and slowly brings the car to the nearest landing door. The direction of the car depends on the minimum amount of power required to bring it to the landing door. Once the nearest floor is reached, the elevator doors open and audio/visual indicators indicate it is safe to exit. After a predetermined time, the elevator door closes again and the power to the elevator drive is turned off.

3. Automatic rescue device ( ARD) power supply design

Figure 1 illustrates the connection from a traditional ARD to the elevator system. The three-phase mains supply is connected to the traction drive via a main contactor. The power contactor is interlocked with the ARD contactor, which connects the output of the ARD to the traction drive. The interlock ensures that both contactors do not open at the same time, thereby avoiding a short circuit between the mains and the ARD output. The single-phase output from the mains power supplies the remaining components of the elevator system such as controllers, door motor controls, brakes, and safety chains through contactors. ” This contactor is also interlocked with the ARD’s single-phase output contactor. During normal operation, the ARD charges the backup battery; its inverter output is disconnected from the elevator system.

Figure 1: Traditional ARD system connections in elevators

The ARD system shown in Figure 2 has an AC/DC charger power stage to charge the battery. A DC/DC converter steps up the battery voltage to a high voltage, and a DC/AC inverter produces an AC output that powers the elevator traction drive and elevator controls. Circuitry continuously monitors the AC power input for outages and single-phase conditions and enables or disables the required power stages within the ARD.

Figure 2: Traditional ARD system

Another approach is to use a bidirectional DC/DC converter, as shown in Figure 3 and Figure 4. Energy can be transferred in both directions. In ARD, the converter is connected directly to the DC link of the elevator traction drive. During normal operation, the converter works like a battery charger, charging the battery from the DC link. When there is no mains supply, the converter works like a boost converter, powering the DC link from the battery. Another inverter stage within the ARD generates a single-phase AC voltage for the control unit.

Figure 3: ARD system with bidirectional DC/DC converter connected to the elevator system

Figure 4: ARD with bidirectional DC/DC

Comparing the two approaches, Table 1 shows how the bidirectional DC/DC converter approach provides more benefits.

Table 1: Comparison of traditional ARD and ARD with bidirectional DC/DC converter

Typical battery voltages for elevator ARDs are 24 V, 36 V, 48 V and 60 V. The nominal DC link voltage of a three-phase 400 V AC powered elevator traction drive is approximately 600 V. The Isolated Bidirectional DC/DC Converter Reference Design is a 2kW, 48V to 400V digitally controlled bidirectional power stage used as a half-bridge battery charger and reverse current fed full-bridge boost converter. The design is scalable to different power levels and input battery voltages by simply redesigning the transformer and selecting appropriately rated MOSFETs.