Depending on the installation, some escalators may go long periods of time with little or no load. During these periods, it is possible to reduce the speed of the escalator motor to save energy. Reduced-Speed Mode can be set to lower the speed of the escalator motor after a defined amount of time. For example, when an escalator has operated unloaded for 10-15 min., the VFD can reduce the speed of the escalator motor to 20-50% of rated speed. The VFD continues to operate in reduced speed mode until an approaching passenger is detected by a light curtain or pressure mat. The VFD then smoothly ramps the motor up to rated speed. Figure 1 shows the energy-saving benefits of using a reduced-speed mode in an escalator application featuring a 15kW induction motor in various control modes. The largest incremental savings result from using a VFD to operate the motor at a reduced speed during periods of inactivity.

Both open- and closed-loop control modes are available when using a VFD. KEB offers an enhanced control mode, Asynchronous Sensorless Closed Loop (ASCL) for applications that wish to run open-loop but would like performance characteristics similar to closed-loop applications . ASCL uses a control algorithm based on the motor data to automatically measure and model the motor characteristics without encoder feedback . This provides excellent control in both stable and dynamic motor operation, accurate speed control and high-speed stability without the added cost and complexity of an encoder. In addition, both closed-loop and ASCL control modes offer PF improvement compared to line-fed or open-loop-controlled escalator applications.

The PF is a unitless number that represents the ratio between the power being put into useful work (measured in watts) and the total electrical power being transferred (measured in volt-amps). A unity PF of 1 is ideal and indicates all power being delivered is being usefully consumed by the escalator motor. Figure 2 shows the benefits of using a VFD compared to a line-fed induction motor. The most noticeable benefits will be when the VFD is in closed-loop or ASCL operation and when the motor is partially loaded – a scenario common in many escalator applications.

However, even when the motor is at full load, the PF is improved by 10% when using a VFD. Line-fed escalators experience high starting currents – sometimes as high as 500-700% of motor rated current. As illustrated in Figure 3, a low PF will also contribute to a higher input current for a given amount of power output  at the motor shaft. Again, the largest benefit is realized under partial load. In this case, the no-load RMS current with a VFD is more than SO% lower than that of the line-fed motor. This higher input current should be avoided, because it requires electrical components to be oversized to accommodate the transfer of unused reactive power. Also, there will be more resistive conductor losses due to the transfer of the additional reactive power. Finally, a low PF will cause higher heating in electrical components, which can lead to reduced operating lifetime.

Regenerative drives are great candidates for overhauling escalator applications in which the induction motor is acting as a generator. Traditionally, braking resistors were used with VFDs to dissipate the regenerated energy as heat. Depending on the braking resistor location, the extra heat produced may require additional cooling or a larger controller cabinet. This could cause problems when designing the escalator control panel, if space is at a minimum. As an alternative to braking resistors, a line- regenerative drive (pictured right) can be connected to the VFD through a DC bus connection. The regenerative unit operates by measuring the line voltage and frequency. Once the DC bus voltage exceeds a predetermined threshold, the drive modulates, allowing current to flow back to the line, where it is consumed by other electrical loads (e.g., in the building or installation). Depending on the application, a regenerative drive can be expected to pay for itself, reduce overall power consumption and provide net cost savings over its operational lifetime.

Escalator applications with high gravitational potential energy (e.g., high-capacity escalators moving many people with long downward travel distances) make great candidates for line regeneration. Gravitational potential energy can be described by the equation:

Gravitational Energy = mass x gravity constant x height of travel