Designing a smoother ride for elevators

Stephen Scales of HID Limited explains how Hitachi has addressed the issue of a smoother elevator ride with a new inverter that offers significant advances for lift engineers.

There are few tougher tasks for the drives designer than to create a smoother elevator ride.

When Hitachi designed its new generation of AC inverters, the company addressed some of the specific needs of the lift and elevator industry.

The reason the company placed such importance on this application area is because of the challenge presented by moving the most sensitive load of all - people.

Indeed, if there is one instrument by which can be measured inverter performance, and the subsequent ride quality of a lift, it's by the perception of the people being carried.

Lifts are also challenging to drives companies since their performance varies considerably according to load.

For instance, a typical lift has a counterweight of about half its full load.

This means when raising an empty carriage, the motor is regenerating and only goes into load when lowering the empty carriage.

Conversely, as soon as the carriage is sufficiently full to exceed the counterweight, the lift regenerates on decent and is under load when raising.

Before examining the technical features that have been incorporated into Hitachi's new drive, it is worth examining how the best current lift installations operate.

Using a modern inverter, the operating process starts when the lift call button is pressed, by the lift controller sending a run signal to the drive via its RUN FWD/REV input.

The drive then applies current to motor, but there is no torque available until a minimum frequency of, say 0.5Hz, for a good quality sensorless vector inverter.

It is here that whoever is tuning the drive must apply a "guesstimate" of the frequency at which the brake release output is given - usually a relay output that delivers the arrival signal to close the brake relay and release the brake.

Because it is necessary to guess the point at which there is sufficient torque to release the brake, it is common for the setter to overestimate by as much as a few Hz.

One reason for this is that the characteristics of the lift vary according to the load and it is impossible to know whether the lift will be full, half full or empty when it is called to a floor.

The result of this is the motor snatches the cable as the brake is released since it is already accelerating to speed and a customary jolt is experienced in many lifts as they begin their accent or decent.

The motor then ramps to full speed - usually using an S-curve acceleration - whereupon, depending on the tightness of the control through the drive, there is a degree of under or overshoot of desired speed.

As the lift approaches the selected floor, proximity switches are used to slow and then stop the lift.

The first of usually two such switches signals the drive to decelerate to the creep speed (say, 5Hz) whereafter the second switch triggers the final deceleration to stop.

As soon as the drive reaches 0.5Hz a relay signal is output to the lift controller to open the relay and apply the brake.

Again, there is often a jolt or judder as the brake is applied against the still energised motor.

The above scenario is true even of the most advanced open loop flux vector inverters and indeed, Hitachi was the first to offer such a drive and that had lift arrival signals as standard.

But the smoothness of the ride still relied on the skill of the engineer tuning the drive.

In fact, the only way to get a really smooth ride was to install a closed loop drive control with feedback devices from the motor - a solution that remains the most common control system in high speed lifts.

Breakthroughs that have been brought about by Hitachi's new SJ300 inverter are its ability to generate full torque at 0Hz and the fact that the drive can apply intelligence to communicate accurately with the lift controller to effect brake application and release.

Significantly faster processors and improved control algorithms have brought about full torque at standstill speed, in what Hitachi has labelled its "Zero Hertz Domain", What this delivers to the lift application is the ability to ramp from zero to full speed with a completely smooth take off.

The Hitachi SJ300 drive knows when it has reached holding torque and sends a dedicated brake release signal to the lift controller to close the relay and release the brake.

The lift control can then pass a signal back to the drive to confirm the brake has released, before the drive applies current sufficient to ramp the motor to full speed.

There is no jolt on brake release.

The same is true for the deceleration cycle as the lift approaches the desired floor.

Again, the drive decelerates as before, but can now reach a complete standstill before the brake is applied.

Once more, the judder to a stop as the brake is applied is totally eliminated.

At the heart of the SJ300 lies a new state of the art power module that uses Trenchgate technology developed in Japan as part of a collaborative research and development programme in which Hitachi was the major partner.

This technology achieves 20-30% lower losses that traditional planar IGBTs (the power transistors) at 50% higher current density.

150% torque at 0Hz is achieved in open loop control without motor shaft oscillation.

At 0Hz close current control excludes excessive motor heat.

The ability to create torque in this way enables the new drive to deliver exceptional starting and stopping for applications such as lifts, as well as hoists and conveyors, where snatch is eliminated.

The technology centres on advanced new control algorithms that can be processed using Hitachi's own new 32-bit RISC processor at the heart of the drive's CPU.

The SJ300's vector control algorithm differs from the method used in Hitachi's previous flagship drive the J300, in that the new program uses slip frequency together with real, rather than assumed voltage values to make its control calculations.

This has resulted not only in full torque at 0Hz but a significantly improved open loop speed resolution of an impressive ñ0.5%.

Other unique features of the new drive include: on line auto tuning with compensation for the changing electrical characteristics of the motor as it warms with use; the ability to control by sensorless flux vector multiple motors from a single drive; on line adjustment of parameters such as PI and flux vector values; controlled braking; fieldbus compatibility; feedback card for closed loop control and direct floor approach; PLC functionality and interface with Hitachi's own EH150 PLC; removable I/O terminals, DC bus capacitors and fans for ease of installation and maintenance.

Back to top