Yaskawa gives elevators a lift

Mark Butters of Yaskic (UK), explains howawa Electr the company has improved ride comfort in traction lifts by providing inverter speed control.

It has long been recognized that ride comfort is the one selling point that can differentiate one elevator from another. As a result, over the last decade, inverter manufacturers have attempted to develop their products to provide a smoother ride in hotels, offices and tower blocks. If any proof of the efficiency of inverter use in such applications is required it can be provided with one statistic: in the past nine years, Yaskawa Electric has installed more than 100,000 inverter units in elevators worldwide. With statistics like that there is no need for sloganeering or marketing speak.

At present, the majority of traction lifts in the UK have single speed or two-speed motors, with or without variable voltage systems (thyristor control). The ride comfort of these lifts can be greatly improved when their drive systems are modernized using inverter speed control. This decreases energy consumption by up to 50% and, due to soft start and stop, maintenance costs and wear and tear are reduced. The inverters that are most commonly used in these applications are IGBT models, which offer high switching frequency and low acoustic noise. Over the last decade, since these inverters have been in use, the general purpose IGBT 7.5kW inverter has dropped in price by around 60% and continues to drop annually by a further 5%. As a result, many lift manufacturers now offer lift drive modernization packages, consisting of lift controller, inverter, braking resistor, EMC filter and motor contactors.

In order to best evaluate how to modernize each kind of lift, it might be profitable to examine the way they operate.

Single speed motors can be found in old lifts operating at speeds of up to 0.6m/s. These systems employ high slip lift motors, to obtain as high a starting current as possible. The lift starts and stops using contactors direct on-line. To reduce starting jerk, a large flywheel is installed. This flywheel stores energy, which is dissipated later and thus contributes to the low efficiency of such systems. A flywheel reduces starting jerk and stores this energy to be dissipated later.

Two speed lifts employ a pole change, or dual wound lift motor, and a flywheel to reduce starting jerk with direct on-line start. The low-speed winding produces a 25% rated speed, the same speed at which the brake operates. This means that the starting and stopping shock is lower than in a one-speed lift. When modernizing these lifts only high speed motor windings are used.

Both of these lifts use on-line starting, which means they share a number of common disadvantages. These include jerky starting and stopping, high starting current, load dependent leveling accuracy and excessive energy wastage through the flywheel.

AC variable voltage systems employ a pole changing lift motor with 4/16 poles. For braking, DC current is applied to the motor low-speed winding. The variable voltage system varies the R.M.S. (root mean square) voltage to the motor. As a result of this voltage reduction, the user experiences an increase in motor slip, which ultimately means speed reduction.

	n=motor speed f=frequency s=slip
Slip losses are proportional to slip.

P_slip = P_L·s
P_L = air gap power
P_slip=slip losses

(Speed Torque Characteristics Relevant to this section )

Furthermore, at low frequencies the firing angles of the thyristors are large and the harmonic content of the voltage becomes very high. As a result, the motor gets hot and the system efficiency drops. The motor efficiency of variable voltage systems is very low. Consequently, force cooling of the motor is necessary, which results in additional energy consumption in the system. Variable voltage systems were widely installed from the mid-eighties to the early nineties.

Inverter drives
The motor speed is proportional to the supply voltage frequency.

	n=motor speed f=frequency p=poles s=slip
Slip losses are proportional to slip.

Basic principle of an inverter
The basic principle of an inverter is that an input rectifier converts the 3-phase AC main voltage into a DC voltage, using pulse width modulation. This DC voltage to 3-phase AC voltage supplies the motor with variable frequency and variable R.M.S voltage.

The inverter provides stepless speed control down to zero speed while the starting current can be limited using pre-set parameters (these are typically 150% motor rated current). Thus, it provides shock-less operation at brake opening, acceleration, deceleration and brake closing. The auto tuning function reads all motor equivalent circuit data and, with this data, the inverter provides the correct flux to the motor in all speed and load ranges. Essentially, this is Vector Control.

The modernization of the lift allows almost the entire system to be retained, replacing only the controller and its constituent parts: the inverter, EMC filter, braking resistor and motor contactors. The controller selects the direction and pre-set speeds (including top speed, leveling speed and service speed) of the lift. Furthermore, the lift controller switches off the inverter's output transistors, in order to avoid switching the motor contactor while running. The inverter opens and closes the brake through the relay output and provides a digital output to allow contactor operation and door opening.

(diagram of lift relevant to this section)
(typical lift sequence relevant to this section)

This entire modernization process gives many advantages, such as improved ride comfort; reduced maintenance costs, improved leveling accuracy, reduced peak current and dramatically improved efficiency. The alternative, direct on-line starting of single or two-speed lifts, provides only limited ride comfort. Low efficiency worm gears allow inverter brake operation during DC injection (zero speed), even with open loop control. After brake opening, the inverter provides an ideal speed profile, which can be freely programmed (S-curve, acceleration and deceleration times).

As a general rule of thumb, and taking the two-speed AC system as a benchmark of 100%, the AC variable voltage system draws 70% and inverter system draws 50%.

An example of energy saving Using Schroeder's formula, one can calculate the daily energy consumption of a lift:

Ed [kWh] = R·ST·TP
00000000________
00000000003600

Ed = daily energy consumption in kW
R = motor rating in kW
ST = daily number of starts
TP = Typical trip time in seconds (10.5s for two speed lift)

Example of a two-speed lift, measured at 7.5kW motor, using 1440 starts per day:

Ed [kWh] = 7.5·1440·10.5
000000000___________
00000000000003600

Over 260 working days this means 11476 kW
11476kWh *0.299 £/kWh= £1752 per year.

Modernizing the lift by fitting an inverter would result in energy savings of approximately 40%. This gives an overall saving of £431.00 per year.

Buildings and peak currents
If an induction motor is started direct on-line, then the inrush current for each start is 5-8 times the motor rated current. If the same motor is driven with an inverter, then the starting current can be controlled, for example at 1.5 times the motor rated current. Reduced peak current means smaller building fuses and reduced costs for emergency generating sets.
Installing an inverter causes the brake to open and close without friction, which can increase the lifetime of the brake components considerably. Smooth acceleration and deceleration also increases the lifetime of the gearbox, bearings and all other moving parts.

One of the main reasons for selecting an inverter for lift drive systems' modernization is, particularly from the contractor's perspective, the leveling accuracy. Typical leveling accuracy of an empty lift fitted with an inverter is below ±5mm, while a two-speed lift has a leveling accuracy greater than ±10mm. Furthermore, the speed of single-speed lifts can be increased, if the mechanical and electrical components permit, by up to 10%.
A further advantage of modernization is that the process of fitting the inverter will inevitably reveal any mechanical problems. These can then be addressed to produce good lift performance and an extended lifetime for the inverter.
Before modernisation of the lift drive system using an inverter, the mechanical parts should be thoroughly examined and, if necessary, repaired. The existing massive hand wheel should be replaced with a plastic one. If this flywheel mass cannot be reduced, then the acceleration rate must be decreased, or an inverter with a higher current rating must be selected. A further alternative solution is to use the stall prevention function of the inverter.

Lift Motor
The older lift motors typically have very soft speed/torque curves, with 10 to 15% slip. In order to obtain soft acceleration the flywheels often have to be dismounted.

(high slip motor/levelling speed relevant to this section)

Because of motor slip, the motor speed in the up direction is different to the speed in the down direction. For example, with an empty car going up, with a 4Hz slip and a leveling frequency of 5Hz, without slip compensation, the actual motor speed will be almost nothing during the leveling run. At regenerative run, the motor speed will be (dependent on efficiency) 8Hz, 0.16m/s. This will cause bad leveling accuracy and shock during brake closing. However, with an inverter driven lift, there is a slip compensation function that automatically makes up for this variation. As a result, the leveling speed in the up direction will be the same as in the down direction.

The lift controller sequences in many lifts do not provide a timing sequence for the lift motor contactors. These are normally opened with the start signal and closed after the lift has stopped. However, this sequence causes motor starting without pre-magnetization and thus an unnecessarily high starting current. Generally, when a lift system is running through an inverter, the motor contactors shouldn't operate. If they do, the lifetime of the inverter will be shortened. Inverters have dedicated functions, such as run delay, contactor supervision and motor low current detection, which allow the user to avoid this type of dangerous sequence.

To give the best ride quality, when using inverter speed control in a lift, the drive's parameters must be modified and in most cases a drive expert is needed to fine-tune the drive to the individual lift in question. The selected inverter should be specified with lift software, optimised default settings and an easy to use interface.

Brake Control
The inverter should include a dedicated relay output, in order to control and interface with the mechanical brake. Before opening the mechanical brake, the inverter should confirm that the motor is connected and that a certain amount of current is flowing to it. At any time, when the brake is opened, the inverter should monitor the motor current. If motor current drops (below no load current) the inverter should detect it and operate the mechanical brake without any time delay. Bad contact can cause motor contactor opening during running.

Motor contactor control
Motor contactors should be closed before the inverter gets its run signal, and they should be opened after the inverter has stopped the IGBT switching. In many cases the inverter installation process doesn't provide this sequence to the lift controller. However, the inverter controls the motor contactor operation (digital output) and supervises the contactor, which is closed during running. After getting a run command from the lift controller, the inverter should delay the inverter ready signal (IGBT operation). This time delay ensures magnetic contactor operation, before the IGBTs are switched on. However, if the lift controller sequence is correct, this time delay becomes unnecessary.

Stall prevention
In order to prevent stalling, the flywheel mass (heavy hand wheel) should be reduced. The alternative is an unnecessarily high acceleration and deceleration current. However, the inverter provides a stall prevention function which keeps the output current at a preset value (150% inverter rated current) and extends the acceleration time, thus avoiding high starting current and protecting the motor from stalling.

In order to avoid over-torque, the inverter detects any motor over load conditions and activates the fault output. This will occur when the lift is overloaded or when there is a problem in the gearbox or brake rail guides.

Released April 2002