**6. Case study 2: Wide span gantry crane**

The experimental behavior analysis of some drives is considered on the example of crane with wide span, which in sugar factory serves for continuous transport of sugar beet from the reception position to the factory storage.

The crane with following details has been taken for experimentation with adjustable frequency drive:


Gantry crane for sugar beet storage is designed from the following functional parts:


Electrical Drives for Crane Application 151

difference of estimated electromagnetic torque (*Te*=*Te*1-*Te*2). Proportional gain of load

In order to ensure the stabile operation of the motors during the large external disturbances, especially at low speed when estimation of electromagnetic torque in speed sensorless drives looses on accuracy, it is necessary to limit the correction value *n*\*, as

For the purpose of suggested algorithm verification the trolley load sharing is analyzed. Because of the short distance between left and right side the skew may be neglected. Trolley drive consists of four motors, two on each side (IM1-IM2 on left and IM3-IM4 on right side). Frequency converters are set on speed sensorless vector control mode. Motors have the common reference speed. In the Fig.20a) motors torque without load distribution is shown. At reference speed, in steady state, we can see that even the motors have the same rated power, load torques are different. Estimated motor torque is not applied in control algorithm. Speed between left and right side is different because it depends of motor

sharing regulators is denoted as *KLS* .

characteristics and load, as shown in Fig.20a).

Fig. 19. The principle of load sharing based on estimated torque.

be applied for all kinds of multi motor drives on cranes.

Effect of load sharing is shown in Fig.20b). The approximately equal motors torque on the same leg can be easily seen. Used system enables that speed of every motor is regulated, but also the load difference is controlled. In this way the load difference is being maintained on

Depending on the purpose of drives and needed accuracy of maintaining load distribution, load controller can be with only proportional effect, but also with proportional integrated effect. In our case only proportional controller with *KLS*=1 p.u. is used. Output from the load controller is restricted on only several percentages of maxsimum speed reference (in our example *n*min-max=2%). That is quite enough to provide necessary load regulation and not to "break" the drive speed regulation by too big effect on the speed reference. This solution can

shown in Fig.19.

the desired accuracy.

In the Fig.18 gantry crane with indicated drives is shown. All motors are three phase fed by frequency converters.

Certainly, the most complicated is the gantry drive, from following reasons:


Basic requirements set in front of this drive are: equal load distribution between motors located on the same side, as well as skew elimination between fixed and free gantry leg.

Fig. 18. Gantry crane with indicated drives.
