ABB inverter drives with DTC improve cement kiln control and lower energy use.

Portland cement clinker is made by heating a mixture of raw materials in a rotary kiln at high temperature, before being ground into a fine powder, for use as the binder in many cement products. ABB drives with DTC play an important role in speed control.

The kiln is the most critical element in the cement making process. It is where the chemical processes occur that turn raw mix into clinker. The kiln also consumes the greatest amount of energy within a cement plant. Operation of a rotary kiln provides many challenges.

High starting torque

It comprises a very large and heavy steel cylinder containing substantial amount of material which is rotated with a drive system often having two motors. This means that the starting of the kiln requires high toque that can be 2.5 times the full load torque. At the same time, users need to synchronize the speed and torque of the motors to avoid damaging the drive gears of the kiln.

The kiln rotation is continuous and is never allowed to stop. In addition to the high starting torque, accurate speed regulation and dynamic torque regulation are key requirements. Accurate control is needed to ensure that the kiln is used to its capacity, whilst optimising fuel consumption, maintaining low running costs and meeting environmental standards.

The most common way to rotate the kiln, in modern installations, is to use inverter drives. This enables full control to cope with varying demands such as changing production volumes, and inconsistent quality of the raw mix.

Kiln control is best carried out by controlling each kiln motor separately using an inverter drive. When there are two motors, both are mechanically connected to the same kiln. Thus, the rotation speeds of the motors must be identical and the load must divide equally between the motors.

Drives master-follower connected

The drives are connected in a master-follower arrangement. The master drive is given the speed reference by the automation system. It then sends a speed reference and a torque reference to the follower drive. This arrangement ensures that the speeds of both motors are identical and that the load is equally divided between the motors.

Should a fault occur in any of the two motors or two inverter drives, the kiln can be rotated by the one remaining functional motor/drive pair. In this situation, the master-follower arrangement cannot apply. Also in this case, the automation system controls the one motor/drive pair only.

Direct torque control (DTC)

ABB drives using direct torque control (DTC), with independent control of speed and torque, ensure uninterrupted and stable running, irrespective of the kiln speed or load. This gives accurate speed control as well as dynamic torque control, enabling high starting torque to be produced.

The drives also enable soft start of the kiln, which minimises mechanical stress on the mechanical equipment. It also allows the supply network to be sized according to a low starting current, thus reducing the low voltage switchgear, transformers and cabling costs.

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