ABB AC drives extend rock crusher life and save energy

ABB AC variable speed drives (VSDs) are widely used in mineral processing and cement making, where consistent raw material quality and continuous material flow are essential.  To obtain the correct particle size raw material, different types of crushers are available, and selection depends on the quality and quantity of material demanded. Hammer and jaw crushers are commonly used and VSD offer savings in crusher life and energy consumption.

Raw material quality is determined by its hardness, stickiness and water content. As such, quality variations can be wide and the crusher, therefore, needs to be able to adapt its speed accordingly, while providing accurate crushing to ensure correct particle size.

As crusher motors are typically rated at hundreds of kilowatts, starting needs to be smooth to minimize mechanical stresses and eliminate high starting currents. The power required by the crusher motor will vary according particle size required: the smaller the particle the more electricity is used in crushing it.

Independent motor control

Using AC drives for crusher motor speed control provides an effective way to meet the smooth starting and crusher motor speed variations. The operation of the AC drive(s) is controlled by the operator according to the crushing requirements, with each crusher controlled by its own motor and rive combination

Hammer crusher

If hammer crusher motors run at constant speed, the particle size cannot be controlled, resulting in particles that are consistently too small or too large. If the particles are too small then too much energy is being used during crushing, whereas too large particles will often mean extra crushing is needed at the cement or minerals processing plant.

Variable speed control of the crusher motors using AC drives ensures the correct particle size. With AC drives, the amount of energy used for crushing can be easily optimized. Increasing the motor speed raises the crushing energy providing the ability to create smaller particle sizes.

Crushing raw material causes the hammers to wear. This reduces the crushing effect such that small particle sizes are difficult to obtain, compared to the sizes possible with new hammers. Increasing the crusher motor’s rotating speed compensates for this weaker crushing effect, thereby extending the lifetime of the hammers and prolonging the maintenance interval of the crusher.

Jaw crusher

Unlike hammer crushers, variable speed control of jaw crusher motors does not affect the particle size. This is due to the different mechanical structure of the two crusher types. The energy needed in crushing depends on the raw material quality – variable speed control of the crusher motor enables increased crushing power whenever needed and constant throughput of crushed material.

Soft starting eases remote power

Many crushers are located in remote areas, where electricity transmission lines are long and the network is weak. With such supply networks, soft starting is important to avoid high current peaks caused by starting motors as this can create major voltage fluctuations that can trip electrical equipment.

With AC drives, motor starting is smooth, resulting in minimal mechanical stresses and elimination of high starting currents. The power factor of an AC drive is close to unity, meaning that only active power is drawn from the mains, so reactive power compensation equipment is not needed.

Should the crusher jam, an AC drive allows the direction of motor rotation to be smoothly changed to unblock the crusher. Change of rotational direction is only possible with rotor equipped crushers, like hammer crushers.

Synchronized motor speed balances loading

With double motor crushers, using AC drives in master-follower mode ensures that the speeds of both motors are identical and that the load is equally divided between the motors. Master-follower also eliminates undesirable dynamic effects, prolonging the maintenance intervals.

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