Case Study

The Coolmore quarry in Cork is operated by John A Wood. The quarry uses a Sheepbridge single rotary crusher which was originally fitted with a 160kW slip ring motor and starter combination. The replacement equipment was required to provide extremely reliable control together with significant energy savings at start up.

The electricians at John A Woods had initially calculated that a 200kW AC motor would be required to cope with the load. After detailed tests and analysis however, The Reliance Bearing and Gear Company, working with Invertek Drives Ireland, was able to recommend a 160kW squirrel cage motor controlled by an Optidrive P2.

The first challenge was reducing energy consumed on start-up. Traditionally, slip ring motors are commonly used on high inertia loads because of their superior starting torque, efficiency and their ability to start high inertia loads.

Starting this type of high inertia load direct-on-line with a standard squirrel cage motor typically takes up to 550 per cent of rated current for 60 seconds. Starting the same machine with a slip ring motor requires only 200 per cent increase in current for around 20 seconds. In this instance the Optidrive P2, together with a squirrel cage motor, was able to start the system in only 40 seconds whilst taking only 27 per cent of motor rated current. This provides an extremely energy efficient solution.

As the stone feeds into the crusher a peak load is created, which could have led to the drive tripping on over current. In addition, the crushing drum can undergo very short periods of light load in-between crushing one large stone and starting on the next one. This could cause the drum to speed up and the drive to trip on over voltage. Reliance Bearing and Gear were able to overcome these challenges by fitting a brake resistor and adjusting the perfomance of the Optidrive P2 vector speed controller.

The next challenge was to guarantee that if the machine was not running at the rated operating speed for any reason it would not fill with material and cause a blockage. The Optidrive P2 was programmed to provide control signals to the feeder line. The result was that if the drive tripped or was not at full speed the feeder would then also stop, the momentum of the rotor would crush the material in the crusher and the machine would be cleared.

Finally, the spin start feature on the drive was enabled to provide rapid and smooth re-start in the event of losing the incoming power supply. The spin start feature is able to detect the speed of the motor and then match the drive to that speed before accelerating to its final operating frequency.

Stephen Tighe of Invertek Drives Ireland said: "The Optidrive P2 has delivered excellent energy savings in this application whilst maintaining extremely high levels of performance and reliability. We would certainly expect this success to lead to increased demand for Optidrive P2."