How a 24.5 MW Compressor Motor Guarantees Production in the Petrochemical Industry

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TURCK GmbH

EDL Anlagenbau Gesellschaft mbH

ProMinent GmbH

Siemens AG

60 Ton Motor

The cracker plant contained an enormous 19 MW induction motor with a direct online starter supplied by Siemens, which drives the raw gas compressor. This is now replaced by an even larger motor, a 60 ton, 24.5 MW motor specially manufactured by Siemens in Berlin. “This is a real monster. As far as I am aware, there are no other induction motors with a direct online starter as big as this anywhere else in the world,” says Tobias Andersson, Chief Electrical Engineer for this Borealis project who wrote the specifications. “This is my baby, and it was very satisfying when the motor was delivered,” he continues.

Andersson was also present at the Siemens factory in Berlin when the motor was being custom-manufactured.

The motor was delivered to Stenungsund in September 2011, two years after the draft specification was drawn up. It took just over one year to build the motor and it was split into three parts because of its size. “It’s a real piece of craftsmanship,” says Mats Hagman, technical support employee working for Siemens Large Drives.

The motor has been specially manufactured based on Borealis’ requirements for guaranteed operational availability: it had to be able to operate for as long as possible and to tolerate external disturbances.

“We are extremely demanding in terms of availability. If the motor were to stop, it would take a week or more to restore plant operation,” says Samu Salo.

Fewer Weak Points

Therefore, it was not appropriate to have a synchronous motor, as these are more sensitive to voltage dips that can occur in connection with lightning strikes. A synchronous motor with its magnetizing equipment has a number of sensitive components that can fail. An induction motor has fewer weak points where it can fail, and is therefore much more suitable for continuous operation. Admittedly, induction motors are less efficient than synchronous motors, but for Borealis, reliability was the most important issue.

Challenges

A direct-starting squirrel-cage induction motor was selected; a motor with a direct on line starter also reduces the number of weak points where things can go wrong when compared with a motor with separate starting equipment.

However, an induction motor has a high starting current, which makes it more difficult to start than a synchronous motor. One of the challenges of the development work was therefore to limit the starting current without having too much of a negative impact on the performance.

The result was an innovative solution. “An induction motor requires five or six times its normal current at the instant that it starts. The electricital grid for the whole of Stenungsund experiences a dip for up to 30 seconds when the motor is started. The big challenge was for the motor to still start despite a drop in the grid voltage,” says Tobias Andersson.

The grid is used as the starting network. Instead of using a starting transformer, a block transformer was connected directly to the motor in order to reduce the starting voltage and therefore lower the starting current. However, this causes a loss of torque. As a consequence, a motor had to be designed with a very high starting torque and also a low starting current.

“It’s a tricky equation with such a large motor, where there are requirements concerning starting current and starting torque, and at the same time the short-circuit output must be considered,” says Tobias Andersson.

The motor’s external features were also specially adapted. More on the next page.

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