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  • IEC 60034-31:2009

  • 26 –



Applications involving frequent starts and stops and/or mechanical braking

High-Efficiency motor designs typically reduce I²R losses either by a reduced utilization, i.e. by oversizing the motor, and/or by improved conductor material in the rotor (for example die- cast copper instead of aluminium).

However both concepts automatically lead to increased rotor inertia when comparing high- efficiency with lower-efficiency motors of the same output power rating.

In applications where frequent starts and stops are required, the increased rotor-inertia will increase the run-up time and the power-consumption during run-up. It will also reduce the permissible number of starts per hour thereby possibly limiting the application’s throughput.

Furthermore, when braking is performed by a mechanical braking system, both the wear of the brake disc and the braking-time will increase with rotor inertia.

The run-up losses can be greatly reduced and the permissible number of starts per hour can be increased by using a frequency converter to start the motor instead of on-line starting. The general disadvantage of high-efficiency motors in this field of applications however remains.

NOTE A soft-starter will reduce the run-up torques but will not reduce losses nor improve the efficiency.

7.10 Some

Applications involving explosive gas or dust atmospheres design restrictions exist for electric motors for explosive gas or



Motors with flameproof enclosures (“d”) (according to IEC 60079-1) or type of protection “n” (according to IEC 60079-15) are usually not affected.

Motors with increased safety (“e”) (according to IEC 60079-7) could be limited by the requirements on tE-time, air-gap, startup-current etc. Their energy-efficiency level may be reduced.

Motors constructed for use in explosive dust atmospheres with dust ignition protection by enclosure “t” or “tD” (according to IEC 60079-31 or IEC 61241-1) have additional shaft seals.. Their energy-efficiency level might be reduced.

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