Certified motors offer proven safety

An ABB Automation Tech (Drives and Motors) product story

Edited by the Engineeringtalk editorial team Aug 11, 2008

The ABB approach of achieving certification for its motors ensures customers get reduced initial capital expenditure, lower operating costs and faster starting.

ABB motors has achieved stator and rotor component certification for its high voltage modular and cast iron induction motors up to 7000kW and rotor component certification for its low voltage cast iron motors.

The certification proves that its nonsparking (Ex nA) and increased safety (Ex e) motors meet the latest and toughest IEC and EN standards according to IEC60079-15:2005 and EN60079-15:2005 for Ex nA and IEC60079-7:2006 and EN60079-7:2007 for Ex e.

Compared with uncertified motors, the ABB approach of achieving certification for its motors ensures customers get reduced initial capital expenditure, lower operating costs and faster starting.

This is because uncertified motors need special measures such as prestart ventilation (purging), which requires a higher capacity air compressor, piping and a ventilation control unit, increasing capital expenditure.

Uncertified motors also require purging every time the motor is started, increasing the time taken to get plant up and running.

Reliability is also improved as no additional components are required.

Most importantly, ABB's certified motors offer proven safety, as testing represents the only way to verify that equipment is really safe.

The latest IEC and EN standards specify criteria for risk assessment and gas environment tests for rotor and stator designs to show that the motors are spark-free in all operational conditions.

The standards specify that a series of three tests should be carried out in an explosive gas environment.

The stator tests consist of a sinusoidal voltage test, to check for sparking during normal operation and an impulse test, which checks the stator discharges in case of voltage transients.

The rotor test, which is preceded by a process to simulate ageing, checks for sparking on starting or stalling.

The ABB motors achieved the following standards in the tests: all rotor tests passed (motors up to 15kV) with Group IIA, IIB and IIC gases; stator tests passed up to 15kV with Group IIA and IIB gases; and stator tests passed up to 13.2 V with Group IIC gases (including hydrogen).

Stator sparking consists of partial discharges at the winding surface due to high potential differences and it tends to occur continuously while the motor is running.

Typical causes are high voltages or inappropriate stress grading, degradation of the insulation due to ageing, voltage transients, or dirt in the windings.

Rotor sparking takes place during starting and occurs between the rotor bars and rotor core, especially in the vicinity of the first radial cooling ducts.

It is generally caused by shaking of the rotor bars in the slots, which interrupts current flow between the bars and core.

The shaking is the result of angular and radial forces which act on the rotor during starting.