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A Cast Iron Case - part two

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Oilfield Technology,

This is part two of a two-part article. Part one is available to read here.

Creating a firm foundation

Large HV electric motors are typically designed to be rigidly mounted on a massive, stiff foundation. A poor foundation design can result in vibration issues, particularly in large 2-pole motors. If the foundation is overly flexible, uneven, or of varying rigidity it may lead to installation and alignment problems, unplanned downtime due to high vibration levels, and reduced lifetime. Non-uniform foundation stiffness may cause larger vibrations in the motor frame. An uneven foundation may skew the motor’s structure, excite additional resonances, and induce vibrations. Thus a stable foundation of uniform stiffness is necesssary in order to improve the motor’s reliability, prevent downtime, and extend the unit’s lifespan.

Ease of integration

When choosing a flameproof motor, it is important to look beyond the main technical specifications. For example, the ability to mount the main terminal and auxiliary box in several different positions can offer greater flexibility during installation. This flexibility also minimises the need for spare motors, should they need to be swapped out.

Figure 1. Standardised accessories are available, such as a key phasor (which provides shaft orientation information making maintenance easier), vibration sensors (a total of three are possible) and temperature sensors which read the bearing temperature and improve reliability. Fixed points in the casting for auxiliaries keep the cooling channel clear for optimal airflow and cooling effect. 

The importance of maintenance

Customers have clear specifications in mind when it comes to purchasing a flameproof motor. However, misunderstandings can arise with maintenance as it needs to be a certified process. Although most procedures are the same as with a standard motor (i.e. cleaning and checking its operation), if the motor needs to be opened up then a team certified to perform this procedure is required. Naturally, a motor with a long life between each maintenance issue reduces the likelihood of this scenario occurring.

In some installations motors may be exposed to salt-laden environments that require particular attention to corrosion protection. Furthermore, they may require regular cleaning down to remove corrosive dust. IP55 ingress protection – whereby the ingress of dust is not entirely prevented, but dust does not enter in sufficient quantity to interfere with the operation of the motor – is the normal level of protection specified. In addition, the motor is protected from a water nozzle spray against the machine from any direction. However, where a motor may need hosing down, this could be increased to IP66. Such protection means ingress of dust into the motor is not possible and the motor can also withstand water from heavy seas or powerful water jets.

Flameproof motor and VSD packages

In many applications, using a variable speed drive (VSD) can offer significant improvements in energy efficiency by enabling the process to run at the optimum speed, rather than the fixed motor speed. The VSD controls the motor’s rotational speed by adjusting the fixed frequency of the supply voltage to a variable frequency.

Using a VSD can result in a further heat burden on the motor however, which can be a concern when it is installed in a hazardous area. This additional heating can result from the motor running at a lower speed, which reduces its natural air cooling. Issues can also arise from harmonic currents that cause the motor to draw an increased current, generating extra heat.

When using a VSD, the flameproof motor should therefore be specified as suitable for use in this type of application. The AMDR 450 and 500 motors have been developed for both direct online (DOL) and VSD operation, and are available as part of certified ABB motor-drive packages.

Optimising cost of ownership

Safety is the critical factor when specifying a flameproof motor for pumping and drilling applications in the oil and gas industry. It is vital to review not only the procurement price, but also the cost of ownership (COO) that also considers performance, reliability, ease of integration, and maintenance.

This is part two of a two-part article. Part one is available to read here.

Written by Patrick Tampik and Marco Nardi, ABB.

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