When it comes to downtime, motors are a disproportionally large contributor compared to other causes, simply because motors are used in so many processes, from pumping to material management. So, a failure in a motor will have a serious ripple effect on productivity.
We’re wrapping up our series on Motor Protection. Before we get into the fifth and final instalment, take a moment to catch up on previous articles in the series…
Part 1 - the importance of motor protection
Part 2 - fuses vs circuit breakers
Part 3 - Motor Protection Circuit Breakers (MPCBs)
Part 4 - Motor Circuit Breakers
In the fifth instalment, we tackle coordinated motor starters...
How do motors fail?
The most common failure mode for a motor is when it is under excessive load. Such a condition causes an increased draw of current, which then causes overheating. It’s this overheating that causes the undue wear on motor components that ultimately results in failure.
However, its short circuits in motors that pose the greatest dangers for the equipment, the installation, and for personnel. There are a few possible sources, but any short circuit can cause a sudden and massive surge of current within milliseconds that can melt and destroy contacts, produce electrical arcs and even explosions. Short circuits can not only cause physical damage – they can easily lead to catastrophic fires. In fact, it’s estimated that 60% of fires are due to short circuits.
Standards that address motor short circuits
The International Electro-technical Commission (IEC) has recognized that electrical faults in motor control are a critical situation that needs to be properly addressed to ensure safety of personnel and equipment, as well as productivity of an installation. Therefore, the IEC has developed specific standards and testing procedures to ensure that the combination of devices used to control motors – i.e. motor starters – is safe. This is achieved through “coordination”.
A ‘coordinated starter’ is a tested combination of short circuit protection devices (e.g. fuses or circuit breakers), switching devices, and overload protection devices. The motor starter combination is tested under extreme conditions and is required to clear the fault quickly, without damage to the installation or risk to the operator. This makes it the optimum, tested combination of devices to ensure safety.
There are two types of coordination included in the IEC 60947-4-1 standard: Type I and Type II. There is also an additional level of coordination specified by IEC 60947-6-2, which Schneider Electric refers to as ‘Total Coordination’. Let’s have a look at how each of these are defined.
1.Type I Coordination. This coordination level provides a basic solution, which balances uptime and cost effectiveness. It’s a good choice when machine uptime is not a priority, as this level does not provide continuity of service. Also, there are associated maintenance costs. Before restarting, replacement of the starter is recommended
2.Type II Coordination. This is a high performance solution, providing reduced machine shutdown time and reduced cost of equipment replacement. The motor starter must be ready for reuse after a fault, with only a simple inspection required. Contacts may become welded but are easily separated.
3.Total Coordination. This is the highest possible performance, ideal for mission-critical operations and processes. There is no damage to the equipment, and following the fault the starter must be immediately reusable. No resetting is required, and no inspection is needed. Therefore, the installation can return immediately to operation. This level of coordination is listed under a different standard, as it is defined for combination devices with multiple functions.
Choosing coordinated motor starters
You can see that these levels of coordination enable a range of benefits, from protecting people and equipment, to permitting a greater continuity of service, to reducing maintenance costs by avoiding the time and cost of replacing equipment. Given the increasing costs of downtime and machine or process failures, it’s clear that coordination of motor starters is moving from a choice to a necessity, as they deliver a tested, guaranteed solution.
To begin, you should first determine the level of coordination that suits your specific application and business requirements. Next, you should choose a manufacturer that has ensured that their motor starter complies with coordination standards. This requires testing the combination of components to the associated standard, and accordingly publishing the coordination tables for selection.
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