Andy Turtle, Automation Product Manager at Rugby based WAGO, explains how advanced motor and machine protection can now be integrated directly into the control PLC bringing considerable benefits to both the installer and the end user
Traditionally, motor control and protection components have been sold as individual items (contactors, thermal overload relays and short circuit protection devices etc.) More recently, we have seen a trend towards combining these devices into an integrated system. However, where more exacting protection of motors, machines and processes is required, separate protection relays may be considered, but ruled out, as they are not always easy to integrate and can mean considerable extra upfront cost.
Industrial motor protection
For industrial motor protection, specifying the most appropriate solution depends on a number of key factors.
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These include:
– Type of application (e.g. conveying, fans/pumps or hoisting etc.)
– Specification of the motor and utilisation (current, duty, temperature class, starting frequency etc.)
– Method of starting and control (Direct on line (DOL), frequency inverter, soft start etc.)
However, an important consideration, and one that is often over looked, is preventative maintenance which can have a considerable influence on productivity. By considering this at the outset when specifying the type of motor and machine protection that will be used, substantial cost and efficiency benefits can be realised.
Preventative maintenance
These include:
– Maximising plant ‘up time’ and increasing productivity
– Efficient use of manpower through planned maintenance
– Reliability of production or service provision to ensure customers are not inconvenienced.
Most low cost motor protection devices incorporate an inherent flaw: they only let you know there is a problem when they trip. At this point of course it can be too late, as the machine, or indeed any other part of the process, may be already damaged due to overloading or overheating. Significantly, the process will stop whilst the problem is identified and rectified which can result in production being disrupted with potentially severe cost consequences.
One solution is the advanced motor protection relay (or ‘motor manager’) that monitors the current to the motor and calculates the I2t heating effect. Such devices will also usually feature a pre-alarm function which provides the additional all- important warning of an impending problem.
However, they can be expensive to purchase and difficult to integrate due to their size and their requirement to physically measure current. What’s more, they are not measuring actual motor temperature rise so performance can be compromised in applications where motor starting is arduous, e.g. high ambient temperatures, long starting times or frequent starting.
Furthermore, whist it is often assumed that mechanical problems associated with the machine will be reflected in an increased motor current, this is not always the case.
Solution for mechanical problems
There’s a solution at hand…
To get around this dilemma, the use of physical temperature rise monitoring is an effective solution. Not only can the motor be monitored using probes embedded in the windings, but also other critical parts such as bearings, gearboxes and fluid couplings, etc. can be checked.
It is also possible now, using products such as WAGO’s new thermal protection I/O modules, to integrate this capability approach, many critical areas of the machine and / or motor can be monitored easily within the automation solution as the compact modules can facilitate up to 8 PTC (positive temperature coefficient) thermistor channels.
The 750-1425 8 channel module can monitor up to 6 PTC thermistors per channel, a total of up to 48 in one 12 mm wide I/O module. Even though PTC probes operate at a fixed threshold, they can provide advanced warning of a problem developing as a physical temperature rise will usually indicate deterioration of the equipment.
If a more predictive, proportional temperature monitoring is required to enable even closer protection of very critical areas, then an analogue temperature measurement device such as a thermocouple or RTD (resistive temperature device) can again be connected via a specialised I/O unit (such as the WAGO 750-451 8 channel 2 wire RTD 12mm module).
With these analogue devices integrated in the PLC, setting up one or more pre-alarms is a simple case of including threshold comparators within the machine’s control program. The status of these comparators can then provide alarms or directly apply remedial action such as switching to a standby system, reducing the load or enacting other measures that can prevent a stoppage from occurring.
Recently there has been much discussion about the importance of productivity and how UK businesses are not necessarily as ‘productive’ as those in other countries. In simple terms, productivity can be described as the ratio of output to input. Manufacturers and process operators can clearly benefit from any improvement in this ratio. A key element in terms of input will be the cost of maintenance; conversely lost production directly impacts on output.
Installing equipment that increases productivity will have long term benefits and improve the competiveness of the business by reducing the fixed cost base. Having said that, securing capital expenditure budget is not always plain sailing, so justifying the best returns on the budget available is very important.
When it comes to maintenance, the choices that are made can have significant impact. A long term view will not only help productivity but improve the business’ reputation for being a supplier that can be relied on to make on-time deliveries whilst being competitive on price…
In conclusion, choosing an integrated control and diagnostic monitoring motor control system can provide an extremely cost effective solution for a preventative maintenance approach and provide significant benefits.
Benefits for the installer:
– Cost reductions thanks to space saving, reduced wiring and lower overall equipment costs
– Easier diagnostics and commissioning thanks to a fully integrated control and monitoring solution
Benefits for the end user:
– Possibility of achieving a higher degree of motor and machine preventative diagnostic and remedial action whilst minimising the equipment and staff costs.
– Ability to carry out maintenance at a time that does not impact production.
– Reliability of deliveries to customers and avoidance of penalties associated with contravening a service level agreement.
– Increased productivity.
