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Motor Starter Selection

Not long ago, choosing the best type of starter to use in a control panel was easy, mostly because there was almost no choice! Now, however, there's an almost bewildering array of options. Moeller Electric's Steve Rickard looks at the most useful of these, and explains how to choose the best starter for the job.

A traditional motor starter for use in a control panel is made up of three fuses, a contactor and an overload with, of course, variations on this theme for reversing, star-delta and other more specialised starter types. Starters based on this apparently simple arrangement are, in fact, still widely used, even in new equipment.

There's nothing wrong with this, especially as this arrangement, in many cases, involves the lowest expenditure on starter components. It's worth noting, however, that this doesn't necessarily mean the lowest cost per starter - the traditional design has a lot of components to be mounted, and involves a lot of wiring. The resulting labour costs can easily outweigh the savings on hardware.

Traditional starters also have a number of shortcomings which may not be immediately obvious. For example, any modern starter should be expected to provide Type 1 or 2 protection co-ordination, as defined in BS EN 60947-4-1. Traditional starters can do this, but only if the components used to make them up are correctly chosen.

Control gear manufacturers provide excellent guidance on component choice, but it's all too simple to substitute, for example, fuses of the appropriate rating, but of the wrong type. This is a particular risk if blown fuses need to be replaced when the equipment is in service, as fitting the wrong fuse can seriously compromise safety. Even the actual use of fuses can be problematic. If a fault occurs, and there are no replacement fuses to hand, the result can be a substantial period of costly plant downtime while spares are being obtained.

An attractive alternative to the traditional starter is a version which uses just two components - a motor-protective circuit-breaker and a contactor. The motor-protective circuit-breaker combines short-circuit and overload protection in a single device, entirely eliminating the need for fuses. Often, it also provides facilities for isolation and in certain circumstances the switch can be used for the emergency-stop function in accordance with IEC/EN 60204-1. Now there are only two components, at the most, to mount and wire, resulting in significant savings in labour costs.

Further, the designer now only has to select the right contactor to go with the circuit breaker, and Type 2 co-ordination can be guaranteed. On site, users will rarely, if ever, need to replace any of the starter components, so the chances of this co-ordination being destroyed by the fitting of unsuitable components are negligible.

And, of course, if a trip occurs, the starter can be quickly returned to service simply by resetting the breaker, once the cause of the problem has been located and remedied.

Some control gear suppliers, including Moeller Electric, take this two component starter idea a step further, by providing motor starter combinations where the breaker and contactor connect together directly, without the need for any wiring at all. Not only does this save even more wiring time, it also produces what is, in effect, a compact fuseless motor starter which is a single unit that can be mounted simply by clipping it to a DIN rail.

The starters we have considered so far have all been individual units which require their own power, motor and control connections, but there's a trend towards decentralised and networkable motor starters where the power and control connections are taken care of automatically.

Essentially, these products are similar to the two-component starters already discussed, but they are designed to plug into a base module or chassis. This provides them with their main power supply, which means that individual power connections to the starters are eliminated.

The base module also provides the starters with a fieldbus interface, making them fully networkable. No conventional control wiring is needed and therefore these starters are very easy to integrate into modern PLC-based automation systems, which invariably have support for one or more fieldbus options.

To round off this article, let's try to answer the obvious question – what's the best type of starter to use? It depends, of course, on the application, but in many cases, the best choice is likely to be the two-component starter, made up of a motor-protective circuit-breaker and contactor. Starters of this type are easy to specify and use, they're competitively priced, they offer high levels of safety and they're available in a wide range of ratings.

The networkable starters are, however, likely to be a better choice for PLC-based systems, especially when a panel needs to be particularly compact, or the ability to reconfigure the control system without extensive rewiring is needed. They cost a little more than more traditional starters, but extra cost is offset by their greater convenience and flexibility in appropriate applications.

Note, however, that networkable starters are usually only available with modest power ratings. For drives of more than a few kilowatts, it will probably be necessary to choose a two-component starter and add a separate network interface module.

Finally, what about the good old fuse, overload and contactor starter? As stated earlier, there is nothing actually wrong with this arrangement – it's more that it has very few attractions compared with the more modern alternatives. It has a place, perhaps, in simple systems where the lowest possible component cost is essential, but it's becoming very hard to justify its use elsewhere.

There's no doubt that motor starting has changed in recent years, but only for the better. Designers now have more choice than ever before, allowing them to match their motor starting solutions to the exact technical and budgetary requirements of the application in hand.

This page last updated: 12 June 2007