Motor starters assembled from only two components

Edited by the Engineeringtalk editorial team Aug 1, 2000

Moeller Electric's PKZM0 fuseless motor protection circuit breakers combine the function of isolation, motor switching, short-circuit and overload protection in a single unit

Moeller Electric's PKZM0 fuseless motor protection circuit breakers combine the function of isolation, motor switching, short-circuit and overload protection in a single unit enabling highly efficient motor starters to be assembled from just two basic components: a circuit breaker and a contactor.

The conventional fuse-based approach to protecting motor control systems requires designers to spend a great deal of valuable time and effort in selecting the correct components, such as fuses and fuse links, isolator, overload relay, and contactor.

Duty level, size of prospective fault currents and the type of short circuit protection needed must all be taken into account.

More often than not there is a degree of uncertainty over prospective fault ratings which in turn leads to problems in specifying the right components.

Particular care must be taken to ensure that type 1 or type 2 co-ordination for the whole system is not invalidated.

A three-phase motor using the conventional approach would require no fewer than nine separate components to be specified: 3 x fuse bases, 3 x fuse links, 1 x three-phase isolator, 1 x three-phase contactor, 1 x three-phase thermal overload relay.

Designing the layout of the components with drilling details, wiring diagrams and minimum space requirements makes a significant contribution to a lengthy design process.

Add the time taken to assemble the configuration, fitting the fuse links into the carriers and the overload relay to the contactor, fitting the starter to the mounting plate, preparing the wire links for connecting the fuse bases, isolator and contactor, and the time and the costs quickly mount up! But in the PKZM0 there is now another, better way of achieving the same end result.

The fuseless PKZM0 motor protective circuit breaker's modular design allows designers to quickly and easily select and construct a complete co-ordinated starter combination.

This may comprise a circuit breaker with dedicated contactor module up to 4kW (compact starter), or a circuit breaker plus DILM contactor and MVS kit, and if required, a wide range of accessories.

Direct-on-line, reversing up to 11kW and star delta starters up to 12.5kW at 415V can be formed easily.

Two versions are available to meet type 1 or type 2 co-ordination.

Although type 2 permits welding of the contactor contacts if they can be easily separated, the PKZM0/S00 compact starter has been designed so that no welding occurs, ensuring that the starters are ready to close again directly after switching off the short circuit.

When using the PKZM0/DILM combination all the designer has to do is choose from a comprehensive yet simple to use selection chart, the PKZM0 device in the current range to suit the size of motor and whether type 1 or type 2 co-ordination is required.

For the PKZM0 device chosen the chart gives the type of DILM contactor needed for each starter type, i.e DOL, reversing or star-delta.

It also gives the MVS configuration required to mount the combination.

The MVS wiring system is an extremely simple yet very effective means of reducing the costly time required for mounting, wiring and testing.

Using MVS, the motor starter can be assembled on a workbench into a compact co-ordinated unit, which can then be easily clipped or screwed into the control panel.

With MVS the engineer is guaranteed a correctly wired, short-circuit proof and short-circuit resistant electrical connection of protective and switchgear devices to form a factory-tested motor starter.

Thus with Moeller's PKZM0 motor protection system the designer wins many times over.

Fewer components makes it easier to select the right combinations, reduces the possibility of mistakes and saves valuable space.

Assembly and installation time is greatly reduced.

Taking all these factors together it is easy to see that Moeller's PKZM0 fuseless solution is much more cost effective than the conventional fuse-based approach.