Hot Topic

Moeller Delivers on Motor Protection with Innovative New Approach

[picture of ZEV units] In applications such as centrifuges, crushers and mills, high powered drives are required to start high inertia loads that in turn have extensive run-up times (in excess of ten seconds). The need to ensure that the drive starts first time must be balanced with the overriding need to adequately protect the motor once it is running.

Traditional electronic overload relays contain iron core transformers making the devices heavy, cumbersome and large in size. However, with the introduction of Moeller’s innovative ZEV electronic overload relay, the engineer has the benefit of a protection device which delivers full protection in the event of phase failure, overload or current imbalance in a simpler, lighter and more cost-effective package.

Common problems and the need for overload protection
The cost and availability of motors, particularly those in excess of 45kW, and any resulting downtime, makes motor protection a key factor in any application. In the event of phase failure or phase imbalance the motor windings are subjected to increased temperatures which may well damage the insulation of the turns and so reduce the operational life expectancy of the motor. The generally accepted figure is that a 10% excess temperature rise will reduce the life of the motor by half. Overheating can also be caused by an abnormal rise in ambient temperature and problems with the airflow over the motor. To effectively measure these events thermistors have to be embedded into the actual motor windings. Motor insulation faults generally result in an earth fault. Once the fault has been monitored and identified it is usually too late to protect the motor at which point it is essential to limit the damage to the surrounding area.

An innovative new approach to overload protection
Moeller’s new innovative ZEV electronic overload uniquely incorporates Rogowski coil technology. Rogowski coils have been used for many years in the measurement of a.c. electric current technology, primarily in test laboratories. Like transformers they work by sensing the magnetic field set up by the alternating current passing through a conductor, unlike conventional transformers the windings are wound on an air cored former rather than a ferro-magnetic core. The air-cored former can be flexible or rigid, as long as the cross sectional area and the turn’s density remain constant along its length. The method of terminating the end of the coils is also critical. The coil is simply placed around the conductor and the a.c magnetic field produced by the current induces a voltage in the coil proportional to the rate of change in the current. This is then fed to an electronic integrator to produce an accurate current waveform.

The benefits of the ZEV electronic overload relay
Moeller has incorporated Rogowski coil technology, into a commercially viable electronic overload relay. Designated as the ZEV, this new device brings a host of advantages over traditional protection methods:

Improved motor protection
As well as protecting against phase failure, overload and current imbalance, the ZEV can be configured to detect earth faults and can be connected to external thermistors within the motor. An early warning contact is available which can be used to trigger load shedding in the event of a potential trip current.
Improved fault diagnosis and reductions in downtime
The integrated liquid crystal display and function keys on the motor protective relay enable quick fault diagnosis, simplified start up procedures and easy and precise adjustment of settings. The optimum protection of the motor is ensured by features such as displaying the cooling time after an overload in real time and showing the motor current as a percentage of the full current load. The option of electrical remote resetting removes the need for maintenance personnel to attend the panel to manually reset the relay.
Space savings in the control panel
The ZEV occupies significantly less space in a control panel than conventional current transformers.
Simple installation
The small sensor belts are simply laid around the conductor and attached using a Velcro strip. This cuts out the need for tools and time consuming screw fixing of the current transformers to the mounting plate. The ZEV can easily be retrofitted into existing installations without any motor cable interruption and minimal downtime.
Reduced stock inventory
By using the Rogowski coil technology, the various current transformer types usually required for motor protection are replaced by just one current sensor covering the range 40A to 820A. The ZEV unit automatically adapts to different voltage in the most commonly used control voltage again reducing the amount of stockholding required.

In short the ZEV represents a breakthrough in motor protection technology. By employing a relatively unused technology, that the laboratory has only ever seen the benefits of before, the customer can take advantage of all the features that the ZEV delivers in an increasingly vital application area.

This page last updated: 9 February 2003