DRIVES, MOTORS & CONTROLS FEATURE


REDUCING STRAY CURRENT build-up in electric motors


When a motor is in use it is possible for the shaft to become electrically charged which can cause internal damage. Jerry Hodek of Lammers Exico outlines a simple solution


W


hen a motor is in use it is possible for the shaft to become electrically


charged which can be dangerous and cause internal damage to the motor. Many solutions to this are expensive and add significantly to the total lifetime running costs of the motor. It seems that more and more motors are becoming electrically charged when they are running. One of the main reasons for the increase is the growing use of electronic variable speed drives in conjunction with motors. Drives operate by changing the motor’s


supply voltage from its normal sinusoidal wave form into an asymmetric form which can lead to charge build up across the motor’s internal components. Therefore, drives can cause this problem but do not always do so. There are several other possible ways the problem can arise. Further common causes of circulating currents passing through the shaft and hence bearings and frame include magnetic asymmetry, electrostatic discharge and capacitive coupling between the stator windings and rotor. These are all independent of one another so may be present separately or simultaneously. Less common causes of charging include


supply voltage imbalance, unbalanced circulating currents in the parallel circuits of a three-phase winding, non-insulated through bolts in the rotor or armature, eccentric rotation due to inaccurate centring of the rotor, residual magnetism in the shaft and undetected short circuits in the rotor or stator cores. Such unexpected internal voltages within the motor can lead over time to damage of various motor components. Left uncorrected they will reduce the


operational efficiency, smooth running and working life of the motor. Often the first sign of a motor charging


problem is a rattling noise coming from the main shaft bearing which is often indicative of impending bearing failure. Inspection of the bearing may reveal pitting or fluting of the races and discolouration of the rolling elements and/ or races and the lubrication may have darkened in colour. Pitting is probably caused by the build-up and sudden discharge of very high voltages, an effect similar to spark erosion machining. Fluting is probably then caused by mechanical resonance vibration set up as the ball or roller bearings repeatedly pass over the lips of the pits. Discolouration is due to micro-cratering which is most commonly found in motors used with drives.


ADDRESSING CAUSES OF MOTOR CHARGING Addressing the causes of motor charging is difficult as many of them are often based on tiny inaccuracies of tolerances within the motors. While it is possible for motor manufacturers to reduce these during production and assembly, once the motors leave the factory they may be roughly handled, mounted inaccurately or


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Insulating the shaft rather than the bearing is proving cost effective, practical and popular


become worn due to asymmetric loading. So a better solution is to look for ways to disrupt the flow of stray currents around the body of the motor. In theory this is easily achieved, simply by using an electrically insulated bearing on the non- drive end of the motor’s shaft. However, there are two problems with the insulated bearing solution. First, an insulated bearing is significantly more expensive to buy that a non-insulated one of comparable size and performance. Naturally this extra cost has to be passed on to the purchaser of the motor and it is an unavoidable truth that the motor market is very price conscious and buyers may not appreciate the price/ performance benefits of an insulated bearing motor. In addition, the expected effective


working life of an insulated bearing, while long, is nothing like as long as that of an electric motor. Therefore the insulated bearing will need to be replaced, probably a couple of times during the life of the motor. There are considerable costs associated with removing a motor from its driven machinery, dismantling it, replacing the bearing then reassembling, testing and remounting it and possibly lost production or operating time.


APPLY INSULATION UNDER SHAFT AT NON-DRIVE END Lammers Exico has adopted a solution which effectively stops stray current circulating in the motor while avoiding the need for an expensive insulated bearing. Their answer is to apply insulation under the shaft at its non-drive end. This prevents the creation of a viable electric circuit while allowing the use of standard bearings and shafts, thus avoiding significant extra cost. With the increasing use of variable speed drives across virtually the whole range of driven plant and machinery and ever- more scrutiny of life time costs, the need to reduce stray current problems in motors is increasing. Lammers Exico’s approach of insulating the shaft rather than the bearing is therefore proving cost effective, practical and popular.


Exico Electric Motors T: +44 (0)1933 277930 www.exico.co.uk


CONNECTINGINDUSTRY / IRISHMANUFACTURING IRISH MANUFACTURING | AUTUMN 2017 31


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