FEATURE DRIVES, CONTROLS & MOTORS
PREVENTING 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 be dangerous and can cause internal damage to the motor. Many solutions to this are expensive and also add significantly to the total lifetime running costs of the motor. Jerry Hodek, managing director of Lammers Exico, looks at the details of this often overlooked phenomenon and reveals his company’s simple solution
I
t seems that more motors are becoming electrically charged when running. One of the
main reasons for this 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. It is also important to recognise that there are several other possible ways the problem can arise. Other 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 they 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. Related to this, the lubrication may have darkened in colour. 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 and are in the ‘real world’ they may be roughly handled, mounted inaccurately or 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. The first is that an insulated bearing is very significantly more expensive than a non-insulated one of comparable size and performance. Naturally this extra cost has to be passed on to the purchaser of the motor. The second problem is that the expected effective working life of an insulated bearing is not as long as 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, dismantling it, replacing the bearing, then reassembling, testing and remounting the motor. Lammers Exico has adopted an alternative solution that 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. 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 provide a cost effective and practical solution.
Lammers Exico
www.exico.co.uk
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