Preventing stray current build-up in electric motors
Published: 08 June, 2017
When a motor is in use it is possible for the shaft to become electrically charged, which can be dangerous and also 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.
It seems that more and more motors are becoming electrically charged when they are running. In fact this has always been possible and 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, it is fair to say that drives can cause this problem, but we should also note that they do not always do so.
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 of course 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.
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 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 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.
The second problem is that 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 the motor. (Further, to the cost of this engineering effort, it may be necessary to add the cost of lost production or operating time.)
However, Lammers Exico says it has adopted an alternative solution which effectively stops stray current circulating in the motor while avoiding the need for an expensive insulated bearing. its 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.
For further information please visit: www.exico.co.uk
