July 19, 2016 at 8:44 pm #209Alan Hansford
Risks to equipment come from several sources
Equipment issues spring from a range of sources: starting the motor, stopping and starting the motor too frequently, locked shaft overcurrent, high inertial loads and bearing damage, as well as issues with the electrical power service such as single-phasing and under voltage. Modern “green” motors bring greatly improved efficiency but also require close attention to their management.
Starting an electric motor – causes large electrical currents to flow in the motor windings. At start, the shaft speed is zero, and the motor currents are at their highest. This current is beneficial since it generates massive torque which is needed to get the motor spinning, but these high currents also rapidly heat the motor windings. For systems that start and run reliably, the starting currents and their rapid heating effects are not an issue. The motor will likely enjoy its normal life expectancy.
Frequent starts and stops – can cause the internal motor temperature to build up and eventually damage the winding insulation.
Locked shaft conditions – occur when a fan or conveyor line jams. Much like motor start conditions, the current in motor windings increases dramatically when the shaft is jammed. Internal temperature can very quickly reach damaging levels.
High inertial loads – result when large objects move onto a conveyor line or in various types of pumping operations. The motor will be under a high-current heating condition until that large object or flows are moving at their proper speed. High inertial-loads can cause insulation overheating and motor damage.
Bearing damage – results from lubrication failures, but also from failed electrical insulation. Once motor winding insulation begins to fail, electrical current can be shunted through the motor housing and shaft. This current seeks the easiest path to ground, which is often through the bearings. Current flows through bearings cause pitting and early bearing failure.
Single-phasing – results when electrical power is lost in one of phases of the three-phase motor. The motor will continue to operate with only two phases. The power imbalance causes higher current flow that can trigger overheating.
Under voltage – results when the electrical voltage is lower than required for the motor. Under voltage is an issue in facilities that operate at the limits of their electrical service. Under voltage causes current increases that can trigger overheating and damage.
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