Ages and Maintenance of Variable Load

Ages and Maintenance of Variable Load

Sizing to Variable Load :

1. Industrial motors frequently operate under varying load conditions due to process requirements. A common practice in cars where such variable-loads are found is to select a motor based on the highest anticipated load.
2. In many instances, an alternative approach is typically less costly, more efficient, and provides equally satisfactory operation.
3. With this approach, the optimum rating for the motor is selected on the basis of the load duration curve for the particular application.
4. Thus, rather than selecting motor of high rating that would operate at full capacity for only a short period, a motor would be selected with a rating slight lower than the peak anticipated load and would operate at overload for a short period of time.
5. Since, operating within the thermal capacity of the motor insulation is of greatest concern in a motor operating at higher than its rated load, the motor rating is selected as that which would result in the same temperature rise under continuous full-load operation as the weighted average temperature rise over the actual operating cycle. Under extreme load changes, e.g. frequency starts / stops or high inertial loads, this method of calculating the motor rating is unsuitable since it would underestimate the healing that would occur.
6. Where loads vary substantially with time in addition to proper motor sizing the control strategy employed can have a significant impact on motor electricity use. 
7. Traditionally, mechanical means (e.g. throttle valves in piping systems) have been used when lower output is required.
8.  More efficient speed control mechanisms include multi-speed motors, eddy-current couplings, fluid couplings, and solid-state electronic variable speed drives.

Maintenance :

1. Inadequate maintenance of motors can significantly increase losses and lead to unreliable operation. A example, improper lubrication can cause increased friction in both the motor and associated do transmission equipment.
2. Resistance losses in the motor, which rise with temperature, would increase. 
3. Providing adequate ventilation and keeping motor cooling ducts clean can help dissipate heat to reduce excessive losses.
4. The life of the insulation in the motor would also be longer : for every 10 ° C increase in motor operating temperature over the recommended peak, the time before rewinding would be needed is estimated to be halved.

A list of good maintenance practices to help ensure proper motor operation is as follows :

1. Inspecting motors regularly for wear in bearings and housings (to reduce frictional losses) and for dirt / dust in motor ventilating ducts (to ensure proper heat dissipation).
2. Checking load conditions to ensure that the motor is not over or under loaded . A change in motor load from the last test indicates a change in the driven load, the cause of which should be understood.
3. Lubricating appropriately. Manufacturers generally give recommendations for how and when to lubricate their motors. Inadequate lubrication can cause problems, as noted above. Over-lubrication can also create problems, e.g. excess oil or grease from the motor bearings can enter the motor and saturate the motor insulation, causing premature failure or creating a fire risk.
4. Checking periodically for proper alignment of the motor and the driven equipment. Improper alignment can cause shafts and bearings to wear quickly, resulting in damage to both the motor and the driven equipment.
5. Ensuring that supply wiring and terminal box are properly sized and installed. Inspect regularly the connection at the motor and starter to be sure that they are clean and tight.

Age

1. In India, most motor cores are manufactured from silicon steel or de-carbonized cold-rolled steel the electrical properties of which do not change measurably with age. 
2. However, poor maintenance, e.g. inadequate lubrication of bearings, insufficient cleaning of air cooling passages, etc. can cause a deterioration in motor efficiency over time.
3. Ambient conditions can also have a detrimental effect on motor performance. 
4. For example, excessively high temperatures, high dust loading, corrosive atmosphere, and humidity can impair insulation properties; mechanical stresses due to load cycling can lead to misalignment. However, with adequate care, motor performance can be maintained.