Motor running cost calculation
In many applications it is worthwhile replacing motors even when considerable working life remains. Motors can run without problems for 20 years or more with good protection and routine maintenance.
However, if they are running inefficiently, it is worthwhile replacing them as running costs are much more than first costs. Motors can be considered as consumable items and not capital items, considering the current energy prices. The importance of running cost can be seen from and Table. The following points may be noted:
Motor rating (kW) | 7.5 | 7.5 | 37 | 37 |
Efficiency, p.u | 0.86 | 0.88 | 0.92 | 0.93 |
Power input (kW) | 8.72 | 8.52 | 40.22 | 39.78 |
Running hours/year | 6000 | 6000 | 6000 | 6000 |
Energy input (kWh/year) | 52320 | 51120 | 241320 | 238680 |
Running cost @ Rs. 5 per kWh | 2,61,600 | 2,55,600 | 12,06,600 | 11,93,400 |
Running cost for 10 years (Rs.) | 2,616,000 | 2,556,000 | 12,066,000 | 11,934,000 |
First cost (Rs.) | 15000 | 18000 | 80000 | 96000 |
First cost as % of running cost for 10 years | 0.6 | 0.7 | 0.8 | 0.9 |
Even a small motor of 7.5 kW consumes, at full load, electricity worth Rs.26 lakhs in 10 years. Similarly, a 37 kW motor consumes about Rs.1.2 crore worth of electricity in 10 years.
The first cost is only around 1% of the running coast for 10 years. Hence running costs are predominant in life cycle costing.
Even a small difference in efficiency can make a significant difference in running cost.
When economically justified, motors may be replaced, even if these have been recently installed.
Proper Sizing of Motors for cost saving
It is important to remember that it is the load that determines how much power the motor draws. The size of the motor does not necessarily relate to the power being drawn. For example, a fan requiring 15 kW could be driven by a 15 kW motor; in which case, it is well matched. It could also be driven by a 30 kW motor, and although it would work, it would not be very efficient.
Motors are often oversized because of:
- Uncertainty about load;
- Allowance for load growth,
- Rounding up to the next size;
- Availability
Because motor efficiency curves vary substantially from motor to motor, it is difficult to make a blanket statement as to which motors should be downsized. In general, if the motor operates at 40% of its rated load or less, it is a strong candidate for downsizing. This is especially true in cases where the motor load does not vary much. If you have a 100 horsepower motor that is typically operating at 35 horsepower, for example, but periodically is required to operate at 90 horsepower, it may not make sense to downsize the motor. If your motor operates at 50- 100% of its rated load, it is probably not a good candidate for downsizing, since it is operating near its peak efficiency.
It often makes sense to replace oversized motors even if the existing motor has not failed. Remember, energy costs for a motor over the course of a year can be up to five times the cost of a new motor. This is especially true in cases where the motor is operating at a lower efficiency level due to over sizing.
Of course, there are benefits to over sizing motors in certain cases that should not be overlooked when determining what the proper motor is for a given application. In addition to providing capacity for future expansion, oversized motors can accommodate unanticipated high loads and are likely to start and operate more readily in under voltage conditions. These advantages can normally be achieved, however, with a modest over sizing margin.
The efficiency of motors operating at loads below 40% is likely to be poor and energy savings are possible by replacing these with properly sized motors.
Table gives comparison of cost of owning an oversized motor.
Description | Unit | Motor Rating | Motor Rating | Motor Rating |
Motor Load Requirement | KW | 15 | 15 | 15 |
Motor Rating | KW | 15 | 30 | 55 |
Motor Efficiency at operating load | % | 89 | 89 | 84 |
Input Power | KW | 16.85 | 16.85 | 17.85 |
Input Energy Input Energy Cost @ Rs.5/-(for 6000 hrs/annum) | KWH | 101100 | 101100 | 107100 |
Motor Power Factor | 0.89 | 0.75 | 0.50 | |
Input KVA | 18.93 | 22.44 | 35.70 | |
Energy Difference | KWH | 6000 | ||
Investment | Rs. | 25000 | 55000 | 95000 |
Increase in Investment | Rs. | 30000 | 70000 | |
Increase in Running Cost | Rs. | 24000 |
Oversized Motors lead to the following problems:
Higher investment cost due to larger size.
Higher running cost due to decrease in efficiency.
Higher maximum demand due to poor power factor.
Higher cable losses and demand charges.
Higher switchgear cost.
Higher installation cost.
Higher rewinding cost (in case of motor burnout)