Electrical motors are very often oversized for the real load they have to run. Motors rarely operate at their full-load point. In the European Union, field tests indicate that, on average, motors operate at around 60 % of their rated load.
The maximum efficiency is obtained for the motors of between 60 to 100 % full load. The induction motor efficiency typically peaks near 75 % full load and is relatively flat down to the 50 % load point. Under 40 % full load, an electrical motor does not work at optimised conditions and the efficiency falls very quickly. Motors in the larger size ranges can operate with reasonably high efficiencies at loads down to 30 % of rated load.
●improves energy efficiency, by allowing motors to operate at peak efficiency
●may reduce line losses due to low power factors
●may slightly reduce the operating speed, and thus power consumption, of fans and pumps.
Harmonics caused by speed controllers, etc. cause losses in motors and transformers ( see Section 3.5.2). An EEM takes more natural resources (copper and steel) for its production.
Electric motor drives exist in practically all industrial plants, where electricity is available.
The applicability of particular measures, and the extent to which they might save money, depend upon the size and specific nature of the installation. An assessment of the needs of the entire installation and of the system within it can determine which measures are both applicable and profitable. This should be done by a qualified drive system service provider or by qualified in-house engineering staff. In particular, this is important for VSDs and EEMs, where there is a risk of using more energy, rather than savings. It is necessary to treat new drive application designs from parts replacement in existing applications. The assessment conclusions will identify the measures which are applicable to a system, and will include an estimate of the savings, the cost of the measure, as well as the payback time.
For instance, EEMs include more material (copper and steel) than motors of a lower efficiency. As a result, an EEM has a higher efficiency but also a lower slip frequency ( which results in more rpm) and a higher starting current from the power supply than a motor of standard efficiency. The following examples s how cases where using an EEM is not the optimum solution:
●when a HVAC system is working under full load conditions, the replacement of an EEM increases the speed of the ventilators ( because of the lower slip) and subsequently increases the torque load. Using an EEM in this case brings about higher energy consumption than by using a motor of standard efficiency. The design should aim not to increase the final rpm
●if the application runs less than 1000 − 2000 hours per year (intermittent drives), the EEM may not produce a significant effect on energy savings (see Economics, below)
●if the application has to start and stop frequently, the savings may be lost because of the higher starting current of the EEM
●if the application runs mainly with a partial load (e.g. pumps) but for long running times, the savings by using EEM are negligible and a VSD will increase the energy savings.
The price of an EEM motor is about 20 % higher than that of a conventional one Over its lifetime, approximate costs associated with operating a motor are shown in Figure 3.30:
When buying or repairing a motor, it is really important to consider the energy consumption and to minimise it as follows:
●payback period can be as short as 1 year or less with AC drives
●high efficiency motors need a longer payback on energy savings.
Energy Efficiency (2009) 3.6.2