ENERGY-EFFICIENT MOTORS – NOT NECESSARILY A NO-BRAINER

Induction motors are ubiquitous in industry and invariably

offer energy-efficiency opportunities. These opportunities are

however not necessarily in the form of viable motor

replacement options.

Induction motors are used extensively in industrial facilities, and consequently can be responsible for a significant proportion of energy consumption and demand. Older designs are inefficient, and there are now a range of motors available which require less input power (for the equivalent amount of shaft power) than these older designs. In many countries, the use of high-efficiency designs is becoming mandatory. This is not the case in South Africa, and in my experience the uptake of these motors remains quite low, despite sustained increases in electricity prices. This got me thinking about why this is the case, and in this post I will explore some potential reasons for the relatively poor penetration of these devices, and also what you need to consider when assessing motor replacement opportunities. I won’t get into issues of torque in this post, but will remind you of the fact that a motor’s torque characteristics are an important factor, and the torque requirements of the driven process must be well understood when assessing replacement options.

Motor efficiency refers to the ratio/percentage of input power to output or shaft power for a given motor. The efficiency values you will see on a motor’s nameplate are quoted for full-load conditions. Energy-efficient motors require less input power for a given amount of shaft power. However, it is wrong to speak of a motor having a specific efficiency level. As discussed in a number of previous posts, motor efficiency is a function of load, with load being the proportion of the motor’s capacity that is actually used. If a motor is poorly loaded, replacement with an energy-efficient alternative of the same capacity would yield lower levels of efficiency improvement than replacement with a standard-efficiency motor of the correct capacity. Hence for poorly-loaded motors, correcting the loading problem is a higher priority than motor replacement.

Running hours are important, since the more hours that a motor runs, the more energy can be saved (remember that energy = the product of power and time). The energy savings are the product of the input power differential between the existing motor and its replacement and the running hours: Energy savings = (kW_existing – kW_replacement) x running hours. Clearly, even where a large power differential exists, if running hours are low, energy savings will be low, and the costs of replacement become difficult to justify.

As outlined above, if you can find motors with long annual running hours and which are well-loaded, high-efficiency replacement options should be further investigated. However, motor efficiency is not the only driver of operating cost for induction motors. A further consideration is the difference in power factor between the existing motor and the replacement motor. Power factor is the ratio of real power (in kW) to apparent power (in kVA). The lower a motor’s power factor, the higher the flow of current to the motor for a given real power requirement, and the greater the line losses (also called I²R losses) incurred in operating the motor. For a site that does not have power factor correction systems installed, a motor with a lower power factor (which could be a replacement motor with higher efficiency than the existing motor) will result in increased demand charges as well as some energy losses (these are typically small) due to the increased current flow in the site’s internal distribution system. A reduction in both of these costs can be achieved through the use of local capacitors close to the motor. Sites that have power factor correction systems installed at the point of supply will experience reduced site demand levels, but will not reduce I²R losses in their distribution systems, since excess current will still flow between the capacitor banks and the motor. For such sites, motor efficiency gains are still generally a bigger economic driver than these efficiency losses, particularly when you consider that it is the difference in power factor that is of interest, not only the power factor of the replacement motor.

The above are however not the only important issues when considering motor replacement. Something to bear in mind is that high-efficiency motors tend to operate at slightly higher speeds than standard-efficiency models, due to reduced slip. For fixed speed applications, this can have significant consequences for energy consumption. For example, for centrifugal pumps and fans, flow is proportional to speed, but power varies with the cube of speed. Small increases in speed can result in significant power increases for motors used in these applications. The situation could therefore be one in which the high-efficiency motor uses less energy than a standard-efficiency equivalent would have used for the same output power, but with this benefit negated by operation at a higher output power than was the case before the replacement. Such a situation is only acceptable where the increased power output is actually required, or can be managed - for example through reductions in operating time. How big a problem could this be? Consider a motor replacement option with a speed that is 1.3% faster than a standard-efficiency motor. Input power would increase such that P_final = P_initial x (speed_final / speed_initial)³ = P_initial x (1.013 x speed_initial / speed_initial)³ = P_initial x 1.013³ = 1.04 x P_initial, which is a 4% power increase! This could easily match or exceed the efficiency differential.

 

One final thought is that motors are part of systems, and system efficiency is the product of the efficiency levels of the individual components of the system. No matter how efficient a motor is, if it is driving an inefficient machine or process, replacement of the motor will have a limited impact on the efficiency of the system. For example, an inefficient motor driving a machine producing products in which only 50% of production meets specification with the balance ending up as scrap cannot be considered a high-leverage energy efficiency opportunity.  Not until the scrap problem has been resolved. This highlights the relationship between operational excellence and sustainability on industrial sites, something I will explore more in future posts.

What I've tried to show is that motor replacement on the basis of efficiency improvement is not a straightforward matter, and that hasty replacement without a considered analysis can actually lead to higher operating costs. Motor replacement is certainly not a "no-brainer" and calls into question moves to regulate motor efficiency standards.

Copyright © 2013, Craig van Wyk, all rights reserved