By Steve Meyer, Contributing Editor

Extremely large air handling systems can save considerable power when using either variable-speed controllers or constant-speed, high-efficiency and premium motors instead of standard motors. 

The list of ways in which we can reduce energy usage is a big issue for today’s US government policy makers. The Department of Energy (DOE) has been allocated a $9.8 billion budget for 2008, but it has already spent hundreds of millions of dollars in the last two decades for programs that were intended to make electric motors more efficient, often with limited success. Based on DOE studies, 25% of all the electricity generated in the US powers electric motors, and for industrial plants alone, the number is about 70%.  In view of this, industrial users must look at their plant operations and strategically replace older equipment with newer, more energy-efficient systems.

Government Intelligence
To help reach this goal, President Bush has signed legislation that mandates the use of energy-efficient motors and control systems and allows accelerated depreciation for companies that take advantage of other methods to reduce energy costs in their operations. For example, many utilities provide $15/hp in cash incentives to purchase high-efficiency motors because it costs less than adding plant capacity to the US electrical grid.  The utilities benefit further by selling more capacity to consumers at $0.15/kWh and less to the industrial users who pay only $0.08/kWh.

Also, the DOE offers a cooperative program that allocates 50% of the cost to conduct a detailed energy audit of a plant or building.  This is a definite savings because plant operations and building owners can measure the energy usage and record data concerning their facility that can help them find the best opportunities for return on investment (ROI). And these opportunities include more than just purchasing energy-efficient motors.

Motor efficiency
Manufacturers have gotten just about as much efficiency as they can ever expect to see out of a motor by using the latest materials for stator laminations, improved rotor bars, tighter manufacturing tolerances, and denser copper fill. Small electric motors are typically 80 to 85% efficient, and larger, high-efficiency and premium motors can reach 96 to 98%. Trying to squeeze the last 1 or 2% of efficiency out of a motor is just not possible. 

High-efficiency brushless servomotors and controls from Baldor, Fort Smith, Ark., deliver higher torque in smaller packages, run quieter, and require less maintenance than standard motors for numerous variable-speed applications. The new series of energy-saving motors come in NEMA mountings from ¼ to 3 hp at 1800 rpm, and can be used for material handling, conveyors, mixers, and other general purpose industrial applications.

Motor sizing
System efficiency depends not only on motor qualities, but also on how well the motor matches the load. Big energy returns are a function of how precisely we can match the demand for power to load variations. For instance, pumps and fans typically regulate fluid flow through mechanical valves or dampers, which develop high impedances and place excessively large loads on the constant-speed motors. But fan and pump speeds can be electronically controlled to vary the flow with variable frequency inverters and on-board controllers. By adding intelligence to the power electronics, the system can be configured with advanced algorithms and motor field-current controls to maximize the energy savings regardless of varying load conditions. Although the variable-speed hardware initially costs more than contactors and relays, energy and other kinds of savings come from less electricity usage over time and extended operating life of the components.

In many applications the motor size is selected based upon starting torque requirements. This approach can end up with the installation of an over-sized motor, which more often than not, wastes energy during normal running periods. This problem becomes pretty evident when examining the equation for calculating the torque required for a specific acceleration. The innocent-looking factor is the time (t) required to accelerate the load.  When motors are connected across the line with a relay or contactor, the torque is:

Tacc= WK2 (S2-S1)
                 308 t
Where:  Tacc  =  torque of acceleration, ft-lb
WK2 = inertia of the load (motor and load)
  S2-S1 = change in speed, rpm
  t =  time, s

Motor starting
Across-the-line starting hides the fact that the motor must accelerate to full speed, typically 1750 rpm, within just a few cycles of the ac source frequency.  If the motor attempts to spin the load up in 3.78 s, then the torque requirement and stress on the load is extremely large. This also requires that the motor be sized to adequately start the load given the very short amount of time, which generally means a much larger motor is needed to accelerate the load than to run it.
For example, given a certain motor with a load specified as WK2 , a torque multiplying factor can be calculated when starting across the line. The speed change is specified as 1750 rpm during a time of 3.78 s.  This yields the following torque factor:

Tacc = 1750/(308)(3.78) = WK2 x 1.5

But, by comparison, if the load is allowed to be started in 5.68 s, the required torque multiplier is reduced from 1.5 to 1.00:

Tacc = 1750/(308)(5.68) = WK2 x 1.00

As might be expected, a significant increase in the time allowed to start the motor demands less power than otherwise needed. Mechanical stress on equipment also shrinks proportionally, which increases life expectancy and reduces maintenance costs.

Compared to mechanical damper and vane-controlled fluid flow systems, a variable fan-speed system can save a considerable amount of power by electronically running the fan at lower speeds when needed to reduce the fluid flow.

In a mechanical damper and vane controlled fluid flow system, the fan motor continuously runs at 100% of speed and power regardless of the amount of flow, which also continuously consumes the maximum amount of power.

An extraordinary example of optimally matching the motor to the load and using electronic controls can be seen in new appliances. Many new washing machines do not just get a 4 or 5% reduction in operating costs, they can realize 40% or more in energy efficiency. The savings do not just come from motor technology, but from a new mechanical approach that adds control intelligence to the system, which nets these great energy savings. The starting point in the design was the elimination of the traditional transmission system. A large diameter, aluminum flywheel and belt and pulley drive with a large reduction ratio gives a small ac motor the mechanical advantage needed to run both the low-speed agitation cycle and the high-speed spin cycle. By using an intelligent drive to manage the motion, the control system can agitate the basket, change over to spin cycle, and regulate electric power through the motor, which reduce operating costs by more than 40%. This dwarfs the energy savings that can be achieved through direct improvements on the motor alone.

To ensure that a particular motor can handle a specified load, compare the motor’s speed-torque curve to the load’s speed-torque requirements. As long as the motor torque is above the 100% line and the load is below the 100% line, the motor to load is adequately matched.

With US washing machine manufacturers producing more than 500,000 new machines per month, reducing the energy consumed in these machines have a tremendous national impact. For example, if the average family runs 12 loads of laundry per week using approximately 7.2 kWh per week, this amounts to 374 kWh per year.  If we can successfully reduce the energy consumption of 6 million washing machines per year by the combination of intelligence and better mechanical design, a 40% reduction in energy consumption would result in a 900,000,000 kWh reduction nationally. 

Energy saving hints
Replacements Motors: Unfortunately, sometimes replacement motors are installed based on motor availability, so if the exact motor size can’t be found to replace a failed motor, the next larger motor size will be installed. This can quickly become a significant over-sizing problem because motor frame sizes are limited.  A 10-hp motor might have to be replaced with a 15-hp motor if the 10-hp replacement motor is not available.  This is a 50% oversized motor, and even though the motor won’t use all the rated current because it is not fully loaded, some excess power will be wasted just running it. Before replacing a motor, read its nameplate rating or measure the motor current to determine how well it matches the load. If it is over or undersized, it might be more cost effective to replace it. 

Wire sizing: In industrial machinery applications it is more difficult to find major energy savings. But it might be worth the time to check motor leads with an ammeter to determine if they are sized correctly. Undersized wires and cables have a high resistance that dissipates power and limits the current available to the motor under high loads.

Material handling: If motors in conveyors and material handling equipment are under some type of programmable control, determine if they can be idled when not needed to save energy. 

HVAC: HVAC systems in factories and commercial real estate property and a variety of pump motors that are used daily to manage ambient temperatures greatly affect national energy consumption. But even here, major energy savings are possible. The pumps and fans in these large HVAC systems can be regulated using variable frequency drives, which significantly help reduce operating costs.

Control Band: The error band in certain control systems should be checked for accuracy. In older cooling towers, for example, the water temperature was controlled within a +/- 10° F band. By comparison, a huge amount of energy can be saved when the control system’s temperature sensors and controls are replaced with newer units that can regulate to +/- 1.0° F.

:: Design World ::

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Filed Under: Green engineering, Motion control • motor controls, Motors • servo

 

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