Industry News Display

2012-05-31   Control Engineering

Variable frequency drives offer simple method of boosting energy efficiency of heavy equipment

By: Chris Jaczszolt

For businesses that rely on heavy equipment to produce goods or deliver services, seeking ways to reduce energy consumption makes economic sense. In an era in which a large percentage of the consuming public likes the idea of “going green,” heavy industrial companies also can gain a public relations edge by cutting their energy use. However, it’s not always easy for heavy equipment users to join the green movement.

Most heavy equipment is built to ensure it can operate efficiently at peak loads, and customer demand doesn’t always require running equipment at that level. In many cases, companies must keep equipment online and running even when customer demand is below peak levels because it takes too much time and effort to shut machines down completely and then have to ramp them up again if demand suddenly surges.

One way to cope with this situation is the use of variable frequency drives. A variable frequency drive (VFD) is a reliable electronic device that efficiently operates a three-phase electric motor. These drives or inverters, as they are also called, are used to control motors in building automation, industrial, pumping, agricultural, irrigation, and water/wastewater applications.

The energy saving benefits of utilizing VFDs in fan and pump applications have been well established and documented. The costs of a VFD and its installation for various pump and fan systems can usually be paid back in energy savings within a few years. 

Optimizing motor efficiency

With time and patience, any drive can be fine-tuned to optimize motor efficiency. However, many VFDs are usually capable of optimizing a motor’s efficiency only at one specific load condition. On the other hand, some VFDs now have an energy-savings control feature designed to automatically optimize motor efficiency at any given load point.

With a drive’s automatic energy-savings control, industrial applications can now begin to benefit from energy savings. Not all industrial applications run their motors at rated load. Those applications that do run at rated load generally do not do so on a full-time basis.

A conveyor is one application that definitely can benefit from an energy-savings feature. Conveyors are used in a broad range of industries to move packaged goods, assemblies, process by-products, or any material from one place to another. Airports, mines, cement plants, distribution facilities, assembly, and food processing plants all use conveyors. Conveyors are typically constant torque loads, meaning the required torque to drive a conveyor is independent of speed.

When sizing drives and motors for conveyors, the heaviest load needs to be considered. Quite often, speed, load, and acceleration/deceleration testing is performed to determine the proper drive and motor size. Hence, conveyors are generally oversized to accommodate peak load requirements. As a result, conveyors may experience periods of time where the load placed on the conveyor is not at its maximum capability. There may even be periods of time where the conveyor runs lightly loaded or completely unloaded.

Unloaded operation may occur with conveyors because it is sometimes unjustifiable to continuously start and stop a conveyor. In some cases, such as in large mining conveyors, a long painstaking process would be required to restart a conveyor. In addition to the timing aspects, starting and stopping these conveyors could stress the coupling and joints holding the conveyor belt sections together. Likewise, the belt’s rubber could undergo thermal stress as it cycles from a warm running state to cold state when it is stopped, which could eventually snap or tear the belt. It is in these cases where a drive’s automatic energy-savings control can be used.

Conveyors typically run continuously at a fixed speed, where the only change is the load itself. A fixed amount of torque is always required to overcome the frictional portion (machine drag) of the total load. Therefore, the motor will never be in a no-load state, but the conveyor may run for long periods with very light loads.

A VFD’s energy-savings feature is designed for these lightly loaded conditions. A motor is typically designed for maximum efficiency at rated slip. However, less slip will be required to generate torque at lower load demands. Therefore, slip will decrease as the load placed on the motor is reduced. As slip decreases further and further away from rated slip, the motor becomes less efficient.

By design, VFDs operate motors by applying varying frequency to vary motor speed. The applied frequency is such that the motor generally operates within its normal full-load slip value. However, VFDs are designed to apply rated slip only at rated load. Therefore, a drive will run the motor near its optimum efficiency only while running at rated load. Energy-savings control regulates the drive’s output such that the motor always runs at rated slip to continuously optimize motor efficiency regardless of the load condition.

A drive with its energy-savings function enabled optimizes slip by first determining the amount of power being supplied to the motor. Then, the drive will calculate the amount of power that should be supplied to the motor based on the frequency range, tuned motor parameters, and power measurements. Once the drive has calculated the right amount of slip to run the motor at its maximum theoretical efficiency, the output voltage is adjusted until the calculated amount of slip is achieved. Therefore, energy-savings control improves motor efficiency by regulating the amount of slip through adjustments in the output voltage.

Comparing power consumption

As previously stated, conveyor applications must continuously accommodate for the friction required to move the conveyor. Therefore, a conveyor could feasibly run at 20% of motor rated load without any actual load placed onto the conveyor. The actual percentage of motor rated load will impact the amount of energy savings. The energy- savings function will have its largest impact on motor efficiency at no-load.

Figures 1 and 2 show the power required to run the same load with and without energy-savings control. The series in blue (V/f) designates a drive running a given load with a default constant torque volts-per-hertz pattern. The red series (V/f + ES) has the drive’s energy savings function enabled. The V/f pattern  using energy saving has its voltage continuously adjusted away from the default pattern to optimize slip.

As Figures 1 and 2 show, less power is required to run the same load when energy savings is utilized. The reduction in energy consumption is approximately the same at each load condition regardless of the operating frequency. In the case of our conveyor example, energy savings can be quite significant even at a 20% load. At 20% load, the drive will reduce energy consumption by greater than 10% at both 48 Hz and 60 Hz.

Again, the energy-savings function will provide a larger reduction in power consumption the lighter the load becomes. On the other hand, as we draw nearer and nearer to rated load, the motor will run closer and closer to rated slip. Energy-savings control will have its least effect as the motor runs near or at rated slip, as the motor will already be running near optimum efficiency. 

Energy savings made easy

Additionally, energy-savings control allows for real-time adjustments. By making real-time power measurements, the drive can accommodate motor parameter changes that may have been affected due to motor temperature changes. Therefore, any unexpected load or motor characteristic changes will be automatically compensated for by the drive.

The energy-savings control feature is embedded into many drives today. A drive’s energy-savings control is simple to set up and use. The feature is generally initiated by enabling a parameter setting during the drive’s installation.

Then, a simple one-time auto-tune with the load uncoupled from the motor must be performed. The tune allows the drive to ascertain key motor information for its energy-savings calculations. With a drive’s capability to auto-tune motor data, the type of motor becomes irrelevant.

The benefits from using a drive’s energy-savings control are not limited to conveyor applications. Other applications could include, but are not limited to, spindles and band saws. Essentially, most any application where there are long periods of light loading can benefit from a drive’s energy-savings control feature.

Chris Jaczszolt is Drives Application Engineer with the Drives & Motion Division of Yaskawa America Inc.

For more on saving energy with variable frequency drives, visit the Yaskawa website. This article appears in the June Industrial Energy Management supplement to Control Engineering

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