The Benefits of Variable Frequency Drives

With the cost of energy and water and wastewater services continuing to rise, operators are looking for ways to reduce the costs of these basic, yet crucial to life, utilities. Proposed energy efficiency standards from the US Department of Energy (DOE) and other federal measures could help.

On the water side, DOE has a mandate to develop and enforce national minimum energy efficiency standards for any product if significant savings can be realized in a cost justified and technically feasible manner. During the past year, DOE and stakeholders have been negotiating a regulation for pump efficiency. That rule, now in the Notice of Proposed Rulemaking phase, would apply only to clean water pumps of these types:

  • End suction, close-coupled
  • End suction, framemounted/own bearings
  • In-line
  • Radially split, multi-stage, vertical, in-line, diffuser casing
  • Vertical turbine, submersible

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With the massive infrastructure of existing water distribution systems, the ability to increase energy efficiency comes down to a few items, including pumps and the motors that operate them. Utilities are incentivizing facilities to install variable frequency drives (VFDs) onto their motors and pumps to reduce the electrical surges caused by starting and stopping motors/pumps which benefits the facility through reduced water hammer and lower maintenance costs.

By increasing the efficiency of the pump itself as well as the motor, accomplished by selecting a premium efficiency motor and operating that pump/motor system at constant pressure or constant flow, a VFD can help reduce the energy consumed. But how do we know we will save energy?

This energy consumption is due to the affinity laws; the three affinity laws describe the relationship between speed of the pump (n), flow (Q), pressure head (H), and electrical power consumption (P):

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This shows us that the power to run a pump is relative to the cube of the speed, which means that if we slow down a pump by 20% we enjoy almost 50% in power or energy reduction. So, even small decreases in pump speed can result in energy savings. It is important to note that the maximum speed of the pump may not be the highest efficiency that it can pump when looking at gallons per kWh. It may be experienced at a lower than full speed flow.

But energy consumption may not be the only issue that variable frequency drives may help to resolve. What about non revenue water? Since we can now better regulate pressure, why not reduce our system pressure?

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Prior to the now common use of drives, water distribution systems typically used pressure switches to control pumps. When the system hit a low pressure point, the pump turned on, and when it hit a higher pressure point it was turned off. There might have been two pumps: a smaller one for slow night hours and a larger unit for the higher daily usages. Th is type of control causes pressure fluctuations, but is often common practice. With a VFD, pressure is maintained at a constant set point and, as pressure decreases, the motor/pump increases speed to maintain pressure.

By reducing and maintaining a constant pressure of the system, water leakage can be reduced. For example, a single 1/4 inch hole in a distribution pipe can leak as much as 4 million gallons per year at 50 pounds per square inch gauge or over 5 million gallons per year at 80 pounds per square inch gauge. By keeping the pressure more constant, non revenue water losses can be reduced.

While there are opportunities to reduce energy consumption and losses in water distribution systems which could lead to higher revenue and lower operating costs there are additional savings that may not be initially realized:

  • Protection of pumps and assets
  • Reduction of maintenance cost
  • Limiting the risk for bacteria/contamination of tap water
  • Limiting the risk for road breaks
  • Reducing pipe repair cost
  • Extended service life of network
  • Postponing investment in system upgrades
  • Improved control performance
  • Increased redundancy
  • Usage of dedicated VFD software features thus reduced load on the SCADA system

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Similarly, on the wastewater side of the system, there are more processes, and as a result, a greater number of motors and pumps. Since wastewater pumps operate at different conditions from clean water pumps, they have not been included in the DOE rule, although that does not mean DOE could not develop a rule for wastewater pumps in the future. However, DOE’s schedule for efficiency rulemakings under the present administration appears full and a federal initiative in the near term does not seem likely.

On the other hand, EPA, through its Clean Air Act (section 111d), will require states to develop compliance plans, which could include state energy efficiency plans. Wastewater treatment plants could be included in this. It would not likely state a minimum efficiency for wastewater pumps specifically, but could incentivize plants that reduce their energy use.

So how can wastewater operations work to increase efficiency and reduce costs today? Adding a VFD to the motor and pump can make it more efficient if it is controlled properly, similar to how in clean water applications utility rebates can help with the acquisition and installation costs. Further, the resulting lower energy bill can help to reduce the operating costs, but it’s important to be smart in the control of these systems.

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For smaller facilities, using a larger SCADA system may be out of the question to operate the entire system. But today’s VFDs typically include integral controllers that can be used instead to maintain level, pressure, flow, dissolved oxygen, turbidity, or other processes being monitored. Simply by providing an analog signal to the VFD from the monitoring equipment, the VFD has the ability to maintain a constant process output, and through internal communication modules data can be transmitted to other areas to monitor current status.

Additionally, integral cascade controllers can operate multiple pumps allowing for better overall control and when utilized in a variable pumping system, where pumps of different flow capabilities are installed and staggered to meet flow requirements can increase system efficiency. This type of control, master/follower, can off er the most efficient operation of a pumping system and the highest system efficiency.

In most any size system, the use of VFDs on pump motors means greater flexibility and better process control. By reducing the surges that a plant may see during an event or normal loading, the plant can more closely handle the waste influx by regulating the flows into the plant. VFDs can operate as slave units to the existing SCADA or overseeing system.

Drives can also off er features that help maintain pump efficiency such as a deragging function. Th is feature will rotate the impeller backwards (on a pump that can be operated backwards) to dislodge solids, stringy materials, or other debris to keep the impeller better able to pump at optimum performance.

VFD manufacturers off er different features, but drives designed specifically for the water/wastewater market tend to offer additional features to facilitate motor/pump operation such as deragging, initial ramp and/or check valve ramping, flow counting, flow confirmation, no flow or low flow/dry pump protection, and many others.

Additionally, analog and digital output cards, communication cards, and specific application cards are available to assist in control functions. As the water/wastewater industry continues to modernize and expand and issues like reducing energy consumption and water loss become more important and regulated, VFDs will become more commonplace in many applications.

Modern Variable Frequency Drives are Already Good – But They are Getting Even Better

Variable frequency drives have emerged as a surefire way to reduce energy costs in induction motor systems. From pumps and fans to material handling and industrial processes, VFDs help save many millions of kilowatt-hours around the world each and every year.

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And energy savings are only part of the VFD value proposition. VFDs can help extend the working life of induction motors-by allowing them to operate at lower speeds for significant portions of their lifecycle. VFDs can also improve process control capabilities. In fact, the most advanced vector controlled drives, when paired with appropriate feedback devices in a closed-loop control system, can offer positioning performance close to that of servo systems.

One thing speeding the adoption of VFD technology is the fact that it continues to grow more efficient and reliable due to continuous improvements in the underlying power electronics, such as the insulated gate bipolar transistor (IGBT) technologies developed and employed by Fuji Electric. IGBTs have also seen dramatic improvements in power densities, allowing VFDs to become more compact.

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A related technology trend that’s helping VFDs get better all the time involves the ready availability of low-cost, high-performance processors. More computing muscle allows VFD to run more complex control algorithms at higher speeds, which further enhances the control capabilities of VFDs.

Taken together, technological advances in power electronics and computing power will take VFDs to new levels of performance and cost effectiveness in the coming years. Here’s a look at how these technology trends have transformed some of Fuji Electric’s newly developed VFDs.

General Purpose Performance Boost

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In some ways, today’s VFD technology has already progressed to the point that it meets the vast majority of general purpose application needs. Fluid control applications, such as pumps and fans, are already well served by existing drives. So are many material handling and process control applications.

VFDs have also become much more reliable and efficient over the years. Today’s low voltage drives routinely offer efficiencies in excess of 95 percent up from efficiencies as low as 80 percent just a decade ago.

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As for reliability, modern VFDs typically outlast other components of a motor-driven system. At Fuji Electric, for example, our general purpose VFDs exhibit a failure rate below 0.1 percent even after more than a hundred thousand hours at 40°C.

To say that most applications are well-served by existing VFD technology, however, is not to say that there is no room for improvement. There are growing number of applications that can benefit from improved control performance, in terms of response times or the ability to control speed and torque accurately. Applications that push the envelope for general purpose drives tend to be those that have fast process speeds, braking requirements or impact loads (see Figures 1-5).

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New multi-functional drives have emerged to fill the performance void between lower performing general purpose drives of years past and much more costly servo systems that would be engineering overkill. Think of these drives as high-perfonnance general purpose drives. One recently introduced example of this new class of drive is Fuji Electric’s FRENIC-MEGA drive.

Compared to earlier general purpose drives, FRENIC-MEGA improves control performance and application flexibility with support for not just traditional v/f control but also for three different types of vector control-PG, sensorless and dynamic torque.

When used with the optional PG vector control, the drive’s performance far exceeds what many engineers would expect from a general purpose drive.

The more advanced general purpose drives also share some other technical characteristics.

Improved Reliability:

Avoiding drive related downtime has become more important than ever as general purpose drives take over more control tasks. One route to enhanced reliability is improved increased durability to overload conditions.

Custom Logic:

The more advanced general purpose drive typically support user customizable, sequential logic functions. While not a replacement for dedicated PLC in applications with high I/O counts, this built in logic capabilities can close high-speed control loops and execute time-critical logic that is closely related to the drive application.

Connectivity:

As general-purpose drives move into more difficult process control applications, connectivity via Ethemet TCP/IP, DeviceNet, Profibus and other industry network standards has become essential.

Future Drives Approaching Servo Performance

Moving a notch up the performance spectrum are the true high-performance drives that take positioning and control functionality beyond even the best general-purpose drives. One such drive is the FRENIC-VG, Fuji Electric’s next-generation product.

These FRENIC-VG series drives have a speed response of 600 HZ, or six times better than previous high performance models (See Table 1). Accuracy has improved too. Torque control accuracy is ±3 percent, while speed control accuracy is ±0.005 percent when using a PG card.

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The performance gains are due in part to the VG series’ use of dual processors, which doubled the processing power available to crunch control algorithms quickly. The previous high-performance drives, by contrast, had a single processor.

To make the VG series as adaptable as possible, it supports a lineup of interface cards, including the E-SX high-speed synchronized communications card and a PG interface card. A safety card will be added to the lineup soon as will integrated servo functions. The VG series conforms with common safety standards, including ISO 13849-1 safety standards for EN terminals of inverters and IEC 61508 SIL2 for the optional cards.

The FRENIC-VG series is intended for applications that need tighter control than possible with a general-purpose VFD but still less than a full-blown servo system. Among these applications are those, like steel making equipment, that require precise torque control across the entire speed range. Cranes and heavy-duty material handling systems are also a good fit for the VG series, which can accommodate rapidly changing torque requirements. Additional applications involve industrial machines, such as stamping presses or automotive testing equipment, which require responsiveness at high speeds.

Reforming Capacitors on VFDs

Manufacturers highly recommend reforming capacitors on VFDs that have been stored, without power, for more than 1 year.

Material Required

  • Appropriate single phase or three phase power supply.
    220-230VAC Drives = power supply of 220VAC
    380-480VAC Drives = power supply of 220VAC
    500-600VAC Drives = power supply 300-330VAC
    660-690VAC Drives = power supply 300-330VAC
  • Miscellaneous material for connection of power supply.

Background

All Variable Frequency Drives utilize Electrolytic Capacitors for storage of voltage in the inverter section of the drive. These electrolytic capacitors are made up of basically 3 parts. Two conductive plates (usually one being a metal substance similar to aluminum foil and the other being a porous material impregnated with an acidic substance similar to battery acid) that sandwich a layer of insulator material. The insulator material is generally made up of an oxide that is created when voltage is applied to the capacitor during manufacturing of the capacitor. Subsequent to this initial power up, every time the capacitor receives a charge it “rebuilds” this layer of oxide. As time elapses and there is no voltage applied to “reform” this layer the layer begins to degrade. This layer “thickness” is the determining factor for the voltage rating of the capacitor. If the layer degrades to a certain point one of two failures will occur. 1. The two conducting materials will begin to conduct current at a high rate, and this will cause a boiling of the liquid inside the capacitor. Once this boiling begins the pressure will rise internally and the capacitor will rupture. This failure usually results in a complete destruction of the drive as these capacitors are saturated internally with an acidic mixture. 2. The inrush current will arc across the oxide insulator and create a bridge between the plates. This bridge will short the plates and will cause a direct short in the power circuit that can result in other damage to the electronic components. At a minimum this second failure will require the damaged capacitor to be changed.

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VFD Capacitor Reforming

Procedure

If a VFD has not been powered up for 9 months to 1 year it is REQUIRED that the capacitors be “reformed” prior to the drive being put into service. For 220-230V and 380-480V models apply supply voltage of approximately 220Vac, three-phase or
single-phase input, 50 or 60 Hz, without connecting a motor to the output. This voltage should be applied for a period of 1 hour. After this energizing process, a wait period of 24 hours before installing or utilizing the drive for motor control is vital. Part of the oxide insulation is formed during this “resting” period. For 500-600V, 500-690V and 660-690V models use the same procedure applying a voltage between 300 and 330Vac to the inverter input.

What is a VFD?

You can divide the world of electronic motor drives into two categories: AC and DC. A motor drive controls the speed, torque, direction and resulting horsepower of a motor. A DC drive typically controls a shunt wound DC motor, which has separate armature and field circuits. AC drives control AC induction motors, and-like their DC counterparts-control speed, torque, and horsepower.

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Figure 1. Fixed Speed Fan Application

Application As An Example
Let’s take a brief look at a drive application. In Fig. 1, you can see a simple application with a fixed speed fan using a motor starter. You could replace the 3-phase motor starter with Variable Frequency Drive (VFD) to operate the fan at variable speed. Since you can operate the fan at any speed below its maximum, you can vary airflow by controlling the motor speed instead of the air outlet damper.

 

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Figure 2. Basic Induction Motor Construction

A drive can control two main elements of a 3-phase induction motor: speed and torque. To understand how a drive controls these two elements, we will take a short review of AC induction motors. Fig. 2 shows the construction of an induction motor. The two basic parts of the motor, the rotor and stator, work through magnetic interaction. A motor contains pole pairs. These are iron pieces in the stator, wound in a specific pattern to provide a north to south magnetic field.

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Figure 3. Operating Principles of Induction Motor

With one pole pair isolated in a motor, the rotor (shaft) rotates at a specific speed: the base speed. The number of poles and the frequency applied determine this speed (Fig. 4). This formula includes an effect called “slip.” Slip is the difference between the rotor speed and the rotating magnetic field in the stator. When a magnetic field passes through the conductors of the rotor, the rotor takes on magnetic fields of its own. These rotor magnetic fields will try to catch up to the rotating fields of the stator. However, it never does – this difference is slip. Think of slip as the distance between the greyhounds and the hare they
are chasing around the track. As long as they do not catch up to the hare, they will continue to revolve around the track. Slip is what allows a motor to turn.

Motor Slip:
Shaft Speed = 120 X F
P
– Slip
Slip for NEMA B Motor = 3 to 5% of Base Speed which is 1800 RPM at Full Load
F = Frequency applied to the motor
P = Number of motor poles
Example:
Shaft Speed = 120 X 60 Hz
4
– Slip

Figure 4, Induction Motor Slip Calculation

We can conveniently adjust the speed of a motor by changing the frequency applied to the motor. You could adjust motor speed by adjusting the number of poles, but this is a physical change to the motor. It would require rewinding, and result in a step change to the speed. So, for convenience, cost-efficiency, and precision, we change the frequency. Fig. 5 shows the torque-developing characteristic of every motor: the Volts per Hertz ratio (V/Hz). We change this ratio to change motor torque. An induction motor connected to a 460V, 60 Hz source has a ratio of 7.67. As long as this ratio stays in proportion, the motor will develop rated torque. A drive provides many different frequency outputs. At any given frequency output of the drive, you get a new torque curve.

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Figure 5. Volts/Hertz Ratio

 How Drive Changes Motor Speed
Just how does a drive provide the frequency and voltage output necessary to change the speed of a motor? That’s what we’ll look at next. Fig. 6 shows a basic PWM drive. All PWM drives contain these main parts, with subtle differences in hardware and software components.

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Figure 6. Basic PWM Drive Components

Although some drives accept single-phase input power, we’ll focus on the 3-phase drive. But to simplify illustrations, the wave forms in the following drive figures show only one phase of input and output.

The input section of the drive is the converter. It contains six diodes, arranged in an electrical bridge. These diodes convert AC power to DC power. The next section-the DC bus section-sees a fixed DC voltage.

The DC Bus section filters and smooths out the waveform. The diodes actually reconstruct the negative halves of the waveform onto the positive half. In a 460V unit, you’d measure an average DC bus voltage of about 650V to 680V. You can calculate this as line voltage times 1.414. The inductor (L) and the capacitor (C) work together to filter out any AC component of the DC waveform. The smoother the DC waveform, the cleaner the output waveform from the drive.

The DC bus feeds the final section of the drive: the inverter. As the name implies, this section inverts the DC voltage back to AC. But, it does so in a variable voltage and frequency output. How does it do this? That depends on what kind of power devices your drive uses. If you have many SCR-based drives in your facility, see the Sidebar. Bipolar Transistor technology began superseding SCRs in drives in the mid-1970s. In the early 1990s, those gave way to using Insulated Gate Bipolar Transistor (IGBT) technology, which will form the basis for our discussion.

Switching Bus With IGBTs Today’s inverters use Insulated Gate Bipolar Transistors
(IGBTs) to switch the DC bus on and off at specific intervals. In doing so, the inverter actually creates a variable AC voltage and frequency output. As shown in Fig. 7, the output of the drive doesn’t provide an exact replica of the AC input sine waveform. Instead, it provides voltage pulses that are at a constant magnitude.

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Figure 7. Drive Output Waveform

The drive’s control board signals the power device’s control circuits to turn “on” the waveform positive half or negative half of the power device. This alternating of positive and negative switches recreates the 3 phase output. The longer the power device remains on, the higher the output voltage. The less time the power device is on, the lower the output voltage (shown in Fig.8). Conversely, the longer the power device is off, the lower the output frequency.

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Figure 8. Drive Output Waveform Components

The speed at which power devices switch on and off is the carrier frequency, also known as the switch frequency. The higher the switch frequency, the more resolution each PWM pulse contains. Typical switch frequencies are 3,000 to 4,000 times per second (3KHz to 4KHz). (With an older, SCR-based drive, switch frequencies are 250 to 500 times per second). As you can imagine, the higher the switch frequency, the smoother the output waveform and the higher the resolution. However, higher switch frequencies decrease the efficiency of the drive because of increased heat in the power devices.

Shrinking cost and size

Drives vary in the complexity of their designs, but the designs continue to improve. Drives come in smaller packages with each generation. The trend is similar to that of the personal computer. More features, better performance, and lower cost with successive generations. Unlike computers, however, drives have dramatically improved in their reliability and ease of use. And also unlike computers, the typical drive of today doesn’t spew gratuitous harmonics into your distribution system-nor does it affect your power factor. Drives are increasingly becoming “plug and play.” As electronic power components improve in reliability and decrease in size, the cost and size of VFDs will continue to decrease. While all that is going on, their performance and ease of use will only get better.

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ABB ACS850 VFD

Sidebar: What if you have SCRs?

With the large installed base of SCRs, you might want to know how these operate. An SCR (originally referred to as a thyristor) contains a control element called a gate. The gate acts as the “turn-on” switch that allows the device to fully conduct voltage. The device conducts voltage until the polarity of the device reverses-and then it automatically ‘turns off.” Special circuitry, usually requiring another circuit board and associated wiring, controls this switching. The SCR’s output depends on how soon in the control cycle that gate turns on. The IGBT output also depends the length of time the gate is on. However, it can turn off anytime in the control cycle, providing a more precise output waveform. IGBTs also require a control circuit connected to the gate, but this circuitry is less complex and doesn’t require a reversal of polarity. Thus, you would approach troubleshooting differently if you have an SCR-based drive.

Consider the following when designing or servicing your gear units

  • Mounting Position: The orientation of the gear unit determines the amount of oil. M1 contains the least; M2 and M4 contain the most. Whenever possible, design your system for M1.
  • Input Speed: The faster the oil churns, the higher the churning losses. Therefore, a gear unit with 4-pole motor (~1475 rpm) will run cooler than with a 2-pole motor (~2800 rpm).
  • Synthetic Oil: Synthetic oil is known to reduce friction by 25%, which can be significantly on worm gear units or large gear units. Synthetic oil allows the gear unit to run cooler, doubling the oil service life, which reduces your maintenance interval and costs.
  • Viscosity: The “thicker” the oil, the more resistance it has to flow and the more energy it requires moving. Always use the correct viscosity, considering the type of application and ambient temperature.

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The Truth about Premium-Efficient Motors

1. The motor is only part of the efficiency equation.

As you have seen, a motor is at best one-sixth of the total energy loss potential for an electro-mechanical drivetrain. And, what’s more, it typically isn’t even the most inefficient part. Mechanical devices, such as external transmission elements, have far more inefficiencies than electrical devices do. So, look there first to find your largest energy savings.

2. By revamping your entire drivetrain, you may actually be able to use a
smaller motor and save even more.

Right now, you are probably using a motor of a particular power rating to produce a certain output from your drivetrain. You may be pleasantly surprised to find that, by upgrading your gearbox, drive, and external transmission components, you will have gained enough efficiency that your motor power is now higher than you actually need. Therefore, you may be able to save additional costs by purchasing a lower horsepower motor. For example:

Before: 45 kW load ÷ 53.5% efficiency = 84.1 kW (use 90kW motor)

After: 45 kW load ÷ 72.5% efficiency = 62.1 kW (use 75kW motor)

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SPIROPLAN Right-Angle Gear Units

3. Motors are most efficient when integrated with other drivetrain components
from the same manufacturer.

Systems where the VFD, motor, and gearbox are all engineered by the same company are by nature designed to work well together, eliminating unnecessary inefficiencies and allowing additional energy savings. For example, integrating an SEW-EURODRIVE DRP motor, helical-bevel gear unit, and VFD will provide dramatically higher energy savings than simply replacing the motor.

4. The motor must be well-suited to your application.

Just placing a premium-efficient motor on the line may not automatically solve all your energy problems, even if all the other components are as efficient as possible. Ensure that the specifications of the motor fit your application, especially if you have a high-cycling application that is greater than 10 to 30 cycles/hour. If so, use a premium efficient motor designed for such an application with an appropriately sized integral brake.

Also, where possible, use the smallest motor for the application so that it is loaded near 80% and operates as close to it nameplate efficiency as possible.

5. Mechanical efficiencies matter, too.

Worm gear units, which are common in the industry, have an efficiency range of 50 to 88 percent, depending on the number of starts (teeth) on the worm gear or gear ratio, as shown below.

number of starts —– typical efficiency range
1                                        50-69%
2                                        70-79%
5                                        80-88%

Their poor efficiency is due to sliding gear contact. Since sliding produces friction, much of the energy is wasted through heat. Conversely, helical bevel gear units use rolling friction, so they lose only 1.5% of efficiency for each stage. Thus, a three-stage helical bevel gearbox is 95.5% efficient.

Although helical-bevel gear units are higher in initial cost, they will save money in energy over the lifetime of the system. If you are an end-user, consider specifying helical-bevel gear units the next time you purchase equipment for your plant. It is in your best interest.

6. Gearmotors eliminate even more efficiency losses.

Gearmotors inherently yield tremendous increases in efficiency compared to the average flexible transmission system. Since a gearmotor contains a motor that is rigidly coupled and precisely aligned with the gear unit, the connection is nearly 100% efficient. By eliminating the friction and slippage associated with v-belts, pulleys or chains, you can quickly yield a potential 12-15% increase in efficiency. You will save even more on the replacement and maintenance of belts. And, don’t forget about safety.

7. Oil may be costing you.

Oil plays a role in energy savings because it creates heat as it churns inside a gear unit. And, the amount of heat increases as the oil volume increases. Not only does heat increase your energy bill, it also damages gears and seals. Excessive heat is especially problematic for larger gear units – typically with an output shaft diameter greater than 60mm.