AC Drives

5 Most Frequently Asked Questions About Industrial VFDs

Posted January 25, 2016 by Pepper Hastings

Categories: AC Drives, Blog, Yaskawa Drives

Q: Why does myyaskawa_v1000 motor need a Variable Frequency Drive (VFD)?

A: Sometimes motors don’t need a VFD. But 95% of the time, motors are running in situations where you can/should vary the speed. You may already be varying the speed of your process via chokes or valves.  Putting a VFD on a motor will extend the life of the motor and save on energy costs.

Q: How does a VFD extend the life of my motor?

A: Well, it’s all physics mainly.  Heats kills electronics, and as well as mechanical devices. If you are running your motor full-out, 100%, all the time, then you are inevitably adding unnecessary heat to your system and motor. Just remember that for every 10 degrees the temperature inside of your motor rises above rated spec, you reduce the life of your motor by half! At 20 degrees, you have reduced the life by 75%!

Q: How does a VFD save energy costs?

A: Well it all goes back to the relationship of hp/kw being proportional to the speed cubed.  So if we are running something at 100%, we are using the full amount of hp and kw.  However, if we reduce it to say 50%, we are now only using 12.5% of the hp and kw.  Put two of the 50% systems together and now you have 100% of the load you were looking for at 25% of the energy usage.

Q: How much energy are we talking? Motors don’t use that much energy, right?

A: Wrong, did you know that at very low, conservative numbers, that electric motors consume approximately 25% of the Earth’s energy consumption? At high estimates, it is as much as almost 48%.  Electric motors are the single largest user of electricity in the world, so just think what that translates to your facility.

Q: Yeah but the Return on Investment is years right?

A: Not necessarily.  On the medium voltage side, some of the ROIs can approach 2 to 3 years; however, on the low voltage side, ROIs can be as little as 2 months.  Innovative-IDM can even help you with an energy audit of your facility to help fully develop a VFD ROI proposal.

+Brandt-King-Edited Brandt King is a longtime member of the Innovative-IDM team at the Houston branch. You can reach him at brandt.king@iidm.com

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VFDs and Cooling Towers

Posted January 20, 2016 by Pepper Hastings

Categories: AC Drives, Blog

What are the benefits of running a VFD on a cooling tower? Yaskawa's Joe Keopke knows. Read on.

Variable-frequency drives can adjust fan speed, reverse fan rotation

Used on large industrial projects, such as nuclear power plants, and countless HVAC applications ranging from 15 to 700,000 gpm, cooling towers are everywhere. Because of their increasing popularity and considerable number of applications, cooling towers are available in many different sizes and designs. This is especially true in the HVAC industry, in which the demand to increase efficiency and lower energy costs is great.

One of the most beneficial ways to increase efficiency and lower energy costs in a cooling-tower design is to utilize a variable-frequency drive (VFD). This article will discuss the different aspects of designs in which VFDs are used to improve performance and how this performance differs from conventional cooling-tower designs.

colling tower diagram

Cooling Tower

Figure 1. A mechanical forced-draft cooling tower.

COOLING-TOWER TYPES

FIGURE 2. A mechanical induced-draft cooling tower.

A mechanical-draft tower is a common type of cooling tower used in HVAC applications. Mechanical-draft towers can be broken into two main categories: forced draft and induced draft. A forced-draft tower has a fan mounted on its side, located in the path of entering ambient air (Figure 1). The fan draws ambient air in, pushing/forcing it through the tower and out of the top. Forced-draft towers generally have high air-entrance velocities and low air-exit velocities. An induced-draft tower has a fan mounted on its top, located in the path of exiting air (Figure 2). The fan draws ambient air in from the sides and blows it out of the top. Both designs use one or more fans to provide the flow required to extract heat from process cooling water. VFDs have proved to be beneficial on these fans.

VFD BENEFITS

Cooling Tower Flow

Cooling Tower Flow

Fans regulate airflow to compensate for changes in ambient air and load conditions. In the past, this was achieved by cycling fans on and off, manipulating fan capacity by varying the pitch of fan blades, or using two-speed motors. These methods can have considerable drawbacks and do not leave much room for error.

Across-the-line motor operation can be efficient if a system is designed for a fan to operate at full speed 100 percent of the time. However, that rarely is the case. As conditions change, flow needs to change as well. As a fan cycles on and off, its speed alters dramatically, causing leaving-water temperature to fluctuate, leading to inefficiency and control difficulty.

Fan speed

Fan Speed for cooling tower

Fan Speed Comparison on Cooling Tower

FIGURE 3. Affinity laws.

VFDs are able to adjust fan speed as conditions change while maintaining the exact flow required. They also are able to increase fan speed above 60 Hz to provide additional cooling capacity should the need arise. VFDs have built-in proportional-integral capabilities to automatically adjust fan speed to maintain a given set point, eliminating the need for an external set-point controller or variable-pitch fan. VFDs can reduce the energy consumed by a fan just by slowing the motor. As shown in Figure 3, a fan's power varies proportionally with the cube of its speed, so a small speed reduction causes a large power reduction. For example, if a fan motor runs at 80-percent speed, its energy consumption decreases by 50 percent.

Fan motors consume a large amount of energy because a high inrush of current is required to start the motor every time it is cycled. VFDs eliminate this problem by acting as soft starters, increasing/decreasing speed at a programmable rate. This feature reduces mechanical wear by eliminating stress on the power train caused by across-the-line motor starting. This can increase system life and save maintenance costs.

Fan rotation

VFDs not only save on energy and increase system efficiency, they perform numerous other functions that eliminate the need for additional equipment. For example, VFDs can sense fan rotation. If a fan is rotating in reverse because of windmilling, a VFD can catch it, slow it down, and ramp it back up in the correct rotation. A VFD's ability to reverse fan rotation can be beneficial in cold-weather conditions. Reversing a fan can cause warm air to be blown back into a tower, melting any ice that has developed. These VFD functions typically are performed using mechanical brakes or reversing contactors, which can increase costs and wear while adding mechanical components that can fail.

Automation and motor protection

Using a VFD on a cooling-tower fan also is valuable for automation and motor protection. VFDs have digital and analog inputs, outputs, programmable relays, and numerous serial-communication options that allow for flexibility in tower automation and performance monitoring.

Retrofits

Although it is best to consider VFD capabilities during a tower's initial design, VFDs can be implemented into existing applications effectively. Many, if not all, of the benefits mentioned previously can be utilized; however, there are some considerations to note in retrofit applications. Because many parameters are fixed, there may be tower-design characteristics that need to be worked around rather than avoided completely.

For example, cooling towers have fixed locations, and fan motors may not include inverter-duty features. Depending on where a VFD can be mounted, long lead lengths can be a problem and could damage motor insulation. To protect the motor, an output load reactor or a dv/dt filter can be added to the VFD output. However, it may be more cost-effective to mount the VFD to the side of the cooling tower in an outdoor-duty enclosure. This can solve the long-lead-length issue, making it possible to use the existing motor for the remainder of its life.

Gearboxes also can be considered. Many times a gearbox requires a minimum speed to stay effectively lubricated. If this is the case, a VFD can be programmed with a minimum speed, thus protecting the gearbox.

Because these are just a few of the considerations that may arise in a typical retrofit application, it is best to consult the VFD manufacturer for assistance.

CONCLUSION

VFDs not only can save money, they can make money. Some utilities offer rebates for installing VFDs in new or retrofit work. In many cases, a VFD can pay for itself in less than a year. Upgrading with VFDs is a major part of the U.S. Environmental Protection Agency's effort to improve energy efficiency. Many resources offer comprehensive information on state, local, utility, and federal incentives that promote renewable energy and energy efficiency. Whether in a new design or an existing application, VFDs offer numerous advantages when used on cooling towers.

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7 Things to Know When Choosing an Industrial AC Drive

Posted January 14, 2016 by Pepper Hastings

Categories: AC Drives, Blog

Seven Things To Know When Choosing An AC DriveWhich industrial AC drive should I use? Picking the correct features of a Variable Frequency Drive for your particular application can leave you scratching your head. Here are 7 things to know and should consider when picking the right AC drive for your need.

1. Know the AC drive’s electrical current limit
Your industrial AC drive supplies current to your motor and shut it on and off. Every time your motor shuts off and on, current will spike in your AC drive. Knowing your AC drive’s electrical current limit will ensure that the motor doesn’t burn itself or the AC drive out. It can also let your AC drive shut the motor off in the event the motor begins to draw too much current.
2. Find the motor’s name plate information
The motor your AC drive operates will have a plate of information on it giving you the specs it needs an AC drive to have. Information like horsepower, torque, current draw and etc., are all displayed on the motor so it’s important to match the AC drive to the motor, not the overall system itself.
3. Know your motor’s duty cycle
The AC drive you choose will need to account for the duty cycle of its motor. If the motor is running nonstop or for extended periods of time, it will generate heat and begin to operate less efficiently or even burn up as a result. Picking the right AC drive with the right programming allows it to monitor the motor’s condition and mitigate the productive losses to a minimum.
4. Know what your industrial AC drive communicates with.
Your AC drive needs to chat with other buddies besides the motor. As part of a larger, complete system, your AC drive may need to communicate with a Data Collection System (DCS) or Controlling System. These systems can PC-Based or controlled automatically by a Programmable Logic Controller (PLC). Knowing your systems communication protocol allows you to choose an AC drive with the same or even multiple communication protocols.
5. Know your AC drive’s control method.
Your AC drive can be controlled using a variety of different methods. Depending on whether it will be a local, analog or digital control method, the AC drive will need to be set up differently. Each method requires different wiring and ports, so knowing this beforehand will make selecting an industrial AC drive easier.
6. Know the environmental conditions.
An AC drive will likely operate under less-than-perfect conditions. Take into account how dusty its operating environment will be. How hot will it get? Is there moisture? Industrial AC drives can come built with an Ingress Protection rating, which will give it varying degrees of resistance to adverse conditions. You could also choose a housing cabinet that will protect a standard AC drive.
7. Know your available power.
Knowing what kind of electrical supply you already have will let you select the right industrial AC drive. For example, if you’re using a 120 volt wall outlet, you can select an AC drive that’s capable of operating on that power supply. If it’s more, pick a higher volt AC drive. If it’s lower, pick a lower volt AC drive. -- Troy Hardy, Field Application Engineer

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