Behind the Scenes with VFDs

Ask any system designer to name the top three reasons for specifying a variable-frequency drive (VFD), and you probably won't hear a lot of maintenance cost reduction responses in return. Instead, you'll likely get answers ranging from range of precision and/or control to ease of installation to energy reduction the latter of which tops everyone's list. That's because using a VFD to control the speed

Ask any system designer to name the top three reasons for specifying a variable-frequency drive (VFD), and you probably won't hear a lot of “maintenance cost reduction” responses in return. Instead, you'll likely get answers ranging from “range of precision and/or control” to “ease of installation” to “energy reduction” — the latter of which tops everyone's list. That's because using a VFD to control the speed of a centrifugal fan or pump at 80% of rated speed, for example, can cut energy costs in half. This dramatic energy savings is one reason designers like to use VFDs in today's commercial and residential HVAC systems.

So where does the savings come from? Doesn't adding another piece of equipment increase the costs of maintenance by providing more equipment to maintain? Not necessarily. In fact, converting a process from fixed speed to variable speed can significantly reduce wear and tear on mechanical systems by reducing start/stop cycles. VFDs can also eliminate the need for such active components as vanes, dampers, and valves. Ultimately, you have less equipment to maintain, and longer runtime between failures.

Simply slapping a drive into an existing system, however, isn't going to cut it. To get the desired cost savings, first you need to understand the features offered by the particular drive you have or are planning to buy. Then, you need to think about how to implement those features in a way that will reduce system maintenance and overall operating costs.

Smoothing acceleration. When a load transitions from steady-state speed to accelerating or decelerating, the transition is usually instantaneous. However, the mass of the load doesn't instantly follow. This difference causes a jerking action that puts considerable stress on mechanical components.

VFDs can control acceleration and deceleration along the torque/speed curve to eliminate the jerkiness — and thus reduce the stress on components. This method has long been recognized as an aid in the handling of very light conveyor loads (e.g., a bottling line), extending the life of mechanical components in any application that has fast transitions.

Avoiding overcurrent conditions. Controlling a motor that is already spinning (commonly called a flying start) creates overcurrent challenges. Avoiding the maintenance downtime that could result from an overcurrent trip in those flying-start situations requires an AC drive that can reconnect the drive to a motor already spinning as quickly as possible to resume normal operation with minimal impact on load or speed.

When a drive executes a normal start, it initially applies 0 Hz and ramps up to the commanded frequency. Starting the drive in this mode with the motor already spinning will generate large currents. This can result in an overcurrent trip, if the current limiter does not react quickly enough. The likelihood of an overcurrent trip is even greater, if there is a residual flux on the spinning motor when the drive starts.

Simply preventing an overload trip isn't enough. If done incorrectly, the deceleration and subsequent reacceleration can place extreme mechanical stress on the application. And, of course, this creates a potential for causing premature equipment failure with the attendant downtime and repair costs. This is why a VFD needs a flying start mode.

In flying start mode, the drive responds to a start command by identifying the motor speed and then beginning its output synchronized in frequency, amplitude, and phase to the spinning motor. The motor will then be reconnected at its existing speed, and be smoothly accelerated to the commanded frequency. This process eliminates overcurrent tripping and significantly reduces the time for the motor to reach its desired frequency. Since the motor is “picked up” smoothly at its rotating speed and ramped to the proper speed, little or no mechanical stress occurs.

Skip frequency. All rotating machinery — from motorcycles to industrial fans and pumps — have mechanical resonance points. These are the frequency points at which vibration can rapidly damage that specific equipment. If you're aware of these points and avoid them — by either accelerating beyond or decelerating below them so the motor doesn't run at those points — you can prevent the rapid damage. VFDs with skip features allow you to do exactly that. In fact, most drives offer multiple skip frequency parameters to mitigate different resonance points.

The skip frequencies do not affect normal acceleration and deceleration. The drive output will ramp through the band, uninterrupted. When the operator issues a command to operate continuously inside the established band; however, the drive will alter the output to remain outside the band until a new command is issued.

If you know the mechanical resonant frequencies of your equipment, you can program the drives to “skip” through operation at those frequencies. That is, your equipment will run at those frequencies only momentarily, rather than continuously — just long enough to arrive at a safe frequency of operation. How can you determine what these resonant frequencies are? You may find this information in the equipment manual. A more common method is simply observing the equipment for noticeable changes in heat (for example, at bearings), noise, or motion when the operating frequency changes.

Monitoring the system. While drives don't possess the extensive monitoring capabilities of devices designed specifically for predictive maintenance or monitoring, they do monitor motor current and speed. You can put that information out on your industrial network. A distributed control system or PLC can provide reminders, warnings, and alarms to maintenance personnel.

Using just motor current and speed, a control system can determine a load problem is occurring. It can then call a designated cell phone for intervention before failure occurs. Such a system can also call alternate numbers and take backup actions, which may include more notifications or corrective action.

Overloads and current limits. Almost all drives have a built-in electronic motor thermal overload. When a motor runs outside its safe operating limits, the overload can reduce the output current (or shut off the motor) to prevent thermal damage or outright failure.

Overload software uses an algorithm incorporating motor current, speed, and time as inputs to model the temperature of the motor. This may also be done with thermister feedback directly from devices buried in the motor windings, using actual temperature readings to determine motor stress.

Multi-motor applications (those using one AC drive and more than one motor) require the motor overload to be disabled. The drive can't distinguish the current of each individual motor to provide individual protection. These applications require more advanced monitoring devices that can accept data from multiple sources to alert personnel of impending faults and failures.

A VFD has control of the amount of current it supplies to a motor. By limiting current or shutting down the motor, VFDs can reduce mechanical damage.

Many drives have a feature called an electronic shear pin. This feature is based on the proven concept of the mechanical shear pin. Snow blowers, for example, are equipped with mechanical shear pins — including one on the main driveshaft. If an object — such as your kid's skateboard buried under a foot of a spring snow — jams the rotating blades, the driveshaft shear pin breaks. When it breaks, it disconnects the drive train to the motor. This obviously protects the blower motor. Using shear pins to mechanically disconnect the rotating blades from the motor saves the expense of replacing a damaged motor.

Similarly, an electronic shear pin can define a current limit level that would cause damage. If the torque in the motor ever exceeds the set limit, the drive will automatically shut off the motor.

By limiting torque to a set level, AC drives provide good protection for systems that can become jammed. A common application for this is the chain conveyor. By not allowing a motor to power through the jam, you can use the VFD to prevent chain breakage and damage.

You probably have unused features in your existing motor drives, which means you have untapped cost savings. By taking advantage of the wide array of techniques already available, you can minimize stress placed on valuable plant machinery, increase equipment uptime, and reduce maintenance costs. One last bit of advice when calculating your return on investment for VFDs: Be sure to quantify the maintenance cost savings — especially if you need to submit a capital request. Ask your drive vendor for assistance in obtaining realistic numbers.

Weber is an electrical engineer with Rockwell Automation, Mequon, Wis.

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