The Impact of Variable-Frequency Drives

The Impact of Variable-Frequency Drives

Variable-frequency drives may affect your motors' performance in annoying and costly ways. However, at the same time, they can save energy and help you develop a continuous-flow manufacturing process.

In recent years the use of variable-frequency drives (VFDS) has increased. Engineers value newer, more reliable models for their potential to save energy and improve processes. Some of the benefits are:

Increased energy savings for pumps and fans

Improved process control

Reduced mechanical stress through soft start

Improved electrical system power factor.

Despite the benefits, problems are common. For example, VFDs may cause motor acoustic noise and motor heating Becoming familiar with both the benefits and the pitfalls of using VFDs will ensure you use the units effectively.

Energy savings for pumps and fans

In most facilities, centrifugal pumps and fans run at fixed speeds. An automatic valve, or some other mechanical means, then, varies fluid flow rates. However, if your facility used VFDS, you could change motor speeds electronically. Being able to adjust a pump's or fan's speed to get a desired flow rate can result in significant energy savings.

Figure I illustrates this point. On the basis of the laws of affinity for centrifugal loads, the figure shows how volume of flow is directly proportional to speed (in rpm). Pressure (or head) is proportional to the square of the speed. Input power is proportional to the cube of the speed. So, energy savings occur as the requirement for volume decreases. If, for example, a building-management system calls for operation at 50% volume, it requires only 12.5% of the power needed to run the system at 100% volume. Because power requirements decrease faster than the reduction in volume, there is potential for significant energy reduction at lower volume.

Specifiers generally size centrifugal pumps and fans to handle peak volume requirements that typically occur for short periods. As a result, centrifugal pumps and fans mostly operate at reduced volumes.

Figure 2 shows a bar chart of the typical operating cycle of a centrifugal fan in a variable-air volume (VAV) system. This system operates at below 70% volume more than 87% of the time. Using variable-frequency power for fan or pump duty-cycles of this kind can provide significant energy savings.Figure 1. Affinity laws for centrifugal loads

Figure 2. Typical operating cycle of centrifugal fan in VAV system

Improved process control

Using VFDs to improve process control results in more efficient operating systems. The throughput rates of most industrial processes are functions of many variables. For example, throughput in rubber extrusion or continuous metal annealing depends on, among others, the material characteristics, the cross-sectional area of the material being processed and the temperature of one or more heat zones. If a company uses constant-speed motors to run conveyors on the line, it either must run without material during the time required to change temperature in a heat zone or produce scrap during this period. Both choices waste energy or material.

With VFDS, however, the time needed to change speed is significantly less than the time it takes to change heat-zone temperature. By adjusting the material flow continuously to match the heat zone conditions, a company can operate continuously. The results are less energy use and less scrap.

Reduced mechanical stress:Soft starts

While most engineers select VFDs for energy savings and process improvements, these drives also reduce mechanical stress on process systems by employing soft starts. Starting a motor on line power increases stress on the mechanical system. Belts slip and squeal. Chains jump.

High pressure develops in pipes and ducts. Reduced-voltage and -frequency starting decreases this mechanical stress. VFDs vary output voltage along with output frequency (v/Hz). Output voltage varies with the frequency to control a motor's torque and speed. Controlling torque and speed results in a soft start as the motor's speed accelerates based on a pre-programmed rate. Acceleration time in most VFDs may be varied from 5 to 360 sec. In short, then, variable-frequency drives inherently offer soft starts.

Improved electrical system power factors

Engineers often ask how VFDs affect their facility's power factor. This question is important because many utility companies penalize facilities with poor power factors.

Figure 3 shows a typical power factor triangle. Kilovolt-amperes (kVA) represent the size of a building's power distribution system. Kilowatts (kW) measure the useful work performed by motors and other electrical equipment wired to the power system. It's useful to think of power factor as the measure of a device's efficiency in converting kVA to useful work (kW). Cos 0 represents the power factor. See 0 (or inverse cos 0) gives the angular displacement in the power factor triangle.

To function, all induction machines, including motors, require magnetizing power. A machine's kW and power factor determine the magnetizing power (the vertical segment of the power factor triangle), labeled kVAr (kilovolt-ampere reactive).

VFDs on the market today consist of three main power sections: an AC-to-DC converter, a DC filter and a DC-to-AC inverter. Today's modern PWM (pulsewidth-modulated) drives convert the threephase AC line voltage to a fixed-level DC voltage. They do this regardless of inverter output speed. PWM inverters, therefore, provide a constant power factor regardless designing the motor and the controller installation configuration, for example, by adding a reactor between the VFD and the motor. Newer generation drives and motors minimize annoying motor noise.of output speed and load. A constant, controlled power factor saves energy and money by reducing electrical bills.

Although VFDs provide many benefits in terms of energy savings, other concerns arise for engineers who use these drives with induction motors. Two of those concerns are motor noise and motor heating.Figure 3. A typical power factor triangle

Acoustical motor noise

In some installations, placing a VFD on a motor increases the motor's acoustical noise level. The noise occurs when the drive's non-sinusoidal (current and voltage) waveforms produce vibration in the motor's laminations. The non-sinusoidal current and voltage waveforms produced by the VFD are the result of the transistor switching frequency and modulation in the DC-to-AC inverter. The switching (or carrier) frequency may be a fixed value or (in new technologies) a variable value. The value of the carrier frequency determines the audible motor noise. A factorin the excitation of a motor's laminations is how closely the VFD approximates a pure sine-wave. In general, the higher the carrier frequency, the closer the output waveform is to a pure sine-wave.One method of reducing motor noise is full-spectrum switching. Drive manufacturers accomplish full-spectrum switching by an algorithm within the VFD regulator. The algorithm optimizes motor performance by evaluating motor characteristics, including motor current, voltage and the desired output frequency. The resulting frequency band, though audible to humans, produces a family of tones across a wide frequency band. So, the perceived motor noise is considerably less than it would be with a single carrier frequency.

Motor noise may not present a problem at your facility. Relevant factors include motor locations and the amount of noise produced by other equipment. In most cases, technicians can reduce motor noise by either adjusting the VFD controller or redesigning the motor and the controller installation configuration, for example, by adding a reactor betwwen the VFD and the motor. Newer generation drives and motors minimize annoying motor noise.

Motor heating

Most motor manufacturers design their products according to NEMA standards to operate on utility supplied power. The designers base their motors' heating characteristics and cooling methods on power supplied at fixed voltage and frequency. Two situations affect motor heating and cooling, however. First, variable-frequency power affects fan-cooled motors during speed reduction. Second, maintaining full torque at reduced speeds also affects all motors.

On fan-cooled motors, decreasing the motor's shaft speed decreases the fan's cooling effects by the same amount. If a motor is fully loaded and speed decreases by 50%, the motor must supply full torque with slightly better than half the maximum cooling. With decreasing speeds, this reduced cooling factor eventually will cause the motor to exceed its insulation-temperature rating. This, then, can reduce the life of the motor's insulation or cause the motor to fail. One potential solution is to add a constant speed, separately powered blower to the motor. This approach ensures adequate motor cooling even at low motor speeds.

Fan-cooled motors with centrifugal loads present less of a problem. Pumps and fans, for example, do not require full torque at reduced speeds. So, in these applications, there is less thermal stress on motors. Figure 4 shows how temperature-rise values relate to torque requirements at various speeds. On the basis of Figure 4, then, the load reduction for operating a centrifugal load does not cause the motor to exceed thermal limits defined by the insulation system.

HPPowerTypeFrequency(Hz)Percent SpeedPercenttorqueCurrent(amp)Temp ride(C)Remarks10Sine6010010012.051Line power10VFD15258712.079Load reduction10VFD6108912.5109Over temp limit

50Sine6010010059.162Line power50VFD30508226.872Load reduction50VFD6107051.394Low speed50VFD6106245.066Load reductionFigure 4. Temperature rise values vs. torque requirements

The second major cause of motor heating occurs in applications requiring full torque at full speed. As mentioned earlier, VFDs produce a non-sinusoidal waveform that approximates a pure sine-wave. Because this waveform is not an exact duplicate of a sine wave, the motor generates losses in the waveform's harmonic content. These losses contribute directly to heat generated by the motor.

HPPowerTypeFrequency(Hz)Percent SpeedPercenttorqueCurrent(amp)Temp ride(C)Remarks10Sine6010010012.051Line power10VFD6010010012.555Full Load & speed

50Sine6010010059.162Line power50VFD6010010061.473Full Load & speedFigure 5. Motor losses in waveform harmonic content

For 10- and 50-hp motors operating on VFD-supplied current at full speed and load (see Figure 5,), the current (amps) is about 4% higher than if the motors operated on line power. These increased currents cause increased temperatures in both motors. To compensate for this extra motor heat, consider derating a 1. 15 service factor motor to a 1.0 service factor when operating on VDF power.

Consider the potential problems-more motor noise and higher motor temperatures-when you select VFDS. Remember, though, variable-frequency drives can play important roles in systems aimed at saving energy and improving process control.