Varofoil Controllable Pitch in Motion Fans Compared to Variable Frequency Controlled Fans

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Anissa Norton
6 years ago
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1 Varofoil Controllable Pitch in Motion Fans Compared to Variable Frequency Controlled Fans A look at two accepted technologies..

2 System Characteristics In general, the pressure developed by a fan is a function of the impeller tip speed. The tip speed is a product of the impeller diameter and fan speed. With Varofoil Controllable Pitch in Motion (VPIM) fans, the tip speed is constant. Fans with variable speed control lose pressure capability as a square of the ratio of speed reduction. The If a relatively high fixed pressure system requirement component in a Variable Air Volume System is required, the VPIM fan can provided stable performance down to zero, or near-zero flow if necessary. Speedcontrolled fans can lose stability at low volume flows if fixed pressure components, such as VAV boxes are present in the system. VFD control can also cause problems with motor overheating at very low (10-15% of full) speeds and should not be attempted without coordination with both VFD and motor supplier. Often, special motors with auxiliary cooling means are required. Additionally, VFD use can cause premature bearing failures due to shaft currents set up by electronic harmonics. Special shaft grounding and bearing isolation may be required. Figure 1, VPIM Control result is that the Varofoil VPIM fan retains a high portion of its pressure development over its entire range whereas the speedcontrolled fan loses pressure faster than it loses speed. The illustrations show the effect of speed reduction on fan performance. Figure 2, VFD Control Although a majority of projects do not need a volume lower than that produced on the stable part of the fan curve, it is essential that the minimum flow required be checked against the fan performance, to avoid low duty problems after installation. Noise The noise from a speed-controlled fan reduces as the fan speed reduces. This variation in noise level can make it more noticeable. A Varofoil fan varies its air and motor noise by only a small amount (typically 3 db) across its entire range and the constant level can make it more acceptable compared to a fluctuating noise level. The motor noise from an inverter-controlled fan can be excessive due to harmonics created by the wave chopping which can be amplified and radiated by the structure of the fan. This can be a discrete pure tone, which is particularly noticeable and difficult to remove or attenuate. Some more recent invertors use higher pulse rates and the harmonicas are above the human ear audio

3 range, but not above the range of certain animals (i.e., dogs). If part of the fan has a natural frequency in sympathy with the harmonic frequency, it could resonate, generating additional noise problems. Silencers in a system are designed to reduce the maximum fan noise level and therefore are suitable for both VPIM and VFDcontrolled fans, but may not be capable of attenuating particular discrete frequencies generated in the motor by the VFD. Vibration All fans are suitably balanced at their maximum rotational speed. However, all machines have a natural resonance frequency where damage could occur if the fan is left at that frequency. When a fixed speed fan is started, it is often noticed that, at one point during acceleration the fan vibrates considerably. This is the fan s natural frequency and the phenomenon applies to all types of fans. The Varofoil VPIM fan runs at a fixed speed and can experience this natural frequency only when starting or stopping. A variable speed fan may be cycled into and out of its natural frequency often, and damage could occur. Some VFD manufacturers have acknowledged this problem and incorporate into their programming one or two bands of operating frequencies that can be omitted. This means, for example that the operating frequency could jump from say 23 Hz to 25 Hz, if the fan s natural frequency is 24 Hz, thus avoiding damage. The only drawback to this is that if the system actually requires operation at 24 Hz, the control may oscillate between 24 Hz and 25 Hz, possibly causing instability in the system. Electrical System Effects (in Cables) The Varofoil Fan produces no adverse harmonic effects. Invertors can draw harmonic currents from the electrical supply. It is essential that these harmonics be reduced below a level at the common point of coupling, ie, at the point where the building electrical supply joins the buss feeding the adjacent buildings. Local Code will indicate the maximum level and power companies have made them mandatory. It has been necessary to upgrade some invertors on site to reduce this effect at considerable extra cost. Interference There are no radio frequency interference problems with the Varofoil fan. The high switching frequency in cables from the invertors to the motor on VFD-controlled fans can cause a malfunction in other equipment or radio frequency problems to other users, if not adequately screened. For example, some sensitive military areas ban all invertors due to radio frequency interference. The recommendations of the invertor supplier must be carefully followed, noting that these vary to suit different invertor designs and suppliers. VPIM Impeller Peak Power With Varofoil fans, the peak shaft power is as shown on the curve and in the catalog, together with absorbed power. The peak power is the maximum absorbed power at any point on the fan curve, irrespective of the system duty. Backward curved centrifugal fans, mixed flow fans and all axial fans have non-overloading characteristics That means that, no matter at

4 what point the system curve crosses the fan curve, the motor will not draw more than the peak power for which it was selected. Forward-curved centrifugal fans experience their peak power at maximum volume/zero pressure. For commercial reasons, motors are normally selected for the required operating area of the fan curve and not for the peak power. If all the dampers in a system served by a forward-curved fan were opened to reduce system resistance, the motor may overload and fail. Centrifugal fans are normally belt-driven and belt losses must be included when comparing with direct-drive fans. Direct drive axial flow fan motors are normally airstream-rated (TEAO) enabling more power to be drawn from a smaller motor frame because of additional cooling by the fan s airstream. The net result of these factors is that VFDcontrolled fans experience extra losses at high duties, compared to VPIM fans at the same shaft horsepower. Starting VFD controlled fans, supplied with starter contacts provide an inherently soft start. That is, they start at zero speed and slowly increase speed to the duty required by the control system. Typically, the maximum current drawn is the full-load current. Care should be taken that the invertor/starter is suitable to start the fan against idling rotation, and not just from a standing start. Larger VPIM fans provide larger opportunity for energy savings Varofoil fans are driven by constant speed motors. At lower power (say, up to 20 HP) across-the-line starting can be used without creating too large a voltage drop in the power supply. Above that point, wye/delta starting can be incorporated to reduce the starting current to about one third. Wye /delta starting and auto-transforming starting have been successfully used for many years. The Varofoil fan can have its controls set so that the fan starts and runs up to full speed at minimum blade angle. The controls can then increase blade angle to that required by the system. While this will not decrease the starting current level, it will decrease the length of time that the current is present, because the fan will reach speed faster. An additional benefit is that the motor will experience less heating, extending the life of the insulation system. Low Blade angle cannot be used to reduce cable sizes or overload protection, because it is possible to start the fan with the pitch set to maximum angle. Protection Conventional overload protection in starters, either current sensing or temperature sensing can be used with the Varofoil fan. With VFD drives, complex protection circuits may be needed for the safety of electronic circuits and system power devices. These are normally included in the VFD package, but purchasers must not assume that adequate protection is always provided. Any motor can have either thermistors or thermostats incorporated in the motor winding for overload protection. As a point of interest, if fans are being installed for emergency use, i.e., as smoke exhaust fans,

5 care must be taken that the overload devices are bypassed in the event of an emergency, so that the fan will run throughout the emergency. This applies to VFD protection devices and programming as well. Maintenance Invertors are solid-state electronic device that require minimal maintenance. However, should anything go wrong, it is best to call a specialist to correct the problem. Varofoil fans are mechanical devices, which require regular inspection and limited maintenance. When required, maintenance is provided by the user s own maintenance engineer and does not rely on expensive external personnel. With the Varofoil, service is limited to lubrication of motor bearings (as with VFD-driven units) and checking for smooth operation. Overhaul, i.e. replacement of worn parts, may be required after a long period of service. Reliability Varofoil fans have been installed and operate satisfactorily in over 20,000 systems around the world since their introduction in Overhaul is limited to a time when the mechanism becomes balky, as seen on a 3- month inspection schedule and involves replacing some simple parts, with a down time of no more than 2 or 3 hours. Time-to- Overhaul depends on use, but even with continuous operation, the service interval for 1800 RPM units should be not less than 5 years and could be considerably longer depending on the actual service conditions. VFD controlled fans have been a fact in HVAC installation for many years. The units are quite reliable, with the only area of VFD-Controlled Fan doubt involving the effect of electrical system effects on the motors. Shaft currents induced by harmonic effects can go to ground through the motor bearings and cause premature failure unless protective features are installed in the motor. Installation VPIM fans are limited to the axial type. Invertors can be used on any type of fan, as long as the motor is VFD Compatible. Space needs to be allocated in the area near the fan for the invertors, frequently in a ventilated panel. Smaller VFD units are freestanding, but care must be taken to ensure that they have adequate cooling ventilation. If the distance between the fan motor and VFD exceeds 150 feet, a voltage spike can be generated that may damage the motor winding and power wiring. Retrofit The VPIM impeller cannot be fitted to existing axial fans. A complete new fan is required. Existing fans can be VFDcontrolled, but only if the motor is VFDcompatible. Frequency Boost A VPIM fan can run only at it s motor s synchronous speed, minus any slip. If more performance is required, blade pitch can be increased if the motor has adequate additional power available. An invertordriven fan can be run at increased speed, provided the motor is sized for the additional load and suitable for the overspeed operation. Power Factor With inverter-driven fans, power factor is near unity at all speeds. VPIM fans use a standard squirrel-cage induction motor with power factors ranging from 0.7 to 0.9. If

higher power factors are required, power factor capacitors can be provided. Capital Costs These costs are often difficult to compare. Fan, Motor and VFD prices vary with market segment and geography.

6 higher power factors are required, power factor capacitors can be provided. Capital Costs These costs are often difficult to compare. Fan, Motor and VFD prices vary with market segment and geography. Also, the state of the economy at the time of purchase may depress or inflate prices. As a general rule, VFD-driven fans tend to be cheaper than VPIM fans for installations with less than 15 HP. Energy Savings Each project is unique and it is necessary to do an individual energy evaluation for each situation. The first task in that is to determine the overall duty profile throughout the year. A typical profile may well indicate that a variable flow fan will operate at 100% duty for only 5% of the time and 10% duty 5% of the time and may run the majority of the remainder at 70% duty. Once this profile has been derived the input duty for each of the comparative systems should be assessed and the total input power be determined. Note that with VFD-driven fan, the power delivered to the VFD, rather than to the fan motor should be measured. With the VPIM Fan, the input power is the power to the fan motor. As the load on the VPIM fan is reduced, the blade angle decreases and the motor becomes less efficient. This should be allowed for in the energy calculation. The efficiency of motors operating at reduced loads is sometimes difficult to obtain from Varofoil Fan, showing control attachment at hub motor manufacturers. Losses in the inverter itself (VFD s show this inefficiency as heat) and belt losses in belt-driven fans also need to be included. Taking these factors into account, it is generally accepted that there is greater power savings with the VPIM fan at maximum duty and greater power savings with the VFD-controlled fan at minimum duty. The crossover point varies with the individual fan selected, but is considered to be between 60% and 70% of maximum duty. In other words, if a fan is to run at maximum duty the majority of the time, the VPIM fan will be the more energy-efficient choice. Generally, the energy consumption differences at lower duties, in actual HP, is less than at higher duties. However, if this is critical, a two-speed motor can be used in the VPIM fan to give better energy saving. The fan can be run at the appropriate motor speed, i.e. high speed for summer duty and low speed for winter duty. Summary Although the size and types of fans available from American Fan/Flaktwoods spans the entire range of centrifugal and axial fans and includes all means of flow control, our experience indicates that the Varofoil VPIM fan has many advantages over the other types and should be considered by all variable air system designers.

Selecting the Correct Fan

Selecting the Correct Fan Ryan B. Johnson Air Purification Company Sales Engineer Member, ASHRAE Pikes Peak Chapter rjohnson@airpurificationcompany.com 1 Your Speaker circa 1998 Fan Types Applications