Motors & Drives Information

Discussion Group Answers

• It’s hard to calculate the savings from the details provided. I would expect your power consumption to halve, your motor should need less current and so should run cooler. Remember, though, that the motor fan will run slower, so you would have to check it during operation. Some countries allow tax breaks for investment in energy saving schemes so don't forget to investigate.
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• It would probably depend rather a lot on the characteristics (type) of fan. When you throttle it, does the pressure differential of the fan increase or does it run at a similar pressure ratio with just reduced flow? Throttling flow may increase or decrease drive power. If you plot motor amps at a range of throttled positions, it will give a much better picture of what is actually happening. If motor amps fall as it is throttled more, a variable speed drive will probably not make a huge difference. If motor amps rise sharply with throttled flow, go for the variable speed drive.
• If your throttle valve is having problems, and you are comparing the price of replacing the throttle valve with a VFD, then it is worth investigating. If you are considering replacing the good throttle valve with a VFD, I don’t think you will receive a good payback. When the throttle valve at 30% opening, the fixed speed motor should be at lower load compared to 100% opening. Therefore, changing the fixed speed motor to VFD driven will only reduce the loss of the motor at low load. In my opinion, it would be a good comparison only when a new project is being planned. If you select a VFD, you would save the cost of the throttle valve and some electricity cost due to better efficiency.
• What type of a fan is it and what is your system configuration? Generally, when you reduce the speed of a fan, its pressure-developing capacity also decreases by 1/2nd power of the speed ratio. So you can’t simply use the Q1/Q2 = N1/N2 and P1/P2 = (N1/N2) 3 formulae to assess the power savings. Check the total pressure developed by the fan (with valve wide open) and pressure drop across the fan. The difference between these two readings will give you the required pressure to pump air into the system. Now calculate the required RPM from (N1/N2) 2 = SP1/SP2. Based on this RPM you should be able to calculate the final power consumption.
• Just a reminder here, folks—the performance rules for centrifugal fans in a relatively free-flowing system (air duct with open vanes) are well publicized and understood.
  
  They are 1) flow is proportional to speed; 2) pressure and shaft torque are proportional to the speed squared; and 3) horsepower or kw is proportional to speed cubed. In view of these, we can safely state that the fan, at 30% of full speed will flow 30% of full speed volume, torque and delta p will be .3 x .3 = .09 or 9% of full speed and fan horsepower will be .3 x .3 x .3 = .027 or 2.7% of full speed.
  
  The problem here is that the original question asks for a comparison of energy usage with “throttle valves” , but it doesn't say whether the vanes are on the suction or discharge side of the fan. It makes a lot of difference—from an energy usage standpoint.
• Replacing flow control by a throttle valve with a flow control by using a variable frequency drive will save a lot of power. If this will lead to a cost saving justifying the cost of VFD mainly depends on the time the device is operated at reduced flow. If you operate at reduce flow for one day every year stay with the throttle valve but if you operate with reduced flow for several thousand hours a year, the cost of the VFD will be saved within very short time. Contrary to several “energy saving devices” pushed on the market very aggressively, a VFD in a flow control application is a real “energy saving device”.
• Inlet guide vanes are definitely not an option at the reduced flow rates we are speaking about. They are generally economic if the minimum flow rate is around 80% of maximum flow, below that, efficiency drops to a great extent. The new curve due to inlet control starts from the same shutoff head but with more slope towards maximum flow. So there is no significant difference in performance at lower flow rates. Moreover, it is always possible to run a fan on the locus of BEP by using VFDs. Life of the rotating equipment improves due to low speed but care should be taken to avoid vibration.

# posted by Medical Information : 2:58 AM