The ABC's of Variable Frequency Drives: Application, Benefits and Cost Effectiveness

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Todd Newman
5 years ago
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1 The ABC's of Variable Frequency Drives: Application, Benefits and Cost Effectiveness Presented by: Steve Reed, Kasa Controls & Automation, VP Engineering & Safety Types of Torque Load: CONSTANT TORQUE: Loads that exhibit roughly the same torque requirements regardless of operating speed. VARIABLE TORQUE: Loads that exhibit small torque requirements at lower speeds, and rapidly increasing torque requirements as machine speed increases. Scenario #1: What if I run a leg at a lower speed when no grain is on the leg, between receiving trucks, and then kick it back up to full speed when grain is in the pit or headed to the leg? Since this is a constant torque VFD application the energy savings is linear. So, if you slow down to 80% of full speed then you will get about a 20% energy savings. If you slow down to 50% of full speed then you will get about a 50% savings in energy. However, to move grain, you must run the leg at the appropriate speed. Scenario #2: If I am slow on trucks, will it provide me with any savings if I run a leg at a slower speed all day, with or without grain? Legs are a constant torque load. So yes, slower speed = lower energy. However, to move grain, you must run the leg at the appropriate speed.

2 Scenario #3: Can I use a VFD on leg motor for a creep drive function in lieu of purchasing creep drives? Yes Saving on cost of creep drive purchase. Special considerations should be made including stand-alone creep mode of operation. The drive must start in high torque mode and run at a constant speed (usually 13-20% of full seed) These are parameter settings in the VFD and also require special wiring considerations You can have a JOG pushbutton for indexing and stopping (typically enabled through a key switch located at the MCC). Special procedures must be implemented including LO/TO for exposed hazards! ( (m)(4)) Scenario #4: Can we use the high torque creep drive mode on a VFD to dig out of a plug condition? NO!!!! (q)(1): (Inside) Bucket elevators shall not be jogged to free a choked leg.

3 Scenario #5: We have two 125hp motors & two VFDs. Can we save energy by only running one motor instead of both during slow or no truck times? No The problem is that you are then pulling against the other gear box which adds load. The solution is to run both motors at the slower speed. Scenario #6: We have two 125hp motors & two VFDs. Can we save energy by using VFDs to balance the load? Yes It is normal to have different loads because of differences in motor and gear box setups, belt tensions, etc. Use the VFD load share feature to automatically equally share the load. The load delta could be different from full or empty buckets. Scenario #7: We handle different commodities. Should we run the leg at different speeds for different commodities? Perhaps Confirm optimal leg speed from your leg/cup manufacturer. Cup fill, utilization, and leg amps are all an integral part of this discussion.

4 Scenario #8: Instead of slowing down the motor for idle time between trucks can we just start and stop the receiving leg? Probably The VFD will ramp the leg up to full speed so Power Factor penalties from frequent starting is not an issue. Consider wear and tear on equipment with VFD ramping leg to/from full speed. Many bearing manufacturers will say bearing wear and tear is primarily due to run time and NOT starting and stopping equipment (reduced run time hours). Scenario #9: What effect does changing the conveyor (leg or belt) speed have on slow down or motion switches? Good point! You will need to be able to automatically adjust for 10% and 20% warnings and faults. If not, then you will get nuisance motion alarms.

5 Scenario #10: Should I put a VFD on my receiving pit drag or auger? Probably You can then use the drag or auger to control the flow of grain to get the most out of your leg. This is a constant torque application so energy savings with slower speeds. The drag or auger should last longer because it will run at the appropriate speed rather than always at full speed. Some cases favor a pit gate providing isolation between commodities. Scenario #11: Should I put a VFD on my blend screws? Yes Use a VFD to adjust the speed and rate of flow for blending fines / dust on load out systems. Scenario #12: Should I put a VFD on my reclaim/ bottom rack & pinion gates or receiving rack & pinion pit gates? Probably if you have automation. It will be much easier to accurately hit gate position setpoints with a VFD. The down side is the cost of the VFD on small motors including cable length and load reactor considerations.

6 Scenario #13: Should I put a VFD on my large aeration bin fans? This is a variable torque application so this would save energy. However, you generally want bin aeration fans to run full speed. Confirm adjusting speed with aeration manufacturer. Scenario #14: Should I put a VFD on my large aeration bunker fans? This is a variable torque application with a good chance to save energy if used in conjunction with a wind anemometer. Run enough fans to keep tarp stable, not necessarily all of them. Scenario #15: Are serviceability and support an issue for VFDs in your facility? It is easy for electricians to trouble shoot and replace motor starters. It is not so easy to diagnose, program, and support VFDs. Replacement parts may be very specific.

Benefits of using the Eaton S811+ soft starter to improve uptime and lower maintenance costs

Benefits of using the Eaton S811+ soft starter to improve uptime and lower maintenance costs Why soft start a motor? The most significant benefits of soft starting a motor come from reduced mechanical and electrical stress. This not only translates to lower maintenance costs, but significantly