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Electric Motors, Gearing, Speed Controls, Gyros, Receivers, and Other Electronics Discussion Electric Motors, Gearing, Speed Controls, Gyros, Receivers and Other Electronics Discussion |
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10-16-2012, 02:55 AM | #1 (permalink) |
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Effect of motor kv
I understand that a particular head speed, for a given lipo voltage is achieved by the appropriate gearing and motor kv rating.
One has a choice of either lowering the motor kv, or raising the gear ratio. What is the tradeoff here? Example: 6S Lipo 90% throttle (80% efficiency) 550mm blades 1550 rpm head speed can be achieved with a 1100 kv motor and 14.2 gear ratio or with a 890 kv motor and 11.3 grear ratio. What are the pros and cons of either approach? More specifically is the 890 kv motor likely to run any cooler and/or draw less current? As you may guess this is related to a specific project for a T550 in a scale body - so efficient hover is more important than strong power surges. I already have an 1100 kv motor installed with a 13:1 gear ratio and it gets fairly warm just in pod&boom form.
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Nelson JR DSX9 II; Stretched Atom 500 in Bell 222 body , Stretched TRex550 in 600 size AS350 - . Trex 450 S and 550E for sport . practice.SK720 all round.. |
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10-16-2012, 03:58 AM | #2 (permalink) |
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Nelson
Isn't this the same question you asked 3 weeks ago ? https://www.helifreak.com/showthread.php?t=455376 Colin
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Vario Long Ranger 700e Seahawk 600 UH-1N 500 Baumann EC-145 800+ (coming soon ) |
10-16-2012, 12:49 PM | #3 (permalink) | |
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Quote:
Actually when I wrote this one I had forgotten the details of other one (I did a search for it using my name but it didn't seem to come up so I thought I never posted it) but I did have a vague notion that had not been answered to my satisfaction. After I posted I found the earlier one, but with due respect to your reply there I would like to see some more specific information. Were you implying that kv makes no difference if the HS can be achieved by appropriate gearing? EDIT - It arises because I know of someone with the same airframe and scorpion 890 motor that gets only slightly warm even inside a scale fuse (I don't know his gearing or HS but believe he running 6S). It take your point Colin about the pinion wall thickness. I have a 12T 0.7M 5mm diam steel pinion that I assume is OK.
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Nelson JR DSX9 II; Stretched Atom 500 in Bell 222 body , Stretched TRex550 in 600 size AS350 - . Trex 450 S and 550E for sport . practice.SK720 all round.. |
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10-16-2012, 04:36 PM | #4 (permalink) |
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Join Date: Aug 2010
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Nelson
This as a constant power question (you require some particular HP for your hover, at a given RRPM, and so you have a fixed torque requirement etc). Higher motor RPM implies higher windage losses, and higher friction losses in the bearings. But there may be upsides- more cooling is possible due to the increased motor RPM, assuming a fixed motor can size (i.e. only the winding count is different). So, you cant assume higher efficiency from a cooler motor. According to the data I have seen, there is effectively no efficiency difference between the two scenarios you mention at the motor. Or more to the point, there are far larger deltas in the ESC, cabling, pack IR, etc.
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10-17-2012, 04:15 AM | #5 (permalink) |
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Often the ESC settings will have more effect on motor temp than the motor kV or pinion size. Motor timing and PWN rate can both make for hot motors if there are not suited to the motor.
Generally I use low motor timings to get less power (high power is not required for scale) and more efficient cooler motor running. If you use something Outrunner settings on a CC Ice ESC it can make the motor run hot and potentially damage it. I usually use 8kHz or maybe 12kHz, but this is a balance between a hot ESC (which usually occurs with higher numbers) and a hot motor (which can occur at lower numbers). Scorpion motors also tend to run hotter than some others. When I changed my Scorpion 4025-550kV out for a Xera 4030-470kV (and slightly bigger pinion) the motor temp dropped by about 10°C, which I think was mostly due to the Xera motor rather than the larger pinion, slower motor. I personally usually try to get the slowest motor that I can get pinions for, but I don't have any science behind this. Scale helis with closed cabins often run hot - I have two 12V fans in my Long Ranger. Colin
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Vario Long Ranger 700e Seahawk 600 UH-1N 500 Baumann EC-145 800+ (coming soon ) |
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