MattyB Posted June 7, 2023 Share Posted June 7, 2023 (edited) This recent thread has got me thinking about high voltage servos, what the internals are really like and what performance to expect if you run them on a 4.8V pack... I've often wondered whether or not the physicals (specifically the gearing) of HV servos are different to their LV brethren. Looking at the specs on 2 comparable Hitec servos of the same size and weight where the only difference is the HV capability, the specs appear absolutely identical, just at different input voltages: Hitec D85MG Mighty Micro (std voltage) specifications: Speed Sec/60° (4.8V/6.0V) : 0.17 / 0.13 Torque kg.cm. (4.8V/6.0V) : 3.6 / 4.3 Hitec HS-5087MH HV (HV version of the 85MG) specifications: Speed Sec/60° (6.0V/7.4V) : 0.17 / 0.13 Torque kg.cm. (6.0V/7.4V) : 3.6 / 4.3 I could probably work out what is going on here with enough time, reading and maths(!), but I'm sure others on the forum are more knowledgeable - perhaps some of you have even done some tests. Two key questions: Does this mean the HV servos have different internal gearing? Putting aside whether the electronics could take it for a moment, surely if you apply a higher voltage and everything else were to remain the same, surely both torque and speed would increase? Or am I missing something? Assuming the Hitec figures above are accurate, doesn't this mean HV servos should come with a specific recommendation not to operate them below a minimum of 6V, otherwise the torque ratings will be reduced below that specified*? (Yes, I've checked the manuals - they are pretty much useless when it comes to servos, on the Hitec site there is only one generic doc that is well out of date - it even talks about Nicads, and the max voltage of all servos being 6V!) * - I accept that the fact we tend to massively over-spec our servos these days means operating an HV servo at 4.8V should be absolutely fine in 95% of cases. I am just intrigued about what the actual torque ratings would be vs. the spec sheets. Edited June 7, 2023 by MattyB Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted June 7, 2023 Share Posted June 7, 2023 The servo.speed would suggest that the gearing is the same, so it's maybe the motor KV or motor control electronics that are changed. BTW Hitec do give the performance of the D85MG at 4.8, 6.0 and 7.4v https://www.hiteccs.com/actuators/product-details/D85MG 1 Quote Link to comment Share on other sites More sharing options...
Philip Lewis 3 Posted June 7, 2023 Share Posted June 7, 2023 MKS will give you actual figures at different voltages for all of their servo's, I susoect (but don't know) that they are only giving ratings for voltages they beleive they should be ran at, i.e. don;t run them at 4.8 volts, a motor will obviously run slower the lower the voltage but at a certain point it might not realistically work at all. I use all MKS HV servo's, highly recommended but a tad too pricey for most models. 1 Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted June 7, 2023 Share Posted June 7, 2023 It is quite possible that the motors are indeed the same and actually run at the same voltage. The input voltage being reduced to 6V max electronically inside the servo. Quote Link to comment Share on other sites More sharing options...
PatMc Posted June 7, 2023 Share Posted June 7, 2023 40 minutes ago, Simon Chaddock said: It is quite possible that the motors are indeed the same and actually run at the same voltage. The input voltage being reduced to 6V max electronically inside the servo. If that was the case wouldn't there only be one set of torque & speed figures for the HV version ? Quote Link to comment Share on other sites More sharing options...
MattyB Posted June 7, 2023 Author Share Posted June 7, 2023 2 hours ago, Frank Skilbeck said: The servo.speed would suggest that the gearing is the same, so it's maybe the motor KV or motor control electronics that are changed. BTW Hitec do give the performance of the D85MG at 4.8, 6.0 and 7.4v https://www.hiteccs.com/actuators/product-details/D85MG Well that is officially the weirdest spec sheet entry I've ever seen - don't run it at 7.4V, but if you do here's what it will do jsut before it burns out...! 1 hour ago, Philip Lewis 3 said: MKS will give you actual figures at different voltages for all of their servo's, I susoect (but don't know) that they are only giving ratings for voltages they beleive they should be ran at, i.e. don;t run them at 4.8 volts, a motor will obviously run slower the lower the voltage but at a certain point it might not realistically work at all. Yes, I strongly suspect you are right. If that is the case though it would be sensible for manufacturers of HV servos to make a statement on the "official operating ranges for their servos, not just the "Max - do not exceed" rating. After all, so many of us grew up using exclusively 4.8V AA packs, so in the absence of explicit guidance tot he contrary many experienced modellers will automatically assume they are still good to use for any servo. Quote Link to comment Share on other sites More sharing options...
Nigel R Posted June 7, 2023 Share Posted June 7, 2023 1) tweak feedback pot to work at higher voltage 2) change kv of motor 3) sell "new" and "improved" servo for higher price Quote Link to comment Share on other sites More sharing options...
MattyB Posted June 7, 2023 Author Share Posted June 7, 2023 (edited) 1 hour ago, Simon Chaddock said: It is quite possible that the motors are indeed the same and actually run at the same voltage. The input voltage being reduced to 6V max electronically inside the servo. 1 hour ago, PatMc said: If that was the case wouldn't there only be one set of torque & speed figures for the HV version ? Agreed, I can't see how that could be the case and the figures be different for different input voltages. Perhaps the only way we will be able to answer these questions conculsively is by doing some servo testing at different voltages? Or perhaps there is an independent servo testing dbases somewhere out there on t'interweb that can help... time for a Google! Edited June 7, 2023 by MattyB Quote Link to comment Share on other sites More sharing options...
Philip Lewis 3 Posted June 7, 2023 Share Posted June 7, 2023 8 minutes ago, MattyB said: Yes, I strongly suspect you are right. If that is the case though it would be sensible for manufacturers of HV servos to make a statement on the "official operating ranges for their servos, not just the "Max - do not exceed" rating. After all, so many of us grew up using exclusively 4.8V AA packs, so in the absence of explicit guidance tot he contrary many experienced modellers will automatically assume they are still good to use for any servo. Futaba HV servo's in fact do just that quoting a voltage range which is 4.8 - 7.4 volts although they only give torque and speeds for 6v, 6.6v and 7.4v. Quote Link to comment Share on other sites More sharing options...
MattyB Posted June 7, 2023 Author Share Posted June 7, 2023 (edited) 6 minutes ago, Philip Lewis 3 said: Futaba HV servo's in fact do just that quoting a voltage range which is 4.8 - 7.4 volts although they only give torque and speeds for 6v, 6.6v and 7.4v. That's nearly as illogical as the Hitec spec sheet above - "You cna use 4.8V, but we won't tell you what the specs are so you know what torque you'll be getting..."! 😄 PS - There's nothing on Google or RCGroups related to independent servo tests on this topic, so I've necro'd an old thread on HC servos on the RCGroups Servos forum to see if anyone there has any ideas... Edited June 7, 2023 by MattyB Quote Link to comment Share on other sites More sharing options...
MattyB Posted June 7, 2023 Author Share Posted June 7, 2023 19 minutes ago, Nigel R said: 1) tweak feedback pot to work at higher voltage 2) change kv of motor 3) sell "new" and "improved" servo for higher price Highly possible! Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted June 7, 2023 Share Posted June 7, 2023 What I was considering was an input voltage regulator so as far as the rest of the servo components were concerned it would be unchanged. With my production engineering hat on it is the way I would create a "high voltage servo" with the minimum of change to the existing production line. 😉 Quote Link to comment Share on other sites More sharing options...
Dickw Posted June 7, 2023 Share Posted June 7, 2023 The KST servos I use quote an operating voltage range from 3.8v to 8.4v with torque and speed figures quoted for 3.8v, 6v, and 8.4v. I mostly use them at 8.4v, but the 3.8v is used by some for DLGs that operate with a single (1s) lipo cell. They seem to work OK. Dick Quote Link to comment Share on other sites More sharing options...
Philip Lewis 3 Posted June 7, 2023 Share Posted June 7, 2023 (edited) 1 hour ago, MattyB said: That's nearly as illogical as the Hitec spec sheet above - "You cna use 4.8V, but we won't tell you what the specs are so you know what torque you'll be getting..."! I agree, it would make more sense to quote values at 4.8 and 7.2 nominal figures as everything else is inbetween, at least then you would know the two extremes. having said that I've never owned a NiMh pack anyway so it never bothered me. Edited June 7, 2023 by Philip Lewis 3 Quote Link to comment Share on other sites More sharing options...
Martin McIntosh Posted June 7, 2023 Share Posted June 7, 2023 I have a biplane with four aileron servos, two of which are standard and the other two of the same type HV. On a 6.6V LiFe pack they all operate at the same speed so I can only surmise that the HV ones have a different voltage regulator, almost certainly to allow operation with Lipos. Quote Link to comment Share on other sites More sharing options...
Dickw Posted June 8, 2023 Share Posted June 8, 2023 KST X08 quoted voltage range 3.8v (i.e. single cell Lipo) to 8.4v Torque 3.8v - 1.4Kg.cm 8.4v - 2.8 Kg.cm Speed 3.8v - 0.18 sec/60° 8.4v - 0.09 sec/60° (The 6v figures are also supplied and are part way between.) I wonder if I could tell the difference between 0.18 sec and 0.09 sec just by watching it moving, particularly if the weight and friction of a control surface is added! While the internal control electronics are sure to use a voltage regulator (probably 3.3v) the motor obviously does not have one, and I doubt if any other servo would be different. If you think about it, the motor is an inductor and is being pulsed by the control electronics, which is very similar to the arrangement in a switch mode regulator or the way an ESC controls the main motor in an electric model. Dick Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.