Random stuff, reduction, servo motors, ideas and thoughts

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27 Mar 2021 19:54 #203830 by machinedude
it works but the gains are not as great as the hype would make it seem. it seems only a little better in the low range of speed but that's doing away with the low range step of the pulley system. so to be fair it is a step up. i don't have a need to run fast but it does seem like the torque gets better with speed. so it would probably be more of a drastic difference milling something like aluminum where the speed is wide open.

so the official verdict is I'm on the fence :) it might have something to do with the control board i think it might be limited to about 90 volts and 1000 watts ? i kind of take what i see on the internet with a gain of salt. the only thing i seen for sure was 0-94 volts marked on the linear pot that was on the original electronics.

on a scale of 1 to 10 the giggle factor was about a 2 :)

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28 Mar 2021 13:57 #203901 by machinedude
so after looking at DC drives for a while since i don't know to much about them it appears that 90VDC to 180VDC seems to be the normal operating ranges for single phase power inputs. what i noticed was on a 120VAC input for a 90VDC output the largest HP rating seems to be around 1.5 HP and up to 3.0 HP motors can run on the 180VDC outputs of these drives.

would it be a safe to say that even though the DC motor i have said 2.5 HP @ 120VDC @ 18 A it is more like 1.25 to 1.5 HP because of the basic control board? also since the operating range is @ 90VDC would it also be safe to say the 7100 rpm is lower operating at 90VDC?

does anyone have any input for something like this DC drive? i do think these have trim pots to get closer to the 120DVC operating range would it be a step up in getting the most potential out of a DC motor?

www.galco.com/buy/KB-Electronics/KBMD-240D

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28 Mar 2021 18:39 #203925 by arvidb
If your motor wants to spin at 7100 rev/min at 120 VDC then it will want to spin at 7100/120×90 = 5325 rev/min at 90 VDC.

If it wants to spin at 7100 rev/min at 120 VDC then its velocity constant KV is 7100/120 = 59.166... (rev/min)/V. Or in SI units its KV ≈ 6.2 rad×s⁻¹×V⁻¹.

The torque constant is the inverse of the velocity constant (in SI units): KT = 1/KV ≈ 0.16 Nm/A.

So motor top speed is directly proportional to voltage, and motor torque is directly proportional to motor current. And power is speed [rad×s⁻¹] times torque [Nm]. So yes, if your control board doesn't allow the full voltage and current that the motor can handle, you won't get the full power.

You should try to get a drive that can handle the full motor voltage and that can be current limited to the max current that the motor can handle. This way you will be able to get the full torqe from zero speed up to max speed.

2 hp/120 V ≈ 15.5 A, so a drive that can source 16 A or more would be good.

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28 Mar 2021 18:43 #203926 by arvidb
Also, the motor you have is a permanent magnet motor. The drive you link to seems to be for a motor with a separate field winding (i.e. where the field is produced by electromagnets rather than permanent magnets).

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28 Mar 2021 19:35 #203931 by machinedude
i only seen one of their drives at 16 amps. that particular company has probably about the biggest selection at a decent price point i have seen. i will have to get back on their site and take another look. seemed like the higher amp drives were run from 240 VAC and output 180 VDC which is to much for this motor but most of their drives have trim pots to you can custom tune a few things.

after looking at the original set up it ran at 90VDC but was limited to the 350watt range so i was guessing it was a sub 1/2 hp system i was putting that motor at about .6 Nm vs the 1.8 Nm one i traded up for. so just the motor was a step up alone. makes sense to see where else i can gain from and see how much it will set me back. i did not realize how fast these dc motors were until i salvaged this one and took a closer look. 7,100 rpm is a decent speed for a milling spindle if you can find the right balance of speed and torque

one other thing i will have to do if i push the limits is to get some cooling on the motor which should not be to big of a deal since it has a shaft for it already.

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28 Mar 2021 20:15 #203937 by andypugh

what i noticed was on a 120VAC input for a 90VDC output


Something a bit strange there. If you rectify 120V AC you get 170V (and about 320V if you rectify 230v mains)

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28 Mar 2021 20:37 #203943 by machinedude
i could have very well missed something but that was just how these drives wer listed? i have no idea what they have going on in the inside? i say 120VAC just because that what it actually measures at at the outlet here in the states.

here is the pdf for another type of these drives in a watertight enclosure

This browser does not support PDFs. Please download the PDF to view it: Download PDF

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28 Mar 2021 20:41 #203944 by machinedude
taking a guess it could go through a transformer then go through a rectifier to step the voltage down before it gets converted to DC? i'm not an electronics guy obviously :)

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28 Mar 2021 20:47 - 28 Mar 2021 20:49 #203946 by PCW

what i noticed was on a 120VAC input for a 90VDC output


Something a bit strange there. If you rectify 120V AC you get 170V (and about 320V if you rectify 230v mains)


I think 90VDC rating comes from old SCR style drives that feed the
motor pulsating DC (full wave rectified AC with phase control)

This kind of thing:

www.kb-controls.com/product.sc?productId=10&categoryId=3
Last edit: 28 Mar 2021 20:49 by PCW.

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10 Apr 2021 12:28 #205518 by machinedude
I did test the MC-60 control board and it does only operate at 90VDC so 5,325 RPM is what it works out to and with the reduction on the pulley of the motor it ends up being somewhere around 4,660 RPM so only a slight increase in speed was gained from this upgrade. the torque on the other hand was at least 3 times more. so not as good as i first thought i was going to get with this modification. when the same math was applied to the original motor it worked out to be 4,295 RPM for the top speed. so only a modest gain on the top speed. only other thing i can check to confirm all this is to get myself a handheld tach to see if it checks out to the math calculations.

at some point soon i will have to cut my table top t slots. so cuts 55 inches long will be a good test to see if it gets close to the feed rates for a 3/8 3 flute endmill running at this speed? regardless of out come i will probably take a video and post my findings :) good or bad :)

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