Figuring out motor specifications - Page 1
Forum section: electronics / beginners

author: rthorntn
author_rank: Frequent Contributor
timestamp: July 22, 2023, 01:19:34 am
content: Hi,I recently picked up a used milling machine, it's 30+ years old, online info is basically all over the place, plus it has clearly been modified, I want to figure out the motor power, the specs I can find for similar units of this vintage say 0.75-1HP.So I have a Hopi 9800 and I measure with the mill running, not cutting anything, just spinning, the Hopi shows:A: 5.8V: 231PF: 256HZ: 50W: 354.7 (is this a bug that it can't display over 1000W, 354.7 should read 1354 no?)Can I work it out the approximate motor size from this?Thanks.Richard

author: themadhippy
author_rank: Super Contributor
timestamp: July 22, 2023, 02:36:17 am
content: Quotesomething's wrong there,power factor can only be between 0 to 1.

author: rthorntn
author_rank: Frequent Contributor
timestamp: July 22, 2023, 04:02:44 am
content: Quote from: themadhippy on July 22, 2023, 02:36:17 amMust be .256 the screen has a weird refresh so the text to the left of 256 didnt come out in the photo.

author: IanB
author_rank: Super Contributor
timestamp: July 22, 2023, 04:15:26 am
content: Quote from: rthorntn on July 22, 2023, 01:19:34 amNo bug, the current is the total current, not the active current. If the motor was consuming 1.3 kW while running idle and doing no work then something would be badly wrong.In reality, you don't need to care if the motor is 0.75 hp, 1 hp, or something else. What you need to care about is whether the motor is powerful enough to handle typical milling jobs you want to perform without bogging down. So you need to try out the mill and find out how it goes.

author: srb1954
author_rank: Super Contributor
timestamp: July 22, 2023, 04:28:16 am
content: Quote from: rthorntn on July 22, 2023, 01:19:34 amThis probably a correct power reading and is reasonably consistent with the voltage, current and PF figures within the accuracy of the HOPI.P = V x A x PF= 231 x 5.8 x 0.256= 343WQuoteNot really. All you are seeing is the no load current and power factor readings, which don't necessarily indicate the full load performance. THe power factor is very low because there is minimal load on the motor and the power reading is just the losses in the motor i.e. winding resistance, bearing friction, fan load, windage etcThe best you can probably do with these figures is to compare the catalog no load specifications for other similar sized motors to get some approximate idea of the motor rating.

author: MrAl
author_rank: Super Contributor
timestamp: July 22, 2023, 04:40:09 am
content: Quote from: rthorntn on July 22, 2023, 01:19:34 amHi,Measure the motor size and compare to other motors you can find online for sale. The motor size correlates to the power somewhat. See if you can find motors about the same age though. You should be able to get some idea like that. You don't really have to know that though do you. Try milling some aluminum and then some soft steel with a sharp bit and see what happens.

author: rthorntn
author_rank: Frequent Contributor
timestamp: July 22, 2023, 04:52:34 am
content: Thanks everyone, I assumed amps was actual amps drawn and had no idea what power factor actually meant.It mills steel fine, the motor is just massive for a 1hp and I thought it maybe was replaced at some point, also I might want to replace it with a 3P and VFD...