Anyone familiar with designing electric motors? - Page 1
Forum section: electronics / projects

author: electromotive
author_rank: Regular Contributor
timestamp: February 28, 2019, 08:16:00 pm
content: I'll put this idea out there, and see where it goes. I've wanted to take on an engineering project for quite some time. In particular, I want to engineer my own electric motor. I'm quite familiar with how motors work, but I'm unsure how to scale various components to target a specific output. For example, I'd like to build a 24V DC 1/2HP motor. That's all well and good, but motors have RPM ratings as well. For me this will likely be a multi-year project. I have no end goal in particular, just the satisfaction of having done it. Perhaps it could result in a way of producing a cost effective motor that could be powered by solar in developing countries for pumping wells. Who knows?If anyone could recommend some good books on the topic, I would greatly appreciate it.-Steven

author: Benta
author_rank: Super Contributor
timestamp: February 28, 2019, 09:13:05 pm
content: No big deal.Your starting point is setting up a machine shop.Lathe, mill, centerless grinder, stamping machine, furnace, surface treatment, heat treatment/hardening, precision measuring tools and a couple of other things.Then you're ready to go.

author: calexanian
author_rank: Super Contributor
timestamp: February 28, 2019, 09:58:16 pm
content: Oh, and don't forget the account with the magnet, steel lamination, and wire vendors. Winding and assembling motors (Apart from the coil of wire and battery science class demonstration) is an activity that only works when there is a large scale business behind it. The investment is massive to even make one part. I suggest first visiting an industrial motor repair/rewinding shop. They are hard to get into sometimes, but I have seen a few. it will give you a sense of the scale of resources required.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: March 01, 2019, 01:15:30 am
content: These guys can get you started in something motor-relatedhttp://www.gobrushless.com/

author: Kasper
author_rank: Frequent Contributor
timestamp: March 01, 2019, 04:03:31 am
content: I made a 3 phase ac induction motor in uni from 24V DC power supply. Fun project. I could be wrong but I think it is more electronics and less metalwork than other electric motors. Specially if you make your own vfd.Nothing too challanging in it. Could save a lot of time if you think of easy way to wind it.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: March 01, 2019, 04:39:52 am
content: Start small. Download the free FEMM program to simulate the magnetics. Designing a motor isn't that hard. Designing a good motor is a bit more difficult.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 01, 2019, 05:53:22 am
content: Yeah... not particularly hard if you aren't trying to solve literallyallof the engineering challenges that are faced by a mass-production design, as some have noted...RPM -- is a free parameter. If you go with low RPM, you'll most likely have a motor with many poles, and a very large rotor and stator. Or vice versa at high RPM.This is a key reason why motors generally spin quite fast, when they can. PMDC, series wound, and BLDC motors*, in handheld scale items, typically hit 5-30kRPM. The torque isn't terribly large, but the core doesn't need to be large either, and a few gears are a lot cheaper (and lighter).*BLDC is actually a PMAC machine. (PM = Permanent Magnet, AC/DC. BL = BrushLess, meaning, electronically commutated.)A key design parameter is the Maxwell stress, \$\sigma = \frac{B^2}{2 \mu}\$. When this stress is applied asymmetrically across an air gap, it pushes on the components separated by that air gap. (It has to be asymmetrical, because if it's the same stress at either end of a given unit of space -- as it is for a small segment inside a solid magnetic core, say -- the forces oppose and just compress or stretch the segment, no acceleration.)The fundamental limit of this (for realistic designs) is the magnetic saturation of steel. Above about 1.2T, the amount of magnetization (ampere-turns per meter) required to cause more flux density (tesla, volt-seconds (flux) per meter squared) starts going way up. The in-circuit consequence is more current drawn on the peaks of an AC waveform (where flux is greatest), and the motive consequence is a limitation of maximum torque.The stress of ~1.2T is close to a few atmospheres. The power density of an electric motor is coincidentally quite close to that of a regular compressed air tool (which mostly run on say 4-6 atm).Which is also the limit for a coilgun with a steel armature, a supremely disappointing outcome compared to a potato gun propelled by a tiny squirt of combustible fuel.Applied to a motor, this pressure shows up as a shear force across the air gap, as the rotor and stator fields try to align. Shear around a cylinder, of course, is just another description of torque, so it makes it rotate.I'm... assuming you know a bit of classical mechanics to begin with. If not, keep in mind that torque = force * moment arm, so the larger the rotor/stator is, not only is the air gap longer (more perimeter over which to apply that shear force), but the moment arm (rotor radius) is as well. Also, power = force * velocity (or in the cylindrical case, power = torque * angular velocity), which is why a smaller rotor can deliver the same power if it's made to go much, much faster. And, with pressure limited by materials properties and physics, speed is a valuable ally here!And classical, erm, electrics -- there are in-circuit values (voltage, current, and their ratio, resistance; also, flux is the time integral of voltage, the "area under the curve"), and there are physical values (EMF, flux density, magnetization, MMF..) that start with the in-circuit values, taking geometry into consideration (number of turns, cross-section of the core, air gap length and width..). These are ultimately all simple ratios to each other, which is very convenient: you can always do dimensional analysis to put together a few of these and see what they make.Tim

author: electromotive
author_rank: Regular Contributor
timestamp: March 01, 2019, 06:01:23 am
content: Quote from: Benta on February 28, 2019, 09:13:05 pmI have access to 99% of that. Not on a commercial scale, but a one-off is certainly within the capabilities of my *access*. I'm not at liberty to discuss in detail other than to say my brother works for Vulcan and was one of Paul's trusted few. Another relative uses projects like this to train and test employees (manufacturing components for automakers and others). I'm very lucky in that regard. I'm only seeking to build one as a personal goal. I have no plans for production.Quote from: calexanian on February 28, 2019, 09:58:16 pmIt's probably worth visiting one just for the technical know-how on the project. I doubt they'd take on a one-off job so small, but provided I can advance the design that far, it might be worth a shot in paying for some expert advice on that end.

author: beanflying
author_rank: Super Contributor
timestamp: March 01, 2019, 06:13:24 am
content: Quote from: Alex Eisenhut on March 01, 2019, 01:15:30 amI particular if Dan Sny is still there. Below are a couple of his hand CNC made Microdan range. Left hand one we have pushed well over 300W ( I do have a custom one at 500W from 32g of motor) and the right hand one was generally used with a lower kV winding at 180W and below. Small one is 25mm Stator x 5mm deep large one is 25mm x 10mm. The lamination cutting was outsourced but the rest was his including the tooling for positioning the magnets and all the other Metal parts.These are a couple of unwound frames I have left from when I was the local importer for them. If you can't get hold of Dan via there he was very active on RC Groups.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 01, 2019, 06:22:32 am
content: If you want to go at it from the theory side, Electromechanical Dynamics - Part I Discrete Systems, by Woodson and Melcher gave me a pretty good start. At least when I left the course I thought I knew what was going on.

author: max_torque
author_rank: Super Contributor
timestamp: March 02, 2019, 02:43:23 pm
content: There will be two approaches, and in reality, a mixture of both will be required, and these are Theory and Practice!There is plenty of online resource around eMachines, from magnetic design and simulation to mechanical and electrical architectures etc.These days, there is as much science in the motor's controller as in the motor itself (which is a fundamentally fairly simple device) so you'd want to get up to speed with motor control strategies such as Field Oriented Control etc.One easy , cheap practical way to start is to rewind or remagnet an existing motor, be that a hobby style brushless one of something larger. One demo project i did that worked really well was to use a motor cycle generator as a motor (because they are PM machines), and by adding commutation sensors and a DIY inverter could be used as a good, cheap learning exercise at low voltage and hence intrinsically pretty safe to play with as you learn.Something like this:As the (external) rotor is normally supported directly on the crank shaft you'll have to make a suitable bearing and support system, but if you have access to a lathe (or 3d printer these days!) then that's pretty easy using a pair of cheap deep groove sealed ball bearings.