Sanity check on reversible Universal Motor circuit - Page 1
Forum section: electronics / projects

author: Bakafish
author_rank: Contributor
timestamp: May 26, 2022, 06:40:26 am
content: Hey guys, hoping to have some more experienced eyes take a look at my circuit layout in case I missed something. I have an older drill press that uses a geared universal motor that only runs in one direction. In order to make it reversible, my research showed that the brushes need to be able to be wired in reverse of the normal connection in parallel with the Stator windings. The switch I'm using is an IDEC ASN3220N modular unit that has 3 positions (Reverse is spring return) and two SPST modules, one for the forward one for the reverse. The center position is all contacts open.The relay is an Omron LY3N-AC100 designed for power switching, it's required for space reasons. I could stack more switch modules on the switch to achieve the same goal, but I'm trying to keep everything compact and I would end up with a giant switch that would be awkward to mount.Let me know if anything in the schematic is wrong or unclear. I could make some state diagrams or something, but then you would just be evaluating my possibly bad representations. I made a simulationhttps://tinyurl.com/2h9g4og8, but the simulator has some limitations so it is done using DC, uses two motors to represent the switched polarity and has different switches.

author: Bakafish
author_rank: Contributor
timestamp: June 11, 2022, 05:59:58 am
content: Closing the loop in case anyone wants to make a similar solution.This works fine for me. I had to make minor modifications to decouple the hard wired brushes, the final circuit design is attached. Keep in mind that some universal motors have their brushes biased to enhance a certain rotational direction, this is usually clearly visible by the location of the brushes not being perpendicular to the windings." alt="" class="bbc_img" />

author: Zero999
author_rank: Super Contributor
timestamp: June 11, 2022, 03:19:32 pm
content: It looks very complicated. All you need to do is reverse the connections to either the field, or brushes.

author: wizard69
author_rank: Super Contributor
timestamp: June 12, 2022, 10:36:04 pm
content: I'm not following that schematic at all. As mentioned a wire swap at the brushes should have done it.One thing to consider is that most of the 30 mm type modular switches have "pilot" duty contacts or contacts not rated for motor switching. I would not be relying upon that switch to power off the motor in an emergency. Some brands did have contact blocks with alternative ratings so there is that to consider. For example Allen-Bradley has their Max Duty contact blocks with actual horsepower ratings.

author: Berni
author_rank: Super Contributor
timestamp: June 13, 2022, 09:29:58 am
content: It is quite common to actually have a separate direction selection switch. That way the on/off switch can be a big switch with no confusing middle positions so that you can quickly turn it off if something goes wrong. The direction selector is then a separate smaller switch.In that case the direction selector can be just a normal DPDT switch wired in a cross configuration (Like used when you want to switch a light with 3 or more switches)This does allow you to reverse the motor while running, but you should know not to do that and on a small drill press it won't really do any damage if you do (brushes might not like it very much tho).

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 13, 2022, 10:23:09 am
content: I don't understand your schematic either.First the stator coils are not identifiable in your motor circuit.Also, drawing the relay / switch with the contracts as they are in real live makes a big mess of the wiring, and adding half inductors also does not help to clarify.Reversing the motor field compared to the stator is a common method to make an universal motor reversible, but it does not always work as intended.Sometimes (I've seen this used in hand drills) the brushes are physically rotated related to the stator.Most universal motors are wound symmetrically, but you can't easily see this. Even if the brushes are at 90 degree's compared to the stator, you still can not see how the rotor windings are connected to the commutator.If the motor is not symmetric, you will have a quite noticeable difference in RPM between the two directions.