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Re: (ET) Reversing snowthrower motor

I have some grasp of the theory, and I have done this before on series motors. It's the specifics of this motor that have me a bit puzzled. It should be as simple as switching the connections to both ends of the armature. That leaves the 2 field windings in the same relationship to each other as before. There may be a complication in advanced positioning of the brushes on the commutator, but as this is intended as a short duration (life of construction and duty cycle) and likely not taxing on the motor's capacity after the initial start up surge, it's a risk I can accept.
The problem is, I cannot figure out which posts are the armature connection posts from looking at the motor from the outside.
On closer examination yesterday - in full daylight for a change - I see a metal band secured around the end of the motor which is not the shaft end. Which is probably about inspecting the brushes, which probably makes that the commutator/armature connection end.
I'll have to make time and space next week to pull that band off, and possibly the motor end plate to physically find and follow the inside ends of the connection posts and figure this out.
Perhaps it will be as simple as undoing the existing jumpers and running leads to reverse the connections they make. (But with my luck, probably not.)
Chalking this up to education, and I'll report back afterwards. (5 posts, not 6 - sigh.) 

Darryl

On 9/5/2019 9:19 PM, Chris Zach wrote:
I think the problem is similar to driving the Elec-trak in reverse. An interesting quirk is that you can drive in forward with field weakening, but not in reverse. The reason is this: When your compound motor is spinning in forward, the series field is adding to the shunt field in terms of magnetic polarity. Normally this is pretty minimal, but when the motor bogs down due to load the series field adds to the shunt field and gives the motor more power and less speed. In a way it's the reverse of field weakening.
When you go into reverse and reverse the armature current, the series field is now in opposition to the shunt field. Thus the motor will go a bit faster as it's always in field weakening mode. However when you load down the motor in reverse the series field acts as a field weakening effect and draws more current. This can burn out the motor, which is why I believe FW is disabled in reverse. Note reversing the field (which is what an E15 does) has the same basic effect. 

C

On 9/5/2019 4:18 PM, Darryl McMahon wrote:
Thanks.  I have been web searching for a bit, and they generally say it's feasible to reverse a compound DC motor.  To pick just 1, this one [http://www.industrial-electronics.com/elecy4_3.html] shows a schematic that says if you reverse the armature leads, that reverses the motor, and all is good.  (I'm ASSUMING - and I know the phrase that triggers - that the series and shunt windings are cumulative, as this is a traction application and we would want the extra grunt.)
On the other hand, the actual motors have an arrow on them indicating only CCW (as viewed from the sprocket) operation.  I also recall a motor repair guy many years ago telling me that compound motors should not be run in reverse (no reason given).   So, I figured I would consult the collective wisdom of this list before doing anything potentially dramatic.
Just back in from checking the posts and polarity.  My recollection was that there are no markings on the posts, which is true.  (On series motors they are usually marked A1, A2, S1 & S2).  On one compound motor I had years ago, they were marked A1, A2, S1, S2, F1 & F2 - but posts F1 & F2 were smaller which indicated they were the shunt field.)
I misremembered the posts on this motor.  There are 5, not 6, and they are all the same size.  There are only 2 power connections to the motor, but there are jumpers across two pairs of posts.  Some ASCII art below (use a fixed width font).  Think of this as a 'Top View' where bottom is defined by the base plate. 

              (Negative                          (Positive
              Connection)                        Connection)

                 Post1                          Post2  Post3
                                                ============  Jumper
           ___________________________________________________
           |                                                 |
           |                                                 |
           |                                                 |
        *  |                Motor body                       |
Shaft  *==|                (base below - not visible        |
and    *  |                                                 |
Sprocket  |                                                 |
           |                                                 |
           |                                                 |
           ___________________________________________________

                                                ============  Jumper
                                                Post5  Post4

                                                (No external
                                                connections)
(In hopes of providing some clarity in further discussion, I have arbitrarily numbered the 5 posts CW from top left, but there are no markings on the motor itself around the posts.)  The jumpers are fairly thin metal, which suggests to me they aren't intended to carry as much current as the main power wires, but that's just a guess.
I have never taken one of these apart, but commutator and brushes (armature power) is usually at the external shaft end (no tail shaft on this motor), right?
So, with only one connector post at the shaft end of the motor, which are the armature connectors? 

Darryl

On 9/5/2019 2:22 PM, RJ Kanary via Elec-trak wrote:

         Not authoritative, but a start.........
https://www.quora.com/How-can-the-direction-or-rotation-of-a-DC-compound-motor-be-reversed 


RJ

Wishing his Dad was still alive to answer that question.


On 9/5/2019 1:35 PM, Darryl McMahon wrote:
I have been using a spare ET snowthrower motor over the summer to power some test rig contraptions over the summer, and it's worked like a charm.
Last week, I was asked to try powering something new (possible amphibious drive 'tire'), but the new wrinkle is they want to be able to move forward and reverse.
As I understand it, the  snowthrower motors are compound wound - strongly series but with a weak shunt field to prevent motor speed runaway.  On the blowers and tillers, there are jumpers between what I expect are the 2 sets of field windings.  (I don't have a schematic for these motors.)  Which posts do what on these motors (there are six external connection posts).  For ET use, only two connections are used (pack positive and pack negative).
On a series motor, this can be done by reversing the field or armature connections (but not both).
I'm looking for guidance on the possibility of reversing one of these motors.  If that's doable and not going to damage the motor, I expect I would be putting a set of contactors in place to be able to do this conveniently and repeatedly.  I need the motor to survive so it can go back on a snowthrower in a couple of months. 

Thanks in advance for any documentation or guidance,

Darryl



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--
Darryl McMahon
Freelance Project Manager (sustainable systems)