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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>>
>
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