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From: pimpom on 20 Jul 2010 12:51
Cydrome Leader wrote:
> E <invalid(a)invalid.invalid> wrote:
>>
>> "pimpom" <pimpom(a)invalid.invalid> kirjoitti
>> viestiss?:i2476a$qif$1(a)news.albasani.net...
>>> I'm rebuilding an old machine for a local company by
>>> replacing all
>>> the electronics with my own design. It uses a 24V DC brushed
>>> motor
>>> with an integral reduction gear. The original drive circuit
>>> used a
>>> couple of SCRs for speed control and current was limited to
>>> 3.5A.
>>> My circuit uses high frequency PWM (~22kHz). (The motor
>>> control is
>>> only a small part of the overall design).
>>>
>>
>> Try lower PWM frequency, like 50 Hz or whatever the original
>> SCR
>> circuit used.
>>
>> -ek
>
> This sounds like the best idea. Even at 2 or 3kHz I ran into
> weirdness
> with motors that had capacitors across the leads. They would
> race
> instead of running slow though.

It's not the cap across the motor terminals per se that's causing
trouble. As I said earlier, it runs fine and speed control works
as expected as long as the motor frame is not grounded. (I use a
slotted wheel optical tachometer for feedback and, with a
counter, for duration control).

It's when the motor is mounted on the steel frame of the machine,
connecting it to electrical ground, that things go wrong. The cap
from the negative terminal (also MOSFET drain) to ground must be
playing havoc with the switching output.


From: pimpom on 20 Jul 2010 13:21
E wrote:
> "pimpom" <pimpom(a)invalid.invalid> kirjoitti
> viestiss�:i2476a$qif$1(a)news.albasani.net...
>> I'm rebuilding an old machine for a local company by replacing
>> all
>> the electronics with my own design. It uses a 24V DC brushed
>> motor
>> with an integral reduction gear. The original drive circuit
>> used a
>> couple of SCRs for speed control and current was limited to
>> 3.5A. My
>> circuit uses high frequency PWM (~22kHz). (The motor control
>> is only
>> a small part of the overall design).
>>
>
> Try lower PWM frequency, like 50 Hz or whatever the original
> SCR
> circuit used.
>

I'll consider that option. Thanks.


From: Grant on 20 Jul 2010 16:31
On Tue, 20 Jul 2010 22:21:06 +0530, "pimpom" <pimpom(a)invalid.invalid> wrote:

>Cydrome Leader wrote:
>> E <invalid(a)invalid.invalid> wrote:
>>>
>>> "pimpom" <pimpom(a)invalid.invalid> kirjoitti
>>> viestiss?:i2476a$qif$1(a)news.albasani.net...
>>>> I'm rebuilding an old machine for a local company by
>>>> replacing all
>>>> the electronics with my own design. It uses a 24V DC brushed
>>>> motor
>>>> with an integral reduction gear. The original drive circuit
>>>> used a
>>>> couple of SCRs for speed control and current was limited to
>>>> 3.5A.
>>>> My circuit uses high frequency PWM (~22kHz). (The motor
>>>> control is
>>>> only a small part of the overall design).
>>>>
>>>
>>> Try lower PWM frequency, like 50 Hz or whatever the original
>>> SCR
>>> circuit used.
>>>
>>> -ek
>>
>> This sounds like the best idea. Even at 2 or 3kHz I ran into
>> weirdness
>> with motors that had capacitors across the leads. They would
>> race
>> instead of running slow though.
>
>It's not the cap across the motor terminals per se that's causing
>trouble. As I said earlier, it runs fine and speed control works
>as expected as long as the motor frame is not grounded. (I use a
>slotted wheel optical tachometer for feedback and, with a
>counter, for duration control).
>
>It's when the motor is mounted on the steel frame of the machine,
>connecting it to electrical ground, that things go wrong. The cap
>from the negative terminal (also MOSFET drain) to ground must be
>playing havoc with the switching output.
>
So tip your circuit upside down and pop in a P-channel MOSFET?

Grant.
From: pimpom on 21 Jul 2010 01:08
Grant wrote:
> On Tue, 20 Jul 2010 22:21:06 +0530, "pimpom"
> <pimpom(a)invalid.invalid>
> wrote:
>
>> Cydrome Leader wrote:
>>> E <invalid(a)invalid.invalid> wrote:
>>>>
>>>> "pimpom" <pimpom(a)invalid.invalid> kirjoitti
>>>> viestiss?:i2476a$qif$1(a)news.albasani.net...
>>>>> I'm rebuilding an old machine for a local company by
>>>>> replacing all
>>>>> the electronics with my own design. It uses a 24V DC
>>>>> brushed
>>>>> motor
>>>>> with an integral reduction gear. The original drive circuit
>>>>> used a
>>>>> couple of SCRs for speed control and current was limited to
>>>>> 3.5A.
>>>>> My circuit uses high frequency PWM (~22kHz). (The motor
>>>>> control is
>>>>> only a small part of the overall design).
>>>>>
>>>>
>>>> Try lower PWM frequency, like 50 Hz or whatever the original
>>>> SCR
>>>> circuit used.
>>>>
>>>> -ek
>>>
>>> This sounds like the best idea. Even at 2 or 3kHz I ran into
>>> weirdness
>>> with motors that had capacitors across the leads. They would
>>> race
>>> instead of running slow though.
>>
>> It's not the cap across the motor terminals per se that's
>> causing
>> trouble. As I said earlier, it runs fine and speed control
>> works
>> as expected as long as the motor frame is not grounded. (I use
>> a
>> slotted wheel optical tachometer for feedback and, with a
>> counter, for duration control).
>>
>> It's when the motor is mounted on the steel frame of the
>> machine,
>> connecting it to electrical ground, that things go wrong. The
>> cap
>> from the negative terminal (also MOSFET drain) to ground must
>> be
>> playing havoc with the switching output.
>>
> So tip your circuit upside down and pop in a P-channel MOSFET?
>
I've thought of that as a possible solution, but for various
reasons not a practical one in this case: 1) I don't have a
P-channel MOSFET in stock (I can never remember to include it in
my orders). 2) It's not locally available in my small town.
Placing small orders from another city is not as easy in India as
it is in places like the US. 3) Unless it's absolutely necessary,
I want to avoid having to start the pcb design, construction and
assembly all over again.


From: Grant on 21 Jul 2010 05:22
On Wed, 21 Jul 2010 10:38:54 +0530, "pimpom" <pimpom(a)invalid.invalid> wrote:

>Grant wrote:
>> On Tue, 20 Jul 2010 22:21:06 +0530, "pimpom"
>> <pimpom(a)invalid.invalid>
>> wrote:
>>
>>> Cydrome Leader wrote:
>>>> E <invalid(a)invalid.invalid> wrote:
>>>>>
>>>>> "pimpom" <pimpom(a)invalid.invalid> kirjoitti
>>>>> viestiss?:i2476a$qif$1(a)news.albasani.net...
>>>>>> I'm rebuilding an old machine for a local company by
>>>>>> replacing all
>>>>>> the electronics with my own design. It uses a 24V DC
>>>>>> brushed
>>>>>> motor
>>>>>> with an integral reduction gear. The original drive circuit
>>>>>> used a
>>>>>> couple of SCRs for speed control and current was limited to
>>>>>> 3.5A.
>>>>>> My circuit uses high frequency PWM (~22kHz). (The motor
>>>>>> control is
>>>>>> only a small part of the overall design).
>>>>>>
>>>>>
>>>>> Try lower PWM frequency, like 50 Hz or whatever the original
>>>>> SCR
>>>>> circuit used.
>>>>>
>>>>> -ek
>>>>
>>>> This sounds like the best idea. Even at 2 or 3kHz I ran into
>>>> weirdness
>>>> with motors that had capacitors across the leads. They would
>>>> race
>>>> instead of running slow though.
>>>
>>> It's not the cap across the motor terminals per se that's
>>> causing
>>> trouble. As I said earlier, it runs fine and speed control
>>> works
>>> as expected as long as the motor frame is not grounded. (I use
>>> a
>>> slotted wheel optical tachometer for feedback and, with a
>>> counter, for duration control).
>>>
>>> It's when the motor is mounted on the steel frame of the
>>> machine,
>>> connecting it to electrical ground, that things go wrong. The
>>> cap
>>> from the negative terminal (also MOSFET drain) to ground must
>>> be
>>> playing havoc with the switching output.
>>>
>> So tip your circuit upside down and pop in a P-channel MOSFET?
>>
>I've thought of that as a possible solution, but for various
>reasons not a practical one in this case: 1) I don't have a
>P-channel MOSFET in stock (I can never remember to include it in
>my orders). 2) It's not locally available in my small town.

I live in a biggish small town, can get overnight deliveries from
Sydney, about 1000km away :)

>Placing small orders from another city is not as easy in India as
>it is in places like the US.
> Unless it's absolutely necessary,
>I want to avoid having to start the pcb design, construction and
>assembly all over again.

That's a really good reason. So you add inductor to isolate the
cap, or rip cap out of motor?

I prefer leaving the motors alone, because a replacement down the
track will cause same problem if 'they' forget to take out the cap.

Grant.
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