motor surge current limiting or soft starts

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I have a problem integrating a microprocessor board with an existing
TRIAC based motor speed controller. The speed controller outputs an
AC waveform which is then bridge rectified and fed via a centre-off
reversing switch to a 1/15th horsepower 180 VDC motor.

The 2.5V logic micro board resets when the motor is switched on at full
speed or is reversed at full speed. Various methods of shielding and
tying inputs on the micro board to logic High or Low via resistors has
given some improvements but does not entirely cure the crashes.

Can anyone suggest a startup spike current limiting solution or some
motor speed control circuit with a soft startup.

tia
Mark Harriss


Re: motor surge current limiting or soft starts



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 I had this on a PIC, starting/reversing a 3-phase
 10KW AC motor.  As you have experienced, tying
 inputs to a logic level improved things but was
 not a complete cure.  It did give a hint though
 that lower resistance input tie-downs could be
 the answer.

 So the desperation software sequence ran like this.

 1. Read all uP inputs.

 2. Set all inputs to outputs, at the same logic level.

 3. Do the switching of the motor reversing contactors.

 4. Pause for the nasties to settle, (200mS in my case).

 5. Put the inputs back as inputs.

--
Tony Williams. (posting from s.e.d)

Re: motor surge current limiting or soft starts


Tony Williams wrote:>
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  Thanks for the idea Tony, I could possibly link the manual
speed controller to the logic board to do this by triggering
a outputs only mode.

  My particular application is for a digital angle readout
using a MAXQ2000 micro which can directly drive a 4 1/2 digit
seven segment LCD, IOW a capacitive load which I now have 100K
resistors terminating.

  A snubber across the motor leads does seem to improve things
slightly and the only experiments with axial inductors on the
motor leads made things much worse most likely due to magnetic
leakage, if I can get some toroidals I'll try it again or
even a low value power resistor to see if it helps.

  Short term this may work, for the medium term I'll remove the
reversing switch from the product and long term I'll try and
design a slow start controller possibly PWM.


Re: motor surge current limiting or soft starts



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Inrush limiting thermistors - the kind used for current limiting on
capacitor input power supplies (TV sets and the like).  

Zero crossing solid state relays may also work for light inductive
loads.

DC motors with brushes may also cause EMI that is worse while
starting.  A filter on the motor input will fix that.

-

Re: motor surge current limiting or soft starts


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   I thought they would be ideal except for the 60 second cool down time
which may be a problem.


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The particular TRIAC based speed controller always switches some time
after zero crossing which would prevent this solution.

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The motor is about two feet away, it always seems to crash at start up
or reversing while the motor is spinning which seems to indicate a
current related magnetic surge.

   I've just realized about 2:00 am this morning that there's a mains
side inductor that is axial/ solenoid in construction and is aimed at
the micro board, I'll replace it with a toroidal and see how it affects
things.

I tried a motor side filter based on these same inductors which made
things worse.

Re: motor surge current limiting or soft starts



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The cool down is much less than 1 minute for the size you'd use - more
like five/ten seconds.  If it is the reversing condition that requires
a quick cool down, two could be used one for each direction - soft
start is desirable and if not thermistors could also be incorporated
in the triac control - but that would take more understanding and
effort.

I had this robot - small  lab Cartesian robot on a large table.  All
the gizmos on the table would reset the computer when switched on - we
solved the problem by installing one commercial "brick wall" filter on
the supply to the ancillary machines - just a super filter using many
stages of differential and common mode LC filters.

One of the homoginizers would still occasionally do a reset when it
switched - on the far end of the table with long leads passing by some
of the robot's feedback leads.  It would happen once in 200 times and
wasn't considered severe enough to bother with.  Watchdog timer would
fix things and we'd only lose an hour of time when it happened - late
at night when no one was around.

The homoginizer also had another fluke that was more vexing - the
still liquid in the vessel would fly out when the motor started -
ramping it up fixed the problem.  I went to a radio supply place in
town and bought ten inrush thermistors - six in series made the motor
start slower and fixed the liquid splash problem and also fixed the
rare computer reset problem.
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It would be a less than ideal solution and might not fix your problem
- but one never knows.  Do one?
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\
It is foolish, as a rule, with most motors except steppers and some
synchronous motors to reverse direction on a motor that is already
turning.  That appears as a short to the supply - and draws heavy
current for a time.  In automation applications one almost always lets
the motor coast to a stop before reversing (or electronic braking is
used to quickly stop it) - big motors complain by popping fuses if you
reverse them without a stop first.

Slow Syn made a synchronous motor that turned 600 rpm at 60 cycles -
that particular motor could reverse direction all day long without
hurting it - but it needed a variable frequency drive to change the
speed or a variable sheave pulley type speed adjuster.
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You need an LC network to cut noise - a single inductor is not usually
the answer especially with motor brushes and phase controllers

Most of the crap (EMI/RFI)  that hits processors is common mode - so a
common mode choke with some low inductance capacitors on both sides to
ground and across the line are called for.

Long leads to the motor are an antenna that can radiate EMI - the
filter should be as close to the motor as possible - and another as
close to the controller as possible if the leads are long between
motor and controller - triacs tend to be noisy (AM radio near it will
tell you if it radiates)

Don't ignore snubbers.  Snubbers are just a cap and resistor that go
across switches (motor brushes) to snub the spikes.  As close to the
source of noise as possible - killing noise at the source is 10 X more
effective than trying to eliminate it at the computer.

Snubbers:
When the level of voltage changes suddenly (like a brush leaving one
motor commutator segment and picking up the next on DC motors) the
capacitor absorbs the spike (which is usually very fast - MHZ range).

 A resistor is used in series with the cap, to lower the Q of the
circuit so you don't inadvertently make a tuned circuit with some
inherent inductance in the wiring.  Resistor & cap is placed across
the noise source close to the source.  Point one microfarad, and 100
ohms is a good place to start.  Voltage rating of the non-polar cap
should be two or three times the supply voltage (potted AC rated caps
are most desirable)

You may also need a snubber across the relay or switch that switches
current to the speed controller and/or reversing switch and motor.
Snubbers are cheap and often worth their weight in gold.

Don't overlook a power supply glitch too.  Some supplies allow the
spikes through to the processor - scope out the digital supply when
the motor is changing direction.


-

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For quick fix I'd be inclined to mod' the +/0/- switch, causing it to switch
an extra relay with a power resistor across it.
Something like a 330ohm 10W resistor in line would restrict peak current to
1/2 amp until the relay contacts closed, putting full voltage and current
back onto the motor. Standard relay operating time of 10-20ms is probably
enough for that small a motor. Extra delay by adding a cap feeding the new
relay operating coil.
If the reset always occurred at switch time I'd have fitted big snubber caps
across the changeover relay coils (but it doesn't).
john



--

Re: motor surge current limiting or soft starts



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Hard to say without seeing the setup plus schematics but a software fix
to patch up the effect should IMHO be the last resort. Have you tried
running the motor leads through a common mode choke? How is the ground
between the two boards? If it's just a wire and not a solid plane that
could also spell trouble.

--
Regards, Joerg

http://www.analogconsultants.com

Re: motor surge current limiting or soft starts


Joerg wrote:>
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Hi Joerg, I've haven't tried tried a common mode filter yet, a snubber
did help though. The logic board was placed in a wire earthed PCB copper
foil box with no success and then inside a 12" length of steel 3"x3"
pipe with no effect either which did surprise me, after that I stacked
about 6 Kg (12lbs) of transformer steel around the micro board to
no effect.

    There's no doubt the speed controller board is an ancient and very
noisy design, basically a rectified output DIAC/TRIAC dimmer circuit.

  There is
a solenoid type inductor on the active mains wire which would be
magnetically leaky that I'll substitute with a toroidal inductor to try
and reduce magnetic leakage. I have the mains and motor wiring tightly
twisted in an attempt to cancel the magnetic fields of the internal
wiring but this does not affect the current in the copper tracks on the
speed controller board.

    I have a tightly confined box to place the micro and speed board in
so they are close to each other and overlap slightly, unfortunately the
client did not listen to any advice about using a cast aluminium box
with separate compartments or about making it bigger to have some
separation.

Mark Harriss

Re: motor surge current limiting or soft starts



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Well, as a start I'd try common mode filtering. At least for the motor.
For the rest of the circuit it's hard to tell without schematics and
some detailed photos.

--
Regards, Joerg

http://www.analogconsultants.com

Re: motor surge current limiting or soft starts




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KB have these off the shelf, however rectification is the first step. They
work well.

www.kbelectronics.com





Re: motor surge current limiting or soft starts


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I believe it's based on a very old KB unit no longer made, Homer.

Re: motor surge current limiting or soft starts


The problem could be anything, voltage spikes, current spikes, magnetic
field spikes, switch spark EMI, and probably more.

I'd start with a software fix-- limiting the rate of change of voltage
and current across the motor.
I know it's real cool to be able to "instantaneously" change the
voltage or current directinons, but you have to pay the price of such
abrupt changes-- lots of spikes.   That may help a lot, but won't do a
thing for human-initiated switch flipping-- that will spike and spark
like crazy.

Next I'd try a big snubber across the motor leads-- say 3 ohms and 1uF
for a start.  And maybe a 220 volt varistor to clip the peaks.  And
similarly across the switch contacts.


Re: motor surge current limiting or soft starts


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  The current design has two entirely independent circuits in the same
box. A future design would have a micro reading a pot and outputting PWM
to a big FET.

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   I'm discussing with the client an interim fix of removing the
reversing switch as it's really not at all necessary and earlier models
did fine for 40 years without one, it's more of a "sales feature", this
would eliminate the problem altogether as they then turn it on and vary
speed with a switched pot, slowly starting from a halt.

   That may help a lot, but won't do a
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That's exactly what the problem is caused by: ignoring the instructions
and reversing at speed or setting full speed and turning on via the
reversing switch.






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Re: motor surge current limiting or soft starts



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those drive boards are not designed for proper isolation on the common
lines for your control logic.
   You're getting an in imbalance on the common line which is
causing a momentary short long enough to drive power on your uC section.
   You need input isolation along with isolation on your uC board.
use a xformer supply to operate your uC board and put isolation coupler
on the board that will give you a floating common that you can use for
the drive signal.
    You can also get your self one of those ready made units for that job.
   If you have one of those small low current DC clamp on's , you can
test the common line from the drive to your uC board and i am sure
you'll see a current spike there.


--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
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Re: motor surge current limiting or soft starts


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Hi Jamie,
          The speed and logic boards are not connected in any way except
the common mains supply, one of my early experiments was to run the
logic off a battery supply and it was still affected by the sudden
current draw of the motor turning on and off.



Thanks: motor surge current limiting or soft starts advice


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Thankyou to everyone for the technical advice offered about my
particular motor problem. At this point I'll remove the reversing
switch which is not really necessary and work on PWM speed control
for later models with a soft start circuit.

So far I have tried:

1    Magnetic shielding of the computer using steel (no change)

2    Electrostatic shielding of the computer using an earthed copper
      box (no change)

3    Filtering of the computer power supply, including
      using battery power. (no change).

4    Terminating all tracks on the computer board with resistors
      (about 60% improvement).

5    Twisting the AC and motor wires to cancel magnetic fields
      (no change).

6    Capacitor across motor leads (no change) but speed
      controller did not work properly.

7    Various snubber circuits (slight improvement).

8    Inductors to motor leads to filter current surge (made
      things worse, possibly due to magnetic leakage).

9    Common mode choke across motor leads with and without
      snubber, with and without varistor (slight improvement).

Re: Thanks: motor surge current limiting or soft starts advice



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 Switching a motor from full forward to full reverse
 used to be known as plugging afair, and is a bit
 brutal, both for the motor and for the contactor.

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 A quick soft start possibility is to have a series
 resistor that limits the stall (short circuit)
 current to about 2x or 3x full load current. Say
 to about 1A for your 180Vdc 1/15th hp motor. Use
 the now redundant reversing contactor to short
 out the resistor after a small delay.

 Last minute thought: Did you ever try switching
 the reversing contactor without the motor connected?
 Just in case the contactor was the cause of the
 trouble.

--
Tony Williams.

Re: Thanks: motor surge current limiting or soft starts advice


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Hi Tony,
It'll be even worse in the completed units as it has a steel
flywheel to store energy.

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    The reversing contactor is a DPDT center off switch
I did try it without a motor connected but it had no
effect. It would be nice to have a soft start integrated
with the TRIAC circuit but I haven't worked out how to
do this.

   I'll experiment with a micro reading a POT so that
the midrange area is off and either side of that is
forward or reverse. The $2.50 saved on the reversing
switch should pay for an 8 pin Zilog micro to handle
this.









Re: Thanks: motor surge current limiting or soft starts advice




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Holy crap. That energy has to go somewhere. Can you switch in series
resistance to absorb it?










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