motor surge current limiting or soft starts

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

Reply to
Mark Harriss
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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)
Reply to
Tony Williams

Tony Williams wrote:>

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.

Reply to
Mark Harriss

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.

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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
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Reply to
john jardine

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
Reply to
Joerg

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

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Reply to
Homer J Simpson

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.

Reply to
Ancient_Hacker

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

Reply to
Mark Harriss

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

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

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.

Reply to
Mark Harriss

Joerg wrote:>

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

Reply to
Mark Harriss

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.

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

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.

Reply to
Mark Harriss

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.

It would be a less than ideal solution and might not fix your problem

- but one never knows. Do one?

\ 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.

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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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
Reply to
Joerg

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.

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Reply to
Jamie

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.

Reply to
Mark Harriss

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).
Reply to
Mark Harriss

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.

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.
Reply to
Tony Williams

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

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.

Reply to
Mark Harriss

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

Reply to
Homer J Simpson

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