Keyboard Shortcuts
ctrl + shift + ? :
Show all keyboard shortcuts
ctrl + g :
Navigate to a group
ctrl + shift + f :
Find
ctrl + / :
Quick actions
esc to dismiss
Likes
Search
10 amps to drive steppers
|
Elliot Burke
I bought an old CNC mill and need to retrofit it. It has some ancient 3
phase electronics, huge inductors, and so on to drive the steppers. That will all be replaced with modern electronics- the question is what electronics. Can bigger transistors be put in the Camtronics stepper drivers to handle more current? Are there other sources for high current / low priced chopper drives? Many thanks for all suggestions. Elliot Burke |
|
Matt Shaver
From: "Elliot Burke" <elliot@...>Is this a Bridgeport In the BOSS 3-6 range? If so I am retrofitting one now using the Camtronics board (I hope!) If I read the BP documentation correctly, the desired motor current is 8 amps. Can bigger transistors be put in the Camtronics stepper drivers to handleThe transistors used are: # RDSon(ohms) IDcont(A) IDmax(A) PDmax(W) IRFZ34N 0.04 19 100 21 IRF9Z34N 0.1 -17 -68 56 This data from the DigiKey catalog. So while there is room in IDcont for higher current at 4 and 10 watts apiece (using 10 amps) cooling will be the issue. As the board is laid out now, the eight transistors used for each motor are attached to a common heatsink which is pretty large. I haven't calculated out the surface area/theta's/power relationship, but forced air cooling is a must as the instructions that come with the board say. It may be better in the end to remote mount these FETs on a separate heatsink such as the giant one which already exists in the Bridgeport's electronics cabinet. I asked Dan Mauch about running at higher current and I think he said he had gone up to 8 amps before experiencing problems. Also, IIRC, Dan Falck said in another post he had blown up his FETs at 9 amps. Anyway, I'm in the process of trying this out and I'll let you know how it goes. The only modification to the board needed to allow higher current is to parallel the existing .1ohm sense resistors with another .1ohms which will double the adjustment scale to 10 amps. Wish Me Luck! Matt |
|
"Mo" <[email protected]
know how it goes. The only modification to the board needed to allowhigher current is to parallel the existing .1ohm sense resistors with another.1ohms which will double the adjustment scale to 10 amps.Matt, I wish you luck! It is far nicer to explore at the ends of the envelope when you have a safety chute. Getting the board to run at high Amps is one thing, getting it to run reliably for years is another. My advice is to analyse and improve your protection system to keep the experimenting fun and painless. Do the units have a temperature sensor on the heatsink for over temp protection?(always a good idea when you are relying on forced cooling and fan health) If not consider providing a thermistor or other type temp temp sensor on the heatsink near to the trannys/fets this should be used to drive the enable/disable pin. You may have several of these in key positions. If you need simple circuitry let me me know. Use good quality heatsink compound and really go over the top with the cooling design. The simple baffles in Tim Goldsteins setup caused a 40F drop. Many fans are better than one. If one goes then you still have forced cooling from the other fan(s) On the subject of fans get the airflow specs for the fans you intend to use and go for the highest you can find - I use the Minebea Smartfan 4715ML-012P542-P1 12V 0.5A (commonly found in old Compaq Servers this has an external thermistor which allows it to run as fast as it needs to saving on life, you can replace this with a 5k pot for manual setting or with a 1k resistor for an airflow that will have your Bridgeport hovering around the shop! Get the max operating temps for the mosfets from the manufacturers datasheets or Dan. Run the units for extended periods measuring their temps and gradually increase the current until you don't feel safe going further - need more current then add more cooling and continue your tests. With good cooling you will keep it going well and the attention paid to sensing/shutdown areas will be a great security blanket. Dan may be able to shed some light on these areas. Mo |
|
Dan Falck
Here are some of the problems that I ran into when I tried to hot rod this
toggle quoted message
Show quoted text
circuit: I started with a layout that Kevin Carroll sent to me. This is similar to what Dan is selling, only Dan's is much improved. Kevin said that he had run this design so hot that the mosfets had unsoldered themselves before. He even had a switching arrangement that would reduce current to the motor when it wasn't in use-using AND gates. I built the circuit, milling and drilling the traces with my 2 amp controller. I installed the thing in a big old XT type case, with a 24 volt 15 amp transformer and a HUGE heatsink for the mosfets with two muffin fans blowing on everything. The low current switching sort of worked- but actually created a lot of noise in the circuit. When it was physically pulled out of the circuit, it was audibly quieter. Increasing current was real flakey- missed steps and lots of noise. Talking to Kevin, he had some of the same problems. So, out came the low current switching. As I increased the current above 6 amps (minus the AND gates), the signal was still getting flakey. The circuit was missing steps all over the place. My input signals are shielded using single conductor braided audio cable, grounded on one end only and tied together at a common ground point. I physically moved the cables around and found that by turning cables 90 degrees to other cables things improved somewhat. So, I tucked cables in and wiretied them down the best that I could. Are the mosfets acting as little antennas, emitting lots of noise from any sharp corner on them? Satisfied that I had the noise down as low as I could get it, I proceeded to run the current up. Somewhere along the way I blew an axis card. It was within a day of jacking the current up. I ended up replacing one phase on that axis (two mosfets) and then adjusted the current down to 6 amps on X and Y and 5 on Z. My X and Y have triple stack 34 frame steppers, while the Z has a double stack on it. It's been about a year ago now. This controller has worked trouble free ever since and it gets used every day. Things that I could have done better: Shielding- use better quality shielded cables and then shield them some more. I think I used Radio Shack brand on the previous controller. I will use an etched circuit board next time. I suspect the milled board probably contributed to noise due to ragged edges etc... Layout- I will make sure that signals are miles away from motor leads and power supplies. Cooling- how about three fans on the mosfets? Maybe an air conditioner in the shop right next to it! Anyway, that's my mosfet frying story- no flying pieces or burned fingertips. Dan Falck At 03:02 AM 6/13/99 +0100, you wrote:
From: "Mo" <mo@...>know how it goes. The only modification to the board needed to allowhighercurrent is to parallel the existing .1ohm sense resistors with another.1ohmswhich will double the adjustment scale to 10 amps.Matt, |
|
Tim Goldstein
Mo,
toggle quoted message
Show quoted text
I would be interested in the circuit you are thinking of for an overheat shutoff for this controller. I currently have no need to push higher amps, but I like the idea of getting some protection from a fan failure. Tim [Denver, CO] -----Original Message----- |
|
Jon Elson
Matt Shaver wrote:
Is this a Bridgeport In the BOSS 3-6 range? If so I am retrofitting one nowBe warned, the Bridgepot Boss controllers reduced motor current when standing still. If you run the full 8 A all the time, the motors are likely to overheat. Jon |
|
Jon Elson
From: Dan Falck <dfalck@...>No, the FETs themselves are too small to radiate much RF. The traces connected to them, however, are large enough to radiate. You have to minimize the area inside the loop of the current paths. I did a lot of work minimizing the current loops on my 100 KHz PWM servo amps, and apparently I must have done it right, as I have no RFI, and can even run a radio within a couple feet of the CNC system. (Umm, that is FM, of course. AM radios don't work so well anywhere in my house, due to all the electronic gear. But, it doesn't bother the TV, either.) Things that I could have done better:Twisted pair cables are very good at shielding. Maybe this is why a lot of people use opto-couplers on their stepper drivers. Not only does it provide isolation, but can be quite effective in reducing EMI glitches. I will use an etched circuit board next time. I suspect the milled boardRaggd edges have no effect (below the GHz range, anyway). But layout is REALLY important in any power switching circuitry. Yeah, well I just don't understand why these fets are running so hot. They may not be driven into the proper gate charge to cut conduction losses. My servo amps are switching continuously at 100 KHz, and often idle at about 1 Amp output (plus the 1.5 Amp or so triangle wave going into the filter). The transistors run cool, even with NO fan at all. Just to be sure, I have a whisper fan on them. Jon |
|
Dan Mauch
The kit contains some really nice but large heatsinks. I don't like to run
toggle quoted message
Show quoted text
mosfets much over 130F with heatsinks on them. One of my customers has one of the few units that I sold as complete units. It ran very well for about 11 months but a student diconnected the Z axis motor while running and blew a couple mosfets and a sense resistor.. It is back up running again. Yes you can increase the current by placing 2 .1 ohm sense resistors in parallel for 10 amps but I saw ecessive ground noise at about 9 amps so I deliberately detuned the specs to 5 amps where it runs very reliably. Yes the mosfets could be replaced with higher rated mosfets but you may run into new problems. One note. I NEVER run my stepper motors at the rated current. Why not ? Bacause a stepper motor on draws full current when it is stopped. Who cares that the motor is getting full current when it is stopped. NOT ME. The faster you run the motor the less current draw there is. The reason is that the coils can't charge fast enought at high speed. Additionally ,if you are using a 6 wire motor and wire the coils in series you now only electrically require about 80% of the rated current because the series resistance of the coils has reduced the current requirement. I have tested many motors with many drivers and measure the running torque with a special torque tester and have proven to myself that using 80% of the rated current is better for the motor and the electronics with no degradation in performance. Dan -----Original Message-----
From: Mo <mo@...> To: CAD_CAM_EDM_DRO@... <CAD_CAM_EDM_DRO@...> Date: Saturday, June 12, 1999 7:05 PM Subject: Re: [CAD_CAM_EDM_DRO] 10 amps to drive steppers From: "Mo" <mo@...>driveknow how it goes. The only modification to the board needed to allowhighercurrent is to parallel the existing .1ohm sense resistors with another.1ohmswhich will double the adjustment scale to 10 amps.Matt, the enable/disable pin.circuitry let me me know.further - need more current then add more cooling and continue your tests.discussion of shop built systems in the above catagories. To Unsubscribe: |
|
Dan Mauch
Let me chime in. On some units that I sold to people that live in very hot
toggle quoted message
Show quoted text
climates like india, I included some thermal cutoff switches. I simply mounted one rated at 135F on each heatsink. I wired them into the Vin to the controller. Thermal switches are available from www.meci.com for about $1.30 ea. Dan -----Original Message-----
From: Tim Goldstein <timg@...> To: CAD_CAM_EDM_DRO@... <CAD_CAM_EDM_DRO@...> Date: Saturday, June 12, 1999 8:27 PM Subject: Re: [CAD_CAM_EDM_DRO] 10 amps to drive steppers From: "Tim Goldstein" <timg@...>specs usefor the fans you intend to use and go for the highest you can find - I forthe Minebea Smartfan 4715ML-012P542-P1 12V 0.5A (commonly found in old discussion of shop built systems in the above catagories.manual setting or with a 1k resistor for an airflow that will have your To Unsubscribe: |
|
Tim Goldstein
toggle quoted message
Show quoted text
-----Original Message-----Dan, Pardon me for being a little thick, but what exactly do you mean by "I wired them into the Vin to the controller."? Tim [Denver, CO] |
|
"Mo" <[email protected]
Tim I've got about 1Mb of stuff to send you is it OK to e-mail direct?
toggle quoted message
Show quoted text
Mo I would be interested in the circuit you are thinking of for an overheat |
|
Dan Mauch
The +36 supply to the 5 amp controller. Or if you used +24 then that line.
toggle quoted message
Show quoted text
Dan -----Original Message-----
From: Tim Goldstein <timg@...> To: CAD_CAM_EDM_DRO@... <CAD_CAM_EDM_DRO@...> Date: Sunday, June 13, 1999 9:48 AM Subject: Re: [CAD_CAM_EDM_DRO] 10 amps to drive steppers From: "Tim Goldstein" <timg@...>hot-----Original Message----- wiredclimates like india, I included some thermal cutoff switches. I simplyDan, them into the Vin to the controller."?discussion of shop built systems in the above catagories. To Unsubscribe: |
|
Tim Goldstein
Dan,
toggle quoted message
Show quoted text
That is what I thought you were indicating, but I just wanted to be sure. Seems to me that I would want the fans to keep running after a thermal shutdown to help the temperatures get back down. So, guess that means wiring the fans off the incoming power ahead of the thermal cut-off device. Thanks, Tim [Denver, CO] ----- Original Message -----
From: Dan Mauch <dmauch@...> To: <CAD_CAM_EDM_DRO@...> Sent: Monday, June 14, 1999 7:18 AM Subject: Re: [CAD_CAM_EDM_DRO] 10 amps to drive steppers From: "Dan Mauch" <dmauch@...>discussion of shop built systems in the above catagories. To Unsubscribe: |
|
"Mo" <[email protected]
Any of you guys integrating hi power stepper drive units such as Dans or
others with custom power supplies, should consider 2 areas of disconnection which will cause problems, 1 Driver board driving the phases without a motor connected - can happen inadvertantly - I've been there 2 Reconnecting a driver board with the motor voltage present - the high value smoothing capacitor(s) on your power supply is charged to the full supply voltage and carries a serious amount of charge - unless you take action to get rid of it it will stay charged for a very long time. Both of these situations seriously endanger your valuable boards. To protect against the driver driving without a motor connected, a good idea is to use a plug and socket connection for the motor with a couple of pins more than you need - so if you need 4 pins for your 4 wire motor get a 6pin plug and socket. Use the 2 unused pins as a loopback - shorting them together in the plug which is connected to the motor and the socket which is mounted on the case can then use this short to enable the board. You could have your DC supply voltage going into the socket via one pin, looping back in thro the plug when it is connected and then taken from the second pin of the socket and sent to your board, no motor connected = no DC voltage to board....the boards I use have an output inhibit pin which must be held low 0v for the board to drive the motors - a high or no connection will disable the boaard from driving. I use the above method to feed a ground wire into the socket that then loops back via the motor plug and then back onto that pin. With the other issue of main motor suopply capacitors remaining charged after switch off, you should use a bleed resistor connected across the motor supply voltage - eg across the capacitor (s). The trick here is to get a value of resistor which is low enough to allow the capacitor to discharge quickly thro it but without drawing too much current from the capacitors and getting too hot. I use a 70v supply and a 2.2k resistor gives me about a 30 second disharge time - 2 in series (4.4k) gives about a minute. The amount of current tro the 2.2k = 70/2200 = 30mA which i can afford to waste. But when we calculate the wattage of the resistor it shows 70v x 0.03A = 2.1W that is a big ceramic covered resistor and 2w is hot - very hot, it will burn you if you touch it. Also if you shed 2w you need the resistor to be double that for reliablity e.g a 4watt. So I use 2 2.2k 2watt resistors in series to give 4.4k and it takes a minute to disharge the cap, the current tro is halved to 0.015A therfor the total power dissipation is also halved to 1W. Now I have less heat, more reliability and less current waste and a minute is enough. Be warned, these reistors still get hot so should be kept away from heat sensitive stuff and in the cooling path. Mo |
|
Dan Mauch
I used an A/C fan that comes on as soon as the power comes up. That way if
toggle quoted message
Show quoted text
the thermal switch should trip the fan continues to run. The only problem with this set up is that once it shuts down then when it cools off it starts running the axis again . Dan -----Original Message-----
From: Tim Goldstein <timg@...> To: CAD_CAM_EDM_DRO@... <CAD_CAM_EDM_DRO@...> Date: Monday, June 14, 1999 7:33 AM Subject: Re: [CAD_CAM_EDM_DRO] 10 amps to drive steppers From: "Tim Goldstein" <timg@...>wiring the fans off the incoming power ahead of the thermal cut-off device.line. aboutDan discussion of shop built systems in the above catagories.discussion of shop built systems in the above catagories.wired$1.30 ea.Dan, To Unsubscribe: |
|
Jon Elson
From: "Mo" <mo@...>A properly designed stepper driver shouldn't be troubled by operating with no load. Unplugging or plugging in the motor while the driver is active could cause some arcing that might cause damage, but just running the driver with no motor connected shouldn't be a problem. 2 Reconnecting a driver board with the motor voltage present - the highYes, connecting a discharged capacitor to a charged one can make some big sparks. Both of these situations seriously endanger your valuable boards.This could still cause a spark when plugging in with the power on. With the other issue of main motor suopply capacitors remaining chargedHere's what I did, although it is on a servo system, but the problems are similar. The E-stop signal on my system releases a pair of relays (one a signal size, the other a power contactor). When these relays are released, they connect a power resistor to the servo amps power input, discharging the power supplies. When the relays are engaged, first the signal relay closes, connecting the same power resistor in series with the DC supply and the servo amps. Another contact on this relay connects power through to a time delay circuit. After a 1 second delay, giving time for the resistor to ramp up the voltage in the servo amps, the power contactor closes, delivering direct power to the servo amps. When this relay closes, it also connects the enable input to the servo amps to +12, turning them on. This gets rid of all the high DV/DT transients when powering things up and down, and makes sure that whenever there is a fault, everything is powered down in a controlled manner. This also sees to it that there are no hot resistors smoldering away anywhere, and that the voltages around the servo amps are cut off in a fraction of a second when need be. Jon |
|
Jon Elson
Dan Mauch wrote:
From: "Dan Mauch" <dmauch@...>The thermal switches should be tied to an E-stop circuit, so it kills all axes, and informs the CNC control that there is a problem. Much better to stop all motion than have the other axes continue blindly on their path, wrecking the workpiece, tool or even the machine itself! Jon |
|
"Mo" <[email protected]
From: Jon Elson <jmelson@...>Jon, unfortunately both types of board I have here warn that this is a certainty and I believe the Camtronics boards are the same. I have inadvertantly killed one that way. It must be in the design, as you say. Where a manufacurer warn specifically that their design will be seriously damaged by this - it is good to take the precaution. Here's what I did, although it is on a servo system, but the problems arePerfect power up and enable sequencing. The power bleed resistors are the absolute minimum needed for those that do not have a similar set-up and are still in the process of playing around with their assembly. Jon do you have a schematic for that set-up - if not I could knock something up and put it on Tim's site for members who would like to go that route. Mo |
|
"Mo" <[email protected]
I was asked recently by another member about how to size up a suitable size
of smoothing capacitor for the linear power supply that most steppers use, the linear type with a transformer, bridge rectifier etc.and if there is a formula. The answer is heavily dependent on the current load demanded by the stepper driver(s) that the supply must service. Recently there was a thread here where Dan Mauch gave some guidance in quantifying the current load on the supply. When you have the current load look at the specs for the capacitors and you will gerally see a spec that indicates the max current at a frequency of 120Hz or 100Hz and then another at 20KHz. The one that most interests us is the one at 120Hz or 100Hz. When you rectify the AC voltage from the transformer through a full wave bridge diode, you will get the negative going half cycle flipped positive so we get two positive peak - so our 60Hz AC frequency has become a 120Hz varying DC frequency countries which use 50Hz AC will result in a 100Hz rectified frequency. So the spec will indicate the max current suppliable at frequency. For thsoe interested in a formula to derive the capacitance themselves, it is : Vripple = I load / ( f x C) or C= I load / (f x Vripple) you need to decide what level of ripple voltage you will accept. As the voltage waveform reaches a peak then falls off, the capacitor will start to discharge into the load acting as a reservoir during the dip in rectifier output. Whilst discharging, the capacitor's voltage will drop until the rectifier volatge rises again and starts to recharge the capacitor this causes a not so constant final voltage but one with a ripple. The ripple voltage is the difference between the highest and lowest ebb of the smoothed voltage. if you require a capacitor value C to allow a max ripple of say 2 volts, and need to supply 8A at a rectified frequency f of 120Hz the calculation would be: C=8A / (120Hz x 2V) C= 33,000 uF The current in the above formula is assumed to be constant which of course a stepper chopper load is not and as the voltage drops the driver will hold the phases on just a tad longer until it gets to the trip current for the chopper but armed with an averaged value for load current with greatest nummber of phases driven as in Dans message should be a good start - if the cost difference is not too great then go one size up e.g 47,000 but at least you have a calculated value range to work with now. Mo |
|
Tim Goldstein
Mo or Jon,
toggle quoted message
Show quoted text
Would love to have a schematic of this setup and would definitely be happy to get it posted. Regarding disconnecting a motor on a Camtronics board while it is powered and running, I inadvertently did this once and the board survived with no harmful side effects. I may have been lucky as I only had some small NEMA 23 motors set up at about 1 amp. Tim [Denver, CO] -----Original Message----- |
to navigate to use esc to dismiss