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  #1  
Old Wed 28 March 2007, 20:09
Mike Richards
Just call me:
 
Transformer supply current versus Gecko output current . . . . .

This thread copied from elsewhere:

I built up an extra power supply from a 12A, 24V standard TRIAD transformer (Part No. F-226U). It is being used to drive four 3-amp PK296B2A-SG3.6 Oriental Motor stepper motors via four Gecko G203 stepper drivers. It also has a 11,000mF capacitor.

. . . . . . . The figures show that it is a 288VA transformer (24V X 12A). Using Gecko's formula for four G203 running 3A motors, 4 X 3A X 0.667 = 8A, I should be able to use an 8A (200VA) power supply. . . . .
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  #2  
Old Thu 29 March 2007, 00:10
Gerald_D
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Mike, I have started to question the use of that 0.667 factor that is so automatically applied....

If you have 4 drives set at 3 Amp each, and they are all being pulsed at very low speed, they will draw 12 Amps together. Your DC voltage is 34V. Those 4 drives want to consume 34x12= 408VA. If you could put an ampmeter on the transformer, you should be reading 400÷24= 17A (ac) which means your 12A transformer is way overloaded and getting hot. The 500VA toroid would smile at the 408VA load. (this rough calc assumes transformer/drive efficiencies of 100% - the real situation is a bit worse)

That 0.667 factor cannot apply where all motors run slowly simultaneously. The factor will only work for applications where some motors are standing still most of the time. Or where the motors turn very fast under low load.
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  #3  
Old Thu 29 March 2007, 01:14
Mike Richards
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Gerald, I think you're right. I duplicated the test using the larger toroid transformer and had no heat at all. Later today (it's currently 1:15 a.m. in this part of the world), as a cross-check, I'll reconnect the 12A transformer but only run two drives, which should require about 200VA.

As a general rule, I could probably assume that, on average, 2-1/2 axes would be active on my machine - depending on whether the 2 x-axis motors were active. I do very little work that requires active use of the Z-axis. It usually plunges at the start of a cut and then 24 or 48 or 96 inches later, the z-axis pulls up again. But having a larger than necessary toroid transformer is cheap enough to give peace of mind.
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  #4  
Old Thu 29 March 2007, 01:52
Gerald_D
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Mike, the definitive answer will come from an ampmeter (or is it ammeter?)...AC before and after the transformer, DC after the rectifier. I know it is a bind to wire in a DC "current"meter, and that smoke may be released, but a clamp/tong meter is easy for the AC. Do you have a "tongtester" (our popular name for it) to check that 17 Amp that I think you have?
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  #5  
Old Thu 29 March 2007, 07:04
Mike Richards
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Gerald, I dug out my Greenlee CM-600 Clamp-on Meter, but got some strange readings. They were much lower than expected. Until I can find another meter to cross check the readings, I'll assume that they are in error.

However, here are the readings:

1.1, 1.6 with 1 motor on, 3 motors on standby
1.4, 2.2 with 2 motors on, 2 motors on standby
1.3, 2.7 with 3 motors on, 1 motor on standby
1.5, 3.6 with 4 motors on

The first number is the current reading of AC Hot (Line) going to the transformer. The second number is one of the AC leads going to the bridge rectifier.

The meter only has two ranges to read current through the clamp: 0-200A and 200A-600A. I'm guessing that the meter can't read these small currents correctly, so I'll see if I can borrow a meter that has a range somewhere around 0-20A.

One thing is certain; this particular transformer is not efficient. The voltage going into the transformer is 120.1VAC and the voltage coming out is 25.6VAC or a 4.69:1 ratio, which shows that the transformer has the correct number of windings. (Dividing 115V by 4.69 would give 24.5V.) If the transformer were 100% efficient, then 4.69A being pulled from the secondary side would cause 1A of current to flow through the primary side. As you can see from the numbers, we're really getting about 2:1 (if the readings from the meter are linear - which I tend to doubt).

As far as heat goes, with one motor on and three motors on standby, I read 122-degrees F. after 1/2 hour. With two motors on, I read 135-degress F. after another 15 minutes. With three motors on, I got 157-degrees F. (69 C.). The amount of heat produced also verifies that the transformer is not efficient.

If I've done the math right with the GeckoMotion program, the motors were running at 4000 steps per second. With 20-tooth spur gears and 3.6:1 gearbox, that would equate to 1.75-ips, meaning that the motor is still in the high torque range where it should draw a lot of current.
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  #6  
Old Thu 29 March 2007, 07:23
Gerald_D
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Some very strange readings indeed...are you sure the tongs are closing properly? Does the output voltage stay consistent with load?

Anyway, there is no doubt that toroids are the better transformer, but I didn't think the non-toroids are so bad! (What does one call a non-toroid?)
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  #7  
Old Thu 29 March 2007, 10:27
Mike Richards
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I just finished making the same AC Current tests with a new Fluke T5-600 meter. Its accuracy is supposed to be 0.1 amp with an error +/- 3%. It's not ideal for measuring low current, but the only meter available at the moment that I can use to cross-check the results.

(The first number is the 120VAC Hot lead, the second number is one of the transformer secondary leads.)

This time the Fluke T5-600 read:
0.8, 0.7 - all motors at auto current reduction
0.8, 1.3 - 1 motor
1.0, 1.8 - 2 motors
1.1, 2.7 - 3 motors
1.2, 3.3 - 4 motors

The Greenlee CM-600 read:
0.9, 0.8 - all motors at auto current reduction
1.0, 1.6 - 1 motor
1.3, 2.1 - 2 motors
1.4, 2.9 - 3 motors
1.5, 3.5 - 4 motors

The original readings with the Greenlee CM-600:
1.1, 1.6 - 1 motor
1.4, 2.2 - 2 motors
1.3, 2.7 - 3 motors
1.5, 3.6 - 4 motors

Even though the numbers are not what I expected, they are consistant. The Greenlee reads a few tenths higher, but its clamps have a distance from inside to inside of 1.3-inches at the point where the measurement is made. The Fluke has a slot rather than a clamp and the sides of the slot are 0.5-inches apart, with a reading area clearly marked on the slots. The accuracy of the Greenlee is 1.5% +0.5A, which probably accounts for the slightly higher readings.

With both meters agreeing, and with LOWER readings (about 0.1A) with the motors spinning 4X faster, I have to conclude that these steppers pull a lot less current than I expected. I believe the test is valid because the steppers reach a temperature of about 125 to 135-degrees F. after prolonged use and the G203 reaches a temperature of 85 to 90 degrees F. when bolted to the top of a 3x3x1.5 inch aluminum heat sink.

To complete the test, I hooked up my Fluke 77 meter in line with the + voltage from the capacitor to the DIN rail star + voltage connection to read the DC current.

0.54A - all motors on standby
1.53A - 1 motor
2.44A - 2 motors
3.50A - 3 motors
4.49A - 4 motors

With all motors off, the power supply voltage is at 33.2VDC with 0.081VAC ripple. With four motors running the power supply voltage is at 31.2VDC with 0.556VAC ripple. That shows that the capacitor is large enough for the job. (I'm always happy to have less than 10% ripple when I use an unregulated power supply.)

The pulse generator is a G100. The stepper drivers are G203s. The steppers are Oriental Motors PK296B2A-SG3.6 steppers wired as unipoloar (half-coil) with 33K current set resistors which should allow the steppers to run at just under 3A.

The GeckoMotion instructions for the final series of tests are:

GPO 1000 0000 0000 0000
GPO 0000 0000 0000 1000
Rx05
Mx3
Ax1000
Vx4000
Ry05
My3
Ay1000
Vy4000
Rz05
Mz3
Az1000
Vz4000
Ra05
Ma3
Aa1000
Va4000
x+250000
y+250000
z+250000
a+250000
x-250000
y-250000
z-250000
a-250000

You'll notice that each motor is started independantly of the other motors so that no motor will be affected by the velocity of the other motors. A final P* instruction showed that all motors had returned to position 0 - and that's not bad after traveling 500,000 steps.
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  #8  
Old Thu 29 March 2007, 11:00
Gerald_D
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Mike, great work! It will take a while to digest ALL those results.

The very low current readings overall is the big surprise. That 12A rated tranformer was never asked to deliver more than 4A, and still it was getting hot. Bottom line on that issue is to stick with toroids.

But, the other thing uncovered here is that the 3 Amp drives only consume about 1.1 Amp each ??? This doesn't make any sense. I am wondering what the DC current readings would be if the voltage was doubled - theoretically it shouldn't change, but will it?

Scratching my head after a postprandial glass of wine.....
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  #9  
Old Thu 29 March 2007, 11:08
Gerald_D
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Might be a good time to visit this post again:
http://finance.groups.yahoo.com/grou...ve/message/567

"7) Load the test motor with the paper towel folded over several times
until it is only 1/2" wide. It makes a great brake-shoe. For bigger
motors you will need the channel-lock pliers to apply sufficient
pressure. You may also want to wet it a little to keep it from
smoking or catching on fire when you do.

8) Slowly load the motor while watching the DC ammeter as you do.
Note the reading the instant the motor stalls. Note the power supply
voltage as well at stall if it is unregulated. This may need to be
repeated several times until you get the hang of it."


Looks like a pliers is needed to get the current draw up to near the limit......
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  #10  
Old Thu 29 March 2007, 17:54
Mike Richards
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"Postprandial!" Today is the first time I encountered that word. (It's a good word with a good definition.)

As to the loading of the motors, my motors are very lightly loaded, since they're just sitting on the test bench with flywheels attached to their shafts. It's clear that my test environment is less than ideal when compared to the real world where a stepper is used to do something. Maybe it's time I actually built a machine instead of testing things. I'll bet that if I looked hard enough I might find some plans that would get me started. I think a know a guy in Africa who said that he was going to offer something like that to anyone interested in building a machine. . .
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  #11  
Old Thu 29 March 2007, 23:20
Gerald_D
Just call me:
 
From Africa you say? Surely you meant somewhere else more, ahem, "advanced"? I mean, those guys probably still use feet and inches...??

Mike, have we got it wrong....does a stepper motor not use max current when it is holding a static position with no load? The G203V are said to drop current to 50% while stationary - yet you see those 4 motors drawing only 0.54 Amp together. Can they hold a position while only drawing such a small current?

Another test if you will indulge...

All motors static, or just one motor coupled up and static,

No G100 connected, ie. no pulses to driver,

DC ammeter in supply to drive,

Check what the current reading does if the motor shaft is loaded, ie. put a little torque on the shaft to move it out of its detent position. See if the current climbs.....

This weekend I'll be taking my tongtester home and experimenting a bit....if you havn't beaten me to it!
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  #12  
Old Fri 30 March 2007, 06:44
Mike Richards
Just call me:
 
Gerald, we've opened a can of worms. I wired up the X-axis motor with the leads of my Fluke 77 in series with the ground wire going to pin 1 on the G203 and measured 137.4mA to 154.0mA, about 1/10th of what I expected to see because the module has a 33K current set resistor to give just less than 3A. (Because the readings were so low, I checked them with the 300mA range on the meter instead of the 10A range.) With an advertised reduced current mode of 50%, I expected to see 1.5A. So, I removed the current set resistor (which is done for 7A motors). The current changed to .46A, when it should have climbed to 3.5A.

With the resistor back in place and a 5-inch pulley attached to the shaft of the motor so that I could 'feel' the shaft turn from its standby position to the next 'detent', I slowly turned the shaft as I watched the meter. At the very most, I saw a 15-mA increase in current just before the shaft slipped to the next position. I expected to see the current increase at that point, but I expected to see a huge increase instead of the 10% increase.

Finally, with the meter reconfigured at the 10A range, I started spinning the motor with the G100 attached. At slower speeds, the current rose to 530mA. At faster speeds (Rx06, Ax100, Vx10000), the current rose to 960mA. With a gloved hand and fast RPMs, I increased friction to the pulley and was able to see a little more than 2.5A current before the motor stalled. At lower RPMs (Rx06, Ax100, Vx100), the motor went from a maximum of 1A to stall with very little increase in friction. I repeated the tests several times with almost no deviation.

At this point I started wondering whether the G203 was faulty, so I re-ran the tests with a G202 stepper driver. This time, at rest I read 260mA with the motor in reduced current mode. With the motor running, I read 530mA, which was identical to the G203 when the motor was running at the same speed.

In about five hours when it is 9:30 in California, I'll try calling Gecko support. If Mariss is there, maybe he can give me some hints.
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  #13  
Old Fri 30 March 2007, 08:39
Gerald_D
Just call me:
 
When I got home from work this afternoon, I confirmed what you were seeing, but then I moved the ammeter onto one of the motor coils. The currents there measure correctly.

It started to make a little sense then.... The Gecko is a constant current driver - loading/unloading the motor cannot change the current output. Then I tried to measure the voltage of the Gecko output while loading/unloading a motor, but got around 200V readings and realised my equipment was not up to the digital waveforms involved.

You found max current draw at a certain combination of speed and torque, and from my mechanical experience, that is how it should be...
Power = Speed X Torque. At zero speed, or zero torque, the power equals zero, except for heat losses. Since the power supply output is a fixed voltage, only its current output can vary to match the Power needed by the motor and drive.

While the motor is still, it needs very low power (only for heat losses) and the constant current device feeds it the designated Amps, but at some very low "voltage" I presume. On the input side of the device (drive) that same power is seen as a low current because the voltage is held high by the capacitor.

It must all be to do with the balance of Power (VA) before and after the Gecko. Before the Gecko there is a constant voltage, after the Gecko there is a constant current. ????

Anyway, I think we have proved the theory that a stepper draws its max current at standstill to be false?
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  #14  
Old Fri 30 March 2007, 09:09
Mike Richards
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Gerald, you've figured it out. I got a reading of 1A at standby when I connected the meter in series with one of the Phase B wires. When I disconnect the current set resistor, the current jumped to 2.6A. Both figures are still about 30% less than I expected (based on the 50% standby current), but much better than before. Edited: Since I'm only reading one of the two coils, it looks like everything is working properly.

The odd thing about this entire experiment is that when I stalled the motor by squeezing on the 5" pulley with a leather glove on my hand, the friction 'seemed' about right. I've run a similar test using a spring-scale to weigh fish and a nylon cord. Those tests showed torque figures right where they should have been.

So, end result = Mariss is still brilliant, Gerald is much better at trouble-shooting that I, and I learned something useful.
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  #15  
Old Fri 30 March 2007, 09:53
Gerald_D
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Mike, it is not a contest! You are a far better diplomat than me. (You can say things that I can't). I am the one that led you up the garden path of sticking an ammeter in there so I have foot part of the bill on this one.

I used a G202 and got the full 7.27 Amps in one coil with no resistor, and 4.70Amp with a 93k resistor. When jumpered for standby reduction, the respective currents were 2.35 and 1.47 Amp (The G202 drops to 33% for standby). So there is still something odd with your G203 readings?
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  #16  
Old Fri 30 March 2007, 11:20
Mike Richards
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Gerald, here's a response from Mariss that he posted on the Yahoo group's gecko forum:

"140mA is just about what I would expect during standby. You didn't
mention if the stopped motor had about half its holding torque when
you turned it by hand.

During standby:

At the beginning of each 20kHz switching period the G203V switches the
supply voltage across the winding. The current thru the winding
increases until it reaches the set value. The winding is then shorted
for the remainder of the switching period.

Current thru the winding rises rapidly when the supply voltage is
across the winding and decays slowly when the winding is shorted. The
power supply delivers the rising current, the supply current is zero
while the winding is shorted.

For you to read 146mA means the supply current is 1.5A at a 10% duty
cycle (5uS on, 45uS shorted).

Mariss


That explains how it all works and seems very reasonable to me.
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  #17  
Old Wed 09 July 2008, 06:19
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
I had forgotten this thread.....the next few posts copied from another thread:

Mike, there is an experiment I want to do one day when I have G201 or G202 drive to hand . . . . . .

Disable its current reduction by changing the jumper, connecting it to a power supply and a stationary motor, and then measuring the AC current supplied by the transformer. Theory says it should be 1.4 times the current limit set by the resistor, but I have a suspicion it is a lot less, probably 1/3 less.

If the transformer current does indeed match the drive current, then the issue of current for square versus round motors is irrelevant. The drive doesn't know what is connected to it. Or, does the transformer current into the drive change according to the motor type, even if the resistor stays constant? I can't measure that because I don't have round motors.
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  #18  
Old Wed 09 July 2008, 06:34
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Gerald,

I've got some old PH299 round motors buried under a lot of other stuff in a storage unit. If possible, I'll dig them out and try your experiment.

-----------------------------------

Here's a more recent post that Mariss wrote on June 4, 2008 at 6:43 pm that gives a brief explanation about the differences between round and square motors:

Re: choosing power supply

I suggest you can pretty much ignore winding resistance and winding
voltage. There are only two electrical specifications that matter:

1) Rated motor phase current. This is the more important spec. Use
this rating to select the drive's current set resistor value.

1a) Series or parallel? You have this choice if you have a 6-wire or
8-wire motor. No choice is needed with a 4-wire motor.

6-wire motor wired full winding = 8-wire motor wired in series. For an
8-wire motor use the motor datasheet specified series current rating.
For a 6-wire motor use 1/2 the datasheet specified current.

6-wire motor wired half winding = 8-wire motor wired in parallel. For
an 8-wire motor use the motor datasheet specified parallel current
rating. For a 6-wire motor use the datasheet specified current.

2) Motor inductance. This is the other important spec. It determines
the maximum practical power supply voltage for your motor. That
voltage is 32 times the square root of the motor inductance in
milliHenries. V = 32 * SQRT mH.

6-wire motors:
full winding inductance = 4 times the half winding inductance.

8-wire motors:
series winding inductance = 4 times the parallel winding inductance.

Explanation:

Motor power output doubles when you double the supply voltage. Motor
iron-loss heating quadruples when you double the power supply voltage.
This means motor heating outraces motor power output, placing a
maximum limit on power supply voltage. This limit can be calculated
from the motor specifications.

Back when most step motors were round, expensive and 6-wire, I came up
with a simple calculation that worked well: Maximum supply voltage
equals 20 times the motor's rated voltage. The underlying principle
always was the motor inductance but the equation hid it and the
necessity of pulling a square root. Times have changed.

Motors are square, inexpensive and mostly 8-wire. They are much better
than the best round motors. What makes them inexpensive also crashes
the old, simple equation; winding fill.

The round motors used nearly 100% wire fill (the windings filled
nearly all the available space on the stator). That kept the
resistance to inductance relationship constant (R = L^2). The newer
motors don't have 100% fill because oftentimes a smaller gage wire is
used. This disconnects the resistance to inductance relationship and
makes the '20 times rated voltage' rule inaccurate.

Mariss
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  #19  
Old Wed 09 July 2008, 06:45
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Mike, I would appreciate you doing those measurements - thanks!
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  #20  
Old Wed 09 July 2008, 07:48
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
If we think of a model that shows how a stepper driver works, sometimes that helps explain why an "inductance" or a "resistance" would act one way with a round motor and another way with a square motor.

As far as I've read, the Gecko stepper drivers use pulse width modulation. That means that instead of sending a steady 5V to the coils of a motor, it sends a series of voltage pulses to the coil. The pulse has an ON time and and OFF time. The longer the ON time is active, the higher the voltage the coils "sees". Since we know that we have a power supply that gives us many times the voltage that the motor can handle, the ON part of the pulse needs to be much shorter than the OFF part of the pulse.

Because of the inductance or resistance of the coil, after a period of time, the voltage is able to force enough current through the coil that the coil's voltage rises to an acceptable level. If the voltage keep pushing current through the motor, before long the voltage would rise to an unacceptably high level and the coil would overheat. That is the reason that for the short pulse.

I've been told that a round motor has characteristics that turn a square pulse into a triangle pulse. The square motor has characteristics that allow the square pulse to remain more square. So, if we think back to our days in geometry and think of the area of a triangle compared to the area of a rectangle, we have a visual model of why a power supply that feeds round motors could be rated at about 66% of the rated current. We also have a visual model of why a power supply that feeds square motors needs to supply more current.

(There are many other factors at play, so the visual model is just that, a model. It is not intended to be mathematically correct. It is only intended to explain an electrical phenomenon or characteristic of the two types of motors.)
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  #21  
Old Wed 09 July 2008, 09:46
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
The rule of thumb for making a CNC power supply only 2/3 of the sum of the motor requirements has been around since before square versus round motors became an issue. I always thought this was because CNC builders never ran all their axes at the same time. I did find it odd that this 2/3 rule was happily applied to 4-motor router tables as well as to 3-motor milling machines and 2-motor lathes.

While running a motor hot the other day, I did put a clamp/tong ampmeter on the transformer output, and the readings were ridiculously low. The max I saw was 0.7Amp when the current limit resistor was set for 2Amp.

Our original ShopBot was supplied with a 300VA 48VDC switch mode supply powering 4 motors rated 2 Amp each. In my ignorance I upped the voltage without thinking of the impact on VA - so my replacement supply was a 300VA toroid transformer output 52VAC / 75VDC. That transformer has been going a good few years and stays close to room temperature. While the rule-of-thumb says the transformer should be 400VA, the 300VA is coping fine. (I have actually bought a 500VA transformer to replace it, but it seems a waste to put that in when the "small" one is coping so well)

I accept the square pulse versus the sawtooth pulse as being the basis for the one type of motor dawing more nett amps than the other, but I wonder what the transformer is seeing on the other side of the capacitor?

When my ampmeter gave me such low readings a couple of weeks back, it set me thinking of a dynamometer and a proper shunt ampmeter . . . . . there is just so much that I don't know!

However, I am cautious of de-rating the old rules-of-thumb until I have seen some solid test results.
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  #22  
Old Wed 09 July 2008, 10:05
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Mike, found this old thread:

http://www.mechmate.com/forums/showthread.php?t=828

I think we need to move the last few posts over there . . .

Having read that one year old thread, I have come to a very solid conclusion: My memory is a disaster! We have been through this exercise before!
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  #23  
Old Sun 13 July 2008, 06:49
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Today I tried another experiment, with a good ampmeter in the transformer's output, before the rectifier . . . . .

I wanted to see how much current is needed for a holding/static motor with quite a high torque on it. This would be the case for 2 or 3 of the MechMate motors when cutting out cabinet parts - only 1 or 2 motors would be moving for the cutting action, the rest will be holding.

The shocker for me was that a loaded static motor and an unloaded static motor draw exactly the same current!

Taking a A2A geared motor, clamping the body in a vice, and then forcing the shaft to turn while the motor is energised, made no difference to the ampmeter reading. With the transformer at 35VAC, an 18k resistor on the G203V, the transformer current draw was 0.38 Amp. Putting a 36k resistor on the G203V raised the transformer current to 0.49 Amp.

This means that a stationary axis will never need more than 12.3 VA, if it has been stationary for more than 1 second. If the current reduction had not yet kicked in, the VA's could be 17.5.

My next experiment will be to put a peak-hold ampmeter onto a hard working MM table and see what that does.

At this this stage I cannot fault the conventional advice of sizing a power supply on two thirds of the motor's total peak current demand.
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  #24  
Old Thu 18 September 2008, 07:50
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Quote:
Originally Posted by Gerald D View Post
My next experiment will be to put a peak-hold ampmeter onto a hard working MM table and see what that does.
Well, the peak-hold meter has been on for a day, and a variety of programs have been air-cut. The max for the day was 119.6VA. The 300VA transformers are about to be ordered . . . . . . . but a few more days of live testing (not air-cutting) will be in order.
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  #25  
Old Thu 18 September 2008, 10:03
domino11
Just call me: Heath
 
Cornwall, Ontario
Canada
Gerald,
Also, don't forget that the max value you saw on your meter was a peak value and not a continuous value. Your 300 VA transformer can do 300VA continuous. You can easily pull more than that for short period peaks easily, that is where your capacitance bank comes in to play. The larger the capacitace bank the more storage you have for peak demands. We routinely use only the capacitance banks in our sonar systems to power the short pings and then charge the capacitor bank back up for the next ping. The power supply by itself could never supply the total peak power the ping requires, even though it is a very short time. An easy way to see how much your supply is drooping under short duration loads may be to actually look for the lowest voltage over time. This would show you how much you are draining your storage bank under the peak loads. Just a thought.
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  #26  
Old Thu 18 September 2008, 10:17
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Heath, the purpose of my testing is to determine an adequate VA rating for transformers to be purchased. . . . . I can't figure out if you are suggesting that I might end up buying a transformer that is under-sized?
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  #27  
Old Thu 18 September 2008, 10:48
domino11
Just call me: Heath
 
Cornwall, Ontario
Canada
Gerald,
No my post was to reinforce your assumption that the 300VA transformer is a good choice. All I meant is that the capacitor bank will hold up the supply for peak demands. By monitoring the output voltage you can see what the max drop in output voltage is under peak loads and use that to see if you need a larger capacitor bank to compensate. You have proven from earlier tests that the constant drain from the steppers is well below the rated continuous capacity of the transformer. The transformer supplies the continuous demand and the capacitor bank supplies the peak demands. You need to address both in a good power supply design.
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  #28  
Old Thu 18 September 2008, 11:43
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
I don't think that the capacitor bank is intended to actually supply the peak loads of the motors....I think their purpose is purely to smoothe the 100-120Hz ripple on the rectified AC. I think the transformer must handle the motor peak loads without the capacitors being drained in any way.
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  #29  
Old Thu 18 September 2008, 14:18
domino11
Just call me: Heath
 
Cornwall, Ontario
Canada
Gerald,
The capacitor bank does both jobs. It does hold up the output voltage when the output from the rectifier dips (ripple), and it will do the same job when the output of the supply tries to dip when you put a load on it. The capacitor bank can respond faster to current demand than the transformer can. If your capacitor bank is increased, the peak current demand can be higher and your output voltage will not droop as much as when you have a lower capacitance. When you are not needing peak current demand, you get the added benefit of a lower ripple voltage.

Am I leading you astray? That was not my intent.
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  #30  
Old Thu 18 September 2008, 17:47
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
A quick and dirty way to see whether you have adequate capacitance is to measure the AC ripple on the DC capacitor. Just set your meter to read AC and then read the + and - outputs on the capacitor. The AC reading will be the amount of ripple.

Customarily, a designer tries to get less than 5% ripple when the current load is steady. For stepper-motors, having ripple of about 10% should not cause any problems - assuming that the lowest voltage stays above the minimum voltage required by the stepper-driver. I run some of my steppers at 26VDC, so I have to be certain that I have enough capacitors to keep the DC voltage from drooping too low.
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