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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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#1
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DC servo motor question
I've got a couple DC servo motors that are in great shape but old
enough that Electro-Craft no longer has any info on them. So I would like to know if there is a way to figure out what max voltage and current might be. Can using the motor as a generator be used as a way to determine the approximate proper voltage and current? Thanks, Eric |
#2
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DC servo motor question
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#4
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DC servo motor question
Can using the motor as a generator be used as a way
to determine the approximate proper voltage and current? That will tell you rpms per volt. Stall current against a dyno will give you torque per amp. There's your characteristics. Now, you have no way to tell what maximums are appropriate, as these have nothing to do with what the shaft does versus what the wires are doing. However, the shaft size roughly indicates the max torque, compare what you have to similar motors in catalogs to get a rough spec. Max voltage has to do with the insulation and other build features, max rpms and max power likewise with heat dissipation and the like, you can't measure these things directly. Many DC motors are 90 VDC or 180 VDC for a max speed of 1800 or 3600 rpm, so if you know or guess max voltage or speed you can determine the other. |
#6
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DC servo motor question
Bob Engelhardt writes:
I'm not very confident of these conclusions, so I'd be glad to hear some feedback. Your conclusions are, well, confused. DC permanent magnet motors are quite simple in the ideal. Voltage is proportional to shaft speed, and current is proportional to shaft torque. Everything else follows. There are maximums to each. And to their product, which is power. |
#7
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DC servo motor question
On Sat, 04 Oct 2008 05:27:58 GMT, wrote:
On Fri, 03 Oct 2008 12:36:27 -0400, Ned Simmons wrote: On Fri, 03 Oct 2008 15:45:37 GMT, wrote: I've got a couple DC servo motors that are in great shape but old enough that Electro-Craft no longer has any info on them. So I would like to know if there is a way to figure out what max voltage and current might be. Can using the motor as a generator be used as a way to determine the approximate proper voltage and current? I'd first compare the motor to motors of similar size and construction to get an estimate of torque rating and max RPM. Then experiment using the relationships Ke = RPM/volt Kt = lb*in/amp to get values for Ke and Kt. Either rotating the motor at a known speed and measuring the output voltage, or powering the motor with a known voltage and measuring RPM will give you Ke. To determine Kt you'll need to control torque and measure current, or vice versa. The easiest way would probably be to drive the motor with an adjustable current source with a torque arm and weight attached to the shaft. Adjust the current to balance the weight with the arm horizontal. Keep in mind that DC motors will have both a continuous and a peak current rating. The continuous current is mainly a function of winding resistance and the ability of the motor to dissipate heat. The ratio between continuous and peak current is generally an indication of magnet material and the resistance of the magnets to demagnetization as a result of high current in the windings. You might see a ratio of 3:1 in low end motor, while a high performance, low inertia motor might be as high as 7:1 or more. -- Ned Simmons Thanks for the reply Ned. These motors came from a lathe that originally had a Bandit control. Later these same lathes were equipped with a Fanuc 5T control and Fanuc motors. If I short the wires on the Electro-Craft motors and spin the shaft by hand there is a lot of drag, but the Fanuc motors have very little drag when the shaft is spun by hand. I'll try powering them both with the same voltage and compare speed and then I'll measure the current drawn with the torque arm setup you described. Thanks Again, Eric PM servo motors are designed for equal performance in either direction of rotation. This means that they are necessarily low speed (5000 Rpm) machines because the brush angle cannot be offset to reduce high speed sparking. Typical rated power is at 3,000 RPM. so the voltage needed to produce 3,000 RPM should be pretty close to the motor rated voltage. Check the stalled resistance. Ohmeter readings can be very unreliable - measure the effective resistance by monitoring the terminal voltage when passing 1 amp or so through a stalled motor .. http://xs.to/xs.php?h=xs432&d=08400&...ng_-002155.jpg shows the average relation between motor rated power,rated voltage and typical winding resistance. It will not of course give a precise answer because individual motor designs can be far from average but it will be a LOT better than eyeball estimates. For a closer figure you need to guesstimate the motor maximum current. Copper has a temperature co-efficient of 0.4% /deg C. If you now increase the stalled current (in stages to allow time for the temperature to stabilise) until the measured R increase by 30% the winding temperature will be about 100 deg C which is pretty safe - High temperature servo motors can withstand up to 150 deg C. This process is not as long winded as it sounds - you can usually get a pretty close estimate with no more than two trial currents. For best accuracy use the brushes to feed the current through the armature and measure the voltage drop directly on the relevant commutator segments. Jim |
#8
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DC servo motor question
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#9
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DC servo motor question
Richard J Kinch wrote:
Your conclusions are, well, confused. Maybe my post just sounds confusing & my conclusions aren't confused at all. Or maybe my post is confusing and you have been confused by it. G DC permanent magnet motors are quite simple in the ideal. Voltage is proportional to shaft speed, and current is proportional to shaft torque. ... Yeah, I know that. It doesn't help. We're looking to find the maximum voltage & current. Without knowing the maximum speed or torque. Bob -- Nota for President |
#10
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DC servo motor question
Bob Engelhardt writes:
We're looking to find the maximum voltage & current. Without knowing the maximum speed or torque. So you want to know a maximum X without knowing something in fixed proportion to maximum X, which is absurd. |
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