#1
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Should I buy these "surplus" motors?
Another Motor question
I have a chance to buy a few stepper motors for next to nothing. Can someone take a look below at the spec. and tell me if I can use them? I am building my control box now and I have every thing but the motors to start. these motors are close by an may get me through until I get my motors I will be using on the MM. Thanks Frame Size: 34 Shaft Diameter (inches): 0.5 Shaft Style: w/flat Second Shaft Diameter: 0.5 Second Shaft Style: Round Torque Rating (oz inch): 595 Current Rating (amps): 3.0 Resistance (ohms): 0.8 Voltage: 2.5 Inductance (mH): 8.5 Winding: bipolar - 4 wire Steps per revolution: 200 Degrees per step: 1.8 Length (inches): 3.15 -------------------------------------------------------------------------- Frame Size: 34 Shaft Diameter (inches): 0.5 Shaft Style: w/flat Second Shaft Diameter: 1.5 Second Shaft Style: Round Torque Rating (oz inch): 920 Current Rating (amps): 7.0 Resistance (ohms): 0.3 Voltage: 1.8 Inductance (mH): 2.4 Winding: bipolar - 4 wire Steps per revolution: 200 Degrees per step: 1.8 Length (inches): 3.83 |
#2
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nope, they are no good.
can you send me the name and phone number of the seller so I can jump on them really quick ? Just kidding. they sound pretty good. get them. they will be useful. Dave |
#3
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The problem, as I see it, is that because the motors are so different, you might have to use two different power supplies. The first motor could use a power supply up to 80VDC since 1000 * SQRT(0.0085) = 92.2. Using Gecko's formula to determine the size of the current limiting resistor, 47 * 3) / (7 - 3) = 35.2K, so a standard 33K resistor would work. The second motor could use a power supply up to about 50V since 1000 * SQRT(0.0024) = 48.9; however, I would run that motor with a power supply of 35V to 40V to keep the heat down. Also, even though the motor is rated at 7 amps, I would limit the current to 6 amps because of heat. You would loose a little torque, but that motor has torque to spare. A 270K or 300K current limiting resistor would give you about 6A.
If I had to use those two dissimilar motors, I would run both motors from a 35VDC power supply before installing two different power supplies. |
#4
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I can buy all 12 motors at a real good price. there is 6 of each one. if you were going to buy only 4 or 5 of the motors which ones would you recommend? Thanks for the replies.
Quote:
Last edited by cbboatworks; Thu 18 October 2007 at 18:41.. |
#5
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I would highly recomend you buy all 6 of one type.
I would buy the larger motor and use 24 volt transformers. that gets you a decent 35 volts. very easy to find 24 volt transformers. I would get all 6 motors just in case you ever needed a spare or two. remember, you can always add a rotary unit to make spindles. needs a stepper. And since you have one extra, just send it along to dave at...... Dave |
#6
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Quote:
Greg |
#7
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The Shopbot machines that I'm familiar with use geared motors that would be in the 300 oz*in range (without gears) and non-geared motors that are about 600 oz*in, so I would favor the first motor in your list. Also, after looking at the specs again, I noticed that the first motor has a voltage rating of 2.5V and an inductance rating of 8.5mH. Using the inductance rating to compute the power supply's voltage gives a much higher number than just multiplying the voltage rating by 20X to 25X. None of the motors that I use have that characteristic. What I'm trying to say is that, based on my own experience, I would try a power supply of about 35V to 40V for the first motor on your list.
Before Mariss posted his formula that uses inductance to compute the maximum power supply voltage for a motor, I used the ball-park figure of 15X the motor's rated voltage to compute the power supply's voltage. For me, that gave a voltage that kept the motor at a reasonable temperature and still gave excellent speed. In your case 15 X 2.5V = 37.5V. As to the second motor on your list, to me it is too big. Although the Gecko stepper drivers are rated for motors drawing up to 7A, I've found that motors with a current rating of 3 to 4.5 amps run the smoothest and still have adequate torque. My favorite motor, the PK299-F4.5 is an exception to that rule. That PK299-F4.5 motor is an eight wire motor that I wire in bipolar parallel mode with the current limiting resistor set for 6A. However, it's bipolar series rating is 3.18A, so perhaps that motor even falls within the 3 to 4.5A catagory if it were wired as if it were a four-wire motor. The sad fact is that by buying untested motors, you might find yourself spending a lot of time and a lot of money experimenting with electrical components before you find the 'perfect' combination of parts that work reliably for you. |
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