# DC servo motor question

• posted

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

• posted

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.
• posted

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

• posted

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.

• posted

Eric, I'm having the same problem with a motor I'd like to use as a servo on my lathe feed. I've come to these conclusions:

- the max current is determined by the motor's insulation. It has a class (A, B, F, or H) which sets the maximum temperature that the insulation can handle (for a given life). Without knowing the rating, you (we) would have to assume the lowest (class A), which has a temperature rating of 105C. The max current, then is that which causes the motor to heat to 105C (loaded).

The empirical approach might be: apply a voltage that gives a "reasonable" no-load speed. Apply a load & measure the current. Allow the temperature to stabilize, monitoring to avoid > 105C. Increase the load & repeat until 105C is reached. This is the maximum (steady) current.

There is also a maximum peak current, which will demagnetize the magnets. I have no idea how to determine that.

- I'm having a bigger problem with the maximum voltage. It seems that the voltage is more of a dependent variable. I.e., a function of the speed desired and load. There is a maximum speed based upon mechanical limits & the maximum voltage is what produces that speed, with a given load. So, under the maximum load (i.e., for the maximum current), the maximum voltage will be greater than the maximum voltage under no load.

There is a voltage limit based upon insulation break down, but I think that it is far greater the the speed limiting maximum voltage.

I'm not very confident of these conclusions, so I'd be glad to hear some feedback.

Bob

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PM servo motors are designed for equal performance in

either direction of rotation. This means that they are

necessarily low speed (

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Jim - thanks for the great info. Question about the max current process: if the motor is stalled during this test, you'd only be heating one winding - is that going to give a good current value that can be used when all windings are in use (running)? In other words, the current required to give a certain temp depends upon the cooling and the cooling will be different when running.

Thanks, Bob

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