Episode # 55: How Servo Drive Configuration Can Limit Motor Performance

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The Motion Control Show

Are you struggling in getting the performance from your servo motor that you are expecting?  Perhaps someone told you to check out your drive configuration to make sure that it is correct.  Today we are going to talk about how the servo drive configuration can adversely affect your servo motor performance.  I am Corey foster at Valin Corporation.  Reach out to us for help at TheMotionControlShow.com or this email address here.  We are always happy to help.  Let's see what we can learn.

Here are some specs that come from a motor configuration file and some software that I am used to using.  If we take a look at the specs here, this motor is rated with a current of 1.4 Amps.  It has a rated speed of 3600 RPM.  And then it has a maximum speed of 4900 RPM, almost 5000 RPM.  Notice there are two different speeds here.  Now let's take a look at what this looks like in the software for this motor with this particular servo drive.  Notice here that we have a rated current which is here on the speed/torque curve.  But then there is also a stall current which in this case is a little bit higher at 1.3 Amps.  Then there is the rated speed which is where the knee of the curve is here.  But then there is a maximum speed, so you have to know which you are talking about.  Are you talking about the rated current or stall current?  Are you talking about the rated speed or the maximum speed?  You need to know which when expecting what you want to get out of it, as well as configuring it. 

In this case, there is a reference system.  It is a torque drive so +/-10 Volts is for the torque.  The 10 Volts is going to equal an output of 4.468 Amps which is a little bit more than the motor is rated for.  So, if you are going up to 10 Volts, you might be expecting to get a little bit more out of this motor than the motor is actually rated for, so you have to be careful here.  But then on the next screen of this particular software, there is a current torque limit of 200% which limits it to 2.8 Amps.  Now remember that the motor can go higher than that.  If we look up here, the motor can go up to…this is a continuous current here of 1.4 Amps and typically servo motors can go up to maybe three times the continuous current…so perhaps we want to set that to 300 so that we can put out that 4.2 Amps.  The drive often times defaults to something that limits the current output to keep from either harming the motor, harming the mechanics, or even harming a human.  So, check this.  This is the default in this case: 300%.  If you are wanting to put out 4.2 Amps, then you have to actually manually change that.  Also notice that the motor speed limit is 100% here to 3667 RPM.  If you are wanting to actually get the fully rated speed of 4980, then you need to increase this motor speed limit to a higher value.

Let's take another look at a different servo drive with a different software package.  This is just looking at the analog torque command scale again of 0 to 10 volts, but notice here how the speed limit function is set to limit to the Limit Speed Value which, in this case, is 1000 RPM.  That's the default.  So if you wanted to go any faster than that, if you want to go to the full 4 to 5000 RPM of the motor, you need to change this, so the limit is the motor’s maximum speed value.  Then on the next screen this has an external torque limit.  In this case, it is set to 3000 times 0.1%.  So it's set to 300% which is 3 times the continuous current or continuous torque.  But perhaps somebody has set that lower.  Or perhaps the version of software defaults that to a lower value.  If this is set to 100% of the current, you might be thinking that's the maximum current, but 100% is typically 100% of the continuous torque or continuous current where the motor, of course, can go two or three times beyond that for its peak current.

I hope that helps.  I am Corey foster at Valin Corporation.  Reach out to us at this email address here or TheMotionControlShow.com.  I hope you get your motor doing what you want it to do!

If you have any questions or are just looking for some help, we're happy to discuss your application with you.  Reach out to us at (855) 737-4716 or fill out our online form.