Episode #48: How to Select an Electric Motor Drive

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The Motion Control Show

We've talked a lot about different technologies in recent episodes.  In this episode, I'm bringing together 5 questions that typically arise when we ask the question: How do I select a drive for my motor?  If you need help, or have any applications you would like to discuss, reach out to us at TheMotionControlShow.com or Valin.com.  I am Corey Foster at Valin Corporation.  I hope this helps.

The five questions that come up, or should come up, are…
  • How do I match a drive to a motor?
  • What communication protocol should I use?
  • What are the effects of power input on drive sizing and selection?
  • Why are there different topologies or architectures?
  • Why does one drive cost more than another?

So, let's start with the first one.  How do I match a drive to a motor?  I really touched base on a lot of this in Episode 32 and discussing the mixing and matching of drives and motors from different suppliers.  But here are a couple reminders:
• How does that drive control the motor?  You have to make sure it's the same technology. 
• How does the motor provide feedback?  If so, what kind?  Again, you need to make sure that they have the same technology and they talk the same language. 
• One other thing to remember to consider is: how is it powered? 

A couple of quick reminders on technologies…

There are AC drives that take regular frequency and amplitude of power in and change that to a variable frequency and voltage for the three phase induction motors.  I talked about that in Episode 39

Then there are servo drives which do a whole lot more.  We discussed that a bit in Episode 40.  Typically, it looks like this where there are three different servo loops: the current loop, the speed loop, the position loop, and a number of other calculations and things in there.  Again, that's Episode 40…at least where we talk about servo motors. 

Then there's the stepper drives which just index through known positions, and that's in Episode 41 where we talk about those motors. 

But there are a couple things I don't really touch base enough on, and that's inductance and switching frequency.  When the inductance goes down of the motor, the switching frequency goes up in the drive, or needs to go up in the drive, in order to be able to control that current well.  That's Episode 37: “What is a Drive?”  I do touch base on that.  We don't talk a lot about that, but that's something that needs to be looked at when matching a drive and a motor together.

What communication protocol should I use?  How is it controlled?  That's Episode 34, mixing and matching the controllers and drives.  But it's also Episode 17: “The Holistic Approach.”  Don't forget to look at the whole picture and make sure you have all the capabilities that you need.  You can't just look from one building block to another.

What are the effects of the power input on drive sizing and selection?  In Episodes 44, 45 and 47, we talk about speed/torque curves and servo motor performance.  A quick summary is that the voltage affects the speed capability of the motor.  The thermal capability, the heat, the heat dissipation, really affects the continuous capability of the motor.  Then the current capability from the drive, what it can supply, affects the peak torque of the motor.

Why are there different topologies or architectures?  Well, here are a couple to consider…

There is what some manufacturers will call a compact drive, where the AC is converted to DC, which then is converted back to the AC-approximated that goes out to the motor.  That's the converter.  Well, you can break that up so that the AC and the DC are a power supply unit.  Then when it's rectified internally here, it's separated into 2 units.  The DC then becomes the AC that goes out to the motor, and that's called an inverter. 

This is one topology where it's one drive.  This is another topology where there's a power supply and then a drive.  It looks like this if you're putting it into a system.  Here we have a power supply and a drive or amplifier in one unit.  Well, sometimes those drives can share the DC power between them, so if one is regenerating, it can give that power over to another drive to power it instead of using the AC power from the line.  Well, if you take that power supply in that drive and you split them up, that allows you to be more modular here, so you can use one power supply to power multiple drives.  In here it's a combination of both, where there's a converter here and sharing it's DC power to just inverters here.  Now, why would you want to do this?  Well, if you have enough axes in the system and there's enough power sharing going on, you can cut the price of the system by maybe 5% just by having one large power supply instead of one power supply for every drive.  You can be more efficient with your energy by sharing the power from one drive to another.  When you're using big drives that can really matter, so those are a couple of different topologies or architectures to think about.

Here's a whole other non-related, but still different, architecture.  If you look here, here's the interface going into a motion controller for a drive and then the motor; this is the amplifier which is the drive.  Now here this feedback from the motor is going to go back to a summing junction so that the command signal can be adjusted going to the amplifier.  On this one here, the servo loop is closed at the motion controller, and this one here has the servo loop closed at the amplifier.  It used to be with the older drives that were just dumb drives, this was very common.  As the drives have become more intelligent, this is definitely more common here.  And sometimes there's actually a combination here where the feedback goes back to the drive and then it passes the information back to the motion controller and the loop is actually closed here.  So, these are just a couple of different examples of servo loop architectures.  In a lot of cases, the motion controller and the drive can be in the same package, at least for single axes.  If you have multiple axes, that motion controller is usually going to be separate from the drives.

Why does one drive cost more than another?  This is something we face a lot if we're trying to come up with a solution where one drive is going to be more expensive.  Well, there's performance.  Some drives just perform better than the others.  They have more processing power.  They have more intelligence and faster processing.  They have more industrial protection from maybe mains filtering or I/O protection or vibration or temperature protection.  They have certifications like CE and UL.  Then there's the power input and output.  A drive that takes in 120 VAC is going to be more expensive than one that just takes in 24 VDC because it has to have that extra power supply internally.  Of course, one that takes in 460 is going to be more expensive.  And one that puts out 20 amps is going to be more expensive than one that only puts out 2 amps.  So, the power supply, the capacitors that are inside it, all comes into that cost.  Then there's simply that quality does matter.  A drive that is just inexpensively made is not going to be as good and robust and as expensive as a drive that is well made. 

Here are four different examples.  They all kind of can run motors for sure in one way or another.  This one here, it may only be $50 or so.  This may be five times that.  However, it has more processing power and intelligence.  Well, this one maybe a couple of times that which is good because it has some industrial protection and certifications.  It definitely has higher input and output power.  Well then, this one maybe even another 50% on top of that or 25% just because it has more processing power and more intelligence and even more industrial protection.  This is a metal case here, where this is a plastic case, whereas this has no case, and this has programming in it for motion control, where this one has to be programmed at a very low level.  So even though you can drive a motor of some sorts with all of these, this is going to be a higher power motor than this one, but this one may be the same size motor, but it has some intelligence this one doesn't.  These may be able to drive the same size motors, but again, this one has more protections than this one, so those all go into just the cost and quality of the drives.

I'm Corey Foster at Valin Corporation.  I hope this helps to understand some of the things that come into play in sizing and selecting drives for your motors.

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.