5 Rules of Thumb When Selecting a Servo Drive

Servo drive selection

Increase your success when selecting a servo drive by following a few rules of thumb.

In the world of servo systems, there are two kinds of problems you might run into:

  1. Problems that are discovered early and can be addressed before they get to the customer.
  2. Problems that are discovered late — after production starts or after they get to the customer. These are the worst.

Wouldn’t it be great if there were a set of guidelines or “Rules of Thumb” to help you avoid problems before it's too late?

Well you’re in luck. We’ve compiled a list of 5 basic rules to keep in mind when selecting a servo drive.

In a hurry and don't have time to read this?  Download this handy cheat sheet!

Scenario

You designed an awesome delivery robot that has been installed at a lot of locations.  Everything seems great until you get a call from a concerned customer, their robots keep getting stuck on an incline.  Uh Oh.

Background: The Autonomous Mobile Robot (AMR) you designed uses servo drives to power the traction wheels.  During development everything checked out and passed with flying colors - at least as far as you could tell.  Same with the initial testing at some customer facilities.  So why is someone now complaining that robots are stalling? What gives? Also, could this be the first of more calls to come?

After some investigation and taking some measurements you see that the servo drives are hitting their current limits.

#1 Make Sure There’s Enough Current

Rule of Thumb

Include an additional 25% of headroom to the current limit above what you think is needed for the application.

Why it’s important

Additional current capacity can ensure uninterrupted operation when the conditions are different from what you expected.  It’s easy to get it wrong, especially when the application environment can be much different from the testing environment.  A little extra current can prevent problems later on.

When it helps

  • When conditions change - in the scenario above a new location had a steeper incline than what the robot was tested for.
    • On a flat surface, hitting the current limit could just mean slower acceleration. On inclines hitting the current limit could mean a stall.
  • When machines get older - the friction can increase over time necessitating more power to do the same thing.  This can happen with any type of machine, not just mobile robots.
  • When the load changes - customers can increase the payload.

One other thing to consider, if you increase the current limit then make sure the motor can handle the additional current!

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#2 Allow for Voltage Fluctuations

Rule of Thumb

Whenever possible, you should select a power supply and servo drive combination that gives you at least 25% headroom between the power supply voltage AND both the undervoltage limit and the overvoltage limit of the servo drive.

Why it’s important

Voltage fluctuations occur for a number of reasons and cause nuisance trips for both under and overvoltage conditions. Increasing the headroom allows for wider swings which increases reliability.

When it helps

Increasing the voltage headroom can help in these situations.

  • When the line voltage at the installation is different from the line voltage where you developed the machine.  "Line voltage" meaning the voltage that is supplied by an AC outlet at your facility.  Differences can be especially pronounced when comparing voltages in different countries.
    • Since many motion control systems use unregulated supplies, any changes in the line voltage will change the voltage that is supplied to the servo drive.
    • Example - for a servo drive that's rated to 20-80VDC:
      • An unregulated 24VDC supply could conceivably be too low in some places
      • An unregulated 80VDC supply could be too high in some places

        Servo drive regeneration scenarios

        Regeneration is the most severe on vertical applications and during heavy deceleration.

  • When regeneration is a factor - Regeneration is that magic moment when the motor changes from consuming  power to becoming a generator.  The flow of energy starts to go backwards causing problems like excess voltage in the power supply.  Without getting into too much detail, the regeneration voltage can go much higher than most people would expect resulting in over-voltage shut downs and in some cases even damaged components.
  • When batteries run low - With battery powered applications you should take into consideration the under-voltage limit and make sure the servo drive will still work when the battery runs low. If the battery is already low, and you suddenly start to draw full power, how far will the voltage drop?

Scenario

You're setting up a system and applying power for the first time then CRACK! There's smoke in the air and a flash of light.  Oops, what happened?

You done goofed, that's what.

From the looks of it, something on the servo drive burned out and the computer isn't working anymore.  After a few calls it looks like you used a non-isolated power supply with a non-isolated servo drive.  The floating ground on the power supply created a huge current that traveled through the servo drive's signal ground and through the controller's ground as well.

Okay, so let's see what happened.  The simplest kind of power supplies consist of a full wave rectifier and a capacitor.  They provide a lot of power at low cost.  They're also non-isolated which means they have a floating ground where the voltage potential between the power supply ground and earth ground is different, usually by over 150V!  Not only is there a voltage difference but there's also a lot of power behind the difference so a pull-down resistor is out of the question.  In fact there's enough power to blow out a fuse or burn the wire or trace that tries to connect the two.  These kinds of power supplies are perfectly fine to use with servo drives with one condition, the servo drives that connect to them need to have isolation between the power ground and signal ground.

We established that the power supply didn't have isolation in this scenario, so let's see what happens when you connect it to a servo drive that also doesn't have isolation.  With a non-isolated servo drive this means the power ground and signal ground are internally connected together (as opposed to an isolated drive where the two grounds are separate).  Also keep in mind that servo drives are connected to other things like cables with shielding, controllers, network cards, computers, logic supplies and more.  For all of these devices to work together their signal grounds are connected together; and with all of these connections there are bound to be multiple paths to earth ground.

Isolation is necessary either in the power supply or the servo drive to prevent shock hazards and damage.

At this point you've probably connected the dots in our scenario, but let's spell it out.  The power supply had a floating ground which was connected to the servo drive's power ground.  Since the servo drive was non-isolated this means the power ground was connected to the signal ground, this in turn was connected to the grounds of all the other components in the system.  Since the power supply ground was floating it created a huge current that took any path it could to get to earth ground causing sparks and destroying any unlucky components in the way.

#3 Make Sure You Have Isolation

Rule of Thumb

To prevent problems at least one of these two things is needed:

  • An isolation transformer between the AC line and the power supply's power ground. AND/OR
  • Optical isolation between the servo drive's power ground and signal ground.

The manufacturers of the power supply and/or servo drive can tell you if they have isolation.  The information should be in the data sheet.

Note this rule doesn’t apply to battery powered systems or for servo drives that take AC power directly.

Power supplies that don't have transformers are less expensive and more powerful. They are safe to use if the servo drive has built-in isolation.

 

Servo drives without isolation can be made smaller and less expensive. They are safe to use if the power supply has an isolation transformer.

Why it’s important

Rectified voltage from an AC supply creates a floating ground.  The voltage difference between the floating ground and earth ground often exceeds 150V.  This voltage difference will create a current path strong enough to destroy the servo drive and other components.  Isolation protects equipment from damage by blocking the current path to ground.

Power supply isolation is common at lower voltages. Servo drive isolation is common at higher voltages.

When it helps

  • Whenever the power comes from an AC source
    • Pay special attention when using power supplies that output voltage in the range of 100 - 200VDC - The reason has to do with how servo drives and power supplies are built:
      • Power supplies rated below 100VDC usually use step-down transformers to lower the voltage.  These transformers also provide isolation.
      • Servo drives rated to operate above 200VDC are usually isolated due to safety and noise concerns.

#4 Don’t Get Too Much Power

large servo drive with tiny motor

An oversized servo drive takes up valuable space and increases the cost of the system.

Rule of Thumb

Yes it’s good to have extra power, but don’t go overboard.

Why it’s important

In most cases too much power won't affect performance, but it can increase costs which might impact profits.  If your system only uses 2A continuous and at most 6A peak then it really doesn't make sense to have a servo drive that outputs 30A continuous and 60A peak.  In this situation you should definitely turn down the current limit to protect the motor, assuming the motor was appropriately sized to the application.

When it helps

  • When you are trying to control costs - why pay for a more expensive drive when you don’t need to?
  • When you need to save space - A more powerful drive is usually bigger, can you afford the extra space?
  • When you need better current control - A drive scaled for much higher current may not have the fine current control needed for the application.

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A closer look at current scaling

One thing a machine designer might not consider is current scaling.  The current loop is the innermost loop and is the foundation to a high performance system.  When you want a system to run really well you need to start with a good current loop.

...Ok, back to current scaling… When we design our servo drives we scale the current feedback to the rating of the servo drive.  What exactly does that mean?  Well, if you take a 50A drive, the full range of output - in this case -50A to +50A needs to be represented with a -10V to +10V signal, or in the case of a digital drive it needs to be represented with a set number of bits, 2^14 bits for example.

For this example this means the scaling would be

(±10V signal) / (±50A output)
=
(20V range) / (100A range)
=
200mV/A for analog or about 160bits/A for digital

So where are we going with this?  The point is the higher the current rating on the servo drive, the more spread out the current feedback needs to be scaled.  As the feedback needs to cover a wider range you start to lose resolution for small current control.  If you have an application that needs low current, you'll get better current control by selecting a servo drive that is more closely rated to the application.

In most cases it really isn't the end of the world if the scaling is way off, but you aren't doing yourself any favors

#5 Always Talk to Technical Support

ADVANCED Motion Controls technical support for recommending servo drives

Save valuable time and reduce mistakes by reaching out to Technical Support.

Rule of Thumb

If you have any questions or you’re faced with a tough decision, reach out to our Technical Support.  Everything from simple questions to complex, we can help steer you in the right direction.

Why it’s important

Time is money and it's usually a lot faster to ask for help rather than try to figure things out yourself.  We pride ourselves with having one of the most responsive technical support teams in the industry, so if you reach out to us we're here to help.  Online we usually respond within one business day.  On the phone we have staff on hand during our normal business hours.

When it helps

  • When selecting a servo drive - Even if you've covered all of the angles and you're sure you picked the right servo drive, it doesn't hurt to reach out and ask someone else to take a look.
  • When you’re not sure about a feature - if there’s a feature that will play a critical role in your design it doesn’t hurt to talk to someone to make sure it will meet your expectations.
  • When you need to check the compatibility with other components - Servo drives connect to a lot of different components.  Reach out to us if you aren’t sure about the feedback, motor, network or other things you plan to hook up to the drive.
  • Whenever you’re stuck - If you’re new to servo drives or new to our products, there’s a learning curve.  Save some time and aggravation and reach out so we can help!
  • When you need help troubleshooting - Whether it's getting something to work the first time, or troubleshooting a machine that suddenly stopped working.  Reach out and let us help you get up and running again.

Secret Bonus Rule: Every application is different, so as the term “rule of thumb” implies, none of these rules are absolute... except calling technical support. Always call technical support.

By Rene Ymzon, Marketing Manager

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