#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
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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!
#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.
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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
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?
#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.
#4 Don’t Get Too Much Power
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.
#5 Always Talk to Technical Support
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.