Industry Articles Display
2019-10-12 Control Engineering
Motor drive advice was offered in a write-in question in the Control Engineering motor drive research, summarized in the September issue.
By: Mark T. Hoske
The 2019 Control Engineering Motor Drives study from Control Engineering, asked respondents, “Based on your experiences and knowledge, what’s the best advice you can offer?” Some of the responses follow, lightly edited for style.
Drive application advice
Applications are critical to reliability and the selection of wiring type and input and output filters.
Check for temperature rise on motor, how harmonic contents spread into the grid, and the overall efficiency at partial load.
It doesn’t matter which brand of drives you choose for your application. Most important are quality of the product, availability, and standardization. Standardization is a big one because it can reduce the stock of spare required and helps support personnel understand the drives used.
Know the specific details of your applications requirements before specifying the drive. For example, know peak velocity usage, not just average velocity. Similarly, know the maximum torque that the application demands. Consider all of this with the motor’s performance in light of the specific drive to be used. Failing to do the pencil-work prior to specifying the drive is almost a guarantee of disappointing performance later on.
Know your application before selecting a drive.
Limit the number of drive suppliers so that you can ensure supportability and the ability of in-house staff to service drives.
Make sure motor is matched to the application and motor drive. Beware of application limitations due to long cable lengths to the motor.
Make sure that the motor/drive selection works well with the PLC controls being used.
No universal drive: be specific on which application you would use it and select accordingly for your best performance and cost effectiveness.
Prepare for dirty power.
Read the spec and test to verify.
Correct drive sizing
As efficiency standards change for induction motors, watch for inrush currents and breakdown torque and breakdown current changes. Inrush currents are rising and can cause challenges with electronic overloads. Know your application, particularly high friction and/or high inertia starting. To get the full use of your available motor torque, the drive must be correctly sized. For very large motors performing heavy-duty cycle work, be certain that the transformer and conductors do not cause large voltage drops when the application is at its greatest demand. For large hammer mills using wound rotor motors and liquid rheostats, we target <5% Voltage drop at the motor at 200% full load amps (FLA). It’s very common for engineers unfamiliar with the application to undersize the transformers and conductors.
Choose the right drive for the motor for optimum performance.
Do the design work up front, such as motor sizing, because it is easier and faster to take the time upfront than to guess and then have to replace a motor or drive later.
They should always be specified to be of the high-efficiency type and be loaded at 70% or higher during normal operations.
Products, parts, support
Replacement parts need to be available.
Be open to new products.
Buy from someone with good technical knowledge and a lot of hands-on experience. Hire and kept good technical support people on staff.
Check compatibility before ordering, if you’re not using similar products.
Check your local supply chain for products before committing to them. There are plenty of vendors out there so don’t be fooled by the sales pitch. If you can’t get the product, or replacement, in a timely manner, then it isn’t worth the money you might save at the outset.
Don’t make assumptions. Do your homework, and don’t be afraid to ask a real person whether something is doable or not.
Don’t pay extra for factory start-up if you have a knowledgeable tech in-house. Factory techs sent to start-ups I’ve participated in have ranged from only marginally knowledgeable to completely unqualified. Not worth it.
Double-check specs and requirements for motor and drive selection.