Driving effortless efficiency – the perfect partnership between rotating load and variable speed drives

Drive and Load - poetry in motion just like Ginger Rogers and Fred Astaire.

When you correctly pair the rotating load with the variable speed drive, then you can get the best in both performance and energy efficiency.

In my mind, I see the good combination of Drive and Load as poetry in motion – a bit like Ginger Rogers and Fred Astaire. They were perhaps the most famous dance partners of all time, a dynamic duo that made dozens of wonderful movies in the 1930s and ’40s. To this day they are still famous for tap dancing to the rhythm, keeping the pace and driving the show. And it all appeared so effortless. They were truly the epitome of effortless efficiency.

However, coming back to our drives, I need to point out one thing. Unlike the name suggests, a variable speed drive is not a source of speed! Without going into the nitty-gritty of Faraday’s law of induction, Maxwell’s equations and the like – Engineers among my readers here will surely be aware that the electric motor produces just torque, while speed is a reaction of the attached load and process to that torque.

So, actually, the term Variable Torque Drive would more accurately and correctly describe what attaching these power electronics would add to the good old motor. Ok, I do realize that from a marketing perspective, it is of course smart to include the value proposition, meaning speed, in the product designation. However, when you are engineering and creating the dimension for a drive to correctly fit its application, it is helpful and sometimes even required to view the task from the correct functional angle with respect to physics and mechanics.

Our drives come with a set of control functions which will govern the way torque, and therefore speed, is controlled dynamically in operation. The speed-control-related functions among them will always have to deal with the load included in the control task or, more specifically, with rotating equipment and with process variables that influence both immediate reaction and behavior over time.

What I mean is that the response depends highly on the system and materials we are controlling. For example, when we apply torque to a stiff block of steel, it will surely linearly accelerate; however, applying torque to an elastic mechanical system with a sluggish process attached at the other end will most likely produce a much more variable and perhaps unpredictable response. Keeping the speed regulated under such circumstances requires an appropriate use and setting of the control functions inside the variable speed drive. Parameterization will therefore directly depend on the load . . . and there is no way around a good technical discussion between drives engineers and process experts to get things right. That’s why I am writing this post: to open a dialogue and stimulate communication.

My rule of thumb to identify driven loads that require wise and careful control strategy selection is as follows: I compare the whole driven equipment to the form-factor of the electric motor. If the load has a similar form-factor to the motor, it’s usually no problem, and the default parameters will do a reasonable job. But when the driven system is comparatively long, but still slim, or when the system has a very large diameter compared to the length of the rotating shaft, I see red flags waving: Alert. In these cases, I know I must talk with the rotating equipment engineers in order to agree on how to control the speed of that system.

My experience with this approach of direct communication has only been positive: both sides, in terms of engineering disciplines, are learning from each other. And as we proceed step-by-step, mutual understanding and trust is built between teams and thus between our customers and ABB. You could say we are communicating our combined brainpower and knowledge in a better way to gain better efficiency.

To tie this all together . . . let’s go back to Ginger and Fred and their effortless dancing: I’d like you to imagine our Variable Speed Drive as being the perfect dancing partner for any critical equipment and important process out there in your industries. And by understanding we are actually dancing arm in arm, we get the feeling for the local rhythm, we get to know the partner and, perhaps most importantly . . . we avoid stepping on each-other’s toes.

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About the author

Pieder Joerg

I have been involved in developing power electronics and large variable speed drives for over 20 years. Over the years I got more and more interested in their application: what are we driving? What’s the advantage of variable speed to the process? Today I’m working with our customers towards the perfect integration of power electronics and control into processes and production facilities. Playing classical music in my spare time, I know very well how important it is to listen to one another, so that the orchestra is more than the sum of instruments.
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