Motors and drives: How to optimize industrial energy use, part 2: Variable frequency drives

In an October 30, 2023, webcast “Motors and drives: How to optimize industrial energy use,” Michael Lyda from Advanced Energy specifies and analyzes how to optimize energy use for motors and drives and other factors to consider.

By Plant Engineering February 29, 2024
Courtesy: CFE Media and Technology

Motor and drive insights

  • Motors and drives can run for a very long time and keep a plant efficient, but it needs the right equipment and knowledge behind it.
  • Understanding the codes and standards behind motors and drives can help engineers enormously in maintaining their equipment.

Over their operating lives, most ac induction motors exhibit common problems, from transients to voltage imbalances and single phasing.

Therefore, being familiar with general troubleshooting techniques is important. To start, it’s vital to understand the operating parameters and forces acting against a motor; to perform tests and make observations based on those parameters; and to remove system components to isolate a problem.

Proper size and power source connection must be verified. Regular measurement procedures, starting with nominal voltage, current and frequency can be the starting point for long-running optimal motor and drive operation.
In an October 30, 2023, webcast, Michael Lyda from Advanced Energy analyzed how to optimize energy use for motors and drives and other factors to consider.

The transcript of this presentation has been provided with minor edits and adaptions for clarity.

Michael Lyda: So that covers our regulations update and now we’ll go over variable frequency drives (VFDs). This is our main topic of the presentation and this is where we’ll be spending the rest of the time. So the content today is broken down into four areas. We’ll cover the basics of VFDs, then we will identify some potential applications to add VFDs. We’ll do an energy savings calculation example. And finally, we’ll discuss maybe the application issues involved with VFDs and also ways that you may be able to mitigate those issues.

Depending on where you are, where you may be, how old you are, things like that, you may have seen a lot of different names or heard VFD represented in many different ways. Variable-speed drive, VFD drive, adjustable-speed drive, inverter, converter is what our colleagues across the pond mainly call it, or just drive.

So what is a VFD? A motor is a rotating apparatus that converts electrical power to mechanical power. Well, likewise, the VFD drive is just a device that connects to the in-between the grid power and the electric motor. And then what it can do is adjust the speed of the electric motor. The VFD is basically just a medium. It’s going to control the speed and potentially torque, depending on the control method, by bearing the frequency and voltage that the motor sees at its terminals.

Needing variable speed for motors and motor applications is not something that’s new and that’s not something that’s just applicable to VFDs. But formally there’ve been other ways that speed control has been completed before VFDs drives were invented. And some of these things definitely are still being used in a lot of places. In some places there are better than VFDs drives, but a few devices pictured on the screen, gearbox, belts and pulleys, baffles, speed reducers, soft starters. And then also even some motors have dual speed functionality from the factory.

Some issues with the older technologies, they may be only designed for single speed. So even with a gearbox, you have your power coming in, the motor’s always going to be spinning at the same speed. The gearbox is going to be reducing that speed down. The motor’s always running at full speed, which is not necessarily energy efficient. If you need the motor to run at one speed constantly, then that’s fine. But if you need the motor to operate at variable speeds, then it’s not an energy efficient way to control the motor. And then things like gearboxes and belts, gearboxes definitely need maintenance, you may need to keep the oil and make sure they’re staying cooling correctly. Belts and pulley systems, they may need adjustments. Baffles, dampers on fans, things like that typically need to be adjusted manually. This is meant to automate somewhat.

So what are some of the advantages and disadvantages of VFDs drives? One of the clear advantages of speed control, it’s very easy to start and stop the motor, so you don’t have that inrush current. When you start an electric motor on grid power, you have about six times the rated current that flows in, in order for it to spool up. With a VFD drive you don’t have inrush current to worry about so it’s a lot easier on your power transformer, a lot easier on your circuit breakers and power panels that are controlling, that are not controlling but feeding into the motor. The inrush current’s less. It’s also less stress on the motor windings when you do inrush because the VFD drive is able to accelerate it from zero speed up to full speed or wherever the target speed is.

That can be set by a parameter on the drive, called acceleration. You also can get improved process control for very precise applications. Another advantage of speed control on drives, especially if you have some type of feedback on the motor shaft, like if you install an encoder and then feed that encoder back into your drive, then the drive is able to see real time what the motor shaft speed is. It’s going to be a lot more accurate than the gear reducer or something like that. Also, high quality VFD is likely you’re going to have a high power factor across your motor loads, the capacitance.

And if you have a facility that’s using all induction motors, your power factor could not be great. You may get a penalty from a utility. Actually working on an energy assessment right now with a customer and has a power factor penalty. One thing you can do is add a capacitor bank, but if you have no VFDs drives in the facility and you have applications that are ripe for adding those drives, that’s another way to solve your low power factor because capacitors and the drives offset that inductance in your induction motors. And then of course the main advantage and probably the things we focus on the most today will be the energy savings potential for your drive.

And then disadvantages of using the drives, obviously it costs money, additional capital investment, more upfront costs. You have to ask people for those, maybe, to spend that extra money, layers of approval. VFDs do add some harmonic distortion into the grid, your portion of the grid maybe in your plant or right around your plant, and that distortion comes from the capacitors and IGBTs, transistors, that they have inside of them. One other item, the nuisance tripping. You can have nuisance tripping in your drives, especially if the drive isn’t programmed correctly or if it’s not sized correctly.