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Which Variable Frequency Drive Approach Is Right for Your Motor?

Knowing when to select integrated, tethered or stand-alone drives

You’ve heard the news, you’ve jumped on the wagon, and now you’re enjoying the ride. A drive-controlled, permanent magnet AC (PMAC) motor is the new standard for the industry — no matter the industry. Whether you call it a variable frequency drive (VFD), a variable-speed drive (VSD), an inverter, or an AC drive, you’ll need one, alongside the PMAC, in every application that comes your way. However, while the name selected to describe the technology does not make a difference, there is a consideration that just might make a big difference in your application. It might seem simple, but considering the difference between a drive that has been integrated into the body of the motor by the manufacturer and a drive that you’ve selected from another manufacturer will be important for your future strategy.

PMAC Motors

First, let’s make sure we’re all on the same page with the technology. The PMAC motor most often works by making use of permanent magnets in or on the rotor and windings in the stator. In order to create rotary motion, the current flowing through the stator windings must rotate. Traditionally, this is done with an alternating current such that the frequency controls the speed. Therefore, in order to control, or at least vary the speed of the motor, an engineer must make use of a system that can vary the frequency of the current flow in the stator. This varying frequency is created by the VFD, VSD, or the myriad other names which permeate the corners of our industry.

However, there is a much more complicated layer to this technology that some engineers prefer to ignore. For example, sometimes you just want your fan to spin at the desired speed, not mess with the configuration of VFDs and motor winding inductance, and just be able to trust that your rotating system is going to be operating as efficiently as possible. However, in other instances, you really want to get deep into the control algorithms, switching frequency, torque optimization and motor inductance parameters because you might be doing something that no one has ever considered before or otherwise is extremely unique. Still another group represented among the readers are those of you who generally exist between these two extremes: you don’t want to be completely agnostic to the drive technology, but you also have no need to invent new drive parameters for the next super motor you’ve created.



Options

You generally have three options. Your choices are: the integrated drive, the tethered drive or the stand-alone drive (see Figures 1–3). The integrated drive usually comes attached to the motor in such a way that you really just need to install one item into your application. This drive has been pre-programmed to be matched to the motor and needs little to no configuration. The tethered drive can give you the benefit of being paired to the motor you’ve purchased, so there’s little if any configuration to worry about, but you also have the freedom to mount the drive anywhere you like as long as you can get the drive connected to the motor. The stand-alone drive has the same requirement of a separate mount but also must be configured to the motor, with time spent being relative to the final performance of your system. That is, the more time you spend getting deep into the VFD parameters and configuration files, the more “matched” your VFD and motor will be.

The topic can create some confusion, so we’ve provided a series of questions to assist you in determining the best drive option for your specific application.

 

Question 1: Do my environment (temperature, humidity, vibration) or space constraints make it difficult to consider the integrated drive approach?

 

Historically, motors have performed better at higher temperatures and humidity than electronics. Therefore, if you are operating in a particularly high ambient temperature, the integrated drive may become a hindrance. Its electronics will oftentimes dictate a lower ambient condition than the motor. For example, at 70C, it may be difficult to find an integrated drive product to meet your cost targets unless you’re below the integral horsepower (IHP) range. However, if you consider a tethered drive wherein the drive is mounted outside of the extreme temperature, you’re likely to find more success. Furthermore, many applications dictate an extremely small space around the motor such that integrating the drive into the same package would hinder your success. Again, in this situation the tethered or stand-alone drive would be more desirable.

 

Question 2: How complicated is your use case? Will you need access to complicated drive parameters, or are you more interested in just using the equipment in its most efficient state?

 

Some applications require an engineer to have access to the deep internal drive parameters such as motor inductance and drive switching frequency. For example, you may desire different torque optimization parameters during different operating conditions in your application to broaden the motor’s capability in the torque-speed landscape. In this case, you would typically need to look at the stand-alone drive category. The engineers working on integrated and tethered drives speak this motor-drive language every day. They are the experts who have matched the drive to the motor in the best and most tested ways possible — sometimes in ways that are considered trade secrets or covered by patents.

 

Question 3: How is the Internet-of-things (IoT) playing into your application? Is it highly critical to have deeply ingrained motor knowledge in your IoT, or are you going to be satisfied with surface-level drive information?

 

IoT is on a lot of minds today. While not everyone has figured out how to monetize it, they know it’s important; drive manufacturers are no different. The stand-alone drive is highly likely to offer the capabilities to integrate into your IoT plan but may lack the in-depth, motor-as-a-sensor knowledge that an integrated or tethered drive may provide. Motor manufacturers are typically keen to their motors’ knowledge and can apply that knowledge into their integrated drives in such a way that your IoT strategy may be better enabled. However, if that integrated IoT knowledge doesn’t aid your application or customers, then the stand-alone drive can suffice.

The Benefits

The three questions above can help you navigate the space. However, maybe solutions still aren’t clear to you because of your application or other considerations. Let’s turn our focus now to the benefits of an integrated and tethered drive as a final step. If these benefits don’t help your situation, then you’ll need a stand-alone drive.

Reliability

Reliability is generally considered higher with an integrated drive because the manufacturer is completing all of their product testing with the same pair they’re selling you. Every agency requirement, long-term life test and extreme storage condition test was completed with the two paired together as a single unit. To a possibly lesser extent, the tethered drive has been tested together with the motor, but if the manufacturer is selling many motors that they can pair with a single drive, they may not have had the same extensive testing together like the integrated drive. We’re not saying that a stand-alone drive paired with your motor will be unreliable, but you’re unlikely to be able to test as extensively as other companies that design integrated products.

Cost

Cost is typically going to be highest when you consider the stand-alone drive and the tethered drive versus the integrated drive, for several reasons. First, two or more companies could be involved when you buy a stand-alone drive. In addition, consider all of the time that you spend in configuration, the cost of cable assembly design and manufacturing, and the two sets of mounting hardware. While it is certainly necessary sometimes, an engineer should expect a higher cost to the stand-alone drive solution due to the added labor for installation, configuration and materials (see Figure 4). With the tethered drive approach, typically one manufacturer manages the system cost for the product, but the customer still needs to consider the extra mounting hardware and the cable assembly. When the drive is integrated, no extra cable assemblies are necessary, and the cost and time required to mount the drive are not needed. Alone, each of these costs may not be significant, but they can add up — especially if you’re considering multiple SKUs across a family of products.

Time

Time is money, but it’s more than that! If you can take it slow, discover the intricacies of your application, and have the bandwidth of your controls engineer, then the stand-alone drive may be a good place to start. You’ll be able to learn quite a bit by experimenting with different control algorithms and expanding the parameter space of the system. However, many companies want an ultrafast time-to-market approach to new products, and the pre-paired aspect of the integrated and tethered drives would provide a faster way to get to the gate.


   Motor Drive
 Installation Materials
Configuration & Installation Labor
 Total System Cost
Integrated
 2.25  2.25  0.2  0.2  4.90
Tethered  2.1  2.1  0.6  0.6  5.30
Stand-Alone  2  2  0.8  0.8  5.60

Summary

We’ve navigated through the three drive options — integrated, tethered and stand-alone — and discussed the reasoning behind each of the methods. Your next step should be determining your application’s requirements regarding the environment, space constraints, complicating factors and IoT. Once you’ve worked through those questions and have decided on an approach, you can develop a cost proposal associated with your choice and make sure it matches your budget. As we’ve surmised, the difference between selecting a drive that has been integrated into the body of the motor by the manufacturer and a drive that you’ve selected from another manufacturer will be important to your future motor/VFD strategy.

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