Benefits of auto-tuning VFDs

Variable frequency drives (VFDs) make applications all over the world work better, at higher efficiency and make the equipment easier to interact with via keyboard displays. Manufacturers of these devices recommend users perform an auto-tune.

An auto-tune helps the VFD control the motor with specific motor settings. Just like the automotive companies not knowing where to put the adjustable seats in a new car, the VFD manufacturer can’t guess what the best default settings for motor information. As a result, they default to an industry standard.

For example, the manufacturer of a 40 HP 480 V drive might default its motor full load amp setting per NEC table 430.250 and set it at 52 A from the factory. That would probably not match the new premium efficiency motor. Setting the amps correctly is the start to getting proper performance. Performing an auto-tune is easy, so let’s discuss the what needs to be done and why, when auto-tuning VFDs.

Figure 1: An auto-tune helps the VFD control the motor with specific motor settings. Courtesy: Yaskawa

VFDs are not like servos

There are other auto-tuning functions. As the name auto-tune implies, any operation that calculates and adjusts things automatically at the user’s behest can be called an auto-tune function. With servo motors and amplifiers (close cousins of the VFD and induction motor), the auto-tuning function will probably have more to do with tuning the servo pair to their loads. The tuning will help adjust gains for positioning and speed loops. The VFD auto-tuning asks the load is detached from the motor when the auto-tune is performed.

The reason for the difference is servo motors and their amplifiers are always bought together from one manufacturer. A simple motor code is all the amplifier needs to know everything about the connected motor. Conversely, the VFD and motor are only sometimes manufactured by a common company. It is more the norm that brand A motor will probably be used with brand B VFD. Because of this unfamiliarity between the motor and the VFD that auto-tuning becomes the bridge by which good motor information can be passed to the drive.

Figure 2: Auto-tune implies any operation that calculates and adjusts things automatically at the user’s behest. Courtesy: Yaskawa

Getting value from auto-tuning: Benefits

Any time spent improving a VFD’s performance better be worth the time invested. If done correctly, a properly auto-tuned VFD will be more efficient (lower current for the same torque) and better performance (more linear and stable operation).

By giving the VFD the actual electrical specifications from the motor nameplate, the VFD will not have to assume generic values and produce excess motor flux and possibly even saturate the motor’s magnetic field, which won’t produce more motor torque but probably will produce more motor losses (heat). The flipside of the equation is how much time does it take to perform an auto-tune?

If there’s a good picture of the motor’s specs, say from the motor nameplate, it will probably take about 90 seconds to enter the information and an additional minute to run the auto-tuning.

Figure 3: Some of the measured values are used to populate a single-phase induction motor circuit model. Courtesy: Yaskawa

What gets changed through auto-tuning

Not every manufacturer does auto-tuning the same, but there are common adjustments they all make. It’s a mistake to assume the VFD is does all the work. The person performing the auto-tune routine is actively involved. When the auto-tune function is initiated on the keypad, the drive user needs to enter typical motor nameplate information. There isn’t really a need to measure or calculate the full-load amps if the user can just enter it. Some specs like slip frequency and leakage inductance are only alluded to on the nameplate and are best calculated by the drive.

Once the user enters the relevant motor information, the drive prompts the user to start the auto-tuning process during which the drive runs and tests the motor to calculate motor specifications not readily found on the nameplate.

After the auto-tune is completed, most VFD makers will allow the user to view a list of modified parameters so it can be simple to do a before and after check of the list to see what was entered and what was measured or calculated. Items that are typically adjusted include:

• Line-to-line resistance
• Motor rated current
• Motor no-load current
• Motor rated slip
• Energy savings coefficient
• Leakage inductance
• Saturation compensation
• Motor iron loss
• Motor-rated power.

Some of the measured values are used to populate a single-phase induction motor circuit model as seen in Figure 3. All three phases should be the same.

For instance, by applying a dc voltage to the stator and measuring the created current with dc current transformers (DCCTs), the drive calculates the line-to-line resistance, which would be R_S in the model. The rest of the model can be calculated using other techniques and measurements and then extrapolated into three phases. This can help the drive better anticipate and compensate for changing loads on the motor and lead to more efficient regulation of motor operation.

Figure 3: Some of the measured values are used to populate a single-phase induction motor circuit model. Courtesy: Yaskawa

What to do when auto-tuning goes wrong

As with any VFD and motor operation, there can be pitfalls that can trip up the auto-tuning.  Manufacturers have a separate list of faults that can occur during an auto-tuning routine. The most typical error happens before the auto-tune begins. Any values input during the auto-tune are checked against the values allowed for the size of drive being auto-tuned and against each other.

If the full-load amps are 100 A, but the entered motor size is 40 HP, the drive will throw a fault to stop the routine. Even if the horsepower and FLA are reasonable, but exceed the rating of the drive, the drive will stop the auto-tune and display an error code regarding a data entry error.

Because a VFD’s auto-tuning routine is to probe the motor and not the load, most auto-tunes that require rotating the motor to make measurements also require the motor and load are disconnected during the auto-tune. The VFD can tell by the amps drawn during rotation whether the load has been detached and may fault with a load fault rather than complete and accept the auto-tuning adjustments.

Figure 5: Any values input during the auto-tune are checked against the values allowed for the size of drive being auto-tuned and against each other. Courtesy: Yaskawa

Types of auto-tunes for VFDs

The most effective version of an auto-tuning routine for VFDs and motors requires the motor shaft to be rotated while unloaded. There just isn’t any replacement for actual measurements that can be made while the motor is turning its own shaft. However, not all VFD and motor applications allow for easy uncoupling. Using sophisticated calculations, some of the motor model information can be estimated using an auto-tune method that does not require motor shaft rotation.

These non-rotational auto-tunes sometimes are referred to a stationary auto-tuning. Some auto-tunes are very basic and require little information to be entered and take only a few seconds. For the most part, the auto-tune will just measure the resistance that is inherent in the windings of the stator and the motor cable.

This goes back to the line-to-line resistance. The advantage of such a simple auto-tune is the VFD can then compensate for the reduced magnetic field strength due to voltage loss.

There also are auto-tuning routines specifically for permanent magnet (PM) motors in many of the same VFDs. Some of what the PM auto-tuning routines do is common with what is done with induction motors. However, even more specific information necessary for a proper auto-tune of a PM motor and wider variety of characteristics are measured, as well.

Figure 6: With drive tools software baselines, they help with seeing any differences in performance and can be a benchmark for future comparisons. Courtesy: Yaskawa

Five VFD auto-tuning tips: How to improve auto-tuning

Most VFD auto-tuning is straightforward and doesn’t require tremendous expertise. However, there are some things that may improve VFD auto-tuning.

1. Some motor characteristics –specifically, resistances – will change as the temperature of the copper windings increases. Since the motor will be warm to hot during normal operations, the auto-tune should reflect that state. The tip is either run the auto-tune multiple times or let the motor run for a while, even unloaded, before performing your rotational auto-tune.
2. Because it is simpler to do a stationary auto-tune, it is tempting to settle for the stationary auto-tune. While convenient, it’s a mistake. Always insist on the uncoupled rotational auto-tune.
3. The auto-tune often can be run before the application is fully assembled. Sometimes, it is easier to do the full rotational auto-tune when the motor is only 20 ft apart and have a relatively short motor cable. Later when the application is fully assembled and the motor is 75 ft away from the drive and already coupled to the load, a simple line-to-line resistance can be run. This non-rotational resistance tune will not affect the motor measurements but will re-test and update the increased resistance due to a longer motor cable. Viewing the modified VFD settings can confirm the new resistance measurement.
4. It also may be helpful to perform pre-tune and post-tune measurements. Try using the VFD’s built-in monitoring displays to complete a table with pre- and post-monitor values. If the user is successfully auto-tuning from factory default settings, all the VFD’s values in the post-tune will be lower than the pre-tune, with the exception of the dc bus voltage, which will mostly stay constant at a steady state.
5. Another way to compare pre- and post-tuning performance is through free trending software offered by many manufacturers with drive tools software. These baselines help with seeing any differences in performance and can be a benchmark for future comparisons.

Auto-tuning is worth the time and money

Auto-tuning is a good best practice when it comes to getting the most from VFDs. The investment is a few minutes of time usually and the differences can be well worth the effort. The better and more accurate the motor model the VFD has, the better and more efficient the motor control will be.

Paul Avery, senior product training engineer, Yaskawa America Inc. Edited by Chris Vavra, web content manager, Control Engineering, CFE Media and Technology, cvavra@cfemedia.com.

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Keywords: variable frequency drives, VFDs

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