In the first part of this two-part series examining frequently neglected, or misunderstood, VFD parameters, we looked at thermal current, pulse-width modulation, deceleration time, and minimum operating speed. In the second part of this series, we explore four more functions that, when fully understood and adjusted for your motor- and application-specific needs, can further improve efficiency and extend the lifespan of your motors.
The frequency jump function typically consists of several parameters. The frequency jump parameters each denote a frequency at which the VFD will not run the load. Many mechanical systems have a frequency – or frequencies – at which the system will excessively – and possibly destructively -- vibrate. For example, if a system has a resonant frequency of 40 Hz, it would excessively vibrate if the motor were running at 40 Hz, possibly shaking parts loose. By properly setting the frequency jump, the VFD will skip 40Hz, and prevent these vibrations. While some equipment manufactures may identify resonant frequencies, they are more commonly found by experience. There may be additional related parameters indicating a band, so that there is a range the VFD will not dwell at.
Every major manufacturer offers a small programmable LCD Human Interface Module (HIM) with their drives. While the default settings may be appropriate for some applications the module is generally programmable to show different values on the display or to customize the user experience . Three of the most valuable settings are covered below. Please note that the actual names of these parameters vary by manufacturer.
Most VFD HIMs have a default factory setting to display the motor speed. Some applications are better served by displaying another value. For example, motor power will change in many mixing applications as the viscosity changes. Displaying the power on the HIM will allow an operator to determine how mixed the product is without bringing up a SCADA display in a control room. The best way to determine the ideal display value is through knowledge of the process, the options, and plant operators’ preferences. Depending on the manufacturer an operator could also view frequency, current, custom messages, or a calculated value.
The value of the HIM can only be optimized if the right process information is displayed in the right units. In some cases, this is simply changing the display to show the value in metric units, such as liters/min instead of gal/min. In other cases, it may be aligning the value to match SCADA and HMI screens to show percent speed instead of RPM.
The password and related settings can be used to limit operation of a VFD from the HIM. While we might think of this for security reasons, there are also operational reasons to restrict control of a motor. For example, it may not be desirable to surprise an operator in the control room by turning on a motor from an HIM in the field. Many manufacturers’ password settings can lock out any unauthorized users, while still retaining visibility on the display parameters. Determining the best use of these security measures is best done as a collaboration with both plant engineering responsible for safety and security as well as operations.
The control characteristics are a group of parameters that help define how the VFD will change frequency and power to meet the programmed setpoints in high-end drives. This can be essential for ensuring that the correct amount of torque is applied at the proper time for your application. Please note that this is NOT to be confused with the settings for those setpoints that are always set up during commissioning. Most manufacturers provide several different control characteristic settings to help with the specific application, with the four most common being:
Volts per hertz (V/Hz) - Controls the magnitude of voltage and current relationship. Good for fan and pump applications where flow is more important than pressure. Will maintain full torque range within about ½ of motor slip (essentially the lag of the stator behind the rotor) but may not be able to maintain torque below 2 HZ.
Sensor-less Vector (SV) -Provides higher starting torque and speed control within ¼ of motor slip. Good for deep well pumps and high constant torque applications. (auto-tune should be used when using this control method)
Flux Vector (Open-loop): Improves upon V\Hz by providing both magnitude and angle control. By sensing the motor flux and orientation, this method provides more precise motor speed and torque control.
Closed Loop vector: this method utilizes a motor mounted encoder to provide shaft position and speed back to the drive. By using this method, a motor can develop full torque at zero speed. Ideal for crane and hoist applications.
Some of these interact with each other, and others preclude the use of different parameters. For example, setting torque control settings negates the ability for auto-tune.
Today, many VFDs include a variety of “smart” features that can help further improve the efficiency and lifespan of the motor. However, since these features vary from drive to drive, we often see that many of these features are underutilized. For example, some drives include sensors to monitor how much power is needed to produce a certain speed. When a drive is first setup a baseline power curve can be established and used to program a maintenance warning if a bearing is wearing and more power is required to produce the same speed.
Some drives may also come with the ability to perform certain actions based on a trigger condition. For example, a drive connected to a pump may sense that the pipeline is starting to experience a blockage based on the amount of power required to move the fluid through the pipe. The “smart” drive could be programmed to temporarily increase the power in an attempt to clear the pipe.
When these manufacturer-specific smart features are understood and implemented correctly, energy efficiency improvements and the ability to proactively perform some maintenance tasks is possible. Additionally, when these features are set in conjunction with the control characteristics discussed above, you can even get more power out of the drive whether you are operating at the low or high end.
While simply getting your VFD installed and running is likely already providing a number of efficiency gains in your operations, tapping the expertise of a systems integrator such as ACE to evaluate the more complex settings in your drives will surely bring additional gains in both efficiency and motor lifespan.
Contact us today to learn how we can work with you and your application, to ensure your VFDs are operating optimally in your process and for your operation.