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10 essential maintenance and troubleshooting tips for VFDs

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10 essential maintenance and troubleshooting tips for VFDs

5 essentials of VFD maintenance

To maximize VFD life expectancy, it’s crucial to support reliability through a proactive maintenance plan. Such programs are tasked with underscoring the two foundational principles of reliability.

  • Enhancing failure-free conditions for the VFD
  • Increasing the probability the VFD will enjoy uninterrupted function under proper conditions.

In order to avoid having a VFD falter, it will be necessary to conduct the five following maintenance essentials.

  1. Keep the VFD clean: Commercial and industrial environments commonly include airborne dust and debris and can negatively impact a VFD. According to FactoryMation, regardless of the VFD chassis type, “It is critical that the heat sink and fans are kept clean.” It’s recommended to periodically wipe, clean and air spray all accumulations from the VFD.
  2. Maintain VFD temperatures: It’s not unusual for VFDs to be housed in enclosures that may experience heat spikes. Include temperature control systems and appropriate air circulation.
  3. Prevent moisture penetrations: Water and electricity do not mix well in industrial settings. Moisture will inevitably cause corrosion, erratic behaviors, and equipment failures. Keep the equipment dry.
  4. Maintain tight connections: Vibrations tend to loosen electrical connections over time. This may result in uneven VFD functions. Check the connections as part of an ongoing maintenance plan.
  5. Conduct visual inspections: The first step to proactive maintenance is to visually inspect VFDs weekly. Have maintenance personnel look for the above-mentioned issues and others that could impact VFD operations. Key elements to visually inspect include the connections, fiber optic cables, emergency circuits, and area cooling systems, among others.

5 VFD troubleshooting tips to consider

While the best way to ensure a VFD does not needlessly falter is proactive maintenance, breakdowns remain an industry reality. These may be caused by not recognizing environmental effects on machinery, hard impacts or other unforeseen circumstances. Industry leaders understand every minute an operation is not functioning at full capacity results in losses. These five VFD troubleshooting tips can help get companies back online.

  1. Check diagnostics display: Today’s VFDs routinely come equipped with troubleshooting functions. The display module can indicate issues such as overheating, irregular voltage, or even a popped fuse. This is often the best place to begin troubleshooting.
  2. Check for moisture: Without making physical contact, take a visual inventory of the machinery and look for moisture accumulations. Water can cause shorts and other damage. Take safety precautions, and do not touch any electrical equipment that may have gotten wet.
  3. Check for dust: If the VFD has not been thoroughly cleaned, it may have grit accumulations inside that caused a malfunction. In such cases, it may be necessary to remove the unit and replace it until the affected VFD can be cleaned and reinstalled. Organizations are wise to have backups in the event of a significant breakdown or issue.
  4. Check VFD connections: While wearing appropriate safety equipment, visually and physically check for loose connections. If the VFD seemed to be delivering uneven output recently, cable or connection problems may be the culprit.
  5. Resonance testing: When vibrations place undue stress on equipment, they tend to become increasingly unreliable. Issues may include impacted welds, and loose bolts, among others. Consider running a diagnostic test to see if the VFD is operating in what is called an “excited state” as a result of resonance. This troubleshooting effort could save you from conducting repeat maintenance and replacement.

A properly maintained VFD can provide decades of cost-effective service. Like any piece of equipment, though, that depends on maintenance and care. Making VFD inspections and maintenance part of facility oversight can improve outcomes.

 

Source: <https://www.controleng.com/articles/10-essential-maintenance-and-troubleshooting-tips-for-vfds/>

Top 10 tips for specifying & using variable frequency drives​

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Top 10 tips for specifying & using variable frequency drives​

How to choose and implement VFDs to optimize efficiency, control, motor life and other parameters

Variable frequency drives (VFDs) have been used to control the speed of three-phase alternating current (AC) induction motors since the 1980s. They also provide adjustable acceleration and deceleration, overload protection and start-stop control of motors. These and other features make VFDs a good choice for fans, blowers, centrifugal pumps, mixers, agitators and conveyors – which require variable horsepower and torque – and for applications in which energy is saved by operating at reduced speed.

VFDs convert single- or three-phase AC input power to direct current (DC), and then invert it back to three-phase AC output power. Before it is converted to the output, the VFD varies the frequency and voltage of the inverted power, allowing it to control motor speed based on the setpoint, which is either set at the VFD or more commonly sent to it by the automation system.

A VFD is often not the best choice for a constant-speed application requiring controlled stops and starts because the electronic conversion of AC to DC to AC results in an efficiency loss of about 4 percent. However, if a variable motor speed is needed, the benefits of a VFD are typically more than sufficient to overcome the efficiency loss. To realize these benefits, consider the top 10 tips for specifying and using VFDs described in this article.

1. Understand & use the benefits & features

The benefits of using VFDs include energy savings, adjustable motor speed and torque, reduced motor inrush when starting, and controlled stopping and reversing. Probably the biggest benefit is reduced energy consumption when operating devices such as blowers, fans and centrifugal pumps at slower speeds.

For example, reducing the speed of a blower to 50 percent reduces the air flow by 50 percent as well, but cuts the power requirements by 87.5 percent. Required fan, blower and centrifugal pump power is proportional to the cube of motor speed, saving significant energy at lower speeds (see Image 1).

The VFD’s ability to vary motor speed allows the optimization of the work required by a machine or process because only the needed speed is provided. This contrasts with running the motor at full speed and throttling the output, which is inefficient, decreases motor life, and increases maintenance for the motor and the throttling equipment.

Torque can also be limited to protect machinery or product from possible damage. These adjustments can be made automatically using a programmable logic controller (PLC) or other controller, or manually using a keypad or potentiometer on the drive.

The VFD can reduce motor starting currents that can be more than eight times the full-load current of a motor. With larger motors, this full-load starting places significant demand on the power distribution system. These high demands can result in a voltage dip or voltage sag when the motor starts at full voltage. A controlled acceleration start, provided by a VFD, addresses this issue.

With VFDs, controlled acceleration reduces starting current and extends motor life. This is especially true in applications that require frequent starting and stopping. Additionally, the VFD eliminates the need for a reversing starter. The controlled acceleration and deceleration provided by a VFD reduce equipment wear and tear and related breakage and loss.

2. Choose the VFD based on load size

Sizing VFDs often requires more than just matching the horsepower rating to the motor. The operating profile of the load it controls must be considered. Constant or variable loads, frequent starts and stops, or continuous operation must also be considered when selecting the equipment.

The torque and highest peak current at any time during operation should be determined. This starts by confirming the motor nameplate full-load amps (FLA). However, rewound motors may have higher FLAs than what is listed on the nameplate.

The VFD should be sized based on peak torque demand instead of just horsepower. Under certain conditions, the motor may demand more power and/or torque, and oversizing may be necessary when dynamic loads or impact loading creates temporary overload conditions. In these and other high-demand situations, the VFD must provide enough current to ensure acceptable motor performance.

For example, more power from a VFD is required to provide additional breakaway torque to start a fully loaded conveyor. While many VFDs can operate for 60 seconds at 150 percent overload, higher overloads may be seen for short durations. Whether the overload duration is short or long, an oversized VFD may be required to provide the headroom required by the application. Even high-altitude installations may require oversizing because less air cooling of the VFD is available.

3. Determine braking options

A VFD may also need a little help when decelerating a load. While it can stop moderate inertia loads, high-inertia loads may cause an overvoltage condition in the drive. For quick deceleration of heavy loads, an external dynamic braking resistor should be considered.

The braking resistor allows the VFD to produce additional braking torque by reducing the voltage generated by a decelerating motor. Without the braking resistor, typical VFDs provide approximately 20 percent braking torque. The external braking resistor can significantly increase the VFD’s braking torque for the fast deceleration of heavy loads, and reduce the heat in the drive caused by frequent starts and stops.

 

4. Interface to the VFD

VFDs are controlled by either hardwired, discrete and analog input/output (I/O); or by digital communications. The discrete inputs to the VFD, usually outputs from a PLC, are used to start and stop the drive, although manual pushbuttons and selector switches can be used as well.

 

Other configurable drive inputs include jog, fault reset, accelerate/decelerate select, preset speed (step) selection, proportional-integral-derivative (PID) control and others. Discrete outputs from the VFD include fault present, frequency attained, non-zero speed and local/remote indication. Some higher-end drives also include frequency outputs for speed reference. A drive’s analog input typically accepts a speed command from a PLC or remote potentiometer. These analog signals are typically 0 to 10 volts DC, 4 to 20 milliamps or something similar.

The drive’s analog output, where available, also has the same signal levels. The analog output can provide a speed reference signal proportional to the motor’s speed. This speed signal can be used to command downstream VFDs’ speeds in a master-follower setup. This configuration can synchronize several VFD motors’ speeds. Alternatively, the analog output can provide analog speed, current and torque signals to the PLC.

5. Understand digital communication options

Digital communication protocols allow commands and information to be communicated between a PLC and the VFD across a single cable, as opposed to the many wires and cables required for hardwired I/O. Protocols range from simple serial interfaces such as Modbus RS-232/RS-485 to more advanced Ethernet and fieldbus communication options such as EtherNet/IP.

These communication interfaces allow the VFD to be controlled by a master device, such as a PLC or other advanced controller. This interface can eliminate the need for hardwired discrete and analog I/O, and enables the monitoring of the drive’s speed, current, fault and other parameters. A serial, RS-232 connection to a drive works well for single drive applications located near the PLC. If multiple drives are needed, an RS-485 network can handle multiple drives in a daisy-chain, multidrop configuration and stretch the communication distance up to 4,000 feet.

An Ethernet interface provides higher performance in terms of speed, bandwidth and network configuration options. Multiple drives can be controlled by a single PLC using industrial Ethernet protocols such as Modbus TCP/IP or EtherNet/IP, simplifying VFD wiring and providing an easy way to remotely configure drives.

6. Apply the right control mode

The application often determines the type of VFD control mode: volts-per-hertz (V/Hz), sensorless-vector or closed-loop. V/Hz controls the ratio between voltage and frequency to vary the motor flux, which supplies the operating torque to the motor. V/Hz drives work well for most applications, such as fans and pumps.

Sensorless-vector VFDs are known for their accurate speed control across a wide speed range without the need for encoder feedback because they work well in open loop. A closed-loop VFD uses encoder feedback for accurate speed control by monitoring actual motor speed and slip information. Sensorless-vector and closed-loop VFDs provide excellent speed regulation, providing tight speed control for paper mills, web handling, printing presses and other converting applications.

7. Define motion profiles

How a VFD’s motion profile is configured depends greatly on the application. Motion profile parameters include motor speed, acceleration, deceleration, ramp linearity, torque control, braking and PID. Most VFDs in the market include these parameters, although PID may only be available on more advanced drives.

These parameters can be accessed and programmed using the operator keypad and display or via digital communications. Careful review of the manual will help users understand these parameters and ensure proper installation, setup and control.

 

8. Outline the installation requirements

Following the installation requirements is important because VFDs generate significant heat while operating. Frequent starting and stopping can cause the drive to heat up an enclosure, requiring ventilation to keep the temperature within drive specifications. The manual provides information to help calculate the expected heat output of the VFD during different operating conditions. A standard induction motor can overheat if run at low speed for an extended period. If low speed operation is required, an inverter-duty-rated motor should be specified.

9. Specify operation parameters

From a control standpoint, the VFD should not be routinely stopped by opening a contactor on the input voltage supply because this reduces its operating life. This should only be done for emergency stop purposes. The drive I/O or communications should control the start-stop in all other instances. These and many other installation and operation procedures are outlined in the manual and should be followed carefully, and the VFD supplier should be contacted with any questions.

10. Handle noise & harmonics

VFDs generate electrical noise and harmonics that may cause damage to motors, equipment, transformers and power wiring. Fortunately, filters and line or load reactors can minimize many problems. Most VFD installation instructions recommend the use of passive harmonic filters, such as AC line reactors and chokes. These devices reduce harmonics and protect VFDs from transient overvoltage on the line side of the drive.

On the line side, active harmonic filters invert the harmonic current waveform and feed it back to the line to counteract the noise generated by the VFD. On the load side, a load reactor protects the motor cable insulation from short circuits and reflective wave damage. Including these reactors in all applications with standard inductive motors and in any application where the VFD-to-motor distance exceeds 75 feet are good design practices.

Source: <https://www.processingmagazine.com/pumps-motors-drives/article/15587066/top-10-tips-for-specifying-using-variable-frequency-drives>

About Control Techniques

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About Control Techniques

Control Techniques, a Nidec brand, is the global drives specialist with a long entrepreneurial history. More recently, we have the support of a large international parent company, Nidec, and its other brands. We are a challenger with a big name in the drives industry. We pride ourselves on the service we provide, not only from Newtown in Wales, but also from our network of 45 drive centres around the world. We are drive obsessed, and our ambition pushes us to be the go-to for drives.

 

History of Control Techniques

We’ve always been a company who dreams big. 

Founded in 1972 as KTK in Newtown, Wales, we took our name from our three founders. Their vision was to provide a new drive that met the needs of the market. The following year they did just that. The KTK 415V DC thyristor drive improved performance, reliability and energy efficiency for motor control. 

As we grew, so did our network. We developed close relationships with our customers. This led to a wave of product innovations including the Commander AC and the Mentor, the world’s first digital DC drive.

In that same year, we became Control Techniques. 

We were closer to industry than ever before. This helped us solve one of the biggest problems in automation. One single programmable drive that could run open-loop, closed-loop, and servo. The first intelligent drive: the Unidrive. 

Not only did the Unidrive control any motor, it could communicate with a broad range of fieldbus. This gave our customers the freedom to improve their systems. Onboard intelligence made it possible to program our drives, giving greater control and enabling new possibilities. 

The year was 1995. The same year Control Techniques joined Emerson. 

Our high performance systems found their niche; where failure wasn’t an option, you’d find our drives. Crane systems, elevators, stage, high performance automation OEMs and factories all specified Control Techniques. Unidrive M, Digitax ST, and Mentor MP took the risk out of industrial breakdown.

Fast-forward to 2016, in Kyoto, Japan. The Nidec Corporation, the world’s largest motor producer saw the possibilities of our customer-focused technology. Eager to build high performance drive control into their business, they began talks. For us, this was the perfect fit.

In February 2017 Control Techniques joined the Nidec Corporation. 

We’re now part of operations in over 40 countries, connected to approximately 300 companies who employ over 110,000 people. 

Nidec’s vision is to be the world’s number 1 for everything that spins and moves. From small precision to supersized motors; we create next-generation drive technology for an interconnected world.

Difference between AC motor and DC motor

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Difference between AC motor and DC motor

There are two types of electric motors , and DC motors. The basic difference between ac motor and DC motor is that AC motor powers on AC current while DC motor powers on DC current, To learn in detail about AC and DC motor comparison table is given below about the difference between AC and DC motor.

What is an AC motor?

The AC  motor’s stator has coils which are supplied with the AC current and produces a rotating magnetic field. The AC  motor’s rotor rotates inside the electric motor’s coils and attached to the output shaft that produces torque by the rotating magnetic field.

 

How does an AC motor work?

There are two types of AC motors. Synchronous motors and asynchronous motors.

The synchronous motor consists of a rotor which is energized by DC supply. The stator has the 3 phase winding from which power can be supplied. Now when these two supplies are given, that is at certain voltages, current is drawn and the coil inside produces the magnetic fields. When the AC rotating field (though the stator does not rotate, the 3 phase field will give the effect of rotation)and the DC field interact, torque is produced which leads to the rotation.
The induction motor or asynchronous varies in just a single part. It does not have a separate DC field. Instead, the rotor rotates by the effect of inductance or transfer of flux. The rotor will try to follow the 3 phase flux in the stator and hence the rotary. This motor is used in fans.

AC motors and DC motors use the same principle of using an armature winding and magnetic field except with DC motors. In AC motors the armature does not rotate and the magnetic field continuously rotates.

In some applications, DC  motors are replaced by combining an AC motor with an electronic speed controller.DC  motors are replaced with an AC  motor and an electronic speed controller because it is a more economical and less expensive solution.

DC  motors have many moving parts that are expensive to replace, and DC electric repair is usually more expensive than using a new AC motor with an electronic controller.

So as far as maintenance is the concern, AC motors are used mostly as easy availability of AC supply DC motor has a constant maintenance problem.

 

Sources: <https://physicsabout.com/ac-motor-and-dc-motor/>

Six primary benefits of VFDs

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Six primary benefits of VFDs are:

1. Keeps starting current in control: A VFD has the capability of starting the motor at zero voltage and frequency, which keeps a check on motor winding flexing and heat generation. This helps in extending the motor life.

2. Reduces power line disturbances: Any voltage sag caused in the power line can adversely affect voltage sensitive devices such as proximity switches, sensors, and computers. Using VFDs eliminates voltage sag.

3. Demands lower power on start: Power required to start an ac motor across the line is substantially greater than with a VFD. When industrial customers start these motors during peak hours of electrical consumption, they are likely to be charged with surge prices. However, with VFD demanding lower starting power, the issue can be addressed.

4. Helps in controlling operating speed and acceleration: Applications such as bottling lines that include easy-to-tip product significantly 

benefit from a gradual increase in power. This allows conveyer belts to smoothly rev up rather than an abrupt jerk to full power. They also allow speed to be remotely adjusted by a controller. Control is speed and acceleration is a big bonus to industries in a production process.

5. Limits and adjusts torque: The drive is capable of limiting and adjusting the amount of torque so the ac motor never surpasses this limit. This protects machinery from damage and protects the process or product.

6. Saves energy and cost: A VFD regulating a pump motor that usually runs less than full speed can cut down energy consumption over a motor running at constant speed for the same period. In addition, it eliminates the need for mechanical drive components, which also helps reduce overall costs.

 

Source: <https://www.controleng.com/articles/vfds-six-benefits-energy-efficiency/>

What is Vector control?

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What is Vector control?

From Wikipedia, the free encyclopedia
Vector control, also called field-oriented control (FOC), is a variable-frequency drive (VFD) control method in which the stator currents of a three-phase AC electric motor are identified as two orthogonal components that can be visualized with a vector. One component defines the magnetic flux of the motor, the other the torque. The control system of the drive calculates the corresponding current component references from the flux and torque references given by the drive’s speed control. Typically, proportional-integral (PI) controllers are used to keep the measured current components at their reference values. The pulse-width modulation of the variable-frequency drive defines the transistor switching according to the stator voltage references that are the output of the PI current controllers.
FOC is used to control AC synchronous and induction motors It was originally developed for high-performance motor applications that are required to operate smoothly over the full speed range, generate full torque at zero speed, and have high dynamic performance including fast acceleration and deceleration. However, it is becoming increasingly attractive for lower performance applications as well due to FOC’s motor size, cost and power consumption reduction superiority. It is expected that with increasing computational power of the microprocessors it will eventually nearly universally displace single-variable scalar volts-per-Hertz (V/f) control.