Frequency inverters are used in any application in which there is mechanical equipment powered by motors; the inverters provide extremely precise electrical motor control, so that motor speeds can be ramped up and down, and maintained, at speeds required; doing so utilizes only the energy required, rather than having a motor run at constant (fixed) speed and utilizing an excess of energy.

Since motors consume a majority of the energy produced, the control of motors, based on demands of loads, increases in importance, as energy supplies become ever more strained. Additionally, end users of motors can realize 25 – 70% energy savings via use of motor controllers. (Despite these benefits, the majority of motors continue to be operated without inverters.)

Here are six additional benefits users realize when operating motors with inverters:

1. Controlled Starting Current – When an AC motor is started “across the line,” it takes as much as seven-to-eight times the motor full-load current to start the motor and load. This current flexes the motor windings and generates heat, which will, over time, reduce the longevity of the motor. An frequency inverter starts a motor at zero frequency and voltage. As the frequency and voltage “build,” it “magnetizes” the motor windings, which typically takes 50-70% of the motor full-load current. Additional current above this level is dependent upon the connected load, the acceleration rate and the speed being accelerated, too. The substantially reduced starting current extends the life of the AC motor, when compared to starting across the line. The customer payback is less wear and tear on the motor (motor rewinds), and extended motor life.
2. Reduced Power Line Disturbances – Starting an AC motor across the line, and the subsequent demand for seven-to-eight times the motor full-load current, places an enormous drain on the power distribution system connected to the motor. Typically, the supply voltage sags, with the amplitude of the sag being dependent on the size of the motor and the capacity of the distribution system. These voltage sags can cause sensitive equipment connected on the same distribution system to trip offline due to the low voltage. Items such as computers, sensors, proximity switches, and contactors are voltage sensitive and, when subjected to a large AC motor line started nearby, can drop out. Using an frequency inverter eliminates this voltage sag, since the motor is started at zero voltage and ramped up
3. Lower Power Demand on Start – If power is proportional to current-times-voltage, then power needed to start an AC motor across the line is significantly higher than with a frequency inverter. This is true only at start, since the power to run the motor at load would be equal regardless if it were fixed speed or variable speed. The issue is that some distribution systems are at their limit, and demand factors are placed on industrial customers, which charges them for surges in power that could rob other customers or tax the distribution system during peak periods. These demand factors would not be an issue with an frequency inverter.
4. Controlled Acceleration – A frequency inverter starts at zero speed and accelerates smoothly on a customer-adjustable ramp. On the other hand, an AC motor started across the line is a tremendous mechanical shock both for the motor and connected load. This shock will, over time, increase the wear and tear on the connected load, as well as the AC motor. Some applications, such as bottling lines, cannot be started with motors across the line (with product on the bottling line), but must be started empty to prevent breakage.
5. Adjustable Operating Speed – Use of a frequency inverter enables optimizing of a process, making changes in a process, allows starting at reduced speed, and allows remote adjustment of speed by programmable controller or process controller.
6. Adjustable Torque Limit – Use of an frequency inverter can protect machinery from damage, and protect the process or product (because the amount of torque being applied by the motor to the load can be controlled accurately). An example would be a machine jam. With an AC motor connected, the motor will continue to try to rotate until the motor’s overload.

Generally, there are dozens of function parameters in frequency inverter, or even hundreds for option. In practical application, it is not necessary to set every parameter, most parameters can be keep factory default settings.

1. Frequency inverter acceleration/deceleration time

Acceleration time is the time required by output frequency from 0 Hz to the maximum frequency, deceleration time is from maximum frequency dropped to 0 Hz. Usually use the frequency setting signal rise and fall to determine the acc/dec time. It needs to restrict the rise rate of the frequency in acceleration during the electric motor start period in order to prevent over-current, and limit the frequency decrease rate in order to prevent over-voltage during deceleration period.

Acceleration time setting requirements: limits the speed up current below the frequency inverter over-current capacity, make the over current speed loss don’t cause the frequency inverter tripped; The deceleration time set point is to prevent smoothing circuit voltage excessive, make regeneration overvoltage don’t cause the inverter tripped. Acc/dec time can be calculated according to the load, but to practical experience it’s better to set longer acc/dec time at the debugging, to see if there are over current or over voltage alarms during the electric motor start/stop; Then shorten the acc/dec time gradually base on the principle with no alarm in operation, repeat several times, you can determine the best acceleration and deceleration time.

2. Torque boosting

Also known as the torque compensation, it’s to compensate the torque decrease in low speed for the stator winding resistance of the electric motor, by increase the low-frequency range of V/F. It’s enable the voltage during acceleration goes up automatically to compensate the starting torque when its set to AUTO, to ensure the electric motor accelerates smoothly. Adopting manual compensation, you can get better curve through test according to the load characteristics, especially the load starting characteristics. For variable torque loads, the output voltage maybe too high in low speed by the inappropriate chooses, to cause a waste of electricity energy.

3. Electronic thermal overload protection

This function is set to protect the electric motor from overheating, it calculate the electric motor temperature rise by the frequency inverter internal CPU base on the operation current and frequency, thus to enable overheating protection. This function applies to “one inverter drag one motor” only, it should be installed thermal relay on each motor when you want one frequency inverter drag several motors.

Electronic thermal protection setting value (%) = [electric motor rated current (A) / inverter rated output current (A)] × 100%.

4. Frequency limitation

Means the frequency inverter output frequency maximum and minimum value. Frequency limitation is to prevent the failure of impropriate operation or external frequency setting signal source from outputting frequency too high or too low, it’s a protective function to prevent damage the devices. In the application you can set it according to the actual situation. This feature can also be used for speed limiting, like in some belt conveyors, due to not so many materials being transported, adopt the frequency inverter to reduce the wear and tear of machines and belt, by setting the inverter maximum frequency to a certain value, this allows the conveyor belt running at a fixed, low speed status.

5. Frequency bias

Sometimes it’s called deviation frequency or frequency deviation setting. Its purpose is when the frequency is set by an external analog signal (voltage or current), use this function to adjust the output frequency value when the frequency setting signal in minimum value. Some frequency inverters deviation value is available from 0 to max when the frequency setting signal is 0%, some frequency inverters even can be set on the bias polarity. In debugging, when the frequency setting signal is 0%, the inverter output frequency is not 0 Hz but x Hz, then the deviation frequency can be set to negative x Hz to enable the frequency inverter output frequency 0 Hz.

6. Frequency setting signal gain

This feature is only effective when the frequency set by external analog signal. It is used to compensate the inconsistencies between the external setting signal voltage and the frequency inverter internal voltage (+10v); at the same time it’s convenient for voltage selection of the analog signal, in setting, when the analog input signal is maximum (10v, 5v or 20mA), obtained the output frequency percentage of V/F graphics and treat it as a parameter to set it; Take an example, external setting signal is 0 – 5v, inverter output frequency is 0 – 50Hz, then signal gain value is set to 200%.

Frequency converters can be powerful tools in maintaining processes by using diagnostics to solve frequency inverter performance issues and troubleshoot related processes. An understanding of how the frequency converter interacts with the process can help you improve overall production and product quality (Fig. 1).

Frequency converters are not infallible; sometimes they need to be repaired or replaced. The frequency converter is often the first indicator of a process change or application problem.

Many frequency converters communicate using an LCD or LED display, or through an open interlock or fault indication. In most applications, the frequency converter interacts with operator controls, process control signals, and PLCs. A problem with the interaction between the frequency converter and these external controls may appear to be a frequency inverter issue, when actually the problem is with the process. Discussing process and frequency inverter symptoms with the machine operators often can help determine the problem area.

If the external controls are working correctly, use the frequency converter to identify problems systematically. If the display status indicator does not operate, verify incoming ac power. If the status indicator still does not display after verifying or restoring ac power, then verify control power, and restore it if necessary.

If the frequency converter has been operating successfully, but suddenly fails to start, or if the frequency inverter starts but does not run properly, check to see if the diagnostics status display indicates a fault. The instruction manual for the frequency converter should have a description of faults and troubleshooting steps. Use diagnostics or a keypad control to monitor variables such as incoming voltage, dc bus, carrier frequency, output frequency, voltage, current, and I/O and control status. These parameters are displayed on most common frequency converters. I/O status uses bits to monitor required start conditions to ensure they are enabled and to determine what may be inhibiting start. Control status indicates the source of the speed reference and can be used to verify incoming speed or direction signals.

1. High bus fault

High bus is a common fault caused by external factors. An instantaneous voltage spike in the ac line or an “overhauling load” created by the inertia of the machine can cause a high bus fault. The load continues to rotate faster than the motor’s commanded speed. When this situation occurs, the frequency converter protects itself by tripping on a high bus fault and shutting off the insulated gate bipolar transistors (IGBTs).

If a high bus fault is indicated, ensure that the ac power supply is consistent and that the deceleration time is adjusted to match the capability of the load. If the process requires rapid deceleration, dynamic braking or a regenerative power control circuit may be added.

2. Overcurrent fault

Another common fault is overcurrent. When troubleshooting overcurrent faults, first check all power connections to ensure that they are properly attached. Loose connections or broken conductors frequently are culprits when overcurrent and control problems occur. Loose power connections cause overvoltage and overcurrent conditions, blown fuses, and frequency converter damage. Loose control wiring causes erratic frequency inverter performance, resulting in unpredictable speed fluctuations or the inability to control the frequency converter.

Use an autotuning feature if it is offered on the frequency converter. The autotuning function on many frequency inverters enables the frequency inverter to identify the attached motor, allowing rotor information to be used in the processor algorithms for more accurate current control. The frequency converter also can compensate for flux current, allowing better control of the torque-producing current. Both over and under fluxing the motor can be troublesome.

The second step is to check the mechanical load for worn or broken parts, or excessive friction. Repair or replace components as needed.

Finally, check incoming voltage and acceleration rate. If incoming voltage is too low, or the acceleration rate is set too fast, an overcurrent fault is possible. Decrease the acceleration rate or stabilize incoming voltage to correct this fault.

3. High starting-load current

High current/load readings may indicate mechanical binding or unexplained changes in process speed or load. The power requirements for many pumps and fans increase proportional to the cube of the rotational speed (S3). Running loads just a few revolutions per minute faster can overload a frequency converter.

Components should be checked before startup to avoid an overload situation. Conveyors left loaded during off hours should be unloaded before startup. Clogged pumps should be avoided by cleaning out solids that have settled while the pump was not in use. Avoid ice or moisture that possibly could form on the load. Wet material is heavier than dry and can place more loading on the conveyor, causing motor and frequency converter overload.

One way to reduce a high starting load is to use a frequency converter with an extended acceleration rate. This feature starts a load slowly and smoothly rather than jerking it to a start. This type of start is easier on mechanical components and has lower line requirements because the frequency converter draws only 100% MDASSML 150% of load.

4. Erratic operation

If the frequency converter is functioning erratically, but a fault is not indicated, external factors may be the cause, or the frequency inverter itself may have failed. Understanding the causes of frequency converter faults helps you determine the root cause of the problem. Frequently overlooked root causes are usually instabilities in the process that force the frequency converter to function in harsh conditions.

Visually inspect the frequency converter for burned or overheated components by looking for signs of discoloration or cracking. Burned or cracked components prevent proper frequency converter operation. Replace defective components and test the frequency converter before returning it to operation.

Power quality is another electrical issue that can affect a frequency converter. A change in utility equipment or unexpected power surges, due to electrical storms or system overloads, can affect frequency converter performance.

5. Contamination failure

Contamination is a preventable cause of frequency converter failure. Check the frequency converter for contamination of dust, moisture, or other airborne particles that may be electrically conductive. Tracking or arcing marks across components or circuit board traces indicate evidence of contamination failures. If contamination is excessive, the frequency converter must be isolated from the contamination source by changing the environment or providing an appropriate NEMA-rated enclosure. If there is significant airborne contamination from dust, moisture, or corrosive vapors, the frequency converter must be in at least a NEMA-12 enclosure.

The internal cooling fans and component heatsinks of the frequency converter also should be checked for contamination. Blocked fans force the frequency converter to operate outside of its temperature specification, which can cause premature failure as a result of inadequate cooling. Check the fan for grease and other contaminants that can cause bearings and other parts of the fan to fail. Both the interior and exterior of the frequency converter, including fans, blowers, filters, and heatsink fins, should be cleaned monthly to reduce the risk of failure from contaminants.

Temperature failure

The environment within which the frequency inverter must operate must be within specified temperature limits. Measure the temperature inside and outside the enclosure to ensure that it is within the ambient specifications determined by the manufacturer. Failure to meet the required temperature specifications can lead to premature frequency converter failure because numerous power components rely on adequate cooling for proper operation.

If the ambient temperature is too high, additional cooling should be added to the enclosure or the frequency converter should be relocated to an area where the ambient temperature is within the specification. Low ambient temperatures may cause problems as well. Condensation may form and cause component or frequency converter failure.

Other failures

Many faults are caused by misapplication of the frequency converter. Process changes, such as variations in load or speed; power issues, such as capacity switching by the utility; or changes in environmental operating conditions are not immediately obvious, but could be a major contributor to frequency converter failure. Evaluate the consistency and condition of the process when trying to determine the cause of failure.

If the frequency converter remains inoperative after performing the aforementioned checks, contact the manufacturer. Most frequency converter suppliers have highly trained technical support personnel that can provide the assistance needed to diagnose the problem. The technical support staff can help you select replacement parts or a new frequency inverter if replacement is necessary.

As intelligent devices embedded in the manufacturing process, frequency converters can provide insight into application and equipment performance. By providing the maintenance worker with the information necessary to understand and interpret the problem, frequency converter issues, and sometimes process or operational problems, can be quickly identified so that plant operation can be resumed and productivity improved.

Talking with machine operators

Talking with machine operators can often identify the problem area. Useful questions to ask are:

Dynamic braking and regenerative power

When a process requires a frequency inverter and motor to decelerate rapidly, the motor can actually operate as a generator. The energy stored within the motor in the form of mechanical rotation must go somewhere. A dynamic braking circuit is used to accommodate this energy.

A dynamic braking circuit is a switch that monitors the dc bus and turns on when the bus level exceeds a certain setpoint. Energy is delivered to the resistors, and is expended in heat until the bus level drops below that setpoint.

A regenerative power supply operates as an inverter-in-reverse, which allows synchronization of the IGBT firing circuits with the incoming ac line. Using a regenerative power supply allows the circuit, which is separate from and external to the frequency inverter, to send the excess energy from the motor through the dc bus back to the power source.