Troubleshooting DC Bus Overvoltage Faults

Understanding a way around the “F05” requires understanding how the fault works.

“F05” is the code for a bus overvoltage fault for the 4, 5, and 7-series of variable frequency drives with the 700S featuring an “F24” code and the 755T featuring a and F10107, F11107, F13066, or F13068 code.  To provide a little more detail:  A bus over-voltage fault means that the bus voltage level has exceeded the maximum value specified for the VFD in question, and the VFD engages in a fault to protect DC bus components.  The tables below provide the trip levels for particular 750-series and 5-series drives at 480 Volts input:

All VFD’s have a bus regulation function that will either modify the output frequency of the drive or work in tandem with a dynamic brake to limit bus voltage.  While these fail-safes exist, circumstances can still conspire to create the “F05” fault condition.

Considering the drive application properly is the first step to preventing bus overvoltage faults.  Two misapplication conditions should be explored to make sure a new VFD install will be fault free:

High inertia loads that require a significant amount of motor torque output to either start or stop may create overhauling conditions.  Overhauling increases DC bus voltage by causing the motor to regenerate: the load’s physics causes the system to decelerate more quickly than the controlling drive’s current output calls for, leading to reverse action of the motor rotor.  The reversing motor acts as a generator and sends its voltage backward toward the VFD DC bus.  The following example provides more detail:

A VFD reduces speed from 60Hz to 40Hz.  The deceleration ramp rate is set to 10 Seconds (6Hz/Sec).  This will allow for a speed step-change in 3.33 Seconds.  However, load physics are such that the load changes speed at 3Hz/Sec, causing the system to need 6.66 Seconds to get to 40Hz speed.  The drive’s output, therefore, is reducing at a faster rate than is the load’s actual speed.  This creates a circumstance where the load places torque on the motor rotor leading to an “overhauling” of the drive output.  The motor now acts as a generator.  This state of “returning” energy is called regeneration.

Upsize the drive

The drive’s amp rating may not be high enough for the overhaul energy involved in the application.  Simply purchasing a larger drive can, in many cases, provided the ampacity needed to prevent a drive overvoltage fault.

Extend the Deceleration time

Extending the deceleration time for the drive can allow the drive to absorb returned energy.

Add an Output Reactor

Adding an output reactor may provide enough voltage spike dampening if the regeneration levels are low enough.

Add a Dynamic Brake Resistor

If process times must be kept short and upsizing the drive is unreasonable, then adding a dynamic brake is often the best course of action.  In many cases, a dynamic brake will be called for in the original system design due to the added accuracy and stopping power provided by such devices.

Consider a Regenerative Drive

The PowerFlex 755TR offers regenerative drive capabilities through its active front end technology.  This allows the drive to absorb regenerative energy and redirect it to the mains providing power to the drive.  This allows the drive to provide the same accuracy and power in stopping actions as is normally provided by a dynamic brake though in a more energy-efficient manner.

There are many reasons to choose a PowerFlex 755T, and regeneration is just one of them.  Check out the 755T page on this website for more information.

Cyclical loading describes a condition where a load is constantly oscillating between high current and low current demands. This is typical of lumber planer heads in sawmills or bull wheels on presses. Again, physics does not allow the system to slow as fast as the VFD can reduce output speed frequency.  What differs between cyclical loading and overhauling, notably, is that cyclical loading has an additive property.  Where overhauling caused DC bus voltage to build to the trip point immediately, cyclical loading may cause the DC bus voltage to rise incrementally over several operational cycles until the trip level is reached.

Example:

A planer head runs low current when not in contact with wood. At contact with the material, the output current spikes to maintain the commanded torque/speed output. When the wood exits the planer head, the output current drops, but the system’s speed increases so that it now overhauls the VFD output speed. The drive’s speed regulator clamps to control speed in response and a cyclical regeneration occurs.  The cycle is variable, so there may not be time for the VFD to absorb/dissipate energy. The DC bus can continually build until it reaches the trip point.

Upsize the drive

The drive’s amp rating may not be high enough for the overhaul energy involved in the application.  Simply purchasing a larger drive can, in many cases, provided the ampacity needed to prevent a drive overvoltage fault.

Extend the Deceleration time

Extending the deceleration time for the drive can allow the drive to absorb returned energy.

Add an Output Reactor

Adding an output reactor may provide enough voltage spike dampening if the regeneration levels are low enough.  This is a good option for a cyclical load scenario.

Add a Dynamic Brake Resistor

If process times must be kept short and upsizing the drive is unreasonable, then adding a dynamic brake is often the best course of action.  In many cases, a dynamic brake will be called for in the original system design due to the added accuracy and stopping power provided by such devices.

Consider a Regenerative Drive

The PowerFlex 755TR offers regenerative drive capabilities through its active front end technology.  This allows the drive to absorb regenerative energy and redirect it to the mains providing power to the drive.  This allows the drive to provide the same accuracy and power in stopping actions as is normally provided by a dynamic brake though in a more energy-efficient manner.

There are many reasons to choose a PowerFlex 755T, and regeneration is just one of them.  Check out the 755T page on this website for more information.

Spikes, dips, and transients on the incoming power feed to a VFD contribute to bus overvoltage faults, typically in conjunction with another cause.  Power line rises can build the drive DC bus voltage above nominal moving it closer and closer to the trip point.  This can lead to a fault condition when normal overhauling and or cyclical activity builds up the DC bus voltage level.

Example:

At 480VAC, a VFD’s DC bus voltage is around 675VDC.  Regeneration within the system builds the DC Bus up to around 790VDC, periodically.  This is within the boundaries of allowable operation for the said drive.  When the incoming power rises to 492VAC, however, the DC bus voltage reaches 695 VDC.  During the next regeneration cycle, the combination of the rise in line voltage with regeneration energy builds the DC Bus Voltage to Trip Point.

Extend Deceleration Time

Extending the stopping time for the drive will allow for the dissipation of bus voltage such that line spikes and transients can be accommodated without leading to a fault condition.

Add Line Reactors

A line reactor serves the purpose of conditioning input or output voltage levels.  This allows the DC bus to remain at nominal levels throughout the operation.  Input line reactors will clamp down on power line rises and transients into the drive, while output line reactors will condition regen energy from overhauling or cyclically loaded systems.

If incoming line transients are the principal cause of faults, then a line reactor may be called for.

If a Dynamic Brake is Used

If the line side voltage cannot be controlled or conditioned effectively, then using a dynamic brake to prevent overhauling or cyclical loads from creating a fault condition may be employed.  Take note:  because of the elevated voltage conditions at play, the duty cycle of the dynamic brake chosen will have to be sized for the voltage levels expected as opposed to nominal voltage levels.

Further, the Dynamic brake is best employed for application reasons (overhauling, load cycling).  Using a dynamic brake for power quality reasons may alleviate bus voltage issues, but a more direct approach to power quality is often better suited to the circumstance.

Regenerative Drive Options

A 755TR can be employed to both allow for sophisticated stop control while also accommodating elevated voltage levels.  Furthermore, the active front end technology found in the 755T series removes the harmonics produced by the drive, possibly eliminating the need for PFCC’s.

When using a dynamic brake resistor, attention should be paid to power line rises.  Sustained voltage increases can lead to dynamic brakes miss-firing and overheating.  This circumstance can often require the use of a larger than anticipated dynamic brake.

Example:

At 480VAC, Drive DC Bus Voltage is around 675VDC.  When the power system reaches 525-530VAC sustained, the drive’s DC bus voltage reaches 750VDC.  The attached dynamic brake transistors/external choppers fire at 750VDC.  The brake resistor is, therefore, being exercised during the entirety of the elevated voltage levels.  Transistors, choppers & resistors must be sized for a 100% duty cycle in circumstances such as this or other steps that should be taken to prevent this occurrence.

If a Dynamic Brake is Used

If the line side voltage cannot be controlled or conditioned effectively, then using a dynamic brake to prevent overhauling or cyclical loads from creating a fault condition may be employed.  Take note:  because of the elevated voltage conditions at play, the duty cycle of the dynamic brake chosen will have to be sized for the voltage levels expected as opposed to nominal voltage levels.

Further, the Dynamic brake is best employed for application reasons (overhauling, load cycling).  Using a dynamic brake for power quality reasons may alleviate bus voltage issues, but a more direct approach to power quality is often better suited to the circumstance.

Many facilities employ power factor correction capacitors (PFCC’s) to both save energy costs and meet the power quality requirements of energy providers.  PFCC’s function cyclically, as they turn on and off in response to power bus loading.  The nature of a PFCC is to increase the system voltage to push the power factor towards unity.  This will, in turn, increase the DC bus voltage on VFD’s connected to a common bus.

When dealing with a PFCC, the distance between the drive and the capacitor bank affects the amount of influence that the capacitors have on the DC bus voltage as does the presence of a line reactor.  Moreover, the drive’s operation can influence the operation of the PFCC.  A 6-pulse drive can easily become the principal load taxing the PFCC’s.  Further, the harmonics that VFD’s place on the power system tends to erode the fuses on PFCC’s.

Extend Deceleration Time

Extending the stopping time for the drive will allow for the dissipation of bus voltage such that line spikes and transients can be accommodated without leading to a fault condition.

Add Line Reactors

A line reactor serves the purpose of conditioning input or output voltage levels.  This allows the DC bus to remain at nominal levels throughout the operation.  Input line reactors will clamp down on power line rises and transients into the drive, while output line reactors will condition regen energy from overhauling or cyclically loaded systems.

If incoming line transients are the principal cause of faults, then a line reactor may be called for.  If reflected wave phenomena are leading to motor winding and lead wire damage, then a reflected wave reactor may be implemented.

When a PFCC is in play, a 5% line reactor can limit the harmonics the drive places on the power system.  This has the benefit of protecting the PFCC fuses and lessening the degree to which the drive will exercise the PFCC.  The input reactor will also help condition incoming voltage rises to the drive’s bus.

Regenerative Drive Options

A 755TR can be employed to both allow for sophisticated stop control while also accommodating elevated voltage levels.  Furthermore, the active front end technology found in the 755T series removes the harmonics produced by the drive, possibly eliminating the need for PFCC’s.

Those who regularly install and use VFD’s are familiar with reflected wave phenomena.  Rockwell has produced a video that explains both reflected wave and various mitigation techniques, please watch here for an overview of reflected wave phenomena.  How reflected wave can contribute to a bus overvoltage fault is explained by insulation breakdown within the motor to motor leads:

As the corona effect leads to insulation breakdown, an increase in the amount of reflected wave voltage occurs.  While there are blocking diodes within a PowerFlex drive that protect the IGBT’s, these same diodes transfer the extra voltage to the DC bus leading to bus overvoltage faults.

Add Reflected Wave Reactors

If incoming line transients are the principal cause of faults, then a line reactor may be called for.  If reflected wave phenomena are leading to motor winding and lead wire damage, then a reflected wave reactor may be implemented.  Reflected wave reactors are placed downline of the drive and will work to condition voltage spikes and transients that arise from reflected wavey.  This can work to extend the lifespan of the connected motor and power conductors.

Regenerative Drive Options

A 755TR can be employed to both allow for sophisticated stop control while also accommodating elevated voltage levels.  The active front end technology included in the 755T can eliminate harmonics both in front of and out of the drive, greatly reducing the incidence of reflected wave phenomena.

PowerFlex VFD’s are outfitted with metal-oxide varistors (MOV’s) attached to their internal circuitry by way of a jumper.  The purpose of the MOV is to absorb voltage transients that may occur on the power system.  The MOV’s work on solidly grounded systems (see diagram below) but do not on other forms of system grounding.

If the power system in question is a solidly grounded wye system, then the jumpers should be installed.  Otherwise, they should be removed.

The need to match the MOV to the power system stems from the presence of circulating ground currents on ungrounded or high resistive grounded systems.  These circulating currents travel through the MOV and onto the DC bus, leading to elevated voltage levels.  While these elevated voltage levels may not cause a fault on their own, they may act concurrently with other conditions to create a fault.

Example

The power system serving a drive is classified as “un-grounded” (Delta, impedance ground wye, etc.).  The MOV jumpers are left installed in the drive.  Per Rockwell Automation install instructions, the MOV jumpers should be removed.

About MOV Jumpers

Install MOV jumpers when appropriate.  Remove when appropriate.  Look here for information about MOV jumpers on PowerFlex 520 series drives and here for information regarding the 750 series.

Repeated insults to a VFD’s bus from transients, overhauling and cyclical loads, and, well, everything we have discussed up to this point can degrade the elements of the DC bus.  Said degradation can promote the occurrence of overvoltage faults.

The two components to keep an eye on are the DC bus capacitors and the bleeder/balance resistors.  Repeated overvoltage, even at levels that does not cause a fault, can erode the ability of these components to absorb high voltage levels, leading the bus to pick up the slack and contributing to faults.  A VFD, therefore, can begin experiencing more frequent bus faults as it ages.

Extending the deceleration time, using reactors, and using dynamic brakes may help alleviate bus voltage faults in the face of a deteriorating drive, but such measures will provide aid only for an indeterminate amount of time.  A full solution will require the replacement of the drive and a reconsideration of the application and power system to ensure that future degradation can be mitigated.