Base Block is a VFD operating state in which the inverter's power-semiconductor firing or gate commands are inhibited so that the Variable Frequency Drive stops producing its normal motor output.

The important point is that Base Block does not mean the complete drive has been electrically isolated. The control electronics and DC bus may remain energized while the inverter output stage is blocked.

When this happens, the motor no longer receives normal PWM drive torque and will commonly enter a coast-to-stop condition.

In simple terms: Base Block is not the physical “base assembly” containing the DC link, control board and cooling hardware. It is primarily a drive-output blocking function or operating state.

What happens inside the drive during Base Block?

During normal operation, incoming power is rectified into the DC link and the inverter semiconductor stage switches that DC voltage to create a controlled PWM motor output.

When Base Block becomes active, the final switching stage is inhibited.

Typically:

• Normal PWM motor output is removed.
• Electromagnetic motor torque disappears or falls rapidly.
• The shaft may continue rotating because of inertia.
• The DC bus may remain energized.
• The keypad and control board can remain powered.
• A display message such as bb, BB, Base Block or External Baseblock may appear.

For maintenance purposes, Base Block must therefore never be mistaken for electrical isolation.

What is External Base Block?

Many drives allow a programmable digital input to be assigned as an External Base Block command.

When that input becomes active, the drive immediately disables its motor output.

For example, OtomasyonAVM's Delta VFD-E fault information identifies bb as External Base Block and notes that drive output is turned off while the B.B. input is active.

Yaskawa uses the same basic idea: an active External Baseblock command shuts off drive output and the motor coasts.

This makes Base Block an important troubleshooting item when a drive appears ready or receives RUN but the motor still does not develop torque.

Base Block and Coast Stop

When Base Block removes inverter output, the drive is no longer actively following a deceleration ramp.

The motor therefore commonly coasts to stop according to mechanical inertia, friction and load.

This is different from a ramped stop:

Ramp stop: the VFD continues controlling the motor while reducing output frequency.

Base Block / Coast: inverter output is disabled and the mechanical system slows naturally.

Large fans, flywheels and other high-inertia machines may continue rotating for a surprisingly long time after Base Block is applied.

Does Base Block stop the motor quickly?

Not necessarily.

Base Block can remove motor torque very quickly, but removing torque and physically stopping the shaft are different things.

If fast stopping is required, the system may need controlled deceleration, a braking resistor, DC injection braking or a mechanical brake depending on the machine.

Is Base Block the same as STO?

No.

STO – Safe Torque Off is a safety-related drive function implemented and certified according to the safety architecture of the drive.

External Base Block is commonly a normal programmable control input.

Both may result in loss of motor torque, but that does not make them equivalent.

Base Block: may be an ordinary control function.

STO: can be a safety-related torque-removal function when correctly integrated and validated.

A Base Block input should therefore not automatically be substituted for an emergency-stop safety function.

Base Block is not an Emergency Stop

An emergency-stop function belongs to the complete machine safety architecture.

Depending on the risk assessment, that architecture can include emergency-stop devices, safety relays or safety PLCs, STO inputs, contactors, mechanical brakes and diagnostic feedback.

Connecting a pushbutton to an External Base Block input does not automatically create a safety-rated emergency-stop circuit.

Base Block vs 3-Wire Control

3-wire control defines how normal START and STOP commands are handled.

Base Block is an additional output-inhibit condition.

A drive may therefore still have a valid RUN command while Base Block is simultaneously active.

This explains one common field symptom: the drive says RUN, but the motor produces no torque.

In that situation, programmable-input status, PLC outputs and the external control sequence should be checked.

What happens when Base Block is removed?

The restart behaviour is product- and parameter-dependent.

Some drives can resume according to the existing RUN command when Base Block is removed. Others require a new RUN transition or apply a defined restart sequence.

This behaviour is particularly important in maintained-command systems because removing Base Block can create unexpected restart if the machine logic has not been designed correctly.

Base Block and Speed Search

Base Block is also used internally by some drive-control functions.

During Speed Search, a drive may temporarily block its output before detecting or synchronizing with the speed of a motor that is already rotating.

Older Yaskawa documentation explicitly refers to a minimum Base Block time before the speed-search sequence begins.

This shows that Base Block is more than a simple external STOP command; it can be part of the internal operating sequence of advanced VFD functions.

Base Block during a momentary power loss

Some drives temporarily enter Base Block during a short power interruption.

The motor may continue spinning while inverter firing is disabled. Once suitable supply conditions return, the drive can perform Speed Search or another restart routine to reconnect to the moving motor.

This can be especially useful on high-inertia fans and pumps.

Is “bb” always a fault?

No.

On many drives, bb simply indicates that an External Base Block command is active.

Yaskawa, for example, describes bb as a temporary condition in which drive output is shut off and the motor coasts.

Delta drives use similar terminology.

Before diagnosing an IGBT or motor failure, first inspect the assigned digital input and external sequence.

However, some products can also have a genuine Base Block Circuit Fault. That is a hardware/control-board problem and is different from an intentional External Base Block command.

Is the DC bus still energized?

Usually, yes.

Base Block disables inverter output but does not normally disconnect the rectifier or discharge the DC-link capacitors.

A stopped motor and a BB display therefore do not mean that the drive is safe to touch.

Before servicing a drive, isolate the incoming supply and wait the manufacturer-specified DC-bus discharge time.

Base Block is not electrical isolation.

Base Block vs output contactor

Base Block disables semiconductor switching electronically.

An output contactor physically opens the electrical connection between the VFD and motor.

They are not interchangeable actions.

Opening a motor-side contactor while a VFD is actively producing output can cause faults or stress unless the drive and control sequence are designed for it.

Some Omron drive manuals specifically describe External Base Block as a function used before changing the motor connection from the inverter to a commercial power source.

Does Base Block save energy?

No. Its primary purpose is output control, not energy optimization.

Base Block is also unrelated to cleaning cooling fans or maintaining heatsinks. Cooling fans and thermal management belong to the physical VFD hardware; they are not “components of Base Block.”

Editor's Note – what I check when I see BB in the field

If a drive shows bb, I do not start by blaming the motor or power module. I first open the digital-input monitor and check which input has been programmed for Base Block.

Then I follow the external sequence: PLC output, relay contact, interlock or field wiring. Start with the sequence.

Before clearing Base Block, I also want to know what the machine will do next. If a maintained RUN command is still present, some drives may resume operation when the block is removed.

For machine safety, I treat Base Block as a normal drive-control function unless the actual drive documentation explicitly says otherwise. For safety-related torque removal I look at STO and the complete safety circuit.

And I never assume that a stopped shaft means an electrically dead drive. The DC bus may still be sitting at several hundred volts.

What should you read next?

To understand why the motor continues rotating after output is blocked, continue with Coast Stop.

To separate ordinary output blocking from a safety-related function, continue with Safe Torque Off. For normal START/STOP command logic, see 3-Wire Control.

For reconnecting to a motor that is still rotating, continue with Speed Search.

Related OtomasyonAVM sections

For actual drive products, use AC Drives / VFDs. For braking, filters and other surrounding hardware, continue to VFD Accessories. For machine-level command logic, continue into the PLC section.

External technical reference

Yaskawa's official fault documentation defines External Baseblock as a condition where the drive output is shut off and the motor coasts. Omron inverter manuals likewise describe External Base Block as a command that shuts off inverter outputs and can place the motor in a free-running condition.