12.07.2026 by Viktor Siebert

An Okuma VACIV-D6 spindle drive arrived at our workshop with a fault that was difficult to isolate: Alarm 10 appeared occasionally during machine operation. After restarting, the CNC machine could sometimes be operated again, but the fault returned without any reliable warning.

Sporadic faults in spindle drives are especially problematic for production companies. As long as the system is operating, the fault often cannot be reproduced immediately. If it occurs during machining, the spindle is stopped and the active program is interrupted. In addition to the actual machine downtime, workpieces, tools and firmly scheduled production times may also be affected.

The provided Okuma manual describes Alarm 10 as “Motor cable overcurrent.” This refers to an instantaneous overcurrent event detected within the motor circuit of the VAC drive system. The alarm has alarm level 1a and processing level 1. The system switches off the motor current and allows the motor to coast freely.

When Alarm 10 occurs only occasionally, it is therefore not sufficient to examine only the power section of the spindle drive. The spindle motor, motor cable, all plug connections, insulation to ground and the current detection circuitry inside the drive must also be considered.

Initial Diagnosis of the Okuma VACIV-D6

The inspection began with a complete visual examination of the unit and its individual assemblies. According to the available inventory data, the VACIV-D6 consists of several functionally separated boards and a power section.

Assembly	Board designation	Model designation	Manufacturer No.	Quantity
Control board	VAC IV CPU Board	06011-10030-10	A006-1510	1
Power board	VAC IV GD Board 2	06011-21030	A006-1511	1
Interconnection board	Not specified	E4809-770-077	Not specified	3
Power board	Power Board	E4809-820-001-B	1006-1212	1
Power section	D6	Not specified	Not specified	1

The following areas were checked first:

• Discoloration, thermally stressed areas and damaged conductor tracks
• Contamination between the individual assemblies
• Condition of connectors and interconnection boards
• Contact points between the CPU board, gate-drive board and power board
• DC bus and power-carrying connections
• Power semiconductors and their drive circuitry
• Current measurement and current feedback
• Power supply to the control electronics
• Insulation clearances and possible conductive deposits
• Mechanical condition of assemblies and solder joints

In the case of a sporadic overcurrent alarm, a purely static measurement is often insufficient. A semiconductor or drive stage may operate normally when cold and only become unstable under temperature, higher motor current or rapid load changes.

The Okuma manual distinguishes between normal operating indication and fault operation. Under normal conditions, the M1, M2 and M3 status LEDs are switched off. In the event of a fault, at least one of these LEDs lights or flashes. The fault is additionally shown as a two-digit number on the seven-segment display of the control board. With Alarm 10, the indicators assigned to the power circuit illuminate according to the fault logic.

Meaning of Alarm 10

Feature	Information from the documentation
Alarm number	10
English designation	Motor cable overcurrent
Meaning	Instantaneous overcurrent in the motor circuit
Alarm level	1a
Processing level	1
Drive reaction	Motor current is switched off
Motor behavior	Motor coasts freely
Assigned status indication	IOCM
Affected area	Power circuit, motor cable or connected load

The term “Motor cable overcurrent” does not automatically mean that only the motor cable is defective. The drive detects a current that exceeds the permissible detection range. The technical cause may be located in several different areas:

• Short circuit or temporary connection between two motor phases
• Insulation fault from one motor phase to PE or the motor housing
• Damaged motor cable
• Contaminated or damp motor connector
• Insulation problem in the spindle motor
• Fault in the power section of the VACIV-D6
• Unstable gate drive
• Faulty current detection
• Contact problem between internal boards
• Thermally unstable electronic components

Actual Fault Cause and Technical Assessment

Because the alarm occurred only occasionally, there was no permanently low-resistance connection at the output. A permanent short circuit would normally have triggered the fault reproducibly during power-up or immediately upon the first drive command.

The fault pattern therefore indicated an intermittent disturbance. Such faults may depend on load, motor speed, temperature or the switching state of the power stage.

The diagnosis focused on the following possible fault chains:

• A component inside the power section changes its electrical characteristics as it heats up.
• The current detection circuit temporarily reports an impermissibly high motor current.
• An unstable gate drive causes an incorrect switching event in the power semiconductors.
• A contact problem between the GD board, power board and power section interrupts or distorts drive signals.
• An aged power supply affects the control electronics or the driver stages.
• Conductive contamination or weakened insulation clearances cause a fault only under certain voltage and temperature conditions.

Without an additional inspection of the motor and motor cable, however, an external fault cannot be completely ruled out. If the alarm reappears after installation, the motor insulation, motor cable and plug connections in the machine must also be checked.

The provided manual clearly assigns Alarm 10 to instantaneous overcurrent in the motor circuit. Alarm 11, by contrast, describes a short circuit in one arm of the inverter bridge. This distinction is important for diagnosis: Alarm 10 initially describes the detected current event, while Alarm 11 explicitly refers to a bridge short circuit inside the inverter.

Repair Measures

Since no detailed list of the electronic components actually replaced is available, no specific semiconductors, capacitors or resistors are presented here as having been renewed.

Professional processing of an Okuma VACIV-D6 with this fault pattern nevertheless includes the following work steps:

• Complete dismantling of the accessible assemblies
• Cleaning of the boards and power components
• Inspection of all plug connections
• Testing of the three E4809-770-077 interconnection boards
• Examination of the VAC IV CPU Board 06011-10030-10
• Testing of the VAC IV GD Board 2 with number 06011-21030
• Inspection of the Power Board E4809-820-001-B
• Electrical testing of the D6 power section
• Inspection of the drive signals between the CPU board and gate drive
• Testing of internal supply voltages
• Examination of the current-measurement and protection circuits
• Thermal testing of suspicious assemblies
• Rework of suspicious solder and contact points
• Final insulation and safety inspection

Particular attention was paid to the connection between the control circuitry, gate-drive section and power stage. Reliable control of the power stage requires the switching commands from the CPU board to be transferred cleanly and processed correctly by the GD board.

Even small contact resistances, aged contact surfaces or thermally stressed solder joints can produce faults in an older spindle drive that remain invisible under no-load conditions.

Diagnosis of the External Motor Circuit

Before the Okuma VACIV-D6 is returned to service, the machine should additionally be inspected in the following areas:

Test point	Procedure	Background
Motor cable U, V, W	Check continuity and phase resistances	Detect interruptions and contact problems
Insulation to PE	Disconnect motor and cable from the drive and test appropriately	Rule out ground faults or weakened insulation
Motor connector	Inspect for oil, moisture, coolant and contamination	Conductive deposits may trigger overcurrent
Terminal points	Check screws and contacts	Loose connections cause heating and arcing
Spindle motor	Compare winding resistances	Phase deviations may indicate winding damage
Mechanical load	Inspect spindle, bearings and drive train	Mechanical stiffness increases motor current
Shielding and grounding	Check cable routing and PE connections	Prevent interference with current detection

An insulation test must not be carried out through the connected electronics of the spindle drive. The motor and motor cable must be professionally disconnected from the VACIV-D6 before testing.

Final Inspection and Return Shipment

After completion of the work, the Okuma spindle drive was reassembled and prepared for functional testing. During the final inspection, the assemblies were not assessed only in the powered-on state. The decisive factor was monitoring the drive across different operating and temperature conditions.

The test included:

• Controlled activation of the control voltage
• Checking the POWER, LOSS and CPUALM indicators
• Checking the seven-segment display
• Monitoring the operating-status LEDs
• Enabling the power section
• Testing the motor-circuit drive
• Switching between different speed and load conditions
• Repeated acceleration and deceleration cycles
• Temperature monitoring of the power electronics
• Extended operating phase to detect sporadic faults
• Repeated start and stop sequences

According to the manual, the POWER LED indicates that the VAC drive control supply is present. The LOSS LED indicates that a voltage in the arithmetic circuit is outside specification. The red CPUALM indicator marks a problem in the calculation or control circuit. The other LEDs serve as input and output indicators during normal operation and help isolate the alarm cause in the event of a fault.

The unit was approved for return shipment only after stable operation throughout the complete test period.

Price and Delivery Time for Okuma VACIV-D6 Drive Unit

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Technical Data of the Okuma VACIV-D6

Technical feature	Documented information
Manufacturer	Okuma
Device designation	VACIV-D6
Alternative spellings	VAC IV-D6, VAC IV D6, Okuma D6 Spindle Drive
Device class	Spindle Drive Unit, CNC Drive, Spindle Amplifier
Function	Control and regulation of a VAC spindle motor
Axes	Spindle drive, partly with C-axis function depending on system configuration
Control board	VAC IV CPU Board 06011-10030-10
Control board manufacturer No.	A006-1510
Gate-drive board	VAC IV GD Board 2, 06011-21030
Gate-drive manufacturer No.	A006-1511
Power board	E4809-820-001-B
Power board manufacturer No.	1006-1212
Interconnection boards	E4809-770-077, three units in total
Power section	D6
Diagnostic display	Two-digit seven-segment display
Status indicators	POWER, LOSS, CPUALM, M1, M2, M3 and LEDs 1 to 13
Alarm transmission	Alarm data is transmitted to the NC and displayed on the CRT
Voltage	Not specified in the provided document extract
Output current	Not specified in the provided document extract
Output power	Not specified in the provided document extract
Frequency range	Not specified in the provided document extract
Software version	Not specified
Weight	Not specified
Protection class	Not specified

Operating Conditions

The uploaded documentation does not contain a complete specification of the permissible operating conditions for the VACIV-D6. Therefore, no temperature, humidity or altitude values from other device series are adopted.

Operating condition	Information
Permissible ambient temperature	Not stated in the provided VAC document
Storage temperature	Not stated
Relative humidity	Not stated
Permissible installation altitude	Not stated
Protection rating	Not stated
Installation environment	Control cabinet of a CNC machine tool
Cooling	Adequate air circulation and unobstructed cooling paths required
Contamination protection	Protection from oil mist, coolant, dust and metal particles required
Grounding	Reliable protective-earth and machine-ground connection required

Regardless of the missing numerical values, the spindle-drive environment should be dry, clean and sufficiently cooled. Contaminated cooling paths and conductive deposits increase the thermal and electrical stress on the assemblies.

Interaction with Other Devices

The following system relationships can be derived from the provided documentation:

• Okuma CNC control with CRT or operator panel
• VAC spindle motor
• Motor cable between VACIV-D6 and spindle motor
• Speed or position feedback
• Machine enable and machine-ready signal
• Emergency-stop signal
• Winding-changeover signals
• Torque limitation
• High-speed and low-speed ranges
• C-axis function, where provided by the machine
• Regenerative main circuit
• External contactors for winding changeover

The normal status indication in the manual includes the following input and output signals:

Signal	Meaning
MS ON	Main-circuit charging start command
SFR	Motor power-circuit supply or enable
WCHG	Winding-changeover command
TLMC	Torque-limit command
MRDY	Machine ready
EMR	Emergency stop
AWCHG	Winding changeover completed feedback
HMSCHG	High-speed range magnet or contactor signal
LMSCHG	Low-speed range magnet or contactor signal
ZERO	Zero-speed signal
AGRE	Speed-agreement signal
SRDY	Operation-ready signal

Functional Description

The Okuma VACIV-D6 is a spindle drive for CNC machine tools. Its task is to process the electrical energy so that the connected VAC spindle motor can be controlled according to the commands of the CNC.

The VAC IV CPU Board processes the control and feedback signals. These include speed commands, machine enables, protection messages and status information. The GD board controls the power section. The power board and D6 power section switch the currents required by the motor.

During operation, the system monitors, among other things:

• Motor speed
• Speed command
• Motor current
• Power circuit
• DC bus voltage
• Input voltage
• Phase condition
• Heat-sink temperature
• Motor overload
• Resolver or PG signals
• Communication with the NC
• RAM and CPU functions
• Winding changeover

The manual distinguishes between two alarm groups. Some alarms are detected directly in the VAC drive. These include power-circuit alarms such as motor overcurrent, inverter-bridge short circuit, overvoltage or undervoltage. Other messages are detected by the NC, such as start, communication or file errors.

Alarms detected directly in the VAC drive are shown both on the NC CRT and on the seven-segment display and corresponding status LEDs. Alarms detected only in the NC appear solely on the operator screen.

Operating Reaction of the Alarm Processing Levels

Processing level	Reaction of the VAC system
1	Motor current is switched off immediately, motor coasts freely
2	Speed is reduced, motor is stopped, then current is switched off
3	External reaction is the same as level 2, but with different internal processing
4	Alarm changes to level 3 after 30 seconds

Alarm 10 has processing level 1. For this reason, the motor current is switched off without controlled deceleration.

Alarm and Fault Codes of the Okuma VAC System

The following table is based on the alarm overview in the uploaded VAC manual. The reset information is formulated functionally because the supplied document shows alarm levels but does not list a separate reset button for every code. The cause must be eliminated before resetting.

Alarm	Fault name	Meaning	Reset	Corrective action
01	P.G. count error	Detected PG count exceeds the PBU data	After eliminating the cause and restarting	Check PG, feedback and PBU data
02	Excessive motor speed	Actual motor speed is too high	After standstill and elimination of the cause	Check speed control and feedback
03	APA speed	Detected spindle speed is too high	After eliminating the cause	Check speed detection and control
04	CON speed	Velocity command or feed data is too high	After correction	Check NC command and communication timing
05	DIFF over	Excessive deviation in C-axis mode	After correction	Check C-axis control and mechanics
06	Resolver error	Resolver signal is not output	After repair	Check resolver, cable and input circuit
07	RAM parity error	RAM cannot be read from or written to correctly	Restart after inspection	Check CPU board and RAM section
08	Communication error	Communication between VAC and NC is interrupted or faulty	After communication check	Check cable, connectors and NC
09	Command error	Transmitted command is undefined or cannot be executed	After command correction	Check NC data and program
10	Motor cable overcurrent	Instantaneous overcurrent in the motor circuit	After eliminating the overcurrent	Check motor cable, motor, insulation, power section and current measurement
11	Inverter bridge short	Short circuit in one arm of the inverter bridge	Do not re-enable before the cause is clarified	Check power semiconductors and gate drive
12	Regenerator IGBT short	Overcurrent in the regenerative main circuit	After repair	Check regeneration IGBT and circuitry
13	Power circuit overvoltage	Excessive DC voltage in the main circuit	After voltage has discharged	Check mains supply and regeneration circuit
14	Input voltage drop	Three-phase input voltage is below specification	After stable supply is restored	Check incoming supply, contactor and wiring
15	Open phase	One phase of the three-phase input is missing	After restoring all phases	Check fuses, contactors and terminals
16	Loss of arithmetic circuit power	Control voltage on the control board is too low	After repair	Check internal power supply
17	Power circuit low voltage	DC voltage in the main circuit does not rise	After inspection	Check charging circuit and power circuit
18	INT loop error	Internal interrupt processing is not correct	Restart after diagnosis	Check CPU board and interference influences
19	Motor overload	Motor temperature exceeds permissible value	After cooling and eliminating the cause	Check load, motor and cooling
20	Heat sink overheat	Drive heat-sink temperature is too high	After cooling	Check cooling, fan and heat sink
21	Data setting abnormal	PBU data or online parameters are outside the valid range	After correction	Check data sets and parameters
22	Internal velocity command too fast	Internal speed command is too high	After correction	Check control parameters and command value
23	P.G. error	Magnetic pulse-generator signal is missing	After repair	Check PG, cable and connector
24	P.G. marker latch data error	Marker count exceeds the PBU data	After correction	Check marker, PG and PBU data
25	Cycle over error	Internal VAC cycle error	Restart after diagnosis	Check CPU board and sequence processing
26	Watchdog error	Watchdog timer is not reset	Restart after diagnosis	Check CPU board and power supply
27	A/D access alarm	A/D access occurs during conversion	After repair	Check A/D circuit and CPU board
28	Master CPU error	Hardware or processing error of the master CPU	After diagnosis	Check or repair CPU board
29	Slave CPU error	Hardware or processing error of the slave CPU	After diagnosis	Check or repair CPU board
30	Excessive velocity deviation error	Motor-speed deviation is too large	After eliminating the cause	Check motor, feedback, load and control
31	Winding changeover error	Winding-changeover contactor is not switched on	After repair	Check changeover contactors, drive and feedback
32	RAM error	RAM contents cannot be cleared at power-up	Restart after diagnosis	Check RAM and CPU board

Preventive Measures for the Machine Operator

Maintenance of an older Okuma VACIV-D6 should not begin only after the spindle drive has failed completely.

Measure	Recommendation
Inspect control-cabinet filters	Regularly check for contamination and airflow
Clean cooling paths	Remove dust, oil mist and metal particles
Inspect plug connections	Check for heating, oxidation and loose contacts
Inspect motor cable	Prevent abrasion, crushing and oil exposure
Inspect motor connector	Keep dry and free from coolant
Monitor spindle motor	Observe operating noise, temperature and current consumption
Document alarm history	Record speed, load and operating condition when the fault occurs
Inspect fans	Replace stiff or noisy fans at an early stage
Check grounding	Ensure uniform and low-resistance PE connections
Clean electronics	Only carry out professionally and with power disconnected

In the case of Alarm 10, information about the exact time of occurrence is particularly useful. Relevant details include whether the alarm appears during start-up, at a certain speed, during acceleration, at tool contact or only after a longer operating period.

Conclusion

The occasional Alarm 10 on the Okuma VACIV-D6 is a serious overcurrent alarm in the motor circuit. The cause may be located inside the spindle drive as well as in the connected motor, motor cable or plug connections.

A reliable repair therefore requires more than replacing a single board. Only a combined assessment of the CPU board, gate-drive board, power board, D6 power section, current detection and external motor circuit provides a dependable diagnosis.

Especially with sporadic faults, extended testing under changing operating conditions is decisive. Only in this way can temperature-dependent and load-dependent faults be detected that remain inconspicuous during a brief power-on test.