Yaskawa VS-626M5 Spindle Alarm 10/11/12 Overvoltage Field

1. Problem Summary

The Yaskawa VS-626M5 spindle drive on a 2001 Kitamura Mycenter 1Xi vertical machining center, controlled by a Yasnac J300, raises a cascade of spindle- and servo-related faults when the operator commands speeds above 10,000 RPM and adjusts the spindle override downward. The drive trips with internal Alarm-10 (Converter Fault), Alarm-11 (Main Circuit Overvoltage), and on the converter section LED code 12 (Main Circuit Undervoltage). The Yasnac J300 wraps these into machine alarm 1005 (Thermal Trip / Power Panel / Oil Cooler / Inverter Abnormal).

The fault is not reproducible as a clean drive trip while the motor is uncoupled from the spindle — only under real mechanical load and during active deceleration. Field investigation shows the root cause is regenerative energy returned to the DC bus during override-driven decel, pushing bus voltage from approximately 220 V to a peak of 255 V, just below the converter's stated 262 V trip ceiling. Long-term continuous-on-time and a 2012 repair history compound the diagnosis.

2. Affected Machine and Drive Configuration

The DPC-A10 Power Pac steps the incoming utility down and is shared with another 220 V Yaskawac-controlled machine. Output of the Power Pac sits 218–220 V phase-to-phase with the spindle running or stopped, which is within the 200/220 V nameplate tolerance.

3. VS-626M5 Alarm Code Reference

The VS-626M5 separates the drive into a Converter (input rectifier / DC bus / regen) and an Inverter (output to motor). Each section has its own fault register and alarm LED. The codes confirmed in this incident are:

4. Yasnac J300 Machine Alarm 1005 Mapping

The Yasnac J300 logs Alarm 1005 (Thermal Trip) when the power panel reports:

• Oil cooler abnormal
• Inverter abnormal
• Power panel thermal trip

Reset method per J300 display:

• Oil cooler & power panel thermal trip — thermal reset
• Inverter abnormal — inverter reset

Full recovery requires shutting down all power except for the mains to clear the latched thermal/inverter flags. The spindle oil cooler in this machine is not the failing element; the inverter abnormal sub-condition is the source of the trip.

The spindle drive itself does not always drop into a hard E-stop when the J300 throws 1005. Operators may see the machine alarm while the spindle drive continues to run. This is why the fault is intermittent and difficult to catch without a datalogger or with one technician at the drive while the other runs the override.

5. Symptom Reproduction Procedure

1. Warm the spindle at a moderate RPM (≤6,000) for at least 5 minutes to bring the motor and bearings to operating temperature.
2. Command a spindle speed above 10,000 RPM, in 500–1,000 RPM increments.
3. Verify the drive loads the motor. At 15,000 RPM no-load (uncoupled) the inverter reports 16–24% load, which is within normal no-load expectation but lower than coupled-spindle cutting loads.
4. With the spindle running at the elevated target RPM, rotate the spindle-override knob downward rather than waiting for a controlled ramp. The override-driven decel is the trigger.
5. Observe: an immediate drive fault (Alarm-10/11) or a Yasnac 1005 machine alarm indicates the bus-voltage excursion during regen has crossed the trip threshold.

Below 10,000 RPM the override reduction does not produce enough regen energy to exceed the bus threshold and the drive continues to operate normally.

6. Root Cause Analysis

6.1 DC Bus Voltage During Regeneration

When the override knob drops commanded speed while the spindle is mechanically at a higher speed, the motor acts as a generator. The regen current flows back through the inverter's anti-parallel diodes into the DC bus, charging the bus capacitors. The observed bus excursion is:

The 7 V margin between peak and trip is not adequate to absorb transient overshoot, mains swell, capacitor aging, or commutation spikes. The drive therefore latches Alarm-11 the moment the bus crosses its detection threshold, regardless of whether the trip ceiling itself is exceeded.

6.2 Converter LED Code Mapping

The converter carries its own 7-segment LED for sub-codes that do not necessarily mirror the inverter's alarm register. In this incident the converter LED showed code 12 (Main Circuit Undervoltage) on one event and DC-bus overvoltage on another. The dual mapping is consistent with a converter whose DC-link capacitors are simultaneously:

• Too weak to hold charge through a momentary mains dip (undervoltage latch), and
• Too weak to absorb regen energy without an excessive voltage rise (overvoltage latch).

The Power Pac DPC-A10 has new capacitors installed the prior summer, so the DC-link capacitor aging is internal to the VS-626M5, not in the upstream transformer.

6.3 Mains Supply Behavior

Shop mains varies over a 10 V swing with occasional peaks of 250 V. The house circuits show dimming and flicker consistent with utility-side disturbance, and a datalogger deployed for 24 hours captures both pre- and post-Power-Pac and pre- and post-isolation-transformer waveforms. The combination of:

• Undervoltage dips on the utility feed (flicker),
• Sustained input above the 200 V nameplate minimum (218–220 V measured), and
• Regen current returning to a 220 V bus,

produces a converter that can latch either undervoltage or overvoltage depending on which transient arrives first. Mains sag during heavy load triggers Alarm-12; regen during decel triggers Alarm-11. Alarm-10 is the umbrella converter-fault that the inverter reports when either sub-fault trips the converter.

7. Field Diagnostic Procedure

1. Log mains voltage on both sides of the DPC-A10 with a true-RMS datalogger for at least 24 hours, including a production shift. Record pre-PP, post-PP, and at the cabinet input terminals.
2. Read the inverter alarm history via the digital operator (Yaskawa JVOP-160 or equivalent). The six-slot history typically retains fault codes that the operator display does not show in the alarm menu. If the digital operator only shows three screens instead of six, the alarm display mode is not enabled — confirm operator type and parameter access level.
3. Read the converter LED at the moment of every drive fault. Write the code down — the LED is volatile on some converter firmware revisions and clears at power-off.
4. Measure DC bus voltage at the test points on the VS-626M5 control board with a storage oscilloscope or a Fluke 87V with min/max capture while commanding a 15K → 0 decel via override. Capture the peak.
5. Verify regen resistor integrity. The VS-626M5 typically integrates the regen resistor inside the converter chassis on the smaller kVA ratings. Measure resistance cold, check for cracking, discoloration, or solder-joint failure on the ceramic element.
6. Confirm spindle mechanical state: bearing condition, coupling alignment, drawbar preload. A noisy or preloaded spindle raises regen current at a given override rate and lowers the RPM at which the bus excursion becomes destructive.
7. Check the Y-axis resolver connector seating. A loose resolver on the Y servo is a separate but parallel issue (see §10).

8. Mitigation and Repair Options

8.1 Spindle Speed Cap

The simplest, lowest-risk action is to cap commanded spindle speed at or below 12,000 RPM. Below 12K the override decel does not generate enough regen current to exceed the bus trip threshold, and the drive runs reliably. If production throughput does not require >12K, this is the recommended permanent setting.

8.2 Override Ramp Profile

If >12K is required, configure the Yasnac J300 spindle-accel/decel parameters to impose a controlled decel ramp when the override knob is rotated down, rather than a step change. The J300 exposes accel/decel constants through the spindle parameter page; lengthening decel by a factor of 2–4 reduces peak regen current proportionally and keeps the bus excursion inside the 262 V ceiling.

8.3 External Regen Resistor

For installations that must hold 15K with full override authority, add an external regen resistor bank sized to the spindle's peak regen power. Sizing rule of thumb: P_regen ≈ 0.25 × motor rated kW during rapid decel from max speed. For this 7.5 kW motor, a continuous regen dissipation of approximately 1.5–2 kW with a 5× peak overload rating is appropriate. Verify the VS-626M5 firmware revision supports external regen — early units require an ERF card option.

8.4 Converter DC-Link Capacitor Replacement

If the drive's internal electrolytic capacitors have aged (typical service life 8–12 years at continuous operation), replace the DC-link capacitor bank in the converter section. This restores hold-up time on undervoltage events and reduces bus voltage rise during regen. After replacement, repeat the §7 logging procedure to confirm both Alarm-11 and Alarm-12 clear.

8.5 Mains Conditioning

Address shop-wide mains issues independently of the drive. A line-interactive UPS or a constant-voltage transformer ahead of the Power Pac will clamp the 250 V utility peaks that show up on the 24-hour log. This also protects the other Yaskawac-controlled machine sharing the same transformer.

9. Verification Procedure

1. With mitigation in place, run the spindle to 15,000 RPM under normal load.
2. Apply the full override-reduction range downward in a single motion.
3. Confirm the drive does not raise Alarm-10, Alarm-11, or Alarm-12, and the Yasnac does not raise 1005.
4. Log DC bus voltage during the decel. Target peak ≤ 250 V to leave margin against the 262 V ceiling.
5. Repeat for cold-start and warm-start conditions to ensure no thermal dependency is hidden.
6. Record final spindle-cap parameter or override-ramp parameter values in the machine documentation so the fix does not get reverted by the next setup operator.

10. Related Issue: Y-Axis Servo Humming

The Y-axis servo exhibits an audible hum that the visiting field technician identified as consistent with a loose resolver connection at the motor. Resolver problems on Yaskawa servo motors manifest as:

• Audible single-phase excitation hum
• Position-error alarm during aggressive moves
• Reduced achievable servo gain before oscillation

Immediate mitigation: Reduce the Y-axis servo gain slightly — the hum quiets substantially with a small gain reduction. Permanent fix: Re-seat the resolver connector at the motor with the correct torque and locking mechanism; inspect pins for oxidation; if the resolver itself is damaged (open or shorted winding), replace the resolver assembly before returning gain to nominal.

Address this separately from the spindle drive issue. It is not the source of the spindle alarms but it will eventually drop the machine with a following-error or deviation-excessive alarm.

11. Preventive Maintenance Recommendations

• Annual: Log mains voltage at the cabinet input and at the converter input over a representative production week. Trending prevents surprise trips.
• Every 2 years: Measure DC-link capacitor ESR with the drive powered down and discharged. Replace when ESR rises above manufacturer threshold or when hold-up time drops below specification.
• Every 4 years (or at any major spindle service): Verify regen resistor cold resistance against the nameplate value. A 20% drift indicates end-of-life.
• At every spindle bearing replacement: Verify spindle balance and coupling alignment. A misaligned coupling raises regen current at any given override rate.
• At every Yasnac J300 parameter reset: Document the spindle cap and override ramp values. Operators frequently reset parameters during service events and unintentionally re-enable >12K operation.
• Resolver preventive: At every Y-axis service, torque the resolver connector to spec and apply thread-locking compound on the locking ring.

12. Frequently Asked Questions