Yaskawa Varispeed 622MT2 Fault 7 OL Spindle Drive Burnt Smell

1. Problem Summary

The Yaskawa Varispeed 622MT2 (MT2B series) is an AC spindle drive used on Matsuura vertical machining centers (the 560V in this case study). A field failure mode occurs where the drive displays fault code 7 (OL — Overload) immediately on power-up, before any motion command is issued, and is accompanied by a strong burnt-electrical smell. The motor is not running when the fault latches. This combination of symptoms points to a thermal-management failure compounded by aged power-stage components and/or DC-link capacitor degradation, not to a motor or load-side fault.

This reference covers the diagnostic sequence used in the field to isolate the failure to a clogged cooling matrix / inoperative fan, plus the component-level inspection path for the 2nd board (gate-drive / power-stage PCB), decision criteria for repair versus MT2B-to-MT3 upgrade, and parameter/memory handling for the 626VM3 variant used on the same platform family.

2. Drive Identification and System Role

The Varispeed 622MT2 is a member of Yaskawa's MT2B generation of spindle drives. On Matsuura 560V-class machines, the Varispeed MT2B is dedicated to the spindle motor only — the X, Y, and Z axis servos are handled by separate Yaskawa servo amplifiers. Confirming this role up front is critical because symptoms that mimic an overload on a spindle drive (regen events, inrush, brake-transistor stress) are not shared with the axis drives on the same machine.

For official product family documentation and current Yaskawa AC drive literature, refer to the Yaskawa Industrial AC Drives and Yaskawa Drive Manuals pages.

3. Fault Code 7 (OL) — What It Means on the MT2B

The MT2B seven-segment readout maps single digits to specific protection events. Code 7 = OL (Overload). The drive's internal electronic thermal overload (ETOL) integration has crossed the trip threshold. In this case study the trip occurs on power-up with the motor at standstill, so the trigger cannot be from actual motor current or mechanical load. Possible root causes for a Code 7 trip at zero speed:

• Cooling fan not running, heatsink temperature sensor or thermistor reading high → drive models junction temp above OL threshold.
• DC-link electrolytic capacitors with high ESR / leakage creating ripple that biases the CT (current transducer) output.
• Shorted output stage transistor (E90 Fairchild) drawing DC-link current that the CT integrates as "motor current" even with the gate drive inhibited.
• Leakage path on the gate-driver / power-stage PCB creating a phantom CT signal.

4. Root Cause Analysis: Burnt Smell on First Power-Up

The combined signature of OL fault on power-up + burnt electrical smell + motor not rotating is diagnostic of one or more of the following, listed in order of likelihood on a 20+ year-old MT2B in a machine tool environment:

1. Heatsink cooling matrix coated with coolant/oil residue (gunk) and cooling fan inoperative. With the fan stopped, the internal NTC reaches thermal limit in seconds; the OL integrator trips before any motion. The "cooking" smell is the bonding epoxy and conformal coating of the power devices reaching elevated temperature.
2. DC-link electrolytic capacitor bank with high ESR and/or vented/leaking cans. A leaking cap will leave brown residue and a sharp acrid smell. ESR rise causes ripple current to heat the caps, accelerating the failure.
3. 2nd board transistor failures — Fairchild E90 (or equivalent) along the board edge, sometimes accompanied by 2 resistor pairs, 2 diodes, 2 small signal transistors, the JH183 hybrid gate driver, and the large Darlington output pairs in the power section. The 2nd, 3rd, or 4th channel is most commonly affected.
4. Brake chopper / braking resistor failure producing over-voltage on decel — but this is excluded here because the trip occurs on power-up, not decel.

The field case in this study confirmed cause #1: gunked cooling matrix plus failed fan. Cleaning the matrix and replacing the fan restored normal operation with no memory loss issues specific to the MT2B (unlike Yaskawa servo amplifiers of similar vintage, which can lose absolute position data on power-down).

5. Thermal System Failure: Cooling Matrix and Fan

The MT2B uses a finned heatsink with a forced-air fan that pulls cabinet air through the fins. In a machine tool environment the airflow path collects cutting fluid mist, hydraulic oil, and floor dust. Over years, this builds a felt-like mat on the heatsink that can reduce thermal performance by 70–90 percent. Symptoms of thermal-matrix fouling include:

• OL trip after a few minutes of light load that previously ran for hours.
• OL trip immediately on power-up when the fan is failed.
• Visible discoloration (browning) of the heatsink between fins.
• Strong epoxy / phenolic smell on first run after a cold start.

6. Component-Level Failure Points on the 2nd Board

When the thermal cause is eliminated and the OL still returns, the 2nd board (power / gate-drive PCB) becomes the primary suspect. Reference designators and common failure modes observed on MT2B drives:

7. Diagnostic Procedure: Step-by-Step

1. Isolate the drive. Lock out / tag out the machine. Open the cabinet, identify the spindle drive, and verify by catalog number (CIMR-MT2B or 626MT2 / 622MT2 family plate).
2. Visual and olfactory inspection of the heatsink, fan inlet, and 2nd board. Photograph before disassembly. Note any brown residue, domed capacitor vents, or scorch marks.
3. Disconnect the motor leads (U, V, W) at the drive terminal block. Power the drive. If the OL clears, the fault is motor or cable — substitute a known-good motor or megger the cable. If OL persists, proceed to step 4.
4. Verify the fan is rotating. If not, the OL is thermally driven. Replace the fan, clean the matrix, retest.
5. De-energize, discharge the DC link (wait at least 5 minutes; verify < 5 VDC on the DC-link test points with a DMM set to 600 VDC).
6. Inspect 2nd board for the Fairchild E90 row, the resistor pairs, the diodes, the small-signal transistors, the JH183 hybrid, and the Darlington output stage. Use a thermal camera or freeze spray while powering the drive (use a current-limited bench supply if possible) to locate the shorted device.
7. Check the DC-link bank for ESR rise using an ESR meter or by comparison measurement against a known-good capacitor of the same rating. Replace any capacitor with ESR > 2x its rated maximum or with visible leakage.
8. Check the braking chopper and braking resistor for the over-voltage failure path. With drive powered and motor disconnected, command a decel — if DC bus over-voltage appears, the brake IGBT or resistor is open.

8. Repair vs. Replace Decision Matrix

9. MT2B to MT3 Upgrade Considerations

MT3 spindle drives are widely available on the used market and are a common upgrade for the MT2B. Before dropping one in, verify the following:

1. Spindle feedback device. The MT3 uses an encoder for orientation, not a resolver. If the host Matsuura uses a resolver or a Hall-effect "magneto" sensor, the MT3 will not close the orientation loop. The presence of an I-series encoder (e.g., I80) on the spindle is a strong indicator that the machine is encoder-equipped and an MT3 swap is mechanically compatible.
2. Spindle orient encoder resolution must match the MT3's expected PPR. Matsuura typically uses a 2000 or 2500 PPR incremental encoder on the spindle for the MT2B; confirm against the MT3 parameter manual.
3. Mechanical mounting is generally drop-in but verify chassis dimensions and DC-bus connector keying.
4. Parameter transfer — the MT3 has a different parameter numbering scheme. A complete re-parameterization is required using either a backup of the original MT2B parameters (if saved) or the Matsuura parameter sheet for that machine.
5. Field wiring — the control terminal pinout differs slightly; consult the MT3 manual and rewire the enable, fault, run, and analog reference signals per the new terminal map.

For the current-generation Varispeed family and the latest parameter / firmware references, consult the Yaskawa Drive Manuals portal and the Industrial AC Drives product index.

10. Parameter / Memory Handling on the 626VM3 Variant

The 626VM3 is a related Varispeed model used on the same Matsuura platform. Unlike Yaskawa servo amplifiers (Sigma-series and predecessors), the Varispeed VM3 does not retain absolute machine-referenced data; its user parameters are stored in non-volatile memory that survives power-down. Even so, follow the procedure below before any disconnect:

1. Read all parameters (Pn-xxx) into the handheld or HMI backup file.
2. Label all terminal connections to the drive and the motor lead markers (U/V/W).
3. Discharge the DC link, then remove the drive.
4. On reinstallation, restore the parameter file and verify the orientation / home pulse against a known reference.

11. Verification After Repair or Replacement

1. With the motor leads reconnected, power up and confirm the drive idles without OL.
2. Run the spindle at low RPM (100–500) for 5 minutes. Monitor heatsink temperature with an IR thermometer — should stabilize < 60 °C with a clean matrix and a functional fan.
3. Run the spindle through a full speed ramp (0 → max → 0) and check for any OL, OV (over-voltage, code 6 or 8 depending on revision), or OC (over-current, code 5) events during accel and decel.
4. Issue a spindle orient command and verify the spindle stops at the programmed angle with the encoder Z-pulse in tolerance.
5. Run the spindle under cutting load at the rated kW for 15 minutes. Monitor the drive display for any intermittent faults. Re-torque the output terminals and inspect for discoloration one hour later.

12. Field-Commissioning Quick Reference