Alarm 39-41 occurs when the control loses the aggregate drive-ready signal: CR_AXES RDY at 100:00/32 turns off. When the trip coincides with spindle acceleration or rapid axis movement, the number that matters is the voltage available during peak current demand—not the unloaded voltage measured before or after the event.
Electrical signature
The reported drive indicators point first to the shared power chain. The X and Y drives retained control voltage but showed Enable off, Drive Ready off, and Under Volts on. The Z drive also showed Control Volt off. A simultaneous undervoltage indication across several axes makes an upstream supply interruption, contactor fault, connection resistance, or control-power dropout more likely than independent failures in all three BDS4 drives.
| Quantity or indication | Reported value or state | Where to read it | Diagnostic meaning |
|---|---|---|---|
| Drive-ready input | Off |
CR_AXES RDY, 100:00/32
|
The control has lost the ready chain; this is the alarm trigger, not necessarily the root cause. |
| X and Y control voltage | On | BDS4 indicators | Control power remained present at the observation time. |
| Z control voltage | Off | BDS4 indicators | Check the Z control-power path in addition to the common bus. |
| Drive undervoltage | On for X, Y, and Z | BDS4 indicators | Prioritize shared supply and contactor checks. |
| Measured DC bus | Approximately 300 VDC | DC bus between drives | A steady reading does not exclude a short dropout. |
| Reported shutdown point | 185 VDC | Bus measurement during the event | Capture the minimum voltage while the spindle or axis accelerates. |
Current, resistance, and timing
Spindle acceleration raises current demand. Any resistance in a contactor pole, terminal, connector, or bus link produces a voltage loss according to Vdrop = I × R. Its heating rises as P = I² × R. A connection that passes light-load checks can therefore overheat and pull the bus below the drive threshold only during acceleration.
This is heat, not logic. A burnt or intermittent contact can preserve apparently normal source or phase-to-phase voltage while failing to deliver voltage under load. The undervoltage event can also be too brief for an ordinary digital meter display; the drive detects it and drops ready before the displayed reading changes visibly. Once ready drops, the control removes enable, so an off enable indicator can be a consequence rather than the initiating fault.
Diagnostic approach comparison
| Approach | What it reveals | Limitation | Use in this case |
|---|---|---|---|
| Steady DC-bus measurement | Gross supply loss or sustained sag | May miss a short acceleration-related dropout | Baseline only |
| Voltage across energized contactor contacts | Resistance in an individual closed contact | Must be measured while current is flowing | Primary test for M11 and M20
|
| Minimum capture or suitable recorder | Lowest bus voltage during the fault | Requires correctly rated instrumentation and safe connections | Best confirmation of a transient crossing 185 VDC |
| Thermal comparison | A contact or terminal hotter than adjacent equivalents | Shows heating but not the exact dropout magnitude | Corroborates a resistive connection |
| Cleaning contacts or terminals | May temporarily reduce surface resistance | Does not restore a worn or burnt contact | Diagnostic clue, not a durable repair |
Start with dynamic voltage-drop measurements across the spindle contactor M11 and Kollmorgen supply contactor M20. Each closed contact should show less than about one or two volts from its line terminal to corresponding load terminal. A significant drop on one of the three paths identifies a resistive or intermittent contact even when other voltage readings appear normal.
Dynamic dropout procedure
- Record every BDS4 indicator immediately after
39-41, before cycling power or resetting the alarm. Separate the initiatingUnder Voltsindication from the resulting loss ofEnableandDrive Ready. - With power isolated, inspect
M11,M20, the inter-drive DC bus, control-power wiring, and associated terminals for discoloration, loose hardware, damaged insulation, or overheated connectors. Follow the machine’s energy-isolation procedure because the measured bus is approximately 300 VDC and can remain hazardous after input power is removed. - Connect appropriately rated instruments before re-energizing. Do not strike energized contactors or reposition exposed test leads while the machine is operating.
- Measure from line to load across each closed contact of
M11, thenM20. Reproduce the load by commanding the spindle acceleration that normally triggers the alarm. A reading below about one or two volts is the expected closed-contact result; a significant increase isolates the defective path. - Capture the DC-bus minimum during the same operation. If the bus crosses the reported 185 VDC shutdown point as drive ready disappears, trace upstream through
M20, its supply, and the bus connections. - If spindle acceleration does not reproduce the event, use the rapid Y-axis handwheel cycling that previously duplicated it. This separates a shared drive-supply problem from a fault limited to the spindle command sequence.
- Compare contact and terminal temperatures only after a controlled load cycle. One connection running hotter than equivalent adjacent connections supports the voltage-drop finding.
Fault isolation and correction
Replace a contactor when a specific energized contact shows abnormal voltage drop, intermittent conduction, burning, or concentrated heating. Tighten or repair a terminal only after isolating power and identifying the connection as the resistance point. Cleaning that briefly changes the symptom confirms contact sensitivity but leaves worn contact material as a recurring failure source.
If M11 drops voltage while M20 and the DC bus remain stable, correct the spindle-contactor path. If M20 drops voltage and all drives indicate undervoltage, correct the common Kollmorgen supply path. If the shared bus remains above the shutdown point but the Z drive alone loses control voltage, trace the Z control-power feed and connectors. Suspect an individual BDS4 drive only after its incoming bus, control voltage, enable command, and wiring remain stable while that drive independently removes ready.
Verification and recurring pitfalls
- Repeat the spindle-start cycle at the operating condition that produced the alarm.
- Repeat rapid Y-axis cycling if it was a second reproducible trigger.
- Confirm that
Control Volt,Enable, andDrive Readyremain on and thatUnder Voltsremains off on all three drives. - Verify that
CR_AXES RDYat100:00/32stays on throughout acceleration. - Record the minimum DC-bus voltage and the line-to-load drop across every tested
M11andM20contact.
Avoid clearing the alarm before recording the indicators, relying only on an unloaded bus reading, or replacing a BDS4 drive before testing the shared supply. Also avoid treating normal phase-to-phase voltage as proof that every closed contact can carry acceleration current.
FAQ
What happens if the DC bus reads 300 VDC but Alarm 39-41 continues?
The meter may be missing a short sag. Capture the minimum bus voltage during spindle acceleration or rapid Y-axis cycling and correlate it with the loss of 100:00/32.
What happens if one energized contactor path drops more than one or two volts?
That path has excessive resistance or intermittent contact. Isolate power, inspect its terminals, and replace a burnt or worn M11 or M20 contactor rather than treating cleaning as the final repair.
What happens if M11, M20, and the DC bus all test correctly?
Stop when the test requires exposed live work beyond the facility’s qualified-person procedures or when the alarm cannot be reproduced safely. Document the LED states, bus minimum, contact voltage drops, and state of CR_AXES RDY. Escalate those results to the machine manufacturer or Kollmorgen’s official support channel for drive-level diagnosis.