An older Hurco SM1 retrofit presents four motor leads: red, black, white, and blue. The evidence indicates that the large brush assembly is the DC servo armature and the smaller, separate permanent-magnet assembly is a DC tachometer. Treat the second pair as feedback leads unless motor and drive documentation proves otherwise; do not parallel it with the armature.
Identify the Two Electrical Circuits
The red and black leads are described as the expected main DC motor pair. The white and blue leads terminate at separate brushes on a second rotor approximately one-quarter the size of the main rotor. That construction is consistent with a brush-type DC tachometer, which generates velocity feedback for an early voltage-loop servo drive.
| Observed leads | Observed connection | Evidence-supported interpretation | Wiring decision |
|---|---|---|---|
| Red and black | Main brush assembly | Servo armature | Connect only to the drive's motor output after confirming polarity and documentation. |
| White and blue | Small, separate brush and rotor assembly | Probable DC tachometer | Connect only to a compatible tach-feedback input, or isolate if the installed drive does not use tach feedback. |
Do Not Parallel the Four Leads
Do not wire the white and blue leads in parallel with the red and black armature leads or connect both pairs to the same power output. A tachometer is a feedback generator, not a second armature winding. Applying armature power to its leads could damage the tachometer or drive feedback circuit.
The available evidence does not identify the ElectroCraft motor model, Centroid drive model, terminal assignments, or required feedback architecture. Those unknowns prevent assigning white and blue polarity or naming exact drive terminals.
Determine Whether the Retrofit Uses Tach Feedback
- De-energize the machine and trace each motor lead without relying on color alone. Confirm which pair reaches the main brushes and which reaches the small tachometer brushes.
- Identify the installed servo-drive model and determine whether it operates with an inner tachometer feedback loop or in torque/current mode without motor tach feedback.
- If the drive requires tach feedback, connect the tachometer pair only to its designated tach input and establish polarity using the drive and motor documentation.
- If the drive uses encoder-to-controller feedback and requires no tach input, isolate the tach leads individually. The evidence also describes removing the tach brushes as an option during conversion to later torque-mode drives, but only after confirming that the drive does not require them.
Verify the Feedback Architecture Before Motion
Early voltage-loop systems use two nested loops: tachometer feedback closes the inner drive loop, while encoder feedback closes the outer controller loop. These loops require separate tuning. Later torque/current-mode drives may use no tachometer feedback at the drive because the controller receives position feedback from the encoder.
Before enabling motion, verify that the armature pair reaches only the motor-output terminals, the tach pair reaches only a tach input or remains isolated, and the encoder remains connected to the controller as required by the retrofit design. Do not attempt tuning until the drive's control mode and feedback polarity are established.
FAQ
What are the four wires on an older Hurco DC servo motor?
The evidence identifies red and black as the probable main armature leads and white and blue as leads to a separate, smaller brush-type DC tachometer. Confirm the physical connections and motor documentation before wiring.
Can I connect the Hurco servo tach wires in parallel with the motor wires?
No. Treat the white and blue pair as tachometer feedback, not as another powered winding; connect it only to a compatible tach input or isolate it when the drive does not use tach feedback.
Does a Centroid retrofit need the original DC tachometer?
The evidence does not identify the installed drive model or control mode, so this cannot be decided from the manufacturer name alone. Check whether the specific drive uses an inner tach-feedback voltage loop or operates in torque/current mode with encoder feedback handled by the controller.