FANUC Spindle Motor Encoder Type: How to Read It From the Model Number (And Why It Matters)

A few years ago, a machining center in a mold-making shop in Dongguan went down mid-shift with an SP0751 alarm — spindle orientation failure. The maintenance team spent two hours checking cables, swapping out the encoder unit, and rebooting the controller before someone finally looked at the motor nameplate carefully. The replacement encoder they'd installed was an Mi type. The machine needed MZi. Two hours of downtime, a wasted encoder, and the fix took about four minutes once the root cause was understood.

That situation comes up more often than it should. The good news is that the FANUC spindle motor model number tells you everything you need to know about the encoder type before you order a single part — you just have to know where to look.

The Two Encoder Types: Mi and MZi

FANUC αi series spindle motors use one of two built-in encoder types, and the difference between them has a direct impact on what the spindle can do:

Mi Encoder — No Z-Phase Detection

The Mi type outputs speed and direction feedback but does not include a Z-phase (one-rotation) signal. This means the CNC controller cannot determine the absolute angular position of the spindle from the encoder alone. A spindle motor with an Mi encoder cannot perform spindle orientation or rigid tapping directly — it needs an external one-turn signal source, typically a separate magnetic or optical pickup mounted near the spindle nose or a BZi sensor unit wired into the amplifier.

MZi Encoder — With Z-Phase Detection

The MZi type includes the Z-phase output, giving the CNC a precise once-per-revolution reference pulse directly from the built-in encoder. This allows the drive to calculate exact spindle position without any external sensor hardware. MZi-equipped motors support spindle orientation, rigid tapping, and Cs axis contouring directly — assuming the amplifier parameters are set correctly.

Both types look identical from the outside. You cannot tell them apart by looking at the motor body. The nameplate is the only reliable source of truth — unless you have a test rig and an oscilloscope handy.

How to Read the Encoder Type From the Motor Model Number

FANUC spindle motor model numbers follow a consistent format. Take this example from a βiI series motor:

A06B-1447-B110#0202

The section to focus on is the B-suffix: B110. This suffix follows the format Bxyz, and each digit carries specific information about the motor's physical configuration:

In the example above, B110 breaks down as: x=1 (foot mount), y=1 (keyed shaft), z=0 (Mi encoder — no Z-phase). So without looking at anything else, you know this motor cannot do spindle positioning on its own.

If the motor were B113 instead, z=3, which means MZi — Z-phase included, direct positioning supported.

Quick-Reference: What Each z Digit Means

Real Model Number Examples

Here are some common FANUC spindle motors and how to read their encoder type at a glance:

Notice that the motor body and rated power are identical between, say, A06B-1447-B110 and A06B-1447-B113. The only difference is the last digit of the Bxyz suffix — and the presence or absence of Z-phase in the encoder unit inside.

Three Real Cases Where Getting This Wrong Cost Time and Money

Case 1 — Wrong Replacement Encoder, Dongguan Mold Shop

The shop from the introduction was running a horizontal machining center with a FANUC 0i-MD control and an αiI 22/10000 spindle motor (model A06B-1412-B200 — Mi type). The machine had been doing spindle orientation via an external magnetic pickup sensor wired to the SPM amplifier.

When the encoder failed, the maintenance team ordered a replacement unit based on the physical dimensions rather than the motor model number. The unit that arrived was pulled from a B203 motor — an MZi encoder. After installation, the machine powered up fine and the spindle ran, but the orientation cycle produced SP0750 and SP0751 alternately. The SPM parameters (specifically SP034 and SP035, which define the one-rotation signal source) were set for an external sensor, not the built-in Z-phase. The mismatch between parameter config and installed hardware caused the alarms.

Resolution: The correct Mi replacement encoder was sourced and fitted. Total additional downtime: 6 hours. The lesson the maintenance supervisor took away was simple — always match the z digit, not just the mechanical dimensions.

Case 2 — Rigid Tapping Failure After Motor Swap, Jiangsu Contract Machiner

A job shop in Jiangsu ran a batch of threaded aluminum housings on a FANUC 18i-MB controlled VMC. Mid-batch, the spindle motor failed (bearing seizure). A replacement motor was sourced quickly from a used equipment dealer — same frame size, same rated speed, apparently the same model.

After installation, spindle rotation and orientation worked normally. But the first rigid tapping cycle failed at the bottom of the hole — the spindle overshot the retraction reversal point and a tap snapped. The CNC logged SP0749 intermittently during the tapping cycle.

Comparing the original and replacement nameplates side by side revealed the problem: the original was A06B-1463-B144 (MZi, z=4), the replacement was A06B-1463-B140 (Mi, z=0). The replacement lacked the Z-phase signal the rigid tapping cycle depended on for reversal accuracy. The SPM was reading no Z-pulse and was estimating reversal position from speed feedback alone — which isn't accurate enough for tapping.

The shop replaced the motor with a correct B144 unit. Rigid tapping accuracy returned to spec immediately. Cost: one broken tap, one scrapped housing, and about half a day of diagnostics.

Case 3 — Cs Axis Contouring on a Turning Center, Zhejiang Automotive Supplier

A turning center at an automotive fastener plant in Zhejiang was being upgraded to add Cs axis (spindle contouring) capability for milling off-center features. The FANUC 31i-B control supported this, and the amplifier was already a suitable SPM model.

The site engineer checked the spindle motor model: A06B-1412-B203. z=3, so MZi — Z-phase present, theoretically capable of Cs axis. However, after enabling the Cs axis parameters (SP000 bit settings and axis assignment), the control faulted during the first contouring move with AL-24 (serial encoder communication error).

After some investigation, it turned out the encoder cable — specifically the shielded cable from the motor connector to the SPM — had been rerouted during a previous cabinet renovation and was now running parallel to a 400V power cable for about 1.5 meters without separation. The Mi/MZi distinction was actually fine; the cable routing was killing the serial encoder signal under load. Re-routing the encoder cable with a minimum 100mm separation from power wiring and adding a ferrite core at the amplifier end resolved the fault completely.

Step-by-Step: How to Identify Encoder Type on Any FANUC Spindle Motor

1.    Locate the motor nameplate. It's usually on the non-drive end of the motor body, or on a label applied to the frame. On some older motors it may be partially obscured by a conduit box.

2.    Find the model number in the format A06B-XXXX-Bxyz (followed by a # suffix for options like brake or special shaft).

3.    Isolate the last three digits of the B-suffix — this is your Bxyz block.

4.    Read the z digit (the last one). z=0, 1, or 2 means Mi — no Z-phase, external sensor needed for positioning. z=3, 4, or 5 means MZi — Z-phase built in, direct positioning supported.

5.    Cross-reference with the x and y digits if you need to confirm the mounting style and shaft type (relevant for physical replacement compatibility).

6.    If the nameplate is unreadable (common on motors that have been in service for 15+ years in a coolant-heavy environment), check the SPM amplifier parameter SP034. A value of 0 or 1 indicates the system was configured for an external one-turn sensor (Mi motor). A value of 2 or 3 points to a built-in Z-phase (MZi motor). This is not 100% definitive — someone may have misconfigured it — but it's a useful cross-check.

Ordering a Replacement: What to Specify

When sourcing a replacement encoder unit or a complete replacement motor, the z digit is the most critical matching criterion after the motor frame size and speed rating. A few points to keep in mind:

•       Match z digit exactly. An MZi unit fitted to a machine configured for Mi (external sensor) will cause SP0750/SP0751 unless you reconfigure the SPM parameters. The reverse is also true.

•       The x and y digits affect physical fit (mounting holes, shaft keyway) but not encoder behavior. If you're replacing just the encoder unit inside the motor — not the whole motor — x and y are irrelevant.

•       Encoder part numbers. For αi series: Mi encoder units are typically A860-2020-T301 or similar. MZi units are A860-2020-T321 or A860-2020-T351 depending on motor variant. Always cross-reference the motor model against the Fanuc Spindle Motor αi series parameter manual (B-65280EN) for the exact encoder part number.

•       Used motors: if buying a second-hand spindle motor as a replacement, insist on a photo of the nameplate before purchase. 'Same model, different suffix' mistakes are extremely common in the used CNC parts market.

Encoder-Related Alarm Codes Quick Reference

Frequently Asked Questions

Can I convert a Mi motor to MZi by swapping the encoder unit?

In theory, yes — the encoder unit is a separate component inside the motor. In practice, it depends on whether the motor body has provisions for the MZi encoder mounting and whether the correct encoder cable assembly is available. It's generally easier and more reliable to source a motor with the correct encoder type from the outset, rather than attempting an internal encoder swap in the field.

My machine does rigid tapping fine. How do I know if it's using the built-in Z-phase or an external sensor?

Check SPM parameter SP034 (one-rotation signal selection). If it's set to 0 or 1, the system is using an external sensor — look for a magnetic pickup or BZi sensor somewhere near the spindle. If it's set to 2 or 3, it's reading Z-phase directly from the built-in encoder. You can also monitor the real-time SPM signals through the FANUC diagnostic screens (Diagnosis 0450~0460 range on most controls) to see whether the Z-phase is being received.

The motor model has a suffix like #0202 after the Bxyz part. What does that mean?

The # suffix denotes factory-set options: brake specification, special winding, modified shaft, or other configuration options. For encoder type identification purposes, you only need the Bxyz block. The # suffix doesn't affect encoder type.

What's the difference between βiI and αiI series motors in terms of encoder options?

Both series use Mi and MZi encoder types with the same Bxyz naming convention. The main difference is that βiI series motors are the lower-cost economy line, while αiI and αiIT are the higher-performance options with better thermal ratings and higher maximum speeds. Encoder identification works the same way across all three series.

Where can I find the encoder part number for my specific motor model?

The definitive reference is the FANUC AC Spindle Motor αi series Parameter Manual (document B-65280EN). Chapter 2 lists compatible encoder units for each motor model. Alternatively, contact us with your motor model number — we can look it up and confirm the correct encoder part number and whether stock is available.

Bottom Line

Reading the encoder type off a FANUC spindle motor nameplate takes about ten seconds once you know the pattern. The z digit at the end of the Bxyz suffix is the number to focus on: 0, 1, or 2 means Mi (no Z-phase, needs external sensor for positioning); 3, 4, or 5 means MZi (Z-phase built in, direct positioning ready).

It sounds like a small detail, but as the cases above show, getting it wrong can mean hours of downtime, wasted parts, and some genuinely frustrating fault-chasing. The nameplate is there for a reason — five seconds reading it properly is worth a lot.

If you need a replacement spindle motor encoder or a complete motor unit and you're not sure which variant you need, send us the full motor model number including the Bxyz suffix and we'll confirm the right part before anything ships.

A860-2020-T301 Fanuc encoder working properly

Its motor working perfectly! A06B-0032-B575

FANUC systems commonly use two types of encoders built into their spindle motors: One type lacks Z-phase detection. Spindle motors using this type cannot directly perform spindle positioning; an external one-turn signal sensor is required for this function.

The other type has Z-phase detection. Spindle motors using this type can directly perform spindle positioning. Both types are commonly used in practical applications.

When performing debugging or repair testing on FANUC spindle motors, is there a way to quickly determine which type of encoder is being used?

Experienced personnel can quickly identify the encoder type using the nameplate on the FANUC spindle motor. For example, the βiI 12/10000 motor shown in the image below has the model number A06B-1447-B110#0202. The last digit of B110 in the model number indicates the encoder type; this encoder does not have Z-phase detection.

FANUC spindle motor specifications follow a naming convention. The last few digits of the model number, Bxyz, have special meanings, as shown in the image below. The last digit z corresponds to the encoder type. When z=0,1,2, the encoder type is Mi, meaning it does not have Z-phase detection. When z=3,4,5, the encoder type is MZi, meaning it has Z-phase detection.

The x and y digits in the current specification also represent the installation method and the type of output shaft, respectively. Refer to the image below for confirmation.