Siemens Servomotor Encoder Alignment: Fixing Motor Overheating

Overview

Servomotor overheating after a service intervention is one of the most diagnostic-heavy issues in industrial motion control. A Siemens brushless AC servomotor exhibiting a 90°C frame temperature after a brake-only repair almost always points to one of three causes: degraded winding insulation, increased friction in the bearing or brake assembly, or commutation phasing error between the rotor magnet axis and the position feedback device (encoder or resolver). Because the source case includes a verified 4,000 MΩ winding-to-ground insulation and matched phase resistances, electrical winding health is largely ruled out. The remaining prime suspect is commutation angle.

This article is written for repair-shop technicians and maintenance engineers who handle Siemens 1FK, 1FT, and 1PH-series brushless servomotors and who need a defensible procedure to either confirm that the existing encoder is the root cause of the heating, or hand the motor off to an authorized service center.

The Physics of Commutation Phasing

Brushless servomotors are synchronous machines. The rotor carries permanent magnets; the stator carries a multi-phase winding. To produce torque, the controller must energize each stator phase at the precise electrical angle where the rotor flux linkage is rising (or falling) most rapidly. The angle between the commanded current vector and the actual rotor magnet axis is the commutation error angle, ε.

The torque produced per unit of stator current follows the cosine of that error:

T / I = kT · cos(ε)

where kT is the motor's torque constant in Nm/A. When ε = 0, the controller commands exactly the right field orientation and cos(0) = 1.000. Any positive ε reduces the effective torque-per-amp ratio, and the servo drive compensates by demanding more current for any given load.

Commutation error ε	cos(ε)	Current multiplier 1/cos(ε)	Effect on I²R losses	Effect on winding temperature
0° electrical	1.000	1.00×	Baseline	Nominal
15°	0.966	1.035×	+7% (I² rises 7%)	~3°C rise
30°	0.866	1.155×	+33%	~12°C rise
45°	0.707	1.414×	+100%	~30°C rise
60°	0.500	2.000×	+300%	catastrophic
75°	0.259	3.864×	+1393%	insulation failure
90°	0.000	undefined (sign reversal)	net negative torque	positive feedback runaway

At 60° electrical commutation error, the drive must source 2× the rated current to deliver rated torque. Because stator copper losses are I²R, losses increase by 300%. A motor that nominally runs at 60°C can climb to 110°C with a 60° encoder offset, well past the Class F (155°C) or Class H (180°C) winding insulation limits over time. This is exactly the regime the source motor is entering at 90°C frame temperature.

A subtler but more dangerous regime begins at ε > 90°. Cosine becomes negative, meaning the controller now commands current in the wrong polarity to "pull" the rotor to where it thinks the rotor should be. The drive does not know the rotor has already passed the commanded electrical angle, so it adds more current in the wrong direction, accelerating the rotor past the target. The result is positive feedback, where each correction makes the error worse. Field experience, as documented by Curt Wilson of Delta Tau Data Systems, describes incidents where such runaways could have caused serious injury. Any encoder replacement without a verified phasing procedure is therefore not merely a thermal risk but a personnel safety risk.

Encoder vs Resolver: Which Feedback Device Is Installed?

Before any diagnosis, confirm the feedback type. Siemens servomotors ship with several incompatible feedback families:

• Optical incremental encoders (ERN 120, ERN 180, ERN 1387) with sin/cos 1 Vpp signals and a reference mark
• Absolute singleturn encoders using EnDat 2.1 or EnDat 2.2 protocol (EQN 1325, ECN 1313, EQN 425, EQI 1329)
• Absolute multiturn encoders using EnDat 2.2 (EQN 1337)
• Hiperface encoders (SRS 50, SRM 50, SKS 36, SKM 36) used on Siemens and compatible third-party drives
• DRIVE-CLiQ encoders with integrated electronic rating plate, used on SINAMICS S120 systems with 1FK7 G2, 1FT7, and 1PH8 motors
• Resolvers on 1FK7 or 1PH4 with the resolver option

The behaviors of these devices under replacement differ profoundly. A resolver is an analog rotary transformer. Its output, two sine waves in quadrature, is mechanically determined by the shape of the rotor's coupling teeth. When the resolver is reassembled, it self-aligns to the magnetic axis of the rotor because the resolver stator and the motor stator share the same shaft and housing reference. No phasing procedure is required: the drive reads the resolver position and interprets it as electrical angle directly. This is what the field report refers to when it says the resolver "is automatically recognized by the machine."

An optical encoder, by contrast, requires the encoder's index pulse (or absolute position datum) to be mechanically aligned to a known rotor angle at the time of assembly. That alignment is fixed by the encoder's mounting screws, the shoulder on the encoder shaft, and critically the orientation of the encoder housing relative to the motor housing. If the encoder is installed 30° off in its mounting flange, the controller will command current that is 30° away from the rotor's actual magnetic axis, with the heating consequences detailed above.

Why DIY Encoder Replacement Is a Forbidden Practice on Most Servomotors

Rockwell Automation maintains an explicit knowledge-base answer for the equivalent question on Allen-Bradley MP-Series servo motors: encoder or resolver replacement is not a user-serviceable procedure. The official response directs customers to a local Rockwell Automation representative for factory-authorized repair. The same philosophy applies in principle to Siemens servomotors: the 1FK, 1FT, and 1PH families are matched at the factory to specific drive firmware using a stored electronic rating plate. Replacing the encoder disc, the code disc, the bearing, or the resolver rotor without subsequent factory-grade phasing will void any warranty and may produce the heating or runaway behavior the source describes. See the Rockwell Automation knowledge base article 52453 for the parallel Allen-Bradley policy.

The reasons this practice is forbidden by every major manufacturer are:

1. Encoder-to-rotor mechanical reference. The encoder's zero-position index must be aligned to a known magnetic axis of the rotor. This is a sub-degree tolerance operation that requires either a factory phasing fixture that injects controlled DC into two phases while monitoring the encoder position, or a controlled-rotation indexing procedure that compares back-EMF to encoder output.
2. Electronic rating plate. Modern Siemens motors (1FK7 G2, 1FT7, 1PH8) carry motor parameters in the encoder's non-volatile memory. Replacing the encoder without re-loading these parameters causes the drive to operate the motor with the wrong torque constant, wrong pole count, wrong thermal model, and wrong current limits.
3. DRIVE-CLiQ handshake. On SINAMICS S120 systems, the encoder carries a DRIVE-CLiQ identity. A mismatched encoder logs fault F30035 (DRIVE-CLiQ communication error) and fault F07900 (Drive: motor blocked) on the first run, and may be followed by F30021 (Hardware fault on encoder module) on subsequent attempts.
4. Mechanical runout. The encoder disc must run concentric to the shaft to within 10 µm or better. Field assembly on a workshop bench without a clean-room environment introduces contamination that shortens encoder life from 100,000 h to a few hundred hours.

Why the Source Motor Is Likely Heating from Encoder Phasing

The source case has several diagnostic indicators pointing away from winding problems and toward feedback phasing:

1. Winding insulation resistance of 4,000 MΩ measured phase-to-ground and phase-to-phase is well above the IEEE 43 / IEC 60034-27-1 minimum of 100 MΩ for a stator in service. Insulation is not degrading from the heat.
2. Matched phase resistances (the source notes the coils have the same resistance) rule out turn-to-turn shorts that would have driven localized hot spots.
3. The brake was rewound, not the stator. The stator has not been disturbed.
4. The customer has on hand a second, smaller motor of the same encoder family that is "working," implying the encoder electronics themselves are likely functional, and the problem is the encoder's mechanical orientation to the rotor.
5. The frame temperature has settled at 90°C, which is consistent with a fixed-angle commutation offset. A shorted turn would produce a hot spot at the failure location, not uniform frame heating.
6. A 30° electrical error produces ~33% additional I²R losses; this is consistent with a measured frame temperature rise of ~30°C above ambient, exactly the regime the source describes.

Field-Procedure for Diagnosing the Encoder Phasing

Before committing to encoder replacement, perform these diagnostic checks with the motor coupled to its drive and the brake released.

Step 1: Capture drive-side diagnostics

1. Read the drive's following error (r0061 in SINAMICS, d21 in SIMODRIVE 611).
2. Read the actual torque utilization (r0080 as % of rated).
3. Read the active current (r0078) and compare to rated current (p0305).
4. Read the encoder diagnostics page. For EnDat: read position-1 raw (r0481) and compare to position-2 raw (r0482). If r0482 is drifting against r0481 by more than 0.1° mechanical, the encoder itself is suspect.
5. Check the drive fault log for F31110 (encoder 1 communication error), F31111 (encoder 1 internal error), F07902 (motor model error), or F07935 (encoderless operation activated due to encoder failure).

Step 2: Back-EMF phasing check

With the motor uncoupled and the drive disabled:

1. Spin the rotor by hand at a known slow rate (use a known reference like a 1-rev-per-second count).
2. Use a scope on two phases (e.g., U and V) and capture the back-EMF zero-crossings.
3. Compare the mechanical angle at which each zero-crossing occurs against the encoder's reported position at that instant.
4. If the reported angle leads or lags the back-EMF zero-crossing by more than ±2° electrical, the encoder phasing is wrong.

Step 3: Static torque test

Command a small locked-rotor torque from the drive (5% of rated) and measure the resulting mechanical position. With ε = 0, the rotor should hold position with zero oscillation. With ε > 30°, the rotor will hunt and oscillate at a low frequency and the drive current will climb.

What an Authorized Siemens Repair Center Does Differently

Factory-authorized service centers perform the following procedure during any encoder replacement:

1. Apply controlled DC to two of the three motor phases to lock the rotor at a defined electrical angle.
2. Install the new encoder disc and reference-mark sensor.
3. Adjust the encoder's mechanical position until its index pulse is aligned to the locked-rotor angle.
4. Tighten the encoder mounting hardware to the torque specified in the motor's service manual.
5. Re-load the electronic rating plate data from the Siemens support database to the new encoder.
6. Run a factory commissioning procedure in the drive (p0010 = 5 motor identification, then p0010 = 0 ready) so the drive re-learns the commutation offset.
7. Log the new commutation angle offset (p0431 in SINAMICS S120) for future reference.

These seven steps cannot be performed with hand tools in a workshop. The p0431 offset, in particular, is the parameter that the drive uses to correct for any residual mechanical error, and it must be set to a known value, either zero (if the mechanical alignment is perfect) or the measured offset.

Recommended Course of Action for the Source Case

Based on the diagnostic indicators above, the responsible recommendation to the repair shop is:

1. Do not attempt to swap the encoder between the two motors in the field. The field report rightly observes that doing so "without knowing how to align it" is unsafe.
2. Ship both motors to a Siemens-authorized service center. The center will verify the encoder signal integrity on each motor, compare the position of the new encoder's index pulse to the rotor's magnetic axis, and commission the motor on a Siemens test bench.
3. While the motor is at the service center, request a stator surge test and a winding partial-discharge test. This rules out any latent insulation damage from the original brake failure (brake failures often coincide with the winding being subjected to mechanical shock).
4. After the motor returns, log the new p0431 offset value in the maintenance record and verify that the drive's following error (r0061) is within 0.5° electrical at all speeds.

Specifications and Reference Data

Parameter	Source / Limit	Notes
Winding insulation resistance, minimum	100 MΩ phase-to-ground	IEEE 43 / IEC 60034-27-1; source case measured 4,000 MΩ
Winding resistance balance, max deviation	1% between phases	Source case: matched
Encoder indexing accuracy, mechanical	±0.1° mechanical typical	Sub-degree alignment needed
Commutation angle error tolerance	±2° electrical	Beyond this, drive enters thermal alarm
Frame temperature, Siemens 1FK7 rated	Class F 155°C winding, ~110°C frame	Source case at 90°C frame = approaching limit
DRIVE-CLiQ encoder fault F30035	Communication error	Raised if encoder identity mismatch
DRIVE-CLiQ encoder fault F30021	Hardware fault	Raised on subsequent attempts
Drive fault F07900	Motor blocked / commutation failure	Raised on first run with wrong phasing
Drive fault F31110 / F31111	Encoder 1 communication / internal error	Raised if encoder cable or memory is bad
Commutation offset parameter	p0431	Logged in SINAMICS S120 after any encoder service

Common Pitfalls and Edge Cases

• A motor that "works" after a brake repair but heats uniformly almost always has a commutation offset. Do not be misled by the fact that the motor turns and follows commands.
• A drive that suppresses fault F07902 (motor model error) automatically after some seconds can mask the wrong phasing. Always check r0080 (torque utilization) versus r0078 (active current); if utilization is high but current is even higher, the cosine loss is occurring.
• Replacing only the brake should not move the rotor relative to the encoder, because the brake sits on the non-drive end of the shaft and the encoder sits on the drive end. The only way the encoder-to-rotor phasing can change during a brake repair is if the encoder was removed for any reason and reinstalled with a different angular position. Verify with the repair shop whether the encoder was disturbed.
• Siemens motors with Hiperface encoders use the SRS/SRM families. These encoders have a programmable electronic label; re-using an SRS 50 from one motor on another will report the wrong motor ID to the drive and trigger a topology check failure on SINAMICS S120.
• Multiturn EnDat encoders (EQN 1337) retain position across power cycles. A used encoder from a "second, working" motor will report its own absolute position, not the position of the motor it is installed on, until the drive runs a factory reset.
• A consumer-grade magnetic encoder such as the ZFHOBBY 8.4 V brushless motor referenced in retail listings is not interchangeable with industrial Siemens servomotor feedback. The protocol, supply voltage, resolution, and mechanical stack-up are completely different.

Safety Considerations

A brushless servomotor with a commutation error greater than 90° will produce a positive-feedback torque runaway if commanded to hold position. Under such conditions, the rotor will accelerate to mechanical overspeed, which can:

1. Destroy the encoder mechanically (encoder disc shatter)
2. Shear the rotor magnets from the shaft
3. Drive the load to mechanical destruction
4. Throw debris at high velocity

Always bench-test a motor after any encoder or rotor service, with the rotor uncoupled from the load, behind a protective barrier, with a current-limited bench supply, and with the drive in torque-limited commissioning mode. Never apply full rated current to a motor with unknown commutation phasing.

Verification Checklist Before Returning Motor to Service

1. Insulation resistance phase-to-ground ≥ 100 MΩ (target ≥ 1,000 MΩ) at 500 V DC.
2. Phase resistance balance within 1% between U-V, V-W, W-U.
3. Encoder index pulse visible on scope at one pulse per mechanical revolution, width within specification.
4. Drive following error r0061 ≤ 0.5° electrical across the operating speed range.
5. Active current r0078 matches expected load profile, no elevated idle current.
6. Frame temperature stabilizes below 80°C at rated load after 2-hour thermal soak.
7. No faults logged in drive buffer; F07900, F07902, F31110, F30035 all absent.

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