Checking Servomotor Current on Siemens 840D: MD1719 and MD1708

Overview

On a SINUMERIK 840D (PCU 50) controlling a 6-axis machine, three axes are driven by 1FK7 synchronous servomotors and three axes are driven by proportional hydraulic valves. Field engineers frequently need to confirm the actual motor current during machine operation, both for commissioning acceptance tests and for proving that the drive will not damage a servomotor during high-torque phases such as clamping against a fixed stop. The two machine data words used for this purpose are MD1719 (current absolute actual value, in A RMS) and MD1708 (smoothed current actual value, in %). This reference explains how to read both, how to convert between them, and how to compare them to the nameplate and catalog ratings of a 1FK7105-5AF71-1EA0 motor powered from a SIMODRIVE 611 LT-MODULINT infeed (6SN1123-1AA00-0EA2) and 611 control module (6SN1118-0DJ21-0AA1).

For the official parameter list, refer to the Siemens Industry Online Support portal and the SINUMERIK 840D sl / SIMODRIVE 611 List Manual, which catalogs every drive machine data word (MD1700 – MD1799) and its scaling.

System Configuration and Hardware

The reported installation uses the following Siemens components. All catalog (MLFB) numbers must match the rating plate and the equipment list in the machine documentation before any current reading is trusted.

Drive Machine Data Used for Current Monitoring

Two drive machine data words are used in the field for live current display. Both can be opened on the HMI through the standard Service Display, and both are accessible from the PLC / NCK via the standard axis interface.

The relationship between MD1719 and MD1708 is fixed by the inverter's peak current I_max_inv:

MD1708 [%] = 100 × MD1719 [A] / I_max_inv [A]

For a SIMODRIVE 611 module driving a 1FK7105, the relevant inverter current is the peak phase current the drive can deliver (typically 1.5 – 2 × the rated motor current, but always verify on the rating plate of the 6SN1118-0DJ21 module). The 160 A in the infeed part number is the DC-link limit, not the per-axis phase current.

How to Read the Current on the HMI

1. Select Menu Select → Start-up → Machine Data → Drive MD on the operator panel.
2. Open the axis that drives the 1FK7105-5AF71-1EA0 motor (for example AX3, AX4, or AX5 in a 6-axis configuration).
3. Search for 1719 to display Current absolute actual value in A RMS. Search for 1708 to display Smoothed current actual value in %.
4. Trigger the motion that is suspected of overloading the motor (for example the clamping travel to fixed stop) and read both values simultaneously.
5. Record the steady-state current, the peak during the acceleration phase, and the current after the mechanical stop has been reached.

For a permanent recording, expose MD1719 and MD1708 as analog or numeric displays in the operator screen, or read them cyclically from the PLC into a CSV log. The same values are also available on the SIMODRIVE 611 seven-segment display as the axis number followed by H (Help) and the parameter number when the control module is in diagnostic mode.

Field Reading Example on a 1FK7105-5AF71-1EA0 Axis

Reported field readings from a machine in auto run:

Calculating the inverter peak current implied by the readings:

I_max_inv = 100 × MD1719 / MD1708 = 100 × 63.56 / 53.25 = 119.4 A

This value must be cross-checked with the SIMODRIVE 611 control module rating plate. A reading of 119 A peak in a drive that is rated for, say, 60 A peak would indicate either an incorrect control-module MLFB, a derated drive, or a fault in the current measurement path.

1FK7105-5AF71-1EA0 Motor Specifications

The 1FK7105-5AF71-1EA0 is a high-inertia 1FK7 servomotor. The catalog values from the Siemens product data sheet (see the 1FK7 Configuration Manual on the Siemens support portal) are:

Note: the nameplate on the installed machine in the source thread shows In = 16 A, I0 = 32 A, 23 Nm, which is inconsistent with the catalog data for the same MLFB. Whenever the nameplate disagrees with the catalog, the catalog value for that MLFB takes precedence in the drive commissioning tool; the nameplate value is what the user must verify physically on the motor. Always re-enter the motor data in the SIMODRIVE 611 commissioning software from the nameplate of the actual fitted motor, not from a generic data sheet.

Converting Between Current, Torque, and Power

The motor torque constant relates current and torque linearly in the constant-torque region:

M [Nm] = kT [Nm/A] × I [A RMS]

For the 1FK7105 with kT = 1.50 Nm/A:

Mechanical output power at the shaft is:

P [W] = M [Nm] × ω [rad/s] = M × 2πn / 60

At n = 100 rpm and M = 95 Nm:

P = 95 × 2 × π × 100 / 60 = 994 W ≈ 1.0 kW

At n = 0 rpm (clamping, fixed stop):

P = 0 W (mechanical), but I²R losses in the winding are at their maximum

The thermal model of the 611 control module integrates I² over time, so a high stall current for several seconds is the worst case for winding temperature — which is exactly the situation during travel to fixed stop.

Travel to Fixed Stop and Clamping Torque

Travel to fixed stop (clamping) is a standard SINUMERIK 840D PLC-commanded sequence: the axis moves with a defined velocity (MD37030 or the equivalent in the active function block) against a mechanical stop and then maintains a defined clamping torque (MD37002 / MD1105). The 1FK7 motor and SIMODRIVE 611 drive are well suited to this mode because:

• The drive continuously regulates torque to a fixed setpoint, not position.
• The motor's stall current is internally limited to the inverter peak current.
• The motor's PTC thermistor feeds the I²t model in the drive, which trips to pulse disable before the winding reaches class-F temperature.

When increasing the clamping torque, observe the following field-proven sequence:

1. Record MD1719 with the existing clamping torque.
2. Compute the new torque setpoint, then convert to expected current: I_new = I_old × (M_new / M_old).
3. Confirm that I_new does not exceed the inverter peak current from the 6SN1118-0DJ21 rating plate.
4. Confirm that I_new does not exceed the I0 of the motor by more than the short-time overload factor (typically 2.0 – 2.5 × I0 for S6 duty, 30 s / 10 min).
5. Apply the new clamping torque in MD1105 (or the equivalent machine data for the active release).
6. Monitor MD1719 and MD1708 over a 5 – 10 minute run; the drive's thermal load (the s6 / M_t display) must not reach 100 %.

Thermal Protection: I²t Model and PTC

The 1FK7 motor carries a PTC temperature sensor in the winding. The 6SN1118-0DJ21-0AA1 control module reads the PTC and maintains a software thermal model (load utilization). The load utilization is the quantity that should be observed on the HMI during any sustained torque application. Equivalent diagnostic values for the drive:

The expected I²t load during a clamping cycle on a 1FK7105 with M = 95 Nm and a 10 s hold at 0 rpm is:

P_loss = 3 × I² × R_phase ≈ 3 × 63.56² × R_phase

For R_phase ≈ 0.18 Ω (typical for 1FK7105 stator):

P_loss ≈ 3 × 4040 × 0.18 ≈ 2.18 kW dissipated in the winding during the 10 s hold

Energy in the hold: W = 2.18 kW × 10 s = 21.8 kJ. The thermal mass of the 1FK7105 frame and the natural-cooling surface are sufficient to absorb this energy for a 10 % duty cycle (1 minute clamp every 10 minutes) without reaching the PTC trip threshold. Higher duty cycles or longer hold times will require either forced-air cooling on the motor or a reduction in clamping torque.

Verification Procedure After a Current Reading

1. Confirm the axis is the correct one: cross-check the drive MD axis index with the motor rating plate (1FK7105-5AF71-1EA0).
2. Capture MD1719 and MD1708 at three states: standstill, mid-travel, and full clamping.
3. Compute I_max_inv = 100 × MD1719 / MD1708 and compare with the rating plate of the 6SN1118-0DJ21 control module. The two values must agree within ±10 %.
4. Convert the peak MD1719 into torque using kT = 1.50 Nm/A and confirm the result is below the inverter torque limit (MD1105) and below the I²t alarm threshold for the configured duty cycle.
5. Read the alarm log for 300500 (drive overtemperature) and 300501 (motor overtemperature) over a 30-minute test run.
6. Sign the commissioning record with the captured values and the catalog ratings for traceability.

Field Notes and Common Pitfalls

• MD1719 is phase current RMS, not DC-link current. The DC-link current shown on the 611 infeed module is a different quantity and must not be used for motor thermal load calculation.
• MD1708 is normalized to the inverter peak current, not the motor rated current. A reading of 53 % means 53 % of the inverter's peak rating, which on a 1FK7105 is well above the motor's continuous rating.
• The Service Display update rate is the position-controller cycle, typically 2 – 4 ms on 840D with the default servo cycle. For sub-cycle phenomena use a trace, not the display.
• Always check the firmware of the 6SN1118-0DJ21 control module against the SINUMERIK 840D sl firmware compatibility list before accepting the current reading. Older firmware versions can have different scaling of MD1719.
• If MD1719 reads 0 A while the motor is mechanically loaded, the drive may be in pulse disable (terminal 663 / 65 missing) or the encoder feedback may be lost. Check the alarm log for 300500, 300501, 300502, and 300504.

FAQ