Siemens MM420: Soft-Start Control via PLC Analog Input

Application Overview

The Siemens MICROMASTER 420 (MM420) is a general-purpose variable frequency drive covering 0.12 kW to 11 kW in voltage classes of 1AC 200–240 V, 3AC 200–240 V, and 3AC 380–480 V. It is widely used on conveyor fans, small pumps, mixers, and packaging machinery where a simple open-loop V/Hz profile is acceptable. A typical field question is: can the drive be turned on and off with a single PLC analog output that is driven to 10 V for run and 0 V for stop, while still providing controlled soft-start and soft-stop ramps?

The short answer is that the MM420 requires a discrete ON command (a rising edge on a digital input, a fieldbus control bit, or a BOP key) before the drive will modulate the output stage. The frequency setpoint on the analog input alone cannot release the inverter. The standard, safe way to satisfy the questioner's intent is to:

1. Use a PLC digital output (or hard-wired contact) for the ON command, with an N/C E-stop loop in series.
2. Use the PLC analog output (0–10 V) as the speed setpoint.
3. Configure P1120 (ramp-up) and P1121 (ramp-down) so that 0 → 10 V produces a controlled acceleration and deceleration.

Where the application genuinely cannot free a PLC digital output, an auto-restart workaround using flying restart (P1200, P1210, P1211) is available. The workaround is documented in this article together with the safety implications, and the standard digital-input + analog-setpoint configuration is provided as the preferred path.

Prerequisites and Hardware

For the official MM420 Operating Instructions and Parameter List, see the Siemens MM420 documentation entry (ID 1080492) and the MM420 Getting Started guide (ID 38644522). Motor parameter limits and the V/f curve definitions are also in the IEEE 519 harmonic guidance for sites where the drive is the largest non-linear load.

MM420 Terminal Layout and Signal Wiring

Locate the control terminal strip on the front of the MM420 under the lower cover. The standard terminal map is:

Quick Commissioning Workflow

The MM420 organizes its parameter database so that Quick Commissioning (P0010 = 1) walks through the minimum required parameters in a fixed order. After Quick Commissioning is finished, P0010 must be returned to 0 before changes are accepted at run time.

1. Power up the drive. The BOP shows 0.00 Hz with the drive in Ready state.
2. Set P0003 = 3 (Expert access level) so all parameters below are visible.
3. Set P0010 = 1 to enter Quick Commissioning.
4. Enter the motor data block P0304–P0311 from the motor nameplate.
5. Enter ramp and limit data P1080, P1082, P1120, P1121.
6. Enter the control source P0700, the digital input function P0701, and the setpoint source P1000.
7. Set P0010 = 0 to leave Quick Commissioning.
8. Optionally set P0970 = 1 to copy RAM values to EEPROM.

For factory reset, set P0010 = 30 and P0970 = 1; the drive reboots with all parameters at their 50 Hz or 60 Hz defaults per P0100.

Motor Parameter Configuration (P0304–P0311)

The motor data block is the foundation of correct V/Hz operation. Every value is taken from the motor nameplate; do not measure with a meter.

Selection of the right motor power input mode is controlled by P0100: 0 = kW / 50 Hz, 1 = HP / 60 Hz, 2 = kW / 60 Hz. After Quick Commissioning, the drive will auto-calculate stator resistance (P0350) and IGBT compensation parameters during the next run-up if P0340 = 1 is set.

Control Source and Command Logic (P0700, P0701, P1000)

The three parameters below separate the command source from the setpoint source, which is the only way to mix a digital ON command with an analog speed reference.

P1000 is bit-coded; setting 2 selects AIN1 as the main setpoint and disables the MOP (motor potentiometer) on the BOP. The frequency setpoint at terminal 1/2 is scaled by P2000 (default 50 Hz for the EU motor frequency). The relationship between voltage on AIN1 and the commanded frequency is therefore:

f_cmd = (V_AIN1 / 10 V) × P2000 × scale_factor

where scale_factor = 1.0 when P2000 = 50 Hz and the default analog scaling (P0757/P0758/P0759/P0760) is intact. If the PLC is set to 10 V when "run" is desired and 0 V when "stop" is desired, the resulting setpoint toggles between 0 Hz and 50 Hz. The acceleration and deceleration are then governed exclusively by P1120 and P1121.

Frequency Setpoint and Ramp Configuration

The S-ramp rounding parameters (P1130–P1133) are how the MM420 produces a true soft-start: rather than a constant dv/dt, the drive inserts a sinusoidal transition at the beginning and end of the ramp. For a soft-start application the recommendation is to set P1130 = P1131 = P1132 = P1133 = 1.0 s and verify with a stopwatch or STARTER trace that the motor reaches P1082 in approximately P1120 + (P1130 + P1131) seconds.

Analog Input Scaling and Calibration (P0757–P0761)

The default 0–10 V → 0–100 % scaling is acceptable for a 10 V = full-scale setpoint. The MM420 stores the analog input scaling as a two-point linear interpolation, with a deadband in volts. Defaults are:

If a 4–20 mA PLC output is used instead, set P0756 = 3, P0757 = 4.0 mA, P0758 = 0.0 %, P0759 = 20.0 mA, P0760 = 100.0 %. The MM420's internal burden resistor is enabled by the 0/4–20 mA selection of P0756, so no external resistor is required.

Drive Running Status Feedback (P0731)

The MM420 has one programmable relay (terminals 10–11) that can be assigned to over 30 status bits. The two that are most often confused are:

For the soft-start application, set P0731 = 52.2. The relay closes when the inverter is enabled and is producing output voltage; it opens as soon as OFF1 (ramp-down) is initiated, when a fault occurs, or when the line is removed. The relay rating on MM420 is 30 V DC at 5 A or 250 V AC at 2 A, resistive.

Auto-Restart with Flying Restart (P1200, P1210, P1211)

If the application cannot free a PLC digital output and the operator accepts that the drive will start automatically as soon as the analog input exceeds the deadband, the following parameter block implements a so-called analog-as-on/off mode. The mechanism is:

1. P1200 (flying restart) is enabled so the drive can synchronize onto a spinning motor without over-current trip.
2. P1210 (auto restart) is set to 3 to attempt restart after a line dip or fault.
3. P1211 limits the number of restart attempts to prevent endless cycling.
4. P0701 is left at 1 so a manual edge at DIN1 still starts the drive if it has been wired that way.

Safety Requirements and E-Stop Integration

EN 60204-1 and NFPA 79 both require a category 0 or category 1 stop function. The MM420 implements OFF1 (controlled ramp via P1121), OFF2 (inhibit, free coast), and OFF3 (fast stop via P1135). To wire an E-stop:

1. Wire a normally-closed mushroom-button contact in series with the DIN1 wire back to terminal 9.
2. Assign P0702 = 4 so the second digital input (terminal 7) commands OFF3 (fast stop).
3. Assign P0731 = 52.3 on a second relay (use the analog output on terminals 12/13 with a separate interposing relay) so the PLC knows a fault has latched.

The MM420 has no STO (Safe Torque Off) input; if the application is SIL 2 or higher, an upstream contactor with a safety relay in the 24 V supply to the drive is the standard solution. Refer to IEC 60204-1 for the canonical stop categories and ISO 13849-1 for the safety control architecture.

Complete Parameter List (Copy/Paste Cheat Sheet)

Commissioning and Verification Procedure

1. Confirm motor is decoupled or safe to run. Verify rotation direction with a 5-second jog (BOP green key) before coupling load.
2. Apply the ON command (24 V at terminal 6) from the PLC. The drive should enter Run state. Relay at terminals 10/11 closes within 200 ms.
3. Ramp PLC analog output from 0 V to 10 V. Motor speed should track with the S-curve defined by P1120, P1130, and P1131. Verify with a tachometer or STARTER trace that motor reaches 50 Hz in P1120 + 1 s.
4. Step the PLC analog output from 10 V to 0 V. Motor should decelerate via P1121, then drop into the deadband (P0761) and stop. Relay opens.
5. Trip the E-stop (if wired). Drive should execute OFF3 per P1135, and the fault LED on the BOP should illuminate if a fault is configured to latch.
6. Remove ON command, then reapply. Drive should restart on the rising edge; this verifies that P0701 is correctly assigned.
7. Cycle power with the PLC DO off. Drive should NOT start. Then re-apply PLC DO while also ramping AIN1 from 0 → 10 V; the drive should accelerate normally.

The last two steps confirm that a single failure mode (loss of the ON command) cannot start the machine. This is the most important check before allowing the line to operate the driven equipment.

Fault Codes and Troubleshooting Matrix

Edge Cases and Field-Proven Caveats

• Analog cable induced noise. Long analog runs in a VFD cabinet can pick up common-mode noise that lifts the AIN signal above 0 V, causing a small unintended speed command. Use a 0.5 V deadband (P0761) or a shielded twisted pair bonded to PE at the cabinet end only.
• PLC analog output is 4–20 mA, not 0–10 V. Set P0756 = 3. The 4 mA corresponds to 0 % and the 20 mA to 100 %; the MM420 will alarm F0060 if the loop drops below 3.2 mA (wire break) when this mode is selected.
• Power-on ramp with auto-restart. If P1210 = 3 is set and a fault is latched, the drive restarts as soon as the PLC analog output exceeds P0761. This is desirable for the "analog-as-on/off" workaround, but it also means that a brief analog glitch can restart the motor unexpectedly.
• S-ramp over-shoot at low speed. P1130/P1131 add up to 2 s of extra time to the ramp. If the process has a tight time budget, set both to 0.5 s rather than 2.0 s; the soft-start will still be smooth.
• Direction of rotation. MM420 does not have a parameter to reverse the output stage. If the rotation is wrong, swap any two motor phase wires at the drive output (U2, V2, W2).
• Star/delta motor nameplate. P0304 is the line-to-line voltage the drive should output. If the motor is wired in delta, P0304 = nameplate delta voltage; if star, P0304 = line voltage. Mismatch is the #1 cause of nuisance F0001 trips at start.

FAQ