SIMOCODE Pro vs DOL/Star-Delta Motor Protection Compared

This technical reference compares the Siemens SIMOCODE pro electronic motor management system against conventional Direct-On-Line (DOL) and Star-Delta starters. It covers the pro S, pro C, and pro V basic-unit family, the protection functions each variant provides, the wiring and diagnostic advantages of an integrated device, and a structured selection matrix for choosing between electronic motor management and classical control gear.

1. Motor Feeder Protection: Two Engineering Approaches

A constant-speed low-voltage motor feeder has three primary jobs: start the motor, run it under controlled conditions, and disconnect it safely on fault. Two fundamentally different implementation strategies exist in industrial practice:

• Classical control gear — a discrete thermal or electronic overload relay, contactor logic, a pneumatic or electronic timer relay (for Star-Delta), and dedicated monitoring relays for phase failure, phase sequence, earth-fault, or PTC thermistor functions. Each function lives on a separate DIN-rail device wired point-to-point to a PLC digital I/O card.
• Electronic motor management — a single intelligent device (SIMOCODE pro) that integrates current and (where applicable) voltage measurement, motor protection, starter control logic, and diagnostics, exposing all status and measured values over PROFIBUS or PROFINET.

The classical approach is the lower-cost baseline and remains appropriate for simple, isolated, or non-networked motor starters. The electronic approach adds hardware consolidation, integrated logic, comprehensive diagnostics, and bus-level data exchange — advantages that scale with the number of motors in a plant and the depth of process monitoring required. The decision is rarely a question of “which is better” and almost always about scope, signal density, and total cost of ownership.

2. DOL Starter: Topology, Components, Limitations

A Direct-On-Line starter is the simplest motor feeder:

Components:

• Main contactor KM1 (3RT series or equivalent)
• Thermal overload relay F1 (bimetallic) or electronic overload (e.g., 3RB22 / 3RB23)
• Control circuit: start pushbutton, stop pushbutton, hold-on contact, indicator lamps, and any auxiliary contact logic
• PLC interface: typically 2–4 digital inputs and 1–2 digital outputs per motor

Limitations. A DOL starter applies full line voltage directly to the motor. The starting current drawn is typically 6–8 × rated full-load current (I_FLC), and the starting torque is 1.5–2.5 × rated torque. For large motors, this causes voltage dip on the supply bus and mechanical shock on the driven load. The classical overload relay offers only single-function (thermal) protection; phase failure, phase sequence, earth-fault, and stall protection require additional relays and additional wiring.

3. Star-Delta Starter: Topology, Timing, Limitations

A Star-Delta starter reduces the starting current to roughly one-third of the DOL value. The motor windings are first connected in star (line voltage divided by √3 across each winding, line current divided by 3) and then reconfigured to delta once a transition criterion is met.

Components:

• Three contactors: KM1 (main), KM2 (star), KM3 (delta)
• Mechanical or electrical interlocking between KM2 and KM3 (a short-circuit between phases if both close simultaneously will destroy both contactors)
• Transition timer T1 (pneumatic, electronic, or PLC-based)
• Thermal overload relay sized for delta connection (full line current)

Quantifying the start. For a motor with rated line current I_FLC and direct-on-line starting current I_DOL = 6 × I_FLC:

Star connection reduces phase voltage by √3 and line current by 3:

I_start_star = I_DOL / 3  =  2 × I_FLC  (typical)
T_start_star = T_DOL / 3  =  0.5 × T_rated (typical)

Limitations.

1. Fixed-time transition — the contactor reconfiguration happens at a preset interval regardless of whether the motor has reached a stable speed. Under load variations, this risks switching from star to delta at a current peak, which produces a second transient inrush.
2. Open transition — there is a brief dead time between KM2 opening and KM3 closing. During this interval, the motor disconnects from the supply, then re-energizes in delta. Closed-transition variants add a fourth contactor and resistors to avoid this gap.
3. Reduced starting torque (33% of DOL) — the motor must accelerate the load in star. High-inertia or high-friction loads may stall.
4. Wiring complexity — six motor leads must be brought to the starter, plus the interlock wiring and timer circuit.

4. SIMOCODE Pro System Architecture

SIMOCODE pro is a modular motor management and protection system from Siemens. The architecture separates the function into three layers, each with its own device family member.

The key architectural departure from a classical starter is that the Basic Unit performs the control logic, protection, and fieldbus interfacing in one device. The contactors become pure power-stage actuators; the wires between them and the PLC disappear in favor of one bus cable carrying dozens of status and measured-value signals in both directions.

The Basic Unit connects to:

• A current measuring module (pass-through or wrap-around CT) that feeds phase currents to the protection engine.
• Digital inputs for local commands (start, stop, fault reset, mode selector).
• Relay or solid-state outputs for the contactor coils.
• Optional expansion modules — additional digital I/O, analog 0/4–20 mA inputs/outputs, temperature modules for PTC / Pt100 / Pt1000 / KTY84, ground-fault modules, and a voltage measuring module for the pro V variant.

For more on the system family, see the official Siemens SIMOCODE product page.

5. SIMOCODE Pro Basic Units: S, C, V Comparison

Three Basic Unit variants exist, each targeting a different application and price point. The differences are summarised below.

Key takeaway: Star-Delta control function is available with SIMOCODE pro S and SIMOCODE pro V. The pro C variant is intentionally limited to direct and reverse starters — if you need Y/D switching with electronic current-based transition, the choice is between pro S (lower cost) and pro V (voltage monitoring + larger function set).

6. Protection Function Matrix

Where a classical starter needs a separate monitoring relay for each protective function, SIMOCODE pro consolidates them. The following table lists the standard protection functions and which device family member typically provides them.

Two functions are particularly valuable for medium- and large-motor feeders:

• Contactor welding detection — SIMOCODE pro checks the auxiliary contact state against the requested command. If a contactor has welded closed, the Basic Unit can refuse to issue the next start command and report the fault to the DCS, preventing a direct short-circuit during a star-to-delta transition.
• Hot/cold start counter — the device tracks the number of starts within a rolling time window, with separate limits for cold and hot conditions. This is critical for motors where too-frequent starting damages the windings, and for processes that require a minimum cool-down interval.

7. Star-Delta Control with SIMOCODE Pro V

The most significant functional advantage of SIMOCODE pro in a Star-Delta feeder is current-based transition.

A classical Star-Delta starter uses a fixed timer. SIMOCODE pro V monitors the motor current during the star phase. Once the current has decayed to a configurable threshold (indicating the motor has accelerated close to its running speed), the Basic Unit commands the transition from star to delta. The result is:

• Shorter star phase on lightly loaded motors — less time spent at reduced torque.
• Longer star phase on heavily loaded motors — the motor has more time to accelerate before the transition, reducing the second inrush transient.
• No second current peak — the delta transition occurs at the lowest instantaneous current, not at a fixed wall-clock time.

Configuration in SIMOCODE ES (TIA Portal):
- Starter type:       Star-Delta
- Transition type:    Current-based (instead of time-based)
- Transition level:   typically 1.1 × I_FLC (motor-specific, tune on site)
- Star timeout:       60 s (backstop to prevent indefinite star mode)

The standard logic templates in SIMOCODE ES — the configuration software — cover direct-on-line, reverse, Star-Delta, Dahlander, pole-changing, soft-starter, and valve-actuator starters. The user selects a template, assigns the I/O, and the engine is generated. The control circuit that would normally be built from contactor interlocks, timer relays, and auxiliary contacts is replaced by the Basic Unit's internal logic — and that logic is editable.

8. Communication, Diagnostics, and Predictive Data

The Basic Unit exposes the following data to the PLC / DCS over PROFIBUS or PROFINET:

This is the operational difference that drives the cost-benefit case for SIMOCODE in plants with many motors. Instead of pulling individual wires for each status bit, the engineering effort is concentrated in a single PROFIBUS or PROFINET drop per feeder.

9. Wiring Reduction and Hardware Consolidation

Quantifying the consolidation, a SIMOCODE pro V motor feeder replaces the following classical components with a single Basic Unit + expansion modules:

The result is fewer DIN-rail devices, fewer terminal blocks, fewer wires, and a single point of configuration. Storage and spare-parts holdings shrink. Wiring errors during commissioning decline because the logic that used to be physical wiring is now a parameter set in software.

10. Sizing, Current Measurement, and Trip Curves

The current measuring module is selected by motor full-load current (I_FLC). Verify the following parameters are configured in SIMOCODE ES before commissioning:

Motor parameters (per nameplate):
- I_FLC [A]              rated full-load current
- U_rated [V]            rated line voltage
- Class                   trip class (5, 10, 15, 20, 25, 30)
- Service factor          1.0 or 1.15
- Phase rotation          CW / CCW (for sequence protection)

Trip class determines how long the simulated thermal image allows an overload condition before tripping. Class 10 is the most common for standard induction motors; Class 20 or 30 is used for high-inertia loads where longer acceleration times are expected.

For a Y/D starter, the overload element in the classical approach must be set to delta current (1/√3 of star current, equal to line current). SIMOCODE handles this automatically once the starter type is set to Star-Delta.

Voltage and current relationships for a three-phase motor in star and delta:

Star connection:
  V_phase = V_LL / sqrt(3)
  I_line  = I_phase
  P       = sqrt(3) * V_LL * I_line * cos(phi)

Delta connection:
  V_phase = V_LL
  I_line  = sqrt(3) * I_phase
  P       = sqrt(3) * V_LL * I_line * cos(phi)

For sizing the upstream feeder apparent power at full load:

S_kVA = sqrt(3) * V_LL [V] * I_line [A] / 1000

If a motor draws 360 A on a 400 V three-phase system, S = √3 × 400 × 360 / 1000 = 249 kVA. The same current value interpreted as single-phase would yield S = 400 × 360 / 1000 = 144 kVA — confirm with the project single-line diagram before sizing upstream breakers or transformers.

11. Application Selection Matrix

Use the matrix below to choose between the four most common configurations.

12. Commissioning and Verification

Field commissioning of a SIMOCODE-equipped feeder should follow a fixed sequence so that the protection, control, and bus communication are all verified before energising the motor.

1. Parameter download. From the engineering station (TIA Portal / SIMOCODE ES), load the project-specific parameter set onto the Basic Unit. Verify firmware version compatibility between the project and the installed device.
2. Direction of rotation test (no motor connected). Issue a star or delta close command and verify the contactor outputs follow the configured logic. Confirm the mechanical / electrical interlock between KM2 (star) and KM3 (delta) is wired — SIMOCODE's internal interlock is in the software, but a hardware interlock is still required for fail-safe operation.
3. Current measurement check. With the motor running at no-load, read the I_L1, I_L2, I_L3 values via PROFIBUS / PROFINET diagnostics. Verify the three values are within ~5 % of each other and consistent with the motor's nameplate no-load current.
4. Phase sequence and voltage check (pro V). Verify the rotation direction matches the configured setting and the line voltage matches the rated value within the configured tolerance (default typically ±10 %).
5. Trip tests. Force a trip condition via SIMOCODE ES (e.g., a simulated overload by raising the set current below the actual load, or a phase failure by opening one fuse). Verify the Basic Unit issues the trip, drives the outputs off, and posts the correct fault code to the PLC.
6. Bus communication verification. From the PLC, read the cyclic status words and confirm the expected bits (running, fault, ready, local/remote) are updating. Trigger a start command from the PLC and verify the motor responds within the configured response time.
7. Start counter and operating hours. After the test cycle, confirm the start counter and operating-hours counter have incremented in the diagnostics page.

For verification of the network side, use the standard PROFINET commissioning tools in TIA Portal to confirm the device is online, has no diagnostic alarms, and is producing the expected I/O data length.