basic electric circuit of stationry crusher plant

Basic Electric Circuit of Stationary Crusher Plant

This article outlines the fundamental electrical architecture of a stationary crushing plant, focusing on the power distribution from the incoming utility supply down to individual motor control centers (MCCs). It explains the role of the main switchgear, transformers, variable frequency drives (VFDs), and the interlocking logic that ensures safe sequential start-up and shutdown of crushers, screens, and conveyors. The content is based on standard industrial practices (e.g., IEC 60204-1 and NFPA 70E) and typical single-line diagrams used in aggregate processing.

---

1. Power Distribution Hierarchy

A stationary crusher plant (typically 200–1500 kW installed load) follows a radial distribution system. The basic circuit begins at the point of common coupling (PCC) with the utility grid, usually at 11 kV or 33 kV. A step-down transformer (e.g., 11 kV / 0.4 kV) feeds the main low-voltage switchboard.

Voltage Level	Typical Equipment	Function
HV (11 kV / 33 kV)	Incoming feeder, HV breaker, transformer	Utility connection, isolation, voltage step-down
LV (400 V / 690 V)	Main ACB (air circuit breaker), busbar, MCCs	Distribution to motor feeders, lighting, control
Control (24 V DC / 110 V AC)	PLC, relays, pushbuttons	Interlocking, monitoring, emergency stop

The main LV switchboard contains an incoming air circuit breaker (ACB) with overcurrent and earth-fault protection. From the busbar, individual feeder breakers supply motor control centers (MCCs). Each MCC has a combination of:

• Molded case circuit breakers (MCCBs) for short-circuit protection.
• Contactors for motor starting (DOL or star-delta).
• Overload relays (thermal or electronic) for motor protection.
• VFDs for variable-speed conveyors (e.g., belt feeders).

2. Motor Starting and Protection

Crusher motors (e.g., 250 kW jaw crusher) draw high inrush current (6–8× FLA). To limit voltage dips on the shared busbar, the following starting methods are common:

• Direct-on-line (DOL) – only for motors < 30 kW or where the grid is strong.
• Star-delta – reduces starting current to ~33% of DOL, used for 30–150 kW motors.
• VFD – provides soft start (0–50 Hz ramp) and torque control, used for conveyors and sometimes for cone crushers to adjust throughput.

Protection coordination is critical. The feeder breaker’s instantaneous trip setting must be above the motor’s starting current but below the cable’s short-circuit withstand. Overload relays are set to 1.05–1.15× FLA with class 10 or 20 trip curves.

3. Interlocking and Sequential Control

A stationary plant cannot start all motors simultaneously. The basic circuit includes a hardwired interlock (via auxiliary contacts) and a PLC-based sequence:

1. Start order: Dust collector → discharge conveyor → screen → crusher → feed conveyor → feeder.
2. Stop order: Reverse of start order, with time delays (e.g., 30 s) to clear material.

The electrical circuit uses:

• Permissive contacts: A downstream motor’s contactor auxiliary contact is wired in series with the upstream motor’s start circuit. If the downstream motor trips, the upstream motor stops automatically.
• Emergency stop (E-stop) circuit: A series loop of normally-closed (NC) pushbuttons across the plant. Opening the loop de-energizes the main control relay, which drops out all motor contactors via their holding coils.

4. Grounding and Safety

• Protective earth (PE): All motor frames, cable trays, and MCC enclosures are bonded to the main earth grid (max resistance < 1 Ω).
• Residual current devices (RCDs): Used for lighting and socket outlets (30 mA), not for motor circuits (to avoid nuisance tripping).
• Surge protection: Type 2 surge arresters on the LV busbar protect PLCs and VFDs from lightning-induced transients.

5. Real Case Example – 250 t/h Granite Crushing Plant

Location: Quarry in Karnataka, India
Configuration: Jaw crusher (150 kW) + cone crusher (90 kW) + 2 screens (15 kW each) + 5 conveyors (7.5–30 kW)
Electrical design:

• Incoming supply: 11 kV, 500 kVA transformer.
• Main LV switchboard: 400 V, 800 A ACB with earth-fault relay (50/51N).
• MCCs: 6 feeders, each with MCCB + contactor + overload relay.
• Cone crusher uses a VFD (90 kW) to adjust closed-side setting via motor speed (not hydraulic only).
• PLC (Siemens S7-1200) with HMI for sequence start/stop and fault logging.

Observed issue: The jaw crusher motor (DOL start) caused a 12% voltage dip, tripping the VFD on the feed conveyor (undervoltage).
Solution: Changed the jaw crusher to star-delta starting (reduced dip to 4%) and added a 5-second delay between the jaw crusher start and the VFD start. The plant now operates without nuisance trips.

6. FAQ

Q1: Why is a star-delta starter preferred over a VFD for a crusher motor?
A1: Crushers have high inertia and require high breakaway torque. A VFD sized for the motor’s full load can handle this, but it is more expensive. Star-delta provides a 33% current reduction at a lower cost, and for fixed-speed crushers, it is sufficient. VFDs are used only when speed adjustment is required (e.g., for feed rate control).

Q2: What happens if the main ACB trips during operation?
A2: All motors lose power immediately. The PLC detects loss of voltage (via a monitoring relay) and resets the sequence. However, the crusher bowl may be full of material. The plant must restart in reverse order (discharge first) to avoid jamming. Manual inspection is required before restart.

Q3: How do you protect a VFD from voltage dips caused by other motors starting?
A3: Three options: (1) Add a line reactor (3–5% impedance) on the VFD input, (2) set the VFD’s undervoltage trip threshold lower (e.g., 85% of nominal), or (3) use a DC bus ride-through module. In practice, changing the starting method of the large motor (as in the case above) is the most cost-effective.

Q4: Is it mandatory to have a separate transformer for the control circuit?
A4: Not mandatory, but recommended. A 24 V DC power supply (SMPS) is common for PLC I/O. For hardwired interlock circuits, a 110 V AC control transformer (isolated from the power bus) prevents nuisance trips due to voltage fluctuations and simplifies troubleshooting.

Q5: What is the minimum cable size for a 150 kW motor at 400 V, 50 Hz?
A5: Based on full load current (~270 A) and a 50 m cable run, a 3-core 95 mm² XLPE copper cable is typical (voltage drop < 3%). Always verify with the cable manufacturer’s data and local electrical code (e.g., IEC 60364-5-52).