Motor Power:
This is the most important parameter, which directly determines the selection of components in the cabinet (contactor, thermal relay, circuit breaker/fuse).
Typical application range: Usually used for three-phase squirrel cage asynchronous motors between 7.5kW (10HP) and 200kW (250HP). Below 7.5kW, direct starting is usually more economical and simple; above 200kW, soft starter or inverter may be considered.
Unit: kW or HP.
Motor Rated Voltage:
Determines the voltage level of the main circuit and control circuit. Must match the power supply grid voltage.
Common voltages: 380V (commonly used in domestic industry), 400V, 415V, 440V, 480V (commonly used in Europe and the United States), 220V/380V (Δ/Y connection motor), 660V/1140V (for high voltage occasions such as mining).
Key points: The rated voltage and the corresponding winding connection (Δ or Y) must be clearly marked on the motor nameplate.
Motor Rated Current:
After the motor power and voltage are determined, the rated current is calculated (also refer to the motor nameplate).
Importance: Used to accurately select the current specifications of the main contactor, thermal relay, and circuit breaker.
Star starting current: About 1.7 - 2.5 times the rated current (about 1/3 of the direct starting current).
Triangle running current: Equal to the motor rated current.
Starting Mode:
Star-delta step-down starting: This is the core function of the cabinet.
Transition Time: The time to switch from star starting to delta operation. This time is crucial:
Too short: The motor speed has not yet increased to a high enough level (usually reaching 70-80% of the rated speed), and the current impact is still large during switching, which may damage the contactor contacts and the motor.
Too long: The motor runs at low torque for a long time (the torque of star connection is about 1/3 of that of delta connection), and may be blocked and burned due to excessive load.
Setting basis: Usually set by time relay, which needs to be adjusted according to motor power, load inertia, and load characteristics (light load/heavy load), and the typical range is between 5 seconds and 30 seconds. Sometimes current relays are used to detect when the current drops to close to the rated current and switch (better, but costly).
Control Power Supply Voltage (Control Voltage):
The voltage that supplies power to the control circuit (contactor coil, relay, indicator light, etc.) in the cabinet.
Common voltages: AC 110V, 220V, 380V; DC 24V (commonly used for safety voltage).
Selection: Determined according to site safety requirements, PLC/DCS interface requirements, and standardization requirements.
Protection Level (IP Rating - Ingress Protection):
Indicates the cabinet's ability to protect against external solid foreign matter (dust) and liquid (water).
Common levels:
IP20: Only protects against solid foreign matter larger than 12.5mm, no waterproofing. Suitable for clean and dry distribution rooms.
IP54: Dustproof (limited intrusion), splashproof. Suitable for most industrial sites with dust and slight moisture.
IP55: Dustproof, low-pressure water spray. Suitable for use in harsh environments.
IP65: Dust-tight, water-proof. Suitable for outdoor or high-dust, high-moisture environments.
Selection: Determined by the installation environment.
Cabinet Dimensions:
Height (H) x Width (W) x Depth (D).
Determined by the number of internal components, size, heat dissipation requirements and wiring space. The higher the power, the larger the cabinet.
Duty Cycle:
Describes the start frequency and continuous operation time allowed for the control cabinet.
Common:
S1 - Continuous Duty: Can run continuously for a long time.
S3 - Intermittent Cycle Duty: With periodic starts and stops (duty cycle must be specified, such as S3 40%).
Importance: Frequent starts will generate more heat and affect component life. The number of starts needs to be considered when selecting.

Reduced voltage starting:
Core feature. When starting, the motor winding is connected in star (Y) so that the voltage of each phase winding is reduced to 1/√3 (about 58%) of the rated voltage. The starting current and starting torque are both reduced to 1/3 of that during direct starting (Δ connection). Effectively reduce the starting current impact.
Automatic switching:
The process of switching from star starting to delta operation is automatically controlled by a time relay or current relay without manual intervention.
Basic protection functions:
Short circuit protection: It is realized by a main circuit breaker or fuse.
Overload protection: It is realized by a thermal relay or motor protection circuit breaker to prevent the motor from burning due to long-term overcurrent.
Phase loss protection (optional or integrated): It prevents the motor from being damaged by the phase loss of the power supply. Some thermal relays or integrated protectors have this function.
Undervoltage protection (usually provided by contactor characteristics): When the voltage is too low, the contactor automatically releases to prevent low-voltage operation from damaging the motor. Manual reset and start are required after the voltage is restored.
Mechanical interlock: Mechanical and/or electrical interlocks are usually provided between the star contactor (KM1) and the delta contactor (KM2) to absolutely prevent them from being attracted at the same time and causing a power short circuit (this is a fatal fault!).
Control and indication:
Provide a start button and a stop button (sometimes with an emergency stop function).
Status indicator: power indication, operation indication (star or delta), fault indication (overload, phase loss), etc.
Control mode: Local cabinet door operation is the most basic mode, and a remote control contact (start, stop, fault signal, operation signal) interface is usually reserved to facilitate access to the PLC/DCS system.
Structural characteristics:
Modular design (common): The main circuit, control circuit, and protection components are relatively independent, which is convenient for installation, maintenance, and replacement.
Standard cabinet: Usually standard size GGD, MNS, GCS and other cabinets or their derivative forms are used.
Clean wiring: The internal wiring should be clear, neat, clearly marked, and in compliance with electrical specifications.
Heat dissipation: High-power cabinets need to consider the design of cooling fans or vents.
Grounding: There is a complete protective grounding terminal block.
Applicable load:
Ideal load: Equipment that starts with no load or light load, and the starting torque requirement is not high. For example: centrifugal pumps, fans, compressors (part), conveyors (light load), air handling units, etc.
Unsuitable load: Heavy-duty equipment that requires high torque when starting, such as cranes, crushers, ball mills, piston compressors, etc. The 1/3 torque during star starting may not be enough to start the load.