Variable-frequency and soft-start motor control cabinets

Purpose

This is a 0.4 kV board that starts and stops a motor in a controlled way rather than directly: through a frequency converter or through a soft starter. Main circuits — 380 V, 50 Hz; control circuits — ~220 V.

Direct-on-line starting with a contactor is cheaper, but it draws an inrush current several times the rated one and jerks the shaft. Where that is unacceptable — long cables, a weak network, pumps prone to water hammer, fans and conveyors with high inertia — controlled starting is used.

Which one to choose

• A frequency converter — when the speed has to change during operation: to hold pressure or flow, to match the output to demand, to save energy at partial load.
• A soft starter — when the running speed is constant and only the inrush current and the jerk need to go; the device is cheaper and takes less space.

We have built both. Which one to install is decided by your control diagram: we calculate it from the current and power of the motors, the duty and the cable length to the motor.

What it may include — to project

• an incoming device and short-circuit protection;
• a frequency converter with a motor choke, or a soft starter with a bypass;
• motor protection: a motor protection circuit breaker, a thermal relay, voltage and phase sequence monitoring;
• a remote control keypad with a display;
• local / auto selectors, indicator lamps, start and stop buttons;
• run and fault signals to SCADA.

What we have built

Delivered work is described by content — by what actually sits inside the cabinets.

• Variable-frequency starting, 100 A: four auxiliary-supply cabinets, each with a 30 kW frequency converter and a motor choke and an incoming breaker rated at 100 A; IP54 version, weight up to 80 kg.
• Variable-frequency starting for a pumping station: two wall-mounted cabinets of 1100×650×285 mm, up to 200 kg, IP21, a 400 A incomer — built and tested by an electrical laboratory in 2019, together with the control cabinets and boards of the same site.
• Soft starting, 250 A: a cabinet with a 171 A / 90 kW soft starter; paired with it — a direct-on-line cabinet and an incoming cabinet with an automatic transfer switch on a motorized changeover switch.
• Soft starting, 400 A: two cabinets with 200 A / 110 kW soft starters, one of them with a remote control keypad and display; these are the cabinets of the pumping stations of a fuel terminal.

The gallery shows cabinets with a soft starter — those are the ones we photographed at the plant. Our variable-frequency cabinets are evidenced by passports and test reports; we have no photographs of those orders.

LK Energy Group has been manufacturing switchboards at its own plant in Odesa since 2015 (the company dates from 2005). Every cabinet leaves the plant with a passport and a circuit diagram; an acceptance test report is provided on request.

Cabinet content

The permanent part is the incoming device, the controlled-start device, motor protection and the ~220 V control circuits. The rest is set by the project.

Incomer

a circuit breaker or a switch-disconnector with fuses — rated to the motor current;

Controlled start

a frequency converter or a soft starter; in delivered work — a 30 kW converter, soft starters of 171 A / 90 kW and 200 A / 110 kW;

Motor choke

installed with a converter when the cable to the motor is long: it damps the voltage overshoot on the winding;

Motor protection

a motor protection circuit breaker, a thermal relay, voltage and phase sequence relays;

Control circuits

~220 V, with their own protective devices;

To project

a bypass contactor, a remote keypad with a display, mode selectors, lamps and buttons on the door, terminals for external signals.

Why the choke and the bypass are not an extra

A converter and a soft starter are power electronics and they behave differently from a contactor. A long cable to the motor turns the voltage edge into an overshoot on the winding — that is what the motor choke is for. After run-up a soft starter can be shunted by a contactor: the device stops heating and the motor runs directly from the network. Both decisions are made at the questionnaire stage, not after installation.

The control diagram comes from the customer or the design engineer; we build the cabinet to it and issue the as-built circuit diagram together with the product passport.

Main parameters

The table shows the parameters of delivered work; for your project they are selected anew.

Final values are agreed at the questionnaire stage.

Scope of supply

• the cabinet according to the contract and the questionnaire — 1 pc;
• product passport — 1 pc;
• circuit diagram — 1 pc.

An acceptance test report is available on request.

What is selected for the project

• the type of controlled start: a frequency converter or a soft starter;
• the rating of the device by motor current and power, with a margin for the duty;
• the motor choke and the bypass contactor;
• the algorithm: holding pressure or flow, duty sequence and rotation of units;
• door controls and the set of signals sent to SCADA;
• ingress protection, dimensions and enclosure colour — to order.

How to order

For a quotation send the motor data and the duty: power and current, the number of units, what is being controlled (pressure, flow, speed), the cable length to the motor and where the cabinet is installed.

The lead time is quoted together with the price, after the questionnaire: it depends on the set of devices and their availability.

Not intended for operation

• in environments with conductive dust, caustic vapours and gases that destroy metal and insulation;
• in places subject to heavy shaking, vibration and impacts;
• on mobile installations.

Warranty

Product warranty — 2 years (extended to 5 years by agreement); enclosure metalwork — 10 years*. The design service life to write-off is 25 years, provided that components with a shorter service life are replaced.

*Indicative terms and conditions. The exact warranty period is stated in the supply contract.

A cabinet with a frequency converter or a soft starter is a 0.4 kV board that starts the motor in a controlled way: 380 V, 50 Hz in the main circuits, ~220 V in the control circuits. A frequency converter is used when the speed has to change during operation — to hold pressure, flow or output; a soft starter is used when it is enough to remove the inrush current and the jerk on the shaft. In delivered work: four cabinets with a 30 kW converter and a motor choke on a 100 A incomer, IP54, weight up to 80 kg; two wall-mounted variable-frequency cabinets of 1100×650×285 mm, up to 200 kg, IP21, tested by an electrical laboratory in 2019; a cabinet with a 171 A / 90 kW soft starter on a 250 A incomer; two cabinets with 200 A / 110 kW soft starters on a 400 A incomer, one of them with a remote keypad and display. The starting device, the choke and the bypass are selected from your control diagram. Built at our own plant in Odesa; every cabinet ships with a passport and a circuit diagram. Price on request.