Frequency converter for an electric motor: when needed

Electric motor frequency converter (FC, VFD - variable frequency drive) is a device that smoothly regulates the speed and torque of an asynchronous motor by changing the frequency and supply voltage. For pumps, fans and compressors, installing a frequency converter reduces energy bills by 20–40%, extends the service life of the drive and eliminates water hammer. In this article, Elektromotors engineers tell you when a state of emergency is really needed, which models to choose in Tashkent and how many months it takes to pay for itself.

What is a frequency converter and how does it work

The classic circuit for direct motor starting from the mains gives a starting current of 5–7 × I_nom, impact torque and a fixed rotation speed determined by the mains frequency (50 Hz = 1500 rpm for a 4-pole motor). The performance can only be changed by throttling the output (valve, damper), which releases energy into heat.

The frequency converter operates in three stages:

1. Rectifier - converts 380 V alternating voltage to a constant voltage of about 540 V DC.
2. DC-bus filter - capacitors smooth out ripples.
3. Inverter based on IGBT transistors - generates a three-phase voltage of the required frequency (from 0 to 400 Hz and above) with PWM modulation.

By changing the frequency, we smoothly change the speed of rotation of the shaft. Modern vector VSDs support precise torque control, reverse, DC braking and energy-saving operation.

The main advantages of installing a state of emergency

• Soft start - the starting current is reduced from 5–7 I_nom to 1–1.5 I_nom. The winding, bearings, coupling, and belts are protected. The engine service life increases by 2–3 times. After installing a state of emergency, customers often extend the overhaul interval and are less likely to call for bearing replacement.
• Electricity savings of 20–40% - for centrifugal pumps and fans, power is proportional to the cube of speed. A 20% reduction in speed results in electricity savings of up to 49%.
• Precise control - pressure in the pipeline, air flow, conveyor speed are maintained automatically based on feedback from the sensor.
• Elimination of water hammer in pumping systems - smooth acceleration/braking without sudden changes in pressure.
• Built-in protection - against phase loss, overload, overvoltage, short circuit, jamming. Separate thermal relays are often not needed.
• Noise reduction - the engine and mechanism operate at reduced speeds when not fully loaded.

When a frequency converter is really necessary

Emergency does not pay off everywhere. It makes sense to use if at least one condition is met:

• The drive operates with variable load or flow rate - water supply pumps with pressure regulation, exhaust fans with variable flow rate, boiler room smoke exhausters.
• Requires precise positioning - conveyors, dispensers, winding machines, elevators. On many elevator electric motors in modern buildings in Tashkent, an emergency is required by design.
• Frequent starts - compressors, crushers, mixers. Direct starting 10–20 times per hour destroys the winding.
• High load inertia - centrifuges, flywheels, crane equipment.
• The engine operates in significant underload mode - real power is 30–50% of the nominal. Frequency regulation reduces steel losses.

But for engines with constant load (mills, screws, piston compressors with constant output), a frequency drive will not pay off - a soft start device (SPD) is better, which is 2-3 times cheaper.

Popular PE models on the Tashkent market

The following manufacturers of frequency converters are officially represented in Uzbekistan or supplied through dealers:

For critical industries - water intakes, boiler houses, metallurgy - we recommend Danfoss VLT or ABB ACS580. For general use, the Hyundai N700E works great. Budget Chinese brands (INVT, Veichi) are acceptable in non-critical drives with the possibility of quick replacement.

Economics: how much it costs and how it pays off

The cost of a state of emergency depends on the power, brand and options (braking resistor, EMC filter, IP version). Conditional price range in Tashkent for 2021:

• Hyundai/INVT 7.5 kW - lower market segment.
• Siemens V20 7.5 kW - middle segment.
• Danfoss VLT 22 kW with filter - upper segment.
• ABB ACS880 110 kW - premium segment.

Calculation of payback using a simple example. A 22 kW pump in the water supply of a multi-storey building operates 16 hours a day. When regulated by a valve, the average engine load is about 85%, consumption is ~18.7 kWh/hour. When installing a state of emergency with pressure feedback, the average load drops to 55%, consumption 12.1 kWh/hour. Savings 6.6 kWh × 16 hours × 365 days = 38,500 kWh per year. With a tariff for enterprises, this pays for the frequency device in 14–22 months, and then it works for a plus of 7–10 years.

For fans, the savings are even higher - the cubic dependence of power on speed makes the payback period 9–15 months.

Pitfalls of installing a state of emergency

The introduction of a frequency generator requires taking into account several important points, otherwise you will not get savings, but a burnt-out engine:

1. Winding insulation - The PWM inverter creates voltage pulses with a rise of 50–100 ns. Standard class B insulation in old Soviet engines (AIR, 4A) breaks through in 6–18 months. The solution is to install a modern asynchronous motor marked “inverter duty” or rewind the winding with reinforced insulation wire.
2. Bearings - high-frequency currents through the shaft burn the bearing currents. The solution is conductive lubricants, insulated bearings on the non-drive side, or cable shielding.
3. Cooling at low speeds - the engine’s own fan hardly blows at 30% speed. For long-term operation at low speeds, an independent fan is needed.
4. Motor cable length - a cable longer than 50 m between the drive and the motor requires an output choke or a dV/dt filter.
5. Higher harmonics in the network - the emergency distorts the shape of the current. For enterprises with dozens of frequency generators, AHF filters or active rectifiers (AFE) are installed.

On the existing engine fleet, we recommend that before installing a state of emergency, conduct an audit and, if necessary, a major repair of industrial electric motors with replacement of insulation and bearings. This insures the investment in an expensive converter.

Typical application scenarios in Tashkent

Based on our experience at enterprises in Tashkent and the region, we present three typical cases:

• Vodokanal pumping station, 4 pumps of 55 kW each. Danfoss VLT AQUA unit with pressure regulation. Savings ~32%, payback 14 months. Additionally - reducing the number of water hammers and extending the service life of pipes.
• Garment factory, exhaust ventilation 11 kW × 6 pcs. Hyundai N700E with dust concentration control. Savings 37%, payback 11 months.
• Crane facilities, main lift 22 kW. ABB ACS880 with brake chopper and resistor. Reduced wear of clutches and brake pads, smooth operation, payback for spare parts 18 months.

Conclusion and help from Elektromotors

A frequency converter is a powerful tool for modernizing a drive, but it requires proper selection and integration. An error in the choice of model, cable length or motor insulation class turns savings into additional repair costs. Before purchasing a state of emergency, be sure to audit the drive and update the protective automation.

Do you want to calculate the payback of an emergency for your facility, select a model and prepare the engine for operation with an inverter? Elektromotors engineers in Tashkent will conduct an energy audit, offer the optimal solution and perform a full range of work - from rewinding the winding for inverter duty to commissioning the control cabinet. Contact us.