This Knowledge Centre is intended as a Guide aimed to clarify the main aspects related to the theory and application of frequency inverters, presenting theoretical basics and practical criteria for specific topics, originated from the experience and knowledge of Drive Online engineers and technical bodies in the subjects. The most important and Internationally recognised technical references concerned with such matters are mentioned and recognised in the presentation of this information.

The criteria and content presented here are not permanent. Like every technology, they may change as new equipment is developed and new experiences are Accomplished. So the information within this guide may be changed without notice and therefore it is important that this is only used as a guide. When installing any drive the manufacture guides should be used for more specific technical information and the correct use of their equipment.


Investing in approved energy saving technologies in the UK has a triple benefit. The first is that plant and equipment operate more efficiently and so reduce energy costs. The second is the Enhanced Capital Allowance scheme administered by HMRC allows companies to write off 100% of the cost against that year’s taxable profits. Thirdly, by running plant more efficiently you can reduce your exposure to the Climate Change Levy.

In introducing the 2008 Climate Change Act, the UK Government established the world’s first legally binding climate change target. Its aim is to reduce the UK’s greenhouse gas emissions by at least 80% (from the 1990 baseline) by 2050.


This programme saw the introduction of the Climate Change Levy (CCL) – a tax on all energy such as electricity, gas and coal for use as fuels (that is for lighting, heating and power) by business consumers, including consumers in industry; commerce; agriculture; public administration, and other services.

The CCL is intended to change business behaviour in the UK to reduce energy consumption and/or consider using energy produced from renewable sources such as wind farms, solar energy and hydro power. By reducing energy consumption, companies will minimise the impact of this tax. From 1st April 2014 to 31st March 2015, the CCL on electricity is 0.541 p/kWh and for gas, 0.188 p/kWh.


At the same time, the Government introduced the Enhanced Capital Allowance (ECA) scheme. The ECA scheme offers a 100% First-Year Allowance (FYA) for investments in certain energy saving plant and machinery. If you buy equipment that qualifies, you can write off, for example, 100% of the cost against that year’s taxable profits.

This could save you a lot of money, as well as reduce your business’ energy use, carbon footprint and climate change levy payments.

The ECA energy scheme supports a variety of energy saving technologies, such as energy efficient boilers, lighting, refrigeration equipment, and metering and monitoring systems. This guide explains how the scheme works, what energy saving products qualify, and how to claim an allowance. Qualifying products and technologies are registered on the Energy Technology List (ETL). Only new equipment is eligible for an ECA.


The Energy Technology List is managed by Carbon Trust on behalf of the UK government. It provides information on products that qualify for the government’s Enhanced Capital Allowance (ECA) scheme to allow businesses to write off the entire cost of any green technology against taxable profits.

To find out more about the scheme, browse products on the Energy Technology List, or find out how to get your product approved, visit the Department of Energy and Climate Change’s ETL website.


Motor speed control offers industry the single largest opportunity for saving energy and money.

Most motors run at a fixed speed. By adjusting the speed to more accurately match the requirements of the loads, which generally vary over time, you can enhance the efficiency of motor drive equipment. The potential benefits of speed variation include increased productivity and product quality, less wear and mechanical stress, power factor correction (cos & phi; close to 1), noise reduction along with energy savings of 50% or even more for some types of applications.

One of the most effective ways to save energy is to target your pump and fan applications, firstly because there are so many of them and secondly because the potential energy savings are so great. Fan and pump applications are known as “variable torque” and in a variable torque load the power changes with the cube of the speed, for just a 7% speed reduction (that equates to 3.5Hz on a 50Hz system) the energy saving would be 20%, for a 20% speed reduction (10Hz), the energy saving rises to 50%.

The graph below shows the energy use of a fan system with three different control methods. The energy use of the drive closely follows the demands of the system, because this is the only control method that regulates the output without introducing inefficiencies into the system.

Both the inlet guide vane and the outlet damper control the output by placing a restriction in the way of the flow, with the motor still running at full speed. By contrast, the drive varies the speed of the motor and only uses the energy needed to achieve the required output. The same principles apply to pump systems.

Variable speed drive technology has improved significantly over many years. New switching technologies and switching devices mean new VSD’s can be 10 – 15% more efficient than older models, so targeting existing variable speed technologies also has great benefit.


Cost to run a motor at 100% speed:

_____KW/hp x 0.746 x _____hrs. x ______£/KW/hr = ______ (a)

Cost to run a motor with a Variable Speed Drive:

_____KW/hp x (____% speed)3 x 0.746 x _____ hrs. x _____ £/KW/hr= _____ (b)

Energy Cost Savings:

(a) – (b) = (c)



The Ingress Protection Rating (IP) consists of the letters IP followed by two digits. IP Code classifies and rates the degrees of protection provided against the intrusion of solid objects (including body parts like hands and fingers), dust, accidental contact, and water in mechanical casings and with electrical enclosures.


LevelObject Size Protected AgainstEffective Against
0No protection against contact and ingress of objects
1>50mmAny large surface of the body, such as the back of a hand, but no protection against deliberate contact with a body part
2>12.5mmFingers or similar objects
3>2.5mmTools, thick wires, etc.
4>1mmMost wires, screws, etc.
5Dust ProtectedIngress of dust is not entirely prevented, but it must not enter in sufficient quantity to interfere with the satisfactory operation of the equipment; complete protection against contact
6Dust TightNo ingress of dust; complete protection against contact


LevelProtected AgainstTesting For
0Not Protected
1Dripping waterDripping water (vertically falling drops) shall have no harmful effect.
2Dripping water when tilted up to 15°Vertically dripping water shall have no harmful effect when the enclosure is tilted at an angle up to 15° from its normal position.
3Spraying waterWater falling as a spray at any angle up to 60° from the vertical shall have no harmful effect.
4Splashing waterWater splashing against the enclosure from any direction shall have no harmful effect.
5Water jetsWater projected by a nozzle (6.3mm) against enclosure from any direction shall have no harmful effects.
6Powerful water jetsWater projected in powerful jets (12.5mm nozzle) against the enclosure from any direction shall have no harmful effects.
7Immersion up to 1 mIngress of water in harmful quantity shall not be possible when the enclosure is immersed in water under defined conditions of pressure and time (up to 1 m of submersion).
8Immersion beyond 1 mThe equipment is suitable for continuous immersion in water under conditions which shall be specified by the manufacturer. Normally, this will mean that the equipment is hermetically sealed. However, with certain types of equipment, it can mean that water can enter but only in such a manner that it produces no harmful effects.


The us National Manufactures Association defines enclosure classification types in NEMA standard number 250.

NEMA TypeDefinition
1General-purpose. For indoor use. Provides a degree of protection to personnel against hazardous parts and a degree of protection to internal parts of enclosure against solid foreign objects.
2Drip-tight. Similar to Type 1 but with addition of drip shields to add a degree of protection to internal parts of enclosure with respect to harmful effects of dripping water.
3Weather-resistant. Provides a degree of protection against falling rain and ice formation. Provides a degree of protection to internal parts of enclosure against solid foreign objects, rain, sleet, snow, and external icing.
3RIntended for outdoor use. Provides a degree of protection against falling rain and ice formation. Provides a degree of protection to internal parts of enclosure against solid foreign objects, rain, sleet, snow, and external icing.
4Watertight (weatherproof). Must exclude at least 65 GPM of water from 1-in. nozzle delivered from a distance not less than 10 ft for 5 min. Provides a degree of protection with respect to harmful effects on the equipment due to the ingress of water such as rain, sleet, snow, splashing water, and hose directed water.
4XWatertight (weatherproof). Must exclude at least 65 GPM of water from 1-in. nozzle delivered from a distance not less than 10 ft for 5 min. Provides a degree of protection with respect to harmful effects on the equipment due to the ingress of water such as rain, sleet, snow, splashing water, and hose directed water. Also provides an additional level of protection against corrosion
5Dust-tight. Provide a degree of protection of the equipment inside enclosure against solid foreign objects such as falling dirt and settling airborne dust, lint, fibers, and flyings; and to provide a degree of protection with respect to harmful effects on the equipment due to the water such as dripping and light splashing.
6Submersible. Design depends on specified conditions of pressure and time; provides a degree of protection with respect to harmful effects on the equipment due water such as hose directed water and the entry of water during prolonged submersion at a limited depth.
12General-purpose. Intended for indoor use, provides a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects such as falling dirt and circulating dust, lint, fibers, and flyings; and to provide a degree of protection with respect to harmful effects on the equipment due to water such as dripping and light splashing.
13General-purpose. Provides a degree of protection of the equipment inside the enclosure against solid foreign objects such as falling dirt and circulating dust, lint, fibers, and flyings; provides a degree of protection with respect to harmful effects on the equipment due to water such as dripping and light splashing; and to provide a degree of protection against the spraying, splashing, and seepage of oil and non-corrosive coolants.


IP – NEMA ratings are not directly equivalent between the two standards, but the following table outlines the NEMA ratings that would correspond to the performance required by an IP code. NEMA ratings also require additional product features and tests (such as functionality under icing conditions, enclosures for hazardous areas, knock-outs for cable connections and others) not addressed by IP ratings.

IP CodeMin. NEMA Enclosure ratings to satisfy IP Codes


There are three different main types of earthing arrangements, as defined by BS 7671 and IEC 60364, each with advantages and disadvantages:


T – Terra (lat.)  = connection to earth

N – Neutral     = direct connection to the neutral

I –  Isolated       = no connection/floating


Distribution inside a building must fulfill the requirements of a TN-S system, so no combined PEN conductors may be used.

The TN-S system has the best EMC performance because the neutral and PE conductors are separated. Thus a current to the N does not produce any effects on the voltage potential of the PE. This is the preferred system for VFD applications.

The disadvantage of the TN-S system, which is in general the disadvantage of both TN and TT systems, is that in the case of an earth fault on the line, the protection fuses will stop the operation.


In the TN-C system the PE and N conductors are combined in a PEN conductor. The disadvantage is that a current through the N conductor is also a current through the PE, thus a voltage potential between earth and the chassis of the connected equipment occurs. In a 50 Hz systems, with linear loads, this system does not pose any special issues. But when electronic loads are present, including VFD’s, the high frequency currents that occur can cause faults. Although this system is compatible with VFD’s it should be avoided because of the associated risks. From an EMC perspective the TN-C system is not best.


The TN-C-S system is a hybrid between TN-C and TN-S. From the transformer to the building distribution point the PE and N are common (PEN) – just like in the TN-C system. In the building the PE and N are separated, like in the TN-S. As the impedance of the PEN conductor between the transformer and the building distribution point is typically low, it reduces the negative effects that occur on the TN-C main.


In the TT system the PE at the building is provided by a local earth electrode. The main advantage of the TT system is that the high frequency currents in the PE circuit of the consumer are separated from the low frequency currents in the N conductor. From an EMC perspective this is the ideal system.

However, because of the unknown impedance of the earth connection between the earth of the transformer and the earth of the consumer, it cannot be guaranteed that a line to PE short circuit at the consumer will blow the fuses quickly enough and protect against electrical chock. The disadvantage can be mitigated by using residual current devices (RCD).


In the IT mains the transformer is unearthed and the three phases are floating.

The basis for such a system is the ability of continuing operation after a line to earth fault occurs. Isolation monitoring devices are used for observing the integrity of the isolation between phases and earth. If the isolation is damaged, corrective maintenance can be carried out.

The disadvantage of this system is its poor EMC performance. Any earth noise current will cause the entire system to float with the noise, possibly causing malfunction of electronic equipment. When VFD’s are used on IT mains special considerations have to be taken, for example by disconnecting all capacitors to earth (such as the common-mode capacitors in the RFI filter). Subsequently, conducted emissions will be unfiltered and a lot of high frequency noise can be found on IT mains.


VFD’s generate variable rotating field frequencies at corresponding motor voltages due to variable-width rectangular current pulses. The steep pulse edges contain high-frequency components. Motor cables and inverter drives radiate these components and conduct them into the mains system via the cable. Manufacturers use radio frequency interference (RFI) filters (also called mains filters or EMC filters) to reduce the level of this type of interference on the mains feed.

They serve to protect devices against high-frequency conducted interference (noise protection) and to reduce the amount of high-frequency interference emitted by a device over the mains cable or by radiation from the mains cable. The filters are

intended to limit these interference emissions to a specified statutory level, which means that as far as possible they should be fitted in the equipment as standard. As with mains chokes, with RFI filters the quality of the filter to be used must be clearly stated.

Specific limits for interference levels are defined in the EN 61800-3 product standard and the EN 55011 generic standard.


External climatic conditions and ambient conditions have a distinct effect on the cooling of all electrical and electronic components in a plant room or cabinet.

Minimum and maximum ambient temperature limits are specified for all drives. These limits are usually determined by the electronic components that are used. For example, the ambient temperature of the electrolytic capacitors installed in the DC link must remain within certain limits due to the temperature dependence of their capacitance. Although drives can operate at temperatures down to -10 °C, manufacturers only guarantee proper operation at rated load with temperatures of 0 °C or higher. This means that you should avoid using them in areas subject to frost, such as un-insulated rooms.

You should also not exceed the maximum temperature limit. Electronic components are sensitive to heat.

According to the Arrhenius equation, the lifetime of an electronic component decreases by 50% for every 10°C that it is operated above its design temperature. This is not limited to devices that are installed in cabinets. Even devices with IP54, IP55 or IP66 protection ratings can only be used within the ambient temperature ranges specified in the manuals. This sometimes requires air conditioning of installation rooms or cabinets. Avoiding extreme ambient temperatures prolongs the life of drives and thereby the reliability of the overall system.


Drives dissipate power in the form of heat. The amount of power dissipation in watts is stated in the technical data of the frequency converter. Operators should take suitable measures to remove the heat dissipated by the frequency converter from the cabinet, for example by means of cabinet fans. The required air flow is stated in the manufacturer documentation. Drives must be mounted such that the cooling air can flow unhindered through the device’s cooling fins.

The majority of drives are designed for a maximum ambient temperature of 40°C, which means that the inlet cooling air must not exceed that value.

This in turn suggests that to maintain an acceptable room temperature a suitable level of airflow will be needed.

The basic formula to determine an airflow in cubic meters per hour is to multiply the heat loss in Watts x a constant (3.1 for air at sea-level) and divide by the temperature differential in K.

These requirements are equally valid for drives installed in secondary enclosures.

In the event of manufacturers’ figures being unavailable, the loss can be assumed to be 2.5% of the controlled motor power for each inverter. If transformers are installed in the same room a similar allowance should be made for each transformer.

As a typical example – a standard 6 pulse a.c. drive rated at 5.5kW with the inverter mounted in a switch room with an ambient temperature of 30°C and an anticipated, temperature, differential of 10K will require 0.025 x 5500 x 3.1 / 10 = 43 m3/h.

Note: These values are in addition to those for dissipating losses from switch and control.

The maximum surface temperature of any parts that may be touched must also be considered.

In multi drive installations the diversity of loading should be considered, since this may offer savings in the cooling requirements.

The airflow must be arranged to ensure that no recirculation of the heated exhaust air occurs. It must also be remembered that warmed air expands; therefore if free ventilation is required the outlet should be at least 20% greater cross sectional area than the inlet. In addition to cooling the equipment adequately, the prevention of condensation, is also critically important. For effective prevention a dry transformer motor or inverter enclosure should be kept at least 10K warmer than the surroundings. Generally heaters are sized as follows to the formula

H = Δt x A x k


H = Heater Power (Watts)

Δt = Temperature differential (K)

A = Surface area in m2

k = constant (typically 5.5 W/Km2 for painted sheet steel).


Although some drives can operate properly at relatively high humidity up to 95% relative humidity, condensation must always be avoided. There is a specific risk of condensation when the frequency converter or some of its components are colder than moist ambient air. In this situation, the moisture in the air can condense on the electronic components.

When the device is switched on again, the water droplets can cause short circuits in the device. This usually occurs only with drives that are disconnected from the mains. For this reason, it is advisable to install a cabinet heater in situations where there is a real possibility of condensation due to ambient conditions. Alternatively, operating the frequency converter in standby mode (with the device constantly connected to the mains) can help reduce the risk of condensation. However, you should check whether the power dissipation is sufficient to keep the circuitry in the drive dry.

Where the heat dissipation is sufficient it is recommended that a supplementary panel heaters are fitted.


AC20A Connecting and disconnecting under no load condition.

AC21A Switching of resistive loads including moderate overloads.

AC22A Switching of mixed resistive and inductive loads including moderate overloads.

AC23A Switching of motor, or other.

Also Manufacturers may assign more than one AC duty rating to their devices. For example an AC22 switch can be given a lower current rating when assigned to AC23 duty. Manufacturers may also assign a maximum kW power rating for motors or a kVA reactive power rating for capacitors.


Unit Name

Unit Symbol


Ampere (amp)AElectric current (I)
VoltVVoltage (V, E)

Electromotive force (E)

Potential difference (Δφ)

OhmΩResistance (R)
WattWElectric power (P)
Volt-Ampere-ReactivevarReactive power (Q)
Volt-AmpereVAApparent power (S)
FaradFCapacitance (C)
HenryHInductance (L)
Siemens / mhoSConductance (G)

Admittance (Y)

CoulombCElectric charge (Q)
Ampere-hourAhElectric charge (Q)
JouleJEnergy (E)
Kilowatt-hourkWhEnergy (E)
Electron-volteVEnergy (E)
Ohm-meterΩ∙mResistivity (ρ)
Siemens per meterS/mConductivity (σ)
Volts per meterV/mElectric field (E)
Newtons per coulombN/CElectric field (E)
Volt-meterV·mElectric flux (Φe)
TeslaTMagnetic field (B)
GaussGMagnetic field (B)
WeberWbMagnetic flux (Φm)
HertzHzFrequency (f)






picop10-121pF = 10-12F
nanon10-91nF = 10-9F
microμ10-61μA = 10-6A
millim10-31mA = 10-3A
kilok10 31kΩ = 1000Ω
megaM10 61MHz = 106Hz
gigaG10 91GHz = 109Hz



Volt is the electrical unit of voltage.

One volt is the energy of 1 joule that is consumed when electric charge of 1 coulomb flows in the circuit.

1V = 1J / 1C


Ampere is the electrical unit of electrical current. It measures the amount of electrical charge that flows in an electrical circuit per 1 second.

1A = 1C / 1s


Ohm is the electrical unit of resistance.

1Ω = 1V / 1A


Watt is the electrical unit of electric power. It measures the rate of consumed energy.

1W = 1J / 1s

1W = 1V · 1A


Farad is the unit of capacitance. It represents the amount of electric charge in coulombs that is stored per 1 volt.

1F = 1C / 1V


Henry is the unit of inductance.

1H = 1Wb / 1A


Siemens is the unit of conductance, which is the opposite of resistance.

1S = 1 / 1Ω


Coulomb is the unit of electric charge.

1C = 6.238792×1018 electron charges


Ampere-hour is a unit of electric charge.

One ampere-hour is the electric charge that flow in electrical circuit, when a current of 1 ampere is applied for 1 hour.

1Ah = 1A · 1hour

One ampere-hour is equal to 3600 coulombs.

1Ah = 3600C


Tesla is the unit of magnetic field.

1T = 1Wb / 1m2


Weber is the unit of magnetic flux.

1Wb = 1V · 1s


Joule is the unit of energy.

1J = 1 kg · 1(m / s)2


Kilowatt-hour is a unit of energy.

1kWh = 1kW · 1h = 1000W · 1h


Kilovolt-amps is a unit of power.

1kVA = 1kV · 1A = 1000 · 1V · 1A


Hertz is the unit of frequency. It measures the number of cycles per second.

1 Hz = 1 cycles / s



The electric power P is equal to the energy consumption E divided by the consumption time t:

P is the electric power in watt (W).

E is the energy consumption in joule (J).

t is the time in seconds (s).


Find the electric power of an electrical circuit that consumes 120 joules for 20 seconds.


E = 120J

t = 20s

P = E / t = 120J / 20s = 6W


P = · I


P = I · R


P = V 2 / R

P is the electric power in watt (W).

V is the voltage in volts (V).

I is the current in amps (A).

R is the resistance in ohms (Ω).


The formulas are for single phase AC power.

For 3 phase AC power:

When line to line voltage (VL-L) is used in the formula, multiply the single phase power by square root of 3 (√3=1.73).

When line to zero voltage (VL-0) is used in the formula, multiply the single phase power by 3.


Real or true power is the power that is used to do the work on the load.

P = Vrms Irms cos φ

P      is the real power in watts [W]

Vrms  is the rms voltage = Vpeak/√2 in Volts [V]

Irms   is the rms current = Ipeak/√2 in Amperes [A]

φ      is the impedance phase angle = phase difference between voltage and current.


Reactive power is the power that is wasted and not used to do work on the load.

Q = Vrms Irms sin φ

Q      is the reactive power in volt-ampere-reactive [VAR]

Vrms  is the rms voltage = Vpeak/√2 in Volts [V]

Irms   is the rms current = Ipeak/√2 in Amperes [A]

φ      is the impedance phase angle = phase difference between voltage and current.


The apparent power is the power that is supplied to the circuit.

S = Vrms Irms

S      is the apparent power in Volt-amper [VA]

Vrms  is the rms voltage = Vpeak/√2 in Volts [V]

Irms   is the rms current = Ipeak/√2 in Amperes [A]


The real power P and reactive power Q give together the apparent power S:

P2 + Q2 = S2

P      is the real power in watts [W]

Q      is the reactive power in volt-ampere-reactive [VAR]

S      is the apparent power in Volt-amper [VA]


To convert power in watts (W) to kilowatts (kW).

One kilowatt is equal to 1000 watts:

1kW = 1000W

Watts to kilowatts conversion formula

The power in kilowatts P(kW) is equal to the power in watts P(W) divided by 1000:

P(kW) = P(W) / 1000


Convert 1300W to kilowatts:


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