Types of operation

Constant torque operation

Operation is said to be constant torque when the charge’s characteristics in the steady state are such that the torque required is more or less the same whatever the speed.

This is the operating mode of machines like conveyors, crushers or hoists. For this kind of use, the starter device must be able to provide a high starting torque (1.5 times or more the nominal rate) to overcome static friction and accelerate the machine (inertia).

                      Constant torque operation curve

Operation with torque increasing with speed

The characteristics of the charge imply that the torque required increases with the speed. This particularly applies to helical positive displacement pumps where the torque increases linearly with the speed or centrifugal machines (pumps and fans) where the torque varies with the speed squared.

The power of displacement pumps varies with the speed squared.

The power of centrifugal machines varies with the speed cubed.

A starter for this type of use will have a lower starting torque (1.2 times the motor’s nominal torque is usually enough).

                  Variable torque operation curve

Operation with torque decreasing with speed

For some machines, the torque required decreases as the speed increases. This particularly applies to constant-power operation when the motor provides a torque that is inversely proportional to the angular speed.

This is so, for example, with a winder, where the angular speed needs to drop as the diameter of the winder increases with the build-up of material. It also applies to spindle motors on machine tools.

The constant-power operating range limited by its very nature: at low speed by the available current from the speed controller and at high speed by the torque the motor can provide. The driving torque on asynchronous motors and the switching capacity of DC motors should therefore be checked carefully.

               Decreasing torque operation

The table gives a list of common machines with their torque law depending on speed.

When a machine starts, it often happens that the motor has to overcome a transitory torque, such as in a crusher when it starts with a full hopper. There can also be dry friction which disappears when a machine is running or a machine starting from a cold stage may needs a higher torque than in normal operation when warm.

Passive loads

There are two types of passive charge used in industry:

-          heating,

                    -     lighting.

Heating

Heating is a costly item for industrial premises. To keep these costs down, heat loss must be reduced; this is a factor which depends on building design and is beyond the scope of this guide.

Every building is a specific case and we cannot allow ourselves to give vague or irrelevant answers.

That said, proper management of the building can provide both comfort and considerable savings. For further information, please see the Schneider Electric Electrical Installation Guide or the Cahier Technique 206 available from the Schneider Electric website.

If necessary, the best solution may be found by asking the advice of the electrical equipment supplier’s experts.

Lighting

• Incandescent lighting

Incandescent lighting (trademarked by Thomas Edison in 1879) was an absolute revolution and, for many years afterwards, lighting was based on devices with a filament heated to a high temperature to radiate visible light. This type of lighting is still the most widely used but has two major disadvantages:

-          extremely low efficiency, since most of the electricity is lost in heat consumption,

-          the lighting device has a lifetime of a few thousand hours and has to be regularly changed. Improvements have increased this lifetime (by the use of rare gases, such as krypton, or halogen).

Some countries (Scandinavian ones in particular) plan to ban this type of lighting eventually

• Fluorescent lighting

This family includes fluorescent tubes and fluocompact lamps.

The technology used is usually “low-pressure mercury”.

Fluorescent tubes

These were introduced in 1938. In these tubes, an electric discharge makes electrons collide with mercury vapour, which excites the mercury atoms and results in ultraviolet radiation. The fluorescent matter lining the inside of the tube transforms the radiation into visible light.

Fluorescent tubes dissipate less heat and last longer than incandescent lamps but require the use of two devices: one to start them and one called a ballast to control the current of the arc once they are switched on.

The ballast is usually a current limiting reactor connected in series with the arc.

Fluocompact lamps

These work to the same principle as a fluorescent tube. The starter and ballast functions are performed by an electronic circuit in the lamp, which enables the tubes to be smaller and to be folded.

Fluocompact lamps were developed as an alternative to incandescent lamps: they save a significant amount of power (15W instead of 75W for the same brightness) and last much longer (8000 hours on average and up to 20,000 for some).

     Fluo compact lamps

Discharge lamps

Light is produced by an electric discharge created by two electrodes within a gas in a quartz bulb. Such lamps all require a ballast, usually a current limiting reactor, to control the current in the arc.

The emission range depends on the gas composition and is improved by increasing the pressure. Several technologies have been developed for different functions.

                                         Discharge lamps

Low-pressure sodium vapour lamps

These have the best lighting capacity, but they have a very poor colour rendition because they radiate a monochrome orange light.

Uses: motorway lighting, tunnels.

High-pressure sodium vapour lamps

These emit a white light tinged with orange.

Uses: urban lighting, monuments.

High-pressure mercury vapour lamps

The discharge is produced in a quartz or ceramic bulb at pressures exceeding 100kPa. The lamps are known as fluorescent bulbs and are characterised by the bluish white light they emit.

Uses: car parks, supermarkets, warehouses.

• Metal halide lamps

This is the most recent technology. The lamps emit a colour with a wide spectrum.

The tube is in ceramic to enhance lighting capacity and colour stability.

Uses: stadiums, shops, spotlighting.

LED (Light Emitting Diodes)

This is one of the most promising technologies. LEDs emit light by means of an electric current through a semiconductor.

LEDs are used for many purposes but the recent development of blue or white diodes with a high lighting capacity opens up new avenues, in particular for signage (traffic lights, safety displays or emergency lighting) and motor vehicle lighting.

A LED has an average current of 20mA, with a voltage drop of 1.7 to 4.6 depending on the colour. Such properties are suited to very low voltage power supply, for batteries in particular.

Mains power requires the use of a transformer, which is economically perfectly feasible.

The advantage of LEDs is their low power consumption which results in a very low operating temperature and an almost unlimited lifetime. In the near future, it will be possible to incorporate such a lighting into buildings at the construction stage.

However, a basic diode has a very low lighting capacity. Powerful lighting therefore requires a great many units to be connected in a series.

As LEDs have no thermal inertia, they can be used for innovating purposes such as simultaneous transmission of light and data. To do this, the power supply is modulated with high frequency. The human eye cannot detect this modulation but a receiver with the right interface can detect the signals and use them.

Powering incandescent lamps

• Constraints of direct powering

The resistance of the filament varies widely due to the very high temperatures (up to 2500°C) it can reach during operation.

When cold, resistance is low, resulting in a power inrush current for a few to several dozen milliseconds when the lamp is switched on and which can be 10 to 15 times that of the nominal current.

This constraint applies equally to ordinary and halogen lamps. It requires reducing the maximum number of lamps that can be powered by the same device such as a remote control, modular contactor or relay on ready-made circuits.

Light dimming

This can be achieved by varying the RMS voltage powering the lamp.

Voltage is usually adjusted by a triac used to vary the triggering angle in the mains voltage cycle.

The waveform of the voltage applied to the lamp is illustrated

                               Current waveform

Gradual powering of the lamp also reduces, or even eliminates, the power surge when it is switched on. Note that light dimming:

-          alters the colour temperature,

-          shortens the life of halogen lamps when low voltage is maintained for long periods. The filament is not regenerated so efficiently at low temperature.

Some halogen lamps are powered at low voltage through a transformer. Magnetisation in a transformer can produce power surges 50 to 75 times greater than the nominal current for a few milliseconds. Suppliers also offer static converters which do away with this disadvantage.

Powering fluorescent lamps and discharge lamps

Fluorescent tubes and discharge lamps require control of arc intensity. This function is performed by a ballast device inside the bulb itself. The magnetic ballast (i.e. limiting current reactor is commonly used in domestic appliances.

                       Magnetic ballast

A magnetic ballast works in conjunction with the starter device. It has two functions: to heat the electrodes in the tube and to generate a power surge to trigger the tube.

The power surge is induced by triggering a contact (controlled by a bimetal switch) which breaks the current in the magnetic ballast.

When the starter is working (for about 1 sec.), the current absorbed by the light is about twice the nominal current.

As the current absorbed by the tube and ballast together is mainly inductive, the power factor is very low (0.4-0.5 on average). In fixtures with a large number of tubes, a capacitor must be used to improve the power factor.

This capacitor is usually applied to each light appliance. Capacitors are sized to ensure that the overall power factor exceeds 0.85.

In the most common type, the parallel capacitor, the average active power is 1µF for 10W for all types of lamp.

The parallel capacitor layout creates stress when the lamp is switched on.

As the capacitor is initially discharged, switching on creates causes a power surge.

       Voltage and current waveforms

There is also a power surge due to oscillation in the power inductor/ capacitor circuit.

The electronic ballast, first introduced in the 1980s, does away with these disadvantages.

The electronic ballast works by powering the lamp arc by an electronic device generating a rectangular alternating voltage.

There are low frequency or hybrid devices, with frequency ranging from 50 to 500Hz, and high frequency devices with frequency ranging from 20 to 60kHz. The arc is powered by high frequency voltage which completely eliminates flickering and strobe effects

 Electronic ballast package