ELECTRICAL TERMINOLOGY

Voltage and Current (Amperes)

An AC Generator is designed to produce a voltage level suitable for the load to which it is connected. The Generator control circuits are designed to automatically maintain this voltage level as the load is increased or decreased. Sudden large changes in loading will produce temporary changes in the voltage. The Automatic Voltage Regulator (AVR) is

designed to recover to a stable condition as quickly as possible. The current drawn from the AC Generator is determined by the amount of load connected to it. Current creates a temperature rise in the windings, hence the requirement for drawing air through the AC Generator by means of the cooling fan. If the full load rated current is exceeded on any phase of the main stator windings, it will result in overheating in this winding. Similarly, any

restriction in the flow of air through the machine will result in a rapid increase in the temperature of the windings.

Frequency (Hz) and Speed (RPM)

An AC Generator is a constant speed device, and should not be operated at speeds above 4% of the rated speed, or more than 1% below the rated speed.

Load changes will create temporary changes in the speed, but the engine must be capable of returning to the steady state condition within a few seconds.

The shaft speed requirements for the AC Generator are determined by:-

(a) The frequency, (Hz), requirements of the load

(b) The number of poles, ( main rotors), in the generator

Frequency (HZ)=( N (speed) X P (pairs of poles ))\ (60 (sec’s))

Kilowatts (kW) kilo Volt Amperes (kVA) and Power Factors (pf.)

For an AC Generator to supply power for a load of 1kW, the prime mover (engine) driving the alternator must produce approximately 1.5 horsepower.

 

Kilowatts are calculated by the formula: -

kW =(Volts x Amperes x Power Factor)\ (100)

kVA (kilo Volt Amperes), are calculated by the formula:-

kVA ==(Volts x Amperes)\ (100)

Both equations are multiplied by _3 (1.732) for a 3 phase machine.

Power Factor

The Power Factor (pf), is a measure of wasted current, which is a product of inductive loads such as motors, transformers, (magnetic circuits), and some forms of lighting.

The formula for calculating the Power Factor is:-

pf = (kiloWatts) \ (kVA)

Unity Power Factor (pf 1)

Purely resistive load, i.e. heating, tungsten filament lighting, has a power factor of one, (pf1), and contains very little Wattless (inductive) load, which is power factor zero, (pf 0).

An AC Generator will deliver continuously the rated full load current at any power factor between pf1 (unity) and 0.8. However, the prime mover, (engine), is greatly affected by the power factor. At pf1, the kVA and kW are equal; therefore the engine is supplying 20% more kW load at pf1, than at pf 0.8. It is important, therefore, that this is taken into consideration, when approaching 75% to 100% load current of the Generator, with a power factor higher than 0.8.

Lagging Power Factors

A Generator is designed to deliver the full load current at any power factor between unity and 0.8 lagging. Certain loads have a power factor lower than 0,8 lag, e.g. welding transformers; autotransformer, motor starting, gas discharge lighting. A

reduction in the full load (kVA), rating is required for a continuous lagging pf lower than 0.8.

Leading Power Factors

Capacitive load e.g. some fluorescent lighting, and power factor correction capacitor banks, produce leading power factor current. The latter is required by the Electricity authorities to improve the customers lagging power factor. The capacitor bank size is

measured in kVAr (reactive). A purely Capacitive load can cause the Generator control system,

(AVR), to loose control, creating voltage instability, and possible high voltage from the Generator. This is due to the fact that, unlike most loads, which are pf1, (unity) or lagging pf, a leading pf load current will cause the Generator excitation voltage to decrease, as the load current increases. Eventually the control system will be unable to control the

Generator excitation level, and voltage instability will occur. The degree of instability is determined by the kVAr size of the capacitors, relative to the kVA size of the alternator. Capacitive load can present a problem for mains failure (standby) Generators. When the mains electricity supply fails, all motor,

(inductive), load is disconnected by the individual contactors.

Subsequently, when the Generator is connected to the system,

the load will mainly consist of lighting, and possibly power factor

correction capacitors. In this situation the AC Generator will see a

very low, (leading), power factor, and may become unstable, and/or generate high voltage.

In order to prevent this situation, it is advisable to ensure that the power factor correction capacitors are switched OFF when the generator takes the initial load.

Further advice in this respect may be obtained from Cummins Generator Technologies if required.