Characteristics - Installed power loads

Characteristics

Installed power loads

The examination of actual values of apparent-power required by each load enables the establishment of :

• A declared power demand which determines the contract for the supply of energy
• The rating of the MV/LV transformer, where applicable ( allowing for expected increased load )
• Levels of load current at each distribution board.

Induction motors

Current demand

The reted current in suppliend to the motor is given by the following formulae :






where
In : reted current (in amps)
Pn: nominal power (in kW)
U: voltage between phases for 3-phase motors and voltage between the terminals for single-phase motors (in volts). A single-phase motor may be connected phase-to-neutral or phase-to-phase.

 : per-unit efficiency, i.e. output kW / input kW
 : power factor, i.e. kW input / kVA input.

transactions :

• An examination of the actual apparent-power demands of differnt loads : A necessary preliminary step in the design of a LV installation.
• The norminal power in kW (Pn) of a motor indicates its rated equivalent mechanical power output. The apparent power in kVA (Pa) suppliend to the motor is a function of the output, the motor efficiency and the power factor.

         

Subtransient current and protection setting

• Subtransient current peak value can be very high; typical value is about 12 to 15 times the rms rated value In. Sometimens this value can reach 25 times In.
• If unexpected tripping of the overcurrent protection occurs during starting, this means the starting current exceeds exceeds the nominal limits. As a result, some maximum switchgear withstands can be reached, life time can be reduced and even some devices can be destroyed. In order to avoid such a situation, oversizing of the switchgear must be considered.

Compensaion of reactive-power (kvar) spplied to induction motors

It is generally advantageous for technical and financial reasons to reduce the current supplied to induction motors. This can be achieved by using capacitors without affecting the power output of the motors. The application of this principle to the operation of induction motors is generally referred to as "power-factor improvement" or "power-factor correction".

As discussed in chapter L, the apparent power (kVA) supplied to an induction motor can be significantly reduced by the use of shunt-connected capacitors. Reduction of input kVA means a corresponding reduction of input current (since the voltage remanis constant).

Compensation of reactive-power is particularly advised for motors that operate of long periods at reduced power.

As noted above    so that a kVA reduction will increase (i.e. improve) the value of

.

The current supplied to the motor, after power-factor currection, is given by

where  is the power factor before compensation and   is the power factor after compensation, Ia being the orginal current.

Resistive-type heating appliances and incandescent lamps (conventional or halogen)

The current demand of a heating appliance or an incandescent lamp is easily obtained from the nominal power Pn quoted by the manufacturer.

The power Pn (watts) indicated on the tube of a fluorescent lamp does not include the power dissipated in the ballast.

The current is given by :






where U is the voltae between the terminals of the equipment.
For an incandescent lamp, the use of halogen gas allows a more concentrated light source. The light output is increased and the lifetime of the lamp is doubled.

Note : at the instant of switching on, the cold filament gives rise to a very brief but intense peak of current.