8 | SAMLEX AMERICA INC.
• Electric Motors: At the moment when an electric motor is powered ON, the rotor is
stationary (equivalent to being “Locked”), there is no “Back EMF” and the windings
draw a very heavy surge of starting current (Amperes) called “Locked Rotor Am-
peres” (LRA) due to low DC resistance of the windings. For example, in motor driven
loads like Air-conditioning and Refrigeration Compressors and in Well Pumps (using
Pressure Tank), the Starting Surge Current / LRA may be as high as 10 times its rated
Full Load Amps (FLA) / Maximum Continuous Running Power Rating. The value and
duration of the Starting Surge Current / LRA of the motor depends upon the winding
design of the motor and the inertia / resistance to movement of mechanical load
being driven by the motor. As the motor speed rises to its rated RPM, “Back EMF”
proportional to the RPM is generated in the windings and the current draw reduces
proportionately till it draws the running FLA / Maximum Continuous Running Power
Rating at the rated RPM.
• Transformers (e.g. Isolation Transformers, Step-up / Step-down Transformers, Power
Transformer in Microwave Oven etc.): At the moment when AC power is supplied to a
transformer, the transformer draws very heavy surge of “Magnetization Inrush Current”
for a few millisecs that can reach up to 10 times the Maximum Continuous Rating of the
Transformer.
• Devices like Infrared Quartz Halogen Heaters (also used in Laser Printers) / Quartz
Halogen Lights / Incandescent Light Bulbs using Tungsten heating elements: Tungsten
has a very high Positive Temperature Coefficient of Resistance i.e. it has lower resist-
ance when cold and higher resistance when hot. As Tungsten heating element will be
cold at the time of powering ON, its resistance will be low and hence, the device will
draw very heavy surge of current with consequent very heavy surge of power with a
value of up to 8 times the Maximum Continuous Running AC Power.
• AC to DC Switched Mode Power Supplies (SMPS): This type of power supply is used
as stand-alone power supply or as front end in all electronic devices powered from
Utility / Grid e.g. in audio/video/ computing devices and battery chargers (Please see
Section 4 for more details on SMPS). When this power supply is switched ON, its inter-
nal input side capacitors start charging resulting in very high surge of Inrush Current
for a few millisecs (Please see Fig 4.1). This surge of inrush current / power may reach
up to 15 times the Continuous Maximum Running Power Rating. The surge of inrush
current / power will, however, be limited by the Surge Power Rating of the AC source.
Power Factor, (PF): It is denoted by “PF” and is equal to the ratio of the Active Power
(P) in Watts to the Apparent Power (S) in VA. The maximum value is 1 for resistive types
of loads where the Active Power (P) in Watts = the Apparent Power (S) in VA. It is 0 for
purely inductive or purely capacitive loads. Practically, the loads will be a combination of
resistive, inductive and capacitive elements and hence, its value will be > 0 <1. Normally
it ranges from 0.5 to 0.8 e.g. (i) AC motors (0.4 to 0.8), (ii) Transformers (0.8) (iii) AC to
DC Switch Mode Power Supplies (0.5 to 0.6) etc.
Load: Electrical appliance or device to which an electrical voltage is fed.
Linear Load: A load that draws sinusoidal current when a sinusoidal voltage is fed to it.
Examples are, incandescent lamp, heater, electric motor, etc.
SECTION 2 |
General Information
