Electrical Equipment and Devices

WEEK 9. VOLTAGE CONTROL DEVICES

Sections: Introduction | Effects | Methods of Regulation

Objectives. The primary object of an electrical system is to provide power users with a supply voltage compatible to therir utilization equipment.

Sections: Introduction | Effects | Methods of Regulation

Effects of Unregulated Voltage

1. Resistance loads (stove, iron, heaters)
Low Voltage : Longer heating time
High Voltage: Shorter life of heating elements

2. Motor Loads (vacuum cleaner, washing machines, fans, refrigerators)
Low Voltage: Increased slip (diffrence between nominal speed and synchronous speed divided by the synchronous speed); Increased line current thus decreased efficiency, and motor overheating.
High Voltage: Over-excited mtoro and increased torque thus damage to coupling and appliances

3. Electronic Loads (radio/television, personal computer)
Low Voltage: Poor quality of reception
High Voltage: Short appliance life.

4. Illumination Loads
Low Voltage: Decreased lamp efficiency
Note: a 10% decrease in voltage is equal to a 70% nominal illumination output.
High Voltage: Decrease in life expectancy

Sections: Introduction | Effects | Methods of Regulation

Methods of Regulation

1. STEP-TYPE - using tapped autotransformers, transformers which part of one winding is connected to both the primary nad the secondary circuit associated with that winding. The primary (exciter) winding is both electrically and magnetically connected to the secondary (series) winding.

2. TRANSFORMER LOAD-TAP CHANGERS - a combination of a transformer and the step-type method. Tap changing is usually gang-operated through a three-phase oil switchgear.

3. FIXED / SWITCHED CAPACITOR - used to improve line-voltage profile by connecting a bank of capacitors in shunt. Improving a lagging power factor would reduce line losses and thus improve regulation. Coversely, when used beyond unity, thus leading, will increase the voltage and increase in current.

4. MICROBASED
Major parts:
1. Voltage-Sensing Device - monotrs output voltage and send signal to control circuit.
2. Amplifying/Switching section - delays and transmits the signals.
3. Motor-drive circuit - close relay contacts, actuate switches to change taps.