Capacitance
All electrical circuits have a certain amount of resistance. As we have learned, resistance is an opposition to current flow. Resistance has prima- rily the same effects on an AC or DC circuit. Capacitance, on the other
Chapter 2: Review of Basic Principles — Electrical Principles 25
hand, is the ability to block DC, but appears to allow AC to flow in a cir- cuit. The device that accomplishes this task is known as the capacitor and is shown in Figure 2-12.
If the power applied to a capacitor is DC, then the capacitor tends to charge to whatever voltage is applied. It should also be noted that the DC voltage level remains on a capacitor for a period of time (up to several hours or days for some large capacitor values—50 µF or higher). The capacitor will slowly discharge into the atmosphere over time. It will dis- charge rapidly when connected to a load, such as a resistor that will quickly absorb the energy.
If the power applied to the capacitor is AC, it appears that AC is flowing through. In reality, the capacitor charges and discharges so rapidly that it is common practice to refer to AC as “flowing through a capacitor.”
Drive manufacturers install what are called bleeder resistors across large capacitor circuits to bring the voltage down to a safe level after power- down (e.g., discharge 680 VDC down to less than 50 VDC in 1 minute). The ability of a capacitor to store and discharge energy allows improve- ment in DC drive output voltage regulation (consistency).
In an AC drive, this charging effect also comes in quite handy. The capaci- tor circuit charges and discharges, keeping the flow of voltage constant and improving the quality of the AC output waveform.
The main purpose of a capacitor is to oppose any change in voltage. As expected, the more capacitance in a circuit, the longer the time required for charging and discharging to occur. Figure 2-13 shows the effects of higher capacitance on a rectified AC waveform.
Figure 2-12. Capacitors and their effects on a circuit DC Voltage
With DC, Charge remains
on a capacitor With AC, the capacitor constantlycharges and discharges AC Voltage
+ +&-
+&- _
Note: For illustration purposes, a half-wave rectifier waveform is shown in Figure 2-13. A typical AC drive rectifier output would be “full-wave,” which would double the number of positive half waves and increase the DC voltage output.
Inductance
Inductance is the ability to block AC but allow DC to flow in a circuit. The device that accomplishes this, is known as an inductor and is shown electri- cally in Figure 2-14.
As shown earlier, an inductor produces a definite polarity when connected to DC voltage. The inductor will be an electromagnet with a specific north and south pole.
The main purpose of an inductor is to oppose any change in current. As you recall, any coil of wire will generate a magnetic field. The inductor works by controlling the expanding and collapsing of the magnetic field. When there is a presence of DC voltage, the magnetic field expands. When DC voltage is removed, the magnetic field collapses and creates a surge of energy. It would not be uncommon for an inductor to produce a short burst of 70 volts, after removal from a 6 volt battery. With this principle in mind, it is easy to see why there is an electrical arc at the contacts of any circuit, whenever a voltage is removed from an inductor.
The magnetic field strength is stronger, with larger amounts of inductance, commonly referred to as henries. Typical values would be µh (microhen- ries) or mh (millihenries). Figure 2-15 shows the effects of higher induc- tance on an AC waveform.
Figure 2-13. Effects of capacitance on an AC waveform
Figure 2-14. Inductance and the effects on a circuit
Μνµ Μνµ Minimal Capacitance (10 )F Higher Capacitance (50 )F No Voltage DC Voltage
Magnetic Field Collapsing Magnetic Field Expanding
+
_
N
Chapter 2: Review of Basic Principles — Electrical Principles 27
As to be expected, the more inductance in a circuit, the more time that is needed to expand and contract the magnetic field around the inductor. In addition, inductors have higher amounts of resistance to AC voltage as compared with DC. An inductor may only have 15 Ω of resistance to DC, but 1000-Ω resistance to AC. AC resistance is call impedance and is signified by the letter Z.
Inductors are used in the DC bus Bus circuit of some AC drives to reduce the amount of AC voltage in that circuit. This tends to “purify” the DC, which in turn provides a cleaner output waveform from the drive. Because of the process of reducing or blocking AC, inductors are some- times called chokes.
Power Factor
By strict definition, power factor is a measure of the time phase difference between voltage and current in an AC circuit. When an inductor is used in an AC power system, the current waveform tends to lag behind the volt- age waveform. Figure 2-16 shows in-phase and out-of-phase voltage and current waveforms.
Figure 2-15. Effects of inductance on an AC waveform
Figure 2-16. Power factor—voltage and current waveforms
AC
Voltage VoltageAC
1 mh 25 mh
Voltage
Current
In Phase Voltage and Current
generated by the utility Out-of-phase Phase Voltage and Currentcaused by use of inductors
+ _ Time (Seconds) Voltage Current 0 + _ Time (Seconds) Voltage Current 0
In a purely resistive electrical circuit, the voltage and current waveforms would be synchronized or in-phase. In-phase voltage and current has a unity power factor of 1.0. Unity power is transmitted to customers by the utility. However, inductive loads such as motors cause the current to lag behind the voltage waveform. Once this occurs, the current being con- sumed is out-of-phase with the voltage waveform.
The power factor is calculated by taking the ratio of true power divided by the apparent power in a circuit. True power is the actual power converted to another form of energy by a circuit, and is expressed in watts (W). Apparent power is the power delivered to an AC circuit and is usually expressed in kilovoltamperes (KVA).
Apparent power is the power obtained by taking volts × amperes. Figure 2-17 shows the formula for power factor and unity versus 50% power fac- tor.
The deviation between the voltage and current waveform is called the phase angle or displacement angle. If voltage and current were 90º out-of- phase, the result would be a power factor of zero.
The utility generates power that is 100% or unity. If the operating equip- ment in a factory, such as AC motors, causes less than 100% power factor, the factory will be assessed a penalty by the utility. Essentially, the factory uses devices that cause current to lag voltage, meaning that the factory is wasting energy generated by the utility. The utility must generate more energy to make up for the energy wasted by the factory. If we use this analogy, the graph to the right in Figure 2-17 would indicate wasting 50% of the utility’s energy.
In recent years, the utilities have promoted the use of high-efficiency motors. A high-efficiency motor wastes less energy. Typically, high effi- ciency motors have a higher power factor compared with motors of stan- dard efficiency. However, manufacturers have to make trade-offs between
Figure 2-17. Power factor calculation
1.0 Power Factor - Seen in Resistive Circuits (e.g. Incandescent Lights)
.5 or 50% Power Factor - Seen in Inductive Loads (e.g. AC Induction Motors) Displacement Angle + _ Time (Seconds) Voltage Current 0 + _ Time (Seconds) Displacement Angle 0
Chapter 2: Review of Basic Principles — Electrical/Electronic Devices 29
high efficiency and high power factor because of magnetic and electrical characteristics of the motor.
Utilities require customers to correct for poor power factor, or at the very least, assess a penalty for inadequate power factor ratings. The power fac- tor of motors can be improved by installing devices such as power factor correction capacitors. Capacitance counteracts the effects of inductance. With capacitors used in connection with AC motors, the results are a higher power factor and less waste of power. The voltage and current waveform are approaching the in-phase power generated by the utility.