In this section, we will review the major components that comprise an AC- or DC-drive unit. Drive units are built using components in addition to those discussed here.
Inductors, Relays, Transformers and Contactors Inductors
As discussed in an earlier section, inductance is the ability to block AC and allow DC to flow in a circuit. The main device that accomplishes this phe-nomenon is the inductor with the electrical symbol shown in Figure 2-18.
As seen earlier, an inductor is basically an iron core, with a coil of wire wrapped around it. Inductors are found in many sizes and, as in resistors, the higher the value of an inductor, the greater the opposition to AC flow.
Inductors are rated in units called henries, with typical values in millihen-ries (mH) due to the large unit of measure. (Note: a millihenry is 1/1000 of a henry.)
Inductors are found in various types of drives, both AC and DC, and will be covered in a later section. When inductors are placed in front of drives, they act as a protective device. In the discussion of electromagnetism, it
Figure 2-18. Inductor electrical symbol AC Voltage
Source
“L” is the typical electrical symbol used on drawings.
was stated that Lenz’s Law applies to a conductor moving through a mag-netic field. An EMF is generated that tries to oppose change in the induced voltage that created it. An inductor would therefore oppose any surge (instantaneous energy increase) if it would occur on the input of a drive.
As shown in Figure 2-19, the three-phase inductor reduces any surge cur-rent that could enter the input section of the drive.
DC drives use power components called thyristors (SCRs- Silicon Con-trolled Rectifiers). If an output short to ground condition exists, tremen-dous inrush current into the drive would result. This inrush current would result in severe damage to the drive’s power conversion and logic sections.
When inductors are placed ahead of the drive unit, the input and protec-tive sections of the drive see a limit and delay of short-circuit current. The delay allows time for incoming line fuses to clear and protect the expen-sive power components.
Relays
Relays are considered an electromagnetic device. Basically, a magnetic field is used to energize a coil, which in turn causes an electrical contact to close. Relays consist of two separate circuits—a control circuit and a power circuit. The control circuit contains the magnetic coil and can be operated by DC or AC voltage, depending on the coil make-up. The power circuit is where the electrical connection is made that allows current to flow. Power circuits can make single contact (SPST—single pole single throw) or are multi-contact (e.g., DPDT—double pole double throw). Power circuits are sometimes considered Form C because of the nature of their operation, which look like the letter C. They are also considered “dry” contacts, since they do not source voltage themselves (Figure 2-20).
Figure 2-20a shows the relay is energized. Figure 2-20b shows the relay power circuit when “de-energized.”
As shown, the relay coil is operated from a 120-VAC power source. Typical relay coil voltages are 120 VAC, 240 VAC, 24 VDC, and 12 VDC. The power circuit contacts are typically rated in both DC and AC voltages and
Figure 2-19. AC inductor (line reactor)
DC Drive Module Fuse
DC Motor Fuse
Fuse
Chapter 2: Review of Basic Principles — Electrical/Electronic Devices 31
currents. The relay may indicate a maximum switching voltage of 300 VDC/250 VAC. It would also indicate a maximum switching current rating (e.g., 8A @ 24 VDC, 0.4A @ 250 VDC, or 2000 VA @ 250 VAC).
As the power circuit contacts open, a small amount of arcing occurs. To help preserve and increase the life of the contacts, some type of noise sup-pression is normally used. A MOV (metal oxide varistor) or RC (resistor/
capacitor) network is installed in series with the incoming power circuit.
As shown in Figure 2-20, suppression would be installed if the incoming power circuit is controlling the coil of a main contactor (discussed in the next section).
Contactors
Contactors are essentially higher power relays. They use the same two cir-cuits that a relay uses: the control circuit and power circuit. It is a common practice to indicate the contactor control circuit (coil) separate from the power circuit. Figure 2-21 shows the contactor coil symbol and the sepa-rate power circuit.
As with relays, the coil (control circuit) is designed for various voltages.
Coil voltages of 115 VAC, 230 VAC, and 460 VAC, single phase are com-mon because of their easy connection to incoming line power. The power circuit is designed to handle DC or AC voltage, at a specific current rating.
In the case of DC drives, a main DC Loop contactor is often used to energize the armature circuit of the DC motor. When control-coil voltage is
removed, a positive disconnect is accomplished, completely disengaging DC voltage from the motor.
Figure 2-20. Relay symbol and electrical connections
Figure 2-21. Contactor control and power circuits 24VDC
Coil 24VDC
120VAC Coil 120VAC
MOV RC
Transformers
Transformers are essentially two inductors separated by an iron core. The iron core increases the electromagnetic characteristics and efficiency of the transformer.
The transformer transfers energy from the primary side (input) to the sec-ondary side (output). Figure 2-22 shows the construction of a transformer and its symbol.
There is no physical connection between the coils. Therefore, the trans-former has an inherent capability of isolating the primary side from the secondary side. A transformer that merely transfers the same voltage from the primary to the secondary (e.g., 460 VAC to 460 VAC) is termed an iso-lation transformer. Because of the design, the transformer isolates the sec-ondary from the primary, in the event of a short circuit or line
interference. In this way, a shock hazard will not exist—protecting the equipment and personnel.
Another feature of the isolation transformer is its ability to step up or step down voltage. For example, common three-phase industrial voltages are 240 VAC or 460 VAC. When an isolation transformer is used, the second-ary can step down voltages to 208 VAC or 415 VAC.
One note of importance should be stated here. If voltage is stepped down, the available current is stepped up. For example, if the input voltage is stepped down two times, then the output current will be stepped up two times. The reverse is also true. If the voltage is stepped up two times, then the output current will be stepped down two times. This is because of the fact that power in = power out minus any losses.
The transformer is a very efficient device, with losses due only to heat.
Note: P = I × E, where power (watts), I = intensity of current, and E = electromo-tive force (voltage). If power in = power out, then input I × E = output I × E.
If the voltage is doubled in the output, then the current must be half, in order for the equation to be equal.
Figure 2-22. Transformer and symbol
Primary Secondary
AC Output AC
Input
Symbol
Chapter 2: Review of Basic Principles — Electrical/Electronic Devices 33
Transformers are rated in kVA (kilovolt amps). A transformer with a large kVA or current rating (200 kVA) will be physically larger than one of a smaller rating (2.0 kVA). For example, on a 150-HP, 460-VAC DC drive, a 175-kVA input isolation transformer would be required. A 7-1/2 HP DC drive at 460 VAC would require only an 11 kVA input isolation trans-former.
Current Transformers
Another type of transformer used in conjunction with drive technology is termed a current transformer. Basically, this transformer is a sensing circuit that identifies the amount of current flowing through an incoming power line. The basic characteristics of a transformer apply. Current flowing through a conductor induces a corresponding current into the coil around the conductor. Figure 2-23 shows a current transformer (CT) used to sense input current to an AC drive.
If an extremely high amount of current is detected by the CT and corre-sponding protective circuitry, the drive is shut down to protect itself and the motor. Drive diagnostics and protective features will be explored in the AC- and DC-drive sections.
Figure 2-23. Current transformer use AC Drive
With Diagnostics
Feedback to Drive Diagnostics
3 Ph Input AC
CT CT
M
Capacitors
As discussed in an earlier section, capacitance is the ability to block DC and allow AC to flow in a circuit. The main device that accomplishes this phe-nomenon is the capacitor with the electrical symbol shown in Figure 2-24.
As seen earlier, a capacitor is basically two plates of conducting material, separated by an insulator. The insulator is usually plastic, mica, or even air, and is called a dielectric. Capacitors are found in many sizes and just as in resistors, the higher the value of a capacitor, the greater the opposition to DC flow. Capacitors are rated in units called farads, with typical values in microfarads (µF) due to the large unit of measure. (Note: a µF is 1/
1,000,000 of a farad.)
Capacitors also have another rating—working voltage. WVDC (Working Voltage DC) is the maximum voltage a capacitor can handle without being damaged. In drive power circuits, 800 or 1000 WVDC are typical values.
As mentioned earlier, one of the main uses of capacitors in drive circuitry is to filter out AC waveforms in a DC circuit. Figure 2-24 shows this func-tion, using an electrolytic capacitor in the DC bus circuit. The capacitor’s charging and discharging capability make it a perfect candidate for smoothing out the DC waveform, which in turn, creates a purer output voltage waveform.
Semiconductors
To understand the theory of regulation and power conversion in adjust-able speed drives, it is essential that semiconductor basics be discussed.
Semiconductors are poor conductors under normal conditions. However, once impurity elements are added to semiconductor materials, their atomic structure changes. Semiconductor materials can be P (positive) or
Figure 2-24. Capacitor symbol and use
Motor L1
L2 L3
C
DC Bus
+ _
+ _
+
_
+
Chapter 2: Review of Basic Principles — Electrical/Electronic Devices 35
N (negative), depending on the impurity added at the time of manufac-ture.
Silicon or germanium provide the base material for electronic components such as SCRs (silicon-controlled rectifiers), GTOs (gate turn-off devices), and ICs (integrated circuits). They also form the base for many of the recently developed logic components such as ASICs (application-specific integrated circuits) and DSPs (digital signal processors).
In an earlier section of this chapter we discussed basic electrical principles, with electrons flowing from negative to positive. This is a common
description when discussing the basics of electricity. However, because of the nature of semiconductor circuits, it is conventional to trace current flow from positive to negative. This is termed conventional current flow and will allow for easier understanding of circuit operation. Therefore, for the remainder of this book, we will consider the current flowing in a drive cir-cuit from positive to negative.
A continuing concept throughout discussions of electronics is that elec-trons tend to seek a point of zero potential. This is similar to the concept that water will tend to seek its lowest point. Voltage will cause current to flow from the highest potential (supply) down to the lowest potential (ground). Voltage will continue to force current flow to ground until no difference of potential exists or until the supply is cut off.
Diodes
Diodes may be considered the simplest semiconductor device. As shown in Figure 2-25, current flow follows in the direction of the triangle.
As shown in the figure, current flow would be from positive to negative. A diode allows current flow only in one direction, much like a check valve would allow water to flow in only one direction. If the direction of current flow is from negative to positive, no current will flow. This condition would exist when the power source is AC, which changes direction 60 times/second. A diode has the effect of cutting off the bottom portion of
Figure 2-25. Diode symbol and circuit operation
the AC waveform. In doing so, a diode can basically change AC to DC, as shown in Figure 2-26. The circuit on the right (Figure 2-26) is commonly referred to as a switch mode power supply.
Many modern three-phase AC drives use a six-diode bridge rectifier circuit that changes AC to DC, which is supplied to the DC bus circuit.
This type of rectifier circuit is the most common in use by today’s drive manufacturers and is shown in Figure 2-27.
This diode bridge circuit works continuously to change AC to DC. There is no control of the output voltage value. As shown in the Figure 2-27, a 460-VAC 3-phase input will yield a 650-VDC output (line voltage × 1.414).
If the drive input voltage is 230 VAC, the diode bridge output would be half (e.g., 325 VDC).
Figure 2-26. Changing AC to DC, using diodes
Figure 2-27. Three-phase diode bridge rectifier circuit Load Load
Half-Wave Rectifier Full-Wave Bridge Rectifier
Load
650 VDC Output 460 VDC
3 Phase AC Input
Chapter 2: Review of Basic Principles — Electrical/Electronic Devices 37
Thyristors (SCRs—Silicon-Controlled Rectifiers)
Diodes provide continuous, fixed-voltage output. However, there are many times when a variable-voltage output is needed. When this is the case, SCRs are typically used. SCRs are essentially two diodes connected back-to-back, with a controlling element called a gate. Figure 2-28 shows the schematic symbol for an SCR.
When the gate voltage is positive, with respect to the cathode, current is allowed to flow from the anode (+) to the cathode (-). This operation is commonly referred to as gating on the SCR or in other terms, firing the SCR, full-on. The circuitry associated with SCR gating is called a pulse generator.
Current flow is limited only by the resistance of the circuit, external to the SCR. As with a diode, an SCR does not allow current to flow in the reverse direction (from cathode to anode).
The SCR is made up of positive (P) and negative (N) junctions—actually two P-N junctions back-to-back. When no gate voltage is applied, at least one of the P-N junctions is reversed biased (polarity in the opposite direc-tion). When a junction is reversed biased, no current will flow. When a signal is applied to the gate, both P-N junctions are forward biased (polarity in the direction of current flow). Current then flows in the forward direc-tion.
When the positive gate signal is removed, the SCR still remains in a con-ducting state. The SCR can be “turned off” by cutting off the flow of for-ward current, by applying a negative polarity to the gate, or by reversing the polarity of the voltage on the anode. When polarity to the SCR is reversed, the SCR will block current flow from the anode to the cathode—
basically shut off.
The ability of the SCR to handle high voltages, its small size, and adequate switching speed make it a prime candidate for use in DC as well as AC drives. Switching speed is the ability to quickly turn on and off the device.
SCRs can be “turned on” in 1 to 5 µs. However, it may take as much as 100 µs to turn off the SCR.
Figure 2-28. Silicon-controlled rectifier (SCR) Direction of
Current Flow
Gate Gate
P
P N
N _
+ Anode (+)
Cathode (-)
The basic purpose of the SCR is to control current flow. Figure 2-29 shows how that is accomplished.
The amount of current output from the SCR depends on when the device is gated on. If the SCR is gated on early in the AC cycle, a higher average output voltage is seen, compared with a late gating during the AC cycle.
Many DC drives use SCRs in the power section. The basic purpose of a DC drive is to generate DC output from an AC input. The DC output must be variable to allow the DC motor speed to change.
GTOs (Gate Turn-Off) Devices
A GTO is basically an SCR with a bit more capability. The GTO is a switch-ing device that provides the latchswitch-ing function (turn-on) capability of an SCR and the controlling function (self turn-off) of a transistor. Turning a GTO on and off is accomplished by simply changing the polarity of the gate current. The GTOs switching time and high current capability is very similar to that of the normal SCR. The GTO features a good short-circuit overload rating, when used in conjunction with overload fuses. Figure 2-30 shows the schematic symbol for a GTO.
Figure 2-29. Gating of SCR’s
Figure 2-30. GTO (gate turn off) device
Ave.
DC Output Ave. DC Output
Gate Pulses
(Early Gating) Gate Pulses
(Late Gating)
Gate
Anode ( )+
Direction of Current
Flow
P +
_ P N
Gate Cathode ( )- N
Chapter 2: Review of Basic Principles — Electrical/Electronic Devices 39
GTOs have been successfully applied in AC-drive technology for many years. GTOs can withstand high voltages, between 600–800 V, and have high current capability. Some drive manufacturers still use the GTO for horsepower ranging from 5–1500 HP. The GTO has the ability to be switched on with a positive gate pulse and off with a negative gate pulse without the use of power commutating (switch-off) circuits. A disadvan-tage of GTOs over SCRs is that GTOs have two to three times the power loss because of the voltage drop across the device.
Transistors
A transistor is a connection of three sections of positive and negative semi-conductor material. The P and N materials are connected so that one of the materials are joined or sandwiched between the other two sections.
There are two basis styles of transistors—NPN and PNP. The style is deter-mined by which material is in the middle, between the other two materi-als. All transistors contain three leads: emitter, base, and collector. Figure 2-31 shows the two basic styles of transistors and the schematic symbols for each.
How the transistor is biased will determine the amount of current flow.
The majority of the current flows through the emitter and base to the col-lector junction. Very little current (2–8%) flows through the base itself. In a transistor, the voltage applied to the base–emitter junction controls the amount of current in the collector. In this way, we can use the transistor as a switching device—allowing large amounts of current output to flow, with only a small amount of controlling voltage applied to the base. When the control current is removed from the base circuit, the transistor stops conducting and turns off.
Figure 2-31. Transistor construction and symbols
P P
C C
B B
E E
C C
B B
E C
P N N
N
E
Darlington Bipolar Transistors
A transistor, is a true switch—it can be turned on and turned off by use of a command signal. The power Darlington transistors are essentially two or more bipolar transistors, internally connected in one package. Figure 2-32 shows the schematic symbol for the power Darlington transistor.
The power Darlington transistor has the advantages of the bipolar transis-tor, but provides high gain circuitry, much higher than a normal bipolar transistor. The bipolar and power Darlington transistors both have an advantage over SCR and GTO devices. Circuitry that needed to turn on the transistors are much less intensive than that of SCRs and GTOs. In many cases, base driver circuits can be included on existing drive circuit cards, rather than require separate, more complex cards.
IGBTs (Insulated Gate Bipolar Transistors)
IGBTs have fast become the power semiconductor of choice with drive manufacturers from fractional to well over 1000 HP. The IGBT carries all of the advantages of the bipolar and Darlington transistors, but offers one additional feature—increased switching time.
As you will learn in later sections, the switching time of the power
As you will learn in later sections, the switching time of the power