INDUSTRIAL ELECTRONICS

muneebmalik — Thu, 03 Sep 2009 19:47:54 +0000

Block Diagram of a Variable-Frequency Drive

muneebmalik — Thu, 03 Sep 2009 19:02:46 +0000

The block diagram of a typical variable-frequency drive can be divided into three major sections:

the power-conversion section
the microprocessor control section (CPU) and the control section that includes the external switches an signals to control the drive operations
the power section where AC voltage is converted to DC and then DC is inverted back to 3-phase AC voltage

The block diagram below contains the power section of the VFD as three separate sections to indicate the three main functions:

Explanation of Drive Parameters

muneebmalik — Thu, 03 Sep 2009 18:55:45 +0000

The drive also provides metering parameters. These can be integrated with protection, display, and fault functions. Most drives have displays that are built into the face of the drive. The technician can use the display on the face of the drive to observe the amount of input voltage, the amount of current, the frequency, the temperature, the output voltage, and the last fault that was recorded.

Advanced setup parameters include the type of braking the drive will use. The choice for this parameter is no braking (coasting to a stop) or braking, and the amount of braking voltage and the amount of time the braking voltage should be applied are determined. Another advanced setup parameter is called DC boost voltage, which is DC voltage that can be applied with AC frequency during starting or at times when the motor needs more torque. The DC boost voltage makes the magnetic field in the motor stronger to reduce the amount of slip the motor has. This will allow the motor to provide additional power in applications where more starting torque is needed or when more torque is needed during specific loading conditions.

The drive has the ability to test hundreds of points in its circuit boards for changes in voltage, current, frequency, and temperature. The present value of each of these variables at the input and output stages of the drive can be compared against the value set into the parameter. If the value is exceeded, the drive can indicate each occurrence with a fault code. The drive also changes the state of contacts that can be used to enable a fault indicator lamp or horn. If the fault has occurred, it will be stored in the drive where it can be brought to the display by pressing a series of keys on the front panel of the drive. The serial port connection provides a method to send the fault codes to an external controller such as a PLC where they can be logged with the date and time they occurred and they can also be printed as they occur. Some drives have the capacity to store multiple faults so that the technician can review the last five faults. This provides a means to detect and store multiple faults if more than one problem occurs during the fault condition. Typical fault conditions are low voltage, high voltage, high current, and over-temperature.

Variable-Frequency Drive Parameters

muneebmalik — Thu, 03 Sep 2009 18:53:17 +0000

Variable-frequency drives have had the ability to provide features such as ramp up speed, ramp-down speed, boost voltage, and braking functions since they were first designed. Prior to having a microprocessor, these features and functions were designed into the op amp circuits that controlled the drive and were enabled by the placement of jumpers or the setting of dip switches. After microprocessor chips were integrated into the drive, these features have become programmable and in many cases they have become proportional so that you can ask for braking, but you can limit the braking to 60% for 3 seconds. If you select ramp up, you can select more than one ramp speed. Then you can integrate the selection with an external switch so that the ramp speed can be selected by external conditions. The numbers that are programmed into the drive to select these features are called parameters. The type and number of parameters will vary from drive to drive. The Allen-Bradley 1336 drive has 89 parameters while the Allen-Bradley 1305 drive has 136 parameters. The table below shows a typical list of parameters for a variable-frequency drive.

The drive has a set of standard values to be used for the parameters. These parameters are called the default settings or factory settings. It is important to record the actual parameter settings if they are different from the factory settings, so that the proper settings can be put into the drive if it is ever removed and replaced with a new one. The parameters provide a means to customize the drive to any specific application. Some manufacturers provide software that can be used on a portable computer to save and load the parameters from a drive. The parameters can also be saved and loaded from the PLC program, which means they can also be changed while the system is in operation to provide custom parameters for multiple recipes.

The parameters can be described in groups by their function. Examples of these groups include metering, setup, advanced setup, frequency settings, diagnostics, faults, and process displays. Examples of the setup group include minimum frequency and maximum frequency. This allows the minimum and maximum frequency to be fixed so that the motor does not run less than the minimum value and more than the maximum value.

Another setup parameter is acceleration time, which determines the acceleration ramp. For example, if you select the ramp time as 10 seconds, the drive will increase the frequency proportionally from the programmed minimum frequency to the programmed maximum frequency in 10 seconds. If you wanted the motor to ramp up to speed more quickly, you would shorten the ramp time. If you wanted the motor to ramp up more slowly, you would increase the ramp time. Most drives have more than one acceleration ramp parameter and each ramp is enabled to the input switches that were previously discussed. This means that you could have up to three acceleration ramps that would be selected by switch 1, 2, or 3. The deceleration time is also programmable. Three deceleration ramps are available for this drive.

Another setup parameter available for the drive is overload current limit. This parameter will act like a programmable fuse. You can select any value from 100-115. The other variable that operates with the overload current is the amount of time this current can exist. Since the drive is controlled by a microprocessor, it monitors the current and voltage and turns off the drive if these values become excessive.

AC and DC Drives: Preventive Maintenance

muneebmalik — Thu, 03 Sep 2009 18:48:35 +0000

If your plant makes extensive use of AC or DC drives, it pays to perform regular (scheduled) preventive maintenance on all drives and controllers. Preventive Maintenance can identify problem areas of your systems and quite possibly some minor adjustments and upgrades could prevent major problems and downtime in the future. Properly-trained technicians will attempt to spot potential problems, and recommend corrective action before a failure causes unscheduled and costly downtime. Some preventive maintenance may be done while the equipment is running. Other types of preventive maintenance requires that the drive be shut down with a proper lockout-tagout.

AC/DC Drive Preventive Maintenance may also be farmed out to external contractors. With this service, the contractor may included a checklist of measurements taken:

DC System Data

Drive
Speed Reference
Armature Voltage
Armature Amps
Field Voltage
Field Amps
Tachometer Voltage
Motor R.P.M.
Oscilloscope test

AC System Data

Drive
Speed Reference
Motor Voltage
Motor Amps
Tachometer Voltage
Motor R.P.M.
Oscilloscope test

The contracted service will usually check wiring, motors, blowers and overload circuits. Also they may interview operators to see if there is any tweaking that may be needed to enhance systems operations.

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Insulated Gate Bipolar Transistor Drives

muneebmalik — Thu, 03 Sep 2009 18:42:32 +0000

In newer drives, such as the 1336 PLUS AC Drive shown on the left, the transistors may be replaced by insulated gate bipolar transistors (IGBTs). The IGBTs are used because they can be switched on and off at much higher frequencies — aka the switch frequencies — that do not conflict with other signals. The higher frequencies are also used outside of the audible range for humans, so the drive will not emit a hum that humans can hear. Typical switch frequencies are 3KHz to 4KHz. (With an older, SCR-based drive, switch frequencies are 250 to 500 Hz). As one can imagine, the higher the switch frequency, the smoother the output waveform and the higher the resolution. However, higher switch frequencies decrease the efficiency of the drive because of increased heat in the power devices.

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The diagram below shows two pairs of IGBTs used to produce two phases of a typical output section for a variable-frequency drive using IGBTs instead of bipolar transistors. From this section one can see that one IGBT of each pair is connected to the positive bus and a second is connected to the negative bus. The IGBTs operate similarly to the transistors in that they are cycled on and off at high frequencies within the overall waveform of a sine wave.

Variable-Frequency Drives and Safety-Interlock Circuits

muneebmalik — Thu, 03 Sep 2009 18:32:16 +0000

A contactor on the input of the VFD rather than the output is preferred, although on can make the case for one on the output.

Putting a contactor on the output of the VFD will assure immediate removal of voltage from the motor which is what you want. On the other hand, some drives are easily damaged by switching on their output and that it is possible that the motor could be reconnected to the output of a drive that was operating above zero frequency and this could also damage the drive. (Effectively a direct (non-soft) or full-voltage start on the output of a VFD)

Placing the contactor on the input of the drive would shut off all power to the motor, but there could be a small delay due to the energy stored in the capacitors in the VFD power supply. This would be safe, would not damage the drive and would prevent direct-online switching on the output of the VFD. The drive would always start from zero when the contactor was re-closed. You may have to reset an under-voltage trip on the drive, but that may be automated if required. You may also use soft starts between the VFD and the motor.

There are other limits of installing a contactor on the input side, including possible damage to the DC bus capacitor pre-charge circuit. The pre-charge circuit will be designed for a certain amount of operations/hour. Is it acceptable contactor(s) to use late-make/early-break auxiliary contact on the output, wired back to the drive?

The early-break contact should break around 500ms before the main legs to be effective. Typically-available 10ms early-break aux-contact isolators are ineffective as the residual motor field will not have decayed, resulting in an inductive kick 10ms later when the 3 main legs open. Pffff! goes the VFD output! It is unclear in this situation what a suitable isolator could be. The only practical solution is a lockable isolator, the expectation being that the person with the key understands the need for care.

If one is expecting a lot of switching on the input, then this could be a problem. If an early-break contact on the contactor is used to panic-stop the drive first, then that may be okay. It depends on the timing and also how the drive responds to this sort of treatment. Some are more tolerant than others. Consult the manufacturers of the drive.

In the case of an emergency, nothing can stop the motor faster than the drive itself (most drives having an E-Stop input anyway). How the VFD handles the emergency condition is usually a programmable function: ramp down at max or coast. This assumes, of course, that the drive is undamaged, one would be wise to tie in a line or load contactor to the NO (normally open) output of the drive-status relay. If it’s an emergency, break it on the load side (drive output). Nobody at the inquiry is going to sympathize with your concerns to pamper the thyristors when weighed against the potential risk to human life.

Increasing the Starting Torque of a Single-Phase Motor in Industry

muneebmalik — Thu, 03 Sep 2009 18:30:09 +0000

The torque of a single-phase motor can be changed by adding capacitors to the start or run winding of the motor. When the single-phase motor is used without capacitors, as has been shown in the diagrams presented so far in this section, the motor is called a split-phase motor.

When a start capacitor is connected in series with the start winding and centrifugal switch, the motor is called a capacitor start, induction run (CSIR) motor. When the motor has a start capacitor in series with the start winding and a run capacitor is connected permanently across the run and start terminal, the motor is called a two-capacitor motor or a capacitor start, capacitor run (CSCR) motor. If the motor has only a run capacitor connected permanently across the start and run winding, it’s called a permanent split capacitor (PSC) motor. If the rotor of the single-phase motor is made of copper wire rather than a squirrel-cage rotor, it’s called a wound-rotor motor or repulsion start motor.

The following sections will explain the operation of each of these motors.

Methods of reversing these motors are also presented with their diagrams. At the end of this section methods of troubleshooting each of these types of motors are provided. We hope to provide an understanding of how these motors operate and how their rotation is reversed. It will also help to understand the methods of reconnecting the motor to operate on dual voltage or dual speeds. This information is important when one must connect motor control devices to them such as reversing starters or dual-voltage starters.

Interpreting the Wiring Diagrams and Tables

muneebmalik — Thu, 03 Sep 2009 18:22:57 +0000

Two ways of representing the connections that must be made to the motor terminals. One way shows the connections with an electrical diagram. and the second way is to show a table that indicates which terminal numbers should he connected. When one is ready to make the motor connections, note that the terminals in the motor are marked with numbers which are stamped into the wire material, or identified with a metal tag that has a number on it. The metal tag is crimped on each wire near the terminal end. After one has located all the terminals and have identified them by their numbers, one is ready to make the connections shown in the diagram for your application.

The table lists the terminals that should be connected to the three input- voltage wires. These are identified as L1, L2, and L3, and any terminal number listed in the category under the line number should be connected to that line. Be sure to check the row indicating the speed for which the motor will he wired. The second column lists the wires that are left open. That is, the wires listed in this column shouldn’t be connected to anything. They are supposed to remain unconnected and they should have an insulated wire nut or cap placed over the end of the wire securely so that it does not come in contact with any metal parts of the motor or other energized wires. In some tables any lead that is not listed in another column should be left open.

The third column lists the leads or wires that should be connected together. In other words, the wires listed in this column should be connected together and no power should he connected to these leads. The leads must he secured together with a wire nut and wrapped with insulating tape because they will be energized. In some diagrams no terminals will he listed in this column, which means that all the leads are used in one of the other columns. Be certain to account for every lead before returning all the leads back into the motor and replace the field wiring access cover.

These diagrams are extremely useful when the application one is working with requires the motor to be reconnected on the factory floor. Many times these diagrams are not readily available when one needs them, so this provides that much needed reference. These motors can also be reversed by exchanging two of the three supply voltage lines. This allows the motors to he used in the widest possible number of applications.

If you’re a technician, it’s important to realize that you’ll be required to make these changes yourself or direct someone to make them for you. So you must understand the concept of changing the connections of motor leads to make the motor fit the application

External Control Switches and Contacts in Industrial Electronics

muneebmalik — Thu, 03 Sep 2009 18:21:06 +0000

the application for the drive below is enabled and controlled by external switches or contacts. The National Electric Code (NEC) and local codes will specify the exact number of external controls required for each drive. The block diagram for the AC drive (variable-frequency drive) below shows typical control switches. You can note that at the upper left side of the drive that a NO (normally open) start push-button is connected to terminals TB2-7 and TB2-6, and an NC (normally closed) stop button is connected between terminals TB2-7 and TB2-8. The voltage for these circuits is provided internally from TB2-7. Power must be received continually through the stop push-button, and when the start push-button is depressed a pulse signal will be received and the drive’s start sequence will begin. The start sequence will provide a ramp that is used to start the motor by slowly increasing the frequency and voltage. Anytime the stop button is opened, the drive will be stopped and if a ramp-down sequence is programmed, the drive will come to a gradual stop. If a ramp-down sequence is not programmed, the motor can be stopped with braking, or it can be allowed to coast to a stop.

The second set of switches provides a set of terminals where an external set of contacts from a control relay can be connected to provide a drive-enable function. When the contacts are connected to terminals TB2-11 and TB2-12, power will flow from terminal TB2-12 through the closed contacts to terminal TB2-11 to enable the drive. When the drive is in the enable condition, power is allowed to flow through the drive motor. Anytime these contacts are opened, the drive will become disabled, and the drive will not send voltage to the motor.

The drive can be switched from forward to reverse with a remote switch. The external switch is connected between terminals TB2-12 and TB2-13 to provide the reverse signal to the drive. Anytime the switch is closed and this circuit is made, the drive will reverse the phase sequence to the motor, which provides the same effect as physically swapping the leads U and W. Please not that whenever two leads of a three-phase power supply are switched, the phase relationship between these two leads is changed and the motor will run the opposite direction. This is an important feature of variable-frequency AC drives in that they can reverse the direction of the motor without using expensive reversing motor starters.