Drives and Drive Systems
Electric drives generally include both an electric motor and a speed control unit or system. The term drive is often applied to the controller without the motor. In the early days of electric drive technology, electromechanical control systems were used. Later, electronic controllers were designed using various types of vacuum tubes. As suitable solid state electronic components became available, new controller designs incorporated the latest electronic technology.
In the past, a variety of terms have been used to describe a system that permits a mechanical load to be driven at user-selected speeds. Some of these terms are Variable-Speed Drive, Variable Frequency Drive, Adjustable-Frequency Drive, and Adjustable-Speed Drive.
With these various names, the term "variable" implies a change that may or may not be under the control of the user. "Adjustable" is the preferred term, since this refers to a change directly under control of the user. Lastly, the term "frequency" can only be applied to drives with an ac output, while the term "speed" is preferred since this includes both ac and dc drives. The term most commonly accepted is Adjustable-Speed Drive (ASD). Each of the general types of electric drives can be further divided into numerous variations. Some of the most popular types of electric drives include adjustable voltage DC drives, adjustable frequency AC drives, adjustable voltage AC drives, servo drives, eddy current drives and wound-rotor motor drives. For the purposes of this document, some of the more common electric drives have been highlighted.
Eddy Current Drives
Different Drive Applications
Drive Video Overview
AC Drives, like the ones pictured above from Eaton Cutler Hammer, serve a variety of different industrial and commercial applications.An AC Drive is an electronic device that converts a fixed frequency and voltage to an adjustable frequency and AC voltage source. It controls the speed, torque, horsepower and direction of an AC motor. AC Drive is also a term used for an AC inverter and is sometimes used to describe a particular section of an AC drive. The section uses the DC voltage from a previous circuit stage (DC Bus) to produce an AC current or voltage having the desired frequency.
AC Drives are also referred to as Variable Frequency Drives (VFD's) or Adjustable Speed Drives (ASD's). These drives are gaining in popularity due to the energy savings that can be obtained related to the AC technology. In addition, AC motors are simpler than DC and usually an "off-the-shelf" item compared to DC motors. Advances in technology have made the size, cost, reliability and performance of AC drives very appealing in industrial variable speed applications.
The AC Drive system is very simple. It consists of three components:
- AC Motor - Usually NEMA Design B, squirrel cage induction, 3-phase motor. The AC motor drives the device (fan, pump, etc.) by converting the electrical power to mechanical power.
- Motor Control Section - (also called Inverter section) The motor control section controls the motor's speed by converting utility power into adjustable frequency power.
- Operator Interface - The operator control allows the operator to command the motor to function as desired through the use of motor control inputs and outputs.
A diagram of a modern DC drive would show that much of the circuitry appears similar to an AC drive. The main difference is that the rectifier stage and output stage of the DC drive are shared because the DC drive simply adjusts the DC voltage and current instead of inverting it back to AC. Since the output voltage for the drive is DC, SCRs will be used in rectifier circuits. The newest drives have programmable parameters similar to AC drives in that they set the maximum voltage, current, and speed, as well as supply protection against overcurrent, over temperature, phase loss of incoming power, and field loss.
DC drives are best used in applications where:
- The DC motor exists in a reasonably clean and dry environment, and the use of DPG, DPG-FV, TENV, or TEFC motor enclosures is required.
- The application requires a wide range of changing loads.
- Motor speeds can reach 2500 RPM
- Starting torque is greater than 150% or unpredictable.
- Application HP requirements are medium to large.
The servo drive works in similar fashion to AC and DC drives, although, it is more technologically advanced. They obtain a signal from a control system, amplify the signal, and transmit electric current to a servo motor in order to produce a proportional amount of motion as called for by the original command signal. Usually, the command signal indicates a desired speed, but can also represent a specific position or amount of torque. A sensor attached to the servo motor reports the motor's status back to the servo drive. The servo drive then compares the motor status with the original commanded motor status. From there, the servo drive then alters the voltage frequency to the motor to correct for any differences from the original commanded status.
If the servo system is properly configured, the servo motor will run at a speed that is very close to the velocity specified by the original command signal sent by the servo drive. Although the servo system is extremely accurate, there are many parameters which can be regulated to gain optimum performance for any given application. This is often referred to as performance tuning. Performance tuning is the process in which certain parameters are adjusted. Some of these parameters include:
- Stiffness - also known as proportional gain
- Damping - also known as gain
- Feedback gain
The photo above is the clutch system within an Eddy Current drive and motor system. The cluth consists of both the fixed speed rotor and the adjustable speed rotor.The basic building blocks of an eddy current drive system is, of course, formulated around the concept of eddy current, itself. Eddy current is an electric current induced in conductors when the conductor is exposed to a changing magnetic field. This can cause a circulating flow of electrons, or eddy current, within the structure of that conductor.
Eddy current drive systems contain a fixed speed motor and an eddy current clutch. The eddy current clutch consists of two rotors (a fixed speed rotor and an adjustable speed rotor) separated by a small gap of air. A field coil containing a direct current produces a magnetic field that determines the torque transmitted from one (input) rotor to the other (output) rotor. The controller can regulate the speed by only allowing the clutch enough current to operate at a specific speed.
Eddy current drive systems are found in many applications and industries, including:
- Asphalt and metal forming
- Press stamping
- Water and wastewater
Electric drives with numerical control began to be used in several areas of technology during the 1960's. Their services handled applications such as multi-operation metal-cutting machine tools and automatic and semiautomatic transfer machines. The invention of automated electric drives for the purpose of individual production operations and processes is the basis for the combination of automation of production. Integrated automation will require electric drives with a wider range of power capacities and control functions, improved reliability, and optimum size and weight.
It is common for processes including standard pumps, fans, and blowers, to incorporate the use of drive systems. Also, newer applications including hoists and cranes, conveyors, machine tools, custom machinery, film lines, extruders, process machinery, and textile-fiber spinning machines are among many other uses for drive systems.
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