DC Drives, Variable Speed Drives
DC Drive Characteristics
Constant Torque Applications
Armature voltage controlled DC drives provide constant torque. They can deliver the rated motor torque at any speed between zero and the base speed of the motor. Horsepower varies in direct proportion to speed. Full rated horsepower is developed only at base speed. Full motor torque is available from zero to 100 percent base speed
Constant Horsepower Applications
ARMATURE CONTROLLED DC DRIVES
Certain applications require constant horsepower over a specified speed range. Because an armature voltage controlled DC drive has constant torque characteristics, the drive must be oversized to handle these applications.
For example, a drive required to deliver constant horsepower over a 2:1 speed range must be rated at twice the required horsepower. This is because it only develops 50 percent of its rated horsepower at half speed.
The motor horsepower rating required for any constant torque drive operated in a constant horsepower application can be easily calculated by multiplying the desired horsepower by the ratio of the speed range over which horsepower must remain constant. If 5 HP is required over a 3:1 range, an armature-only controlled drive rated for 15 horsepower (5 x 3) would be required.
FIELD CONTROLLED DC DRIVE
In a shunt-wound DC motor, a reduction in field voltage to less than the design rating results in an increase in speed for a given armature voltage. This also results in a higher armature current for a given motor load. A simple method of accomplishing this type of control is by inserting a resistor in series with the field voltage source. This may be useful for achieving an ideal motor speed for the application.
A more sophisticated method uses a variable voltage field regulator. The variable regulator provides coordinated automatic armature and field voltage control for extended speed range in constant HP applications. The motor is armature voltage-controlled for constant torque, variable HP operation to base speed. The motor is then transferred to field control for constant HP-variable torque operation to maximum speed.
DC Drives, DC Motor Drives, Variable Speed Drives
Common terminology for DC Motor Drive to this day is Variable Speed Drive. Some call it Adjustable Speed Drive or simply DC Drive.
Vast majority of Adjustable, Variable Speed DC Motor Drives of all types draw their power from constant voltage 60 Hz mains and in nearly all motor drives the first stage consists of rectifier which converts the AC power to a crude form of DC power. When a constant voltage DC output is required, a simple diode rectifier is sufficient, however if the output voltage needs to be varied a fully controlled rectifier is used.
Thyristor and diodes based rectifier provides a low-impedence adjustable DC voltage for the motor armature , thereby providing speed control.
For motors up to a few horsepower, armature control can be provided from either single phase or three phase mains. For larger motors three phase mains is always used. DC Drives are differentiated by it ability turn motor bi-directional.
Non-Regenerative (Single Quadrant) Drives
Supplying power from AC mains to a DC motor running in one direction is known as one-quadrant, Non-regenerative drive.
Regenerative (Four Quadrant) Drives
Supplying power from AC mains to a DC motor running in bi-directional is known as four-quadrant, Regenerative drive. DC motor is inherently a bi-directional, if we apply a positive voltage V greater than E, current flows into the armature and the motor turns in one direction. If we reduce V so that it is less than E, the current and torque automatically reverse directions and the motor acts like a generator, converting mechanical (kinetic) energy into electrical energy.
In order to make DC motor bi-directional, all we have to do is to reverse the polarity of the armature. We need power supply which can provide positive or negative voltage and simultaneously handle either positive or negative current.
Regenerative Analog Drive
Contactor wear can be chronic. It will repeatedly shut down an SCR speed control when reversing contactors are used for rapid forward-reverse cycling.
With Polyspede PRD12B you can start, stop, reverse and brake with no contactor wear because there are no contactors. All switching is done electronically.
PRD12B is your best choice where controlled hold-back is required, as in braking high inertia loads, hoisting and unwinding. And the optional plug-in interface the PRD12B is ideal for high resolution positioning in servo systems.
Braking is regenerative. The motor becomes a generator to send the braking energy back to the AC power lines, eliminating costly, energy wasting resistors.
• Contactorless Braking and Reversing
• 115VAC pushbutton or selector switch
• Subcycle fuses protect the power semi-conductor against short circuits in the wiring and motor. All three legs of the AC line plus the DC armature loop are fused RC snubber networks and MOV suppressors guard against voltage transients.
• All signal inputs and outputs are isolated from the AC input lines and also from the armature voltage. This feature not only makes the drive safer to use, but it also direct accommodation of multiple drive options and process instrument inputs.
• Monitoring terminals are provided for reading both motor load and motor speed with commonly available test meters.
2 volts = 100% rated load
5 volts = 100% based speed.
• Unique design allows starting into a rotating motor without faulting.
• Tachometer feedback. Accommodates a 50 V/1000 rpm tachometer on a 1750 rpm motor (standard circuitry)
• Multiple PRD12B drives may be operated in parallel from a master reference source or from a computer generated input signal without the need for input isolation transformers or signal isolation.
• Zero creep deadband. Adjustable from 0 to 1% of full scale input signal with no degradation in performance. No motoring or generating torques are developed until the input signal exceeds the deadband.
• Positive and Negative torque limits are preset at 150% of rated full load motor torque. Terminals are provided for an external torque limit potentiometer.
• Provision for acceleration/deceleration option card. For factory installations or easy field installation. Available in linear or S-curve.
• Dual voltage 230/460 VAC input. Readily field modifiable. The horsepower rating of the drive is proportional to the input voltage.
• Directly accepts 4~20 ma input in lieu of a speed potentiometer. Jumper selected circuitry offers a choice of unidirectional of bidirectional operation.
• Drive shutdown occurs when potentially damaging conditions exist. The fault which caused the shutdown is displayed by an LED. Also the mode of operation prior to shutdown is displayed. The LEDs are circuit board mounted in open chassis units. In enclosed unit, a door mounted fault and mode indicator panel is included.
• The PRD12B is started normally after a fault shutdown; the faults indicators do not require manual resetting. An automatic restart options is available.
• DC loop contactor
• 240 volts field supply (230VAC input only) Field convertible.
• 50 Hertz input, Field or factory convertible from 60 Hz.
• Jog at separate speed setting.
• Automatic Reversing. Pushbutton reversing without a mandatory stop at zero speed.
• Tachometer feedback for use with tachometers and motors other than the 50 v/1000 rpm tachometer and 1750 rpm motor combination mentioned under standard features.
• Field economizing circuit.
• Field regulator. For improved speed stability (within 1%) or for constant horsepower (field control) range.
• Automatic restart after fault shutdown. Includes non-volatile direction memory.
• Self-contained Dynamic Braking.
• Linear acceleration/deceleration control. Independently adjustable, 0.12 to 27 seconds in two ranges.
• S-curve acceleration/deceleration control. Independently adjustable, 0.12 to 18 seconds in two ranges.
• Extended acceleration and deceleration ranges, linear option to 84 seconds or S-curve to 35 seconds.
• Torque programmed drive. Controls torque in lieu of speed. Field convertible. Applicable to winders, unwanders and web handlers.
• Preset speeds circuit board. Seven channels. Programming jumper and potentiometer for each speed.
• External signal control. Ranges are 1~5 ma, 4~20 ma, 10~50 ma, 0~10 VDC and 0~6 VDC.
• DC tachometer follower. 6 VDC through 100 VDC in three ranges.
• Armature voltage follower. 50 VDC to 500 VDC ranges. Allows PRD12B to follow the speed of another motor without requiring a tachometer.
• Position interface. Adapts PRD12B for controlling position rather than speed. For use with valve positioners and related applications.
• Enclosure Gasketed and Sealed. Outside air is circulated over the heat sink only – not through the electronic compartment. Includes a door mounted fault and mode indicator panel.
• Door mounted disconnect and switch. Ordered with optional enclosure.
• Fault and mode indicator panel. Standard on enclosed controls. Shipped loose as an option with open chassis controls. Installs in a standard analog meter cutout.
• Standard or custom remote operator station.