DC Drives: An Introduction (part 2)

DC drives are widely used in applications that require regeneration, precise speed control, dynamic performance, and constant torque over wide speed ranges.

What does a DC drive do (internally)?

• performs power conversion (AC to DC)
• power amplification
• sequencing of waveform signals

Types of DC drives:

• adjustable or variable-speed control
• servo control: used for servomotors
• integral motion control: comprised of feedback and a controller

Types of DC drive interfaces:

• analog or digital
• manual adjustments: potentiometers and dip switches for non-frequent tweaking
• interchangeable module or EPROM: holds motor parameters to match a drive to a motor (and/or drive different motors)
• computer interfaces and self-test diagnostic options

Fundamental requirements of DC drives include:

• requirement of brushless DC motor: types include: sine wave, trapezoidal, hall sensor commutation, no sensor
• requirement of standard DC motor (with brushes)
• requirement of amplifier

Fundamental operating specifications of DC drives include:

• open loop control: uses a tachometer or other device to monitor motor speed as opposed to using position feedback to control motor.
• positive feedback: uses position, force, or other feedback (in addition to speed sensors) to control motor -- this is also called closed-loop control.
• tach or velocity module: gives direct feedback of motor speed
• back EMF (BEMF) or voltage control: can run without a tachometer using, instead, the motor's back EMF voltage
• torque or current control mode: allows constant current control, uses current feedback
• switching frequency: used by drives that use pulse-width modulation (PWM)
• bandwidth: frequency range in which the drive meets its accuracy specs. . Accuracy is diminshed at lower and lower frequencies unless the device is capable of dc response, and at higher frequencies near resonance and beyond, where its output response rolls off. Frequencies in the drive's database are usually the 3dB rolloff frequencies.
• maximum speed: speed attained where there is no external load on the motor
• continuous power: maximum power the drive can continuously output
• continuous output voltage: maximum voltage the drive can continuously output
• maximum continuous output current: maximum current the drive can continuously output (may require cooling)
• peak output current: peak current the drive can output
• operating temperature

Other important features to consider when looking for DC motor drives:

• PWM switching: offer an output that's pulse width modulated or switching
• dynamic braking: uses a shunt resistor or direct short across the armature to reduce mechanical energy
• regenerative braking: converts mechanical energy back to electrical energy and transfers it back to the energy source. Example of such use may be found in hybrid automobiles

Sample applications of DC drives:

• web handling
• winders
• slitters
• extruders
• wire drawing
• converting lines
• plastics production

Simple stand-alone applications must be easy-to-configure in order to control motor speed, voltage or current using standard internal settings. Set-up should be convenient using the drive keypad, or a remote keypad, or Windows-based drive-configuration software. The DC drive should feature extensive diagnostic and communication abilities that enhance system reliability. High-performance DC drive use powerful microprocessors that can also eliminate the need for a PLC. These drives may feature integral functions such as thresholds, timers and logic gates that perform basic control.