DC Or AC Drives?
Enviado por jwilman • 30 de Octubre de 2013 • 2.999 Palabras (12 Páginas) • 184 Visitas
DC or AC Drives?
A guide for users of
variable-speed drives (VSDs)
The annual growth rate for variable-speed drives (abbreviated to VSDs in the following text) is approx. 6 %, while the growth rate for AC drives is around 8 % p.a., with the market's volume for DC drives remaining more or less stable.
This overview is intended to outline to users, plant managers, industrial design engineers or the persons responsible for a particular process the features offered by DC drives as compared to AC drives.
Handling drive jobs: DC or AC drives?
Digital microprocessor-controlled power converter technology, both for DC and AC drives, has now reached a level of technical sophistication which (in purely technological terms) enables almost any drive job to be handled both with DC and AC drives. Nevertheless, the conventional DC drive (in both its 1-quadrant and 4-quadrant variants) will continue to play an important role, for technical and physical reasons alike, when dynamic drives with a constant load torque and stringent requirements for overload withstand capability throughout a large speed setting range are involved.
Main criteria for the user
The first thing a user should do is to objectively check out the options currently available in DC and AC drive technology for his/her specific requirements/processes.
The main criteria applying for this check are:
A. Total purchase costs for the VSD system(s)
B. Current operating costs:
maintenance
process costs/efficiency levels, etc.
space requirements
C. Technological/Innovative aspects:
dynamic response, ramp-up time; 4-quadrant operation; EMERGENCY STOP, etc. space requirements; weight up-to-the-future DC technology
D. Operational dependability, availability of the drives:
international regulations like IEC, EN, CE-EMC; CSA, UL, etc.
environmental conditions; degrees of protection service; "on-the-spot" repairs
E. Any effects on the surroundings:
supply network
EMC
F. Required space for converter and motor
G. Heat dissipation from the control room
Comparison of the basic characteristics of DC and AC drives in industrial applications
The following comparison of basic DC-drive and AC-drive characteristics covers only 6-pulse 3-phase thyristor drives with externally excited DC motors [referred to below as DCs], and 3-phase frequency converters in PWM design (voltage source converters with Pulse Width Modulation) with
asynchronous three-phase motors [referred to below as ACs], in the following typical rating categories:
In a first superficial comparison, hardly any significant differences can be found; however, when scrutinized more closely, differences in the drive features and in the physical method of functioning emerge.
The sections below cover the following points:
the drive motor as the interface to the process
the converter as power controller
4-quadrant drives
any effects on the surroundings
Modernization of existing DC drives
ABB drives: for innovative future-compatibility
Differences between DC and AC motors
For general motor evaluation, many users adopt the following rather simplistic view: the DC motor is complicated and requires a lot of maintenance, which makes it expensive to run; it also has a lower degree of protection. The AC motor, on the other hand, is simple and sturdy, does not need much maintenance, is therefore less expensive, and possesses a higher degree of protection into the bargain. This categorization may well be true for many simple applications; it is nonetheless advisable to subject this sweeping verdict to more detailed scrutiny!
The forced ventilation feature customarily used (approx. 85 % of VSDs ≤250 kW) ensures good dissipation of the rotor losses
originating in the DC motor.
Typical applications for a constant torque over the entire basic speed range:
wire-drawing machines, piston compressors, lift operators, aerial cableways, extruders, ...
Surface ventilation customarily used (approx. 90 % of VSDs ≤250 kW) for AC standard motors substantially reduces heat dissipation. At small speeds, dissipation of the rotor losses is hardly possible at all.
Typical applications where the falling torque characteristic of AC motors is not a disturbance factor at small speeds (Fig. 4):
pumps, fans, etc. with a quadratically increasing load torque ...
A comparison of operating characteristics of DC and AC motors shows that the direct-current motor is advantageous to the asynchronous motor for continuous operation at low speeds and for high setting ranges at constant power.
The possible overload in short-time duty depends not only on the motor parameters but to a high degree on the
...