DC Or AC Drives?

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 dimensioning of the associated DC thyristor converter / AC frequency converter as well .

The larger the speed range in which a motor can output its maximum power, the better the motor in question can be adapted to suit processes which require a constant drive power in a wide speed range.

Typical application: coilers

Sizes, moments of inertia and ramp-up times:

The basic technical and design-related differences between DC and AC standard motors in magnetic-field formation and powerloss dissipation also entail different sizes ( = ^ shaft height H) for the motors and different mass moments of inertia Jrotor in kgm2 for the rotors, with reference to the same torque; see the Comparison Table 1 below.

Comparison Table 1: Mass moments of inertia for the rotors, sizes/shaft heights and weights for

DC and AC Standard Motors (examples)

DC motors have a significantly lower shaft height H and weight than do AC motors, with the mass moment of inertia of the rotor Jrotor consequently being substantially smaller with DC motors as well. But this mass moment of inertia is an important variable for highly dynamic applications, such as test rigs, flying shears, and reversing drives, since it has a marked influence on the ramp-up time ta and the motor's dynamic response in four-quadrant operation (driving and braking modes).

Mass moment of inertia:

Ramp-up time:

Values obtained from empirical feedback:Ramp-up time ta for DC Motors with MA = MN

Values obtained from empirical feedback: Ramp-up time ta for AC Motors with MA = MN

Table 2: Ramp-up times ta based on the above motor data in the basic speed setting range

High speed setting range at constant power (field weakening operation or field control range):

For specialized drive jobs, like coiler drives, test rigs, winders and unwinders, etc., very large setting ranges at constant power are stipulated. In these cases, conventional field weakening operation with an externally excited DC machine makes implementation particularly cost-efficient. This means: the larger the speed range in which a motor can output its maximum power (length of the horizontal section of the characteristic in Fig. 5, from nG to n1), the smaller the overdimensioning factor can be kept

Values obtained from empirical feedback:

A typical value for the field weakening range of DC Motors with a shaft height of 112 ... 225 mm in the rating category of 5 ... 360 kW (M ≤2900 Nm) is 1 : 3.

The maximum value for the field weakening range at compensatedDC motors with a shaft height of ≥250 mm in therating category of 125 ... 1400 kW (M = 2400 ... 24500 Nm) is 1 : 5.

Values obtained from empirical feedback:

Due to the pull-out torque Mk ≈MN x 2,5, the typical value for the field weakening range is only 1 : 1.5 up to a maximum of 1 : 2.5 for all standard AC Motors.

Motor maintenance:

Today, depending on the application involved, the useful lifetime of brushes in DC motors is at approx. 7000 ... 12000 hours (h), thanks to the sophisticated collectors, carbon brushes and optimized field supply units used.

Depending on the mechanical conditions involved, the relubrication intervals for the bearings of DC/AC motors may be shorter than the useful lifetime of the brushes in DC motors.

Degree of protection for motors:

The historical development of the DC motor as an electric variable-speed drive since the beginning of the twenties has meant that DC motors are customarily used with internal/forced ventilation (approx. 85 % of VSDs ≤ 250 kW).

For variable-speed AC drives, asynchronous standard motors have predominantly been utilized since the 70s/80s, which mostly feature surface ventilation (approx. 90 % of VSDs ≤ 250

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