AC versus DC drive

For decades DC drive was a preferred choice for variable speed motors due to its easy and accurate way of speed or torque control. However, the development of AC drives changed the competitive landscape and paved the way for AC drives. What was the reason for this shift in technology? What are the pros and cons of each solution? And finally why are variable speed AC drives so popular and widespread today? We try to address these points in our article about AC versus DC drive.

Many applications require speed or torque control. Not just in these days, but also in the past. Until a variable frequency AC drive was developed and became commercially available, DC drives dominated the field of speed controlled motors. The simplicity and accuracy of speed or torque control of DC machines was simply unmatched. By adjusting the stator flux and armature (rotor) current the speed and torque of the DC machine can be easily controlled. The method is relatively simple and control algorithms are straight forward. No wonder that speed-controlled DC drives enjoyed high popularity – it was well deserved.

Speed and torque control of DC drives

The beauty of DC drive is in its easy speed control. The principle is summarized in the notoriously known equations:

(i) Ui = k ⋅ Φ ⋅ ω

(ii) T = k ⋅ Φ ⋅ Ia

where

Ui … induced (internal motor) voltage

k … machine specific constant

Φ … machine flux

ω … angular frequency (‘speed’)

Ia … armature current

From (i) we can see that when keeping magnetic flux at constant level the induced voltage is proportional to machine speed. Moreover, eq. (ii) teaches us that motor torque is proportional to armature current Ia (in DC machines armature is the rotating part – rotor).

However, DC drives also have certain drawbacks. One of them is the mechanical commutator. As a sliding contact it is a subject of wear and needs to be periodically replaced. Also, commutator-brush system may be source of sparks that are unwanted and in certain applications even restricted (e.g. hazardous area). Finally, DC machine is more demanding than AC motor from manufacturing and assembly point of view.

* To be fair we shall mention that sliprings and brushes are still used in older locomotives and these locomotives operate in a very reliable way. Yet, the mechanical contact is generally considered as a drawback.

In contrast, asynchronous motor is extremely rugged and fairly simple from design point of view. The only problem prohibiting its wide use in variable speed application was the adjustable speed control itself. As soon as modern VFDs removed that obstacle, variable speed AC drive systems could start the expansion.

DC drives

Flux and torque easily controlled by field and armature current.

Separately excited dc motor especially suitable for variable speed control.

Fast response, high performance near zero speed (high torque, no jitter).

Limited high voltage (max. commutator voltage).

Limited max. motor speed (commutator capability).

Development of material of brushes meaning less wear and longer periods between maintenance.

AC drives

Simple and rugged motor structure.

High maintainability and economy (longer maintenance periods and lower purchase price).

Higher output ratings available.

Smaller dimensions, higher power per kilogram ratio (power/mass ratio). Lower weight, lower rotating mass (inertia).

More complex speed control compared to DC drives.

Pros and cons of DC drives

+ simple variable-speed design
+ fast response
+ four-quadrant operation
+ high performance near zero speed
– commutator and brushes
– periodic maintenance
– limitation in medium voltage (for commutator)
– higher manufacturing cost
– not allowed in explosive atmospheres (hazardous area)

Pros and cons of AC drives

+ simple design
+ low manufacturing cost
+ rugged structure
+ minimum maintenance
+ high overload capability
+ smaller dimensions
+ higher output ratings

AC versus DC drive: Summary

Speed control is a fundamental requirement on certain electric drives. In a DC drive system the realization of speed control is easy and the drive has excellent performance from standstill. In contrast, AC motor is very easy to manufacture and extremely robust (no mechanical contact). Main drawback of AC motor drive was the nonexistence of simple speed control. As soon as the last obstacle was removed by modern VFD the way was paved for modern variable speed AC drives. However, DC drives supplied from thyristor based power electronics remain still popular and will stay operational for next couple of years. For new installations, AC drives are preferred and considered as state of the art.