Digital microprocessor-controlled power converter technology

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.

Efficiency and operating point of DC and AC standard motors

The motor constant makes it possible to design the nominal point of DC motors corresponding to the process requirements.
However the efficiency of AC motors is better (> 55 kW: approx. 1 ... 4 % depending on cooling method).
DC motors are often utilized according to insulation class H. Standard AC motors are used to be utilized according to insulation
class B. This results in a possible higher efficency with AC motors.

Reactive-power demand

Both drive concepts (AC and DC) take reactive power from the mains. Its size is negligible in the case of AC drives, and is RPMdependent below the rated speed with DC drives. The AC drive is the more favourable option here.
Values obtained from empirical feedback:
For AC drives, the value for cos ϕ
1
 is in
1-quadrant applications
with diode bridge cos ϕ
1
 ≈ 0.99
4-quadrant applications
with thyristor bridge and
with energy recovery into the mains cos ϕ
1
 ≈ 0.9
Values obtained from empirical feedback:
For DC drives, the value for cos ϕ
1
 is in
1-quadrant applications cos ϕ
1
 ≈ 0...0.9
4-quadrant applications cos ϕ
1
 ≈ 0...0.85

Modernization of existing DC drive

When it comes to the question of whether it is worth while
modernizing an existing DC drive or less expensive to replace
it entirely with an AC drive, there are also various arguments
which need to be assessed:
Basically, there are several options available for a modernization job:
1. Replace the entire DC drive (converter and motor) by a new
DC drive.
2. Replace only the converter cubicle, if the motor is still in
good condition.
3. Replace the converter module by a modern digital unit.
4. Replace the old, analog drive electronics by new, digital
electronics while continuing to use the power section
(recommended only for ratings above 1 MW).
5. Replace the entire drive system with a new AC drive.
When answering the question of what approach constitutes
the optimum solution in a particular case, the following main
criteria are important:
Will the requirements for the drive change in future (load
requirements, environmental conditions)?
In what condition are the individual components of the
system (reliability, age, maintenance outlay)?
How far will the supply conditions change in future?
Before a decision is made to modify a drive from DC to AC
design, the following points should be taken into consideration:
Outlay for new power cabling.
Space requirements for converter cubicles.
Dissipation of energy losses from the switchroom sufficient?
Foundations, mounting for motor sufficient?
Space requirement for new motor.
Duration of conversion work.

Price comparison DC and AC drive systems

(unit + motor or complete switchgear cabinet  + motor)
Based on the present-day development status of DC and AC drive engineering, and taking into account all the
systems' advantages/disadvantages mentioned above, the following guideline figures can be given:
1-quadrant drives <40...80 kW ê AC drives less expensive
4-quadrant drives >40...60 kW
(AC converter +  braking resistor; see Fig. 10); ê DC drives less expensive
Regenerative 4-quadrant drives > 15 kW ê DC drives less expensive