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.

Degree of protection for motors, Ac Dc Motor

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 kW). Thus the process of matching the three-phase standard motors
to the requirements applying for variable-speed drives with AC converters has not yet been concluded.
The fact that AC motors with ratings of up to approx. 1400 kW are supplied in degree of protection IP 54 as standard is a tribute
to their simple and sturdy construction. For drive jobs in hazardous areas, explosion-protected AC motors are used almost
exclusively. This means that the AC motor has won itself a firm position and proved its practical utility most especially in those
sectors of industry characterized by aggressive ambient conditions and a high degree of dirt and dust in the cooling air.

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 n
1
), the smaller the overdimensioning factor
can be kept Pmax(motor)
/ Pmax(load)
.
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 compensated DC motors with a shaft height of ≥ 250 mm in the
rating category of 125 ... 1400 kW (M = 2400 ... 24500 Nm)
is 1 : 5.
Example: compensated DC Motor DMA + 280 K (see Fig. 5)
nG
n
1
nmax
Speed n (rpm) 0 500 1500 2500
Power P (kW) 0 130 130 80
Torque (Nm) 2483 2483 827 305
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.
Example: AC Standard Motor ...450LL12 (see Fig. 5)
nG
nmax
Speed n (rpm) 0 500 1250
Power P (kW) 0 130 130
Torque (Nm) 2483 2483 933

Mains pollution: ac drive

The line currents of DC drives with a 6-pulse thyristor bridge will always contain, in addition to the fundamental wave, the 5th,
7th, 11th and 13th harmonics with empirical values of 22 %, 14 %, 9 %, 7.6 % , referenced to the fundamental wave. In the
case of several DC drives operating simultaneously on the mains, the different phase sequences of the harmonic currents will
produce a "statistical improvement" in the level of mains pollution.
Due to the dimensioning method adopted for the smoothing inductors, harmonic currents with contents of 40 %, 14 %, 9 %
and 7.6 % must be anticipated with AC drives featuring a 6-pulse diode bridge in 1-quadrant drives. Due to the identical phase
angle of the harmonic currents, several drives on the same mains can be regarded as one drive with the same total current.This
also applies for thyristor bridges in 4-quadrant operation.
Input bridges with IGBT switching elements enable the low-frequency harmonics to be substantially reduced, but conversely
create more high-frequency harmonics.

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