17-07-2012, 04:05 PM
Saving energy
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Slip power recovery systems
The slip power recovery (SPR) drive is an
external system connected to the rotor
circuit in place of the external resistors.
The SPR provides speed and torque
control like the resistors but can also
recover the power taken off the rotor
and feed it back into the power system to
avoid energy waste. In usual practice, an
SPR drive consists of two interconnected
power converters as shown in Figure 2.
The rotor converter is connected to the
three-phase rotor winding.
When are Wound rotor motors used?
Some demanding applications that favour
the use of a WRIM over a standard
induction motor or synchronous motor are
listed below:
• Grinding mills that require high torque at
starting as they often sit with a full load of
rocks. The WRIM is used here because the
rheostat provides an inexpensive way of
starting the loaded motor
• Mills that require variable speed
operation
Mill drive example
An example of slip power recovery is the
wound rotor motor shown below, one of
two for powering a new 10,000 kW SAG
mill in Australia as shown in Figure 3. This
installation includes:
• Two 5000kW six-pole WRIMs with
11,000v stators
• A combining gear box driving the mill
• One 10,000kW liquid resistance starter
(rheostat)
• One dual Slip Energy Recovery system
(SPR drive).
Low harmonics at utility
Previous thyristor based systems injected
significant harmonic currents into the
utility system. In contrast, the PWM SPR
converter is switched at a higher frequency
(usually 1500Hz or higher). The harmonic
currents from this type of converter are
centered around the switching frequency
and multiples of it. These currents are
greatly reduced by the impedance (15-20
per cent on the drive rating base) in the
source reactor. If a harmonic study reveals
that the additional filtering is needed,
a simple high pass filter can reduce the
remaining high frequency harmonics. This
type of filter is not sensitive to changes
in the power system because it is closely
connected to the PWM converter, so
future changes are not likely to disturb
it. The PWM converter needs no reactive
power support, so the filter is not sized to
provide that.
Energy consumption
In Case 1 of Table 1, for the given load at
reduced speed, the WRIM with rheostat
uses the most power (P1) because of heat
wasted in the variable resistance. For Cases
2 and 3, the energy savings relative to the
WRIM case are calculated in the table.
For a mill motor rated at 5000kW,
running at 80 per cent speed, the power
saving using the SPR drive, Case 3, is
970kW. Assuming two motors as in the
10,000kW mill example, and an electricity
cost of 7¢/kWh, the annual savings are
US$1,189,600. The savings with the
MV PWM stator drive are a little less
than the SPR case.
Conclusion
In addition to numerous large WRIM
motors on grinding mills, slip power
recovery drives have been applied in
other industries, such as in municipal
water plants (pumping), and in paper
plants (induced draft fans). Many of the
SPR installations have been retrofits to
existing controls for large WRIMs, that
use rheostats for speed control. In these
upgrades the rheostats are often kept for
motor starting.