technical NOTES

• In case of failure of SPRS, plant can run with GRR


• Easy in maintenance due to LV drive concept (690 V)


• Recover the slip power 10% to 15% normally lost in the rotor resistor with GRR.


• Less capital investment compared to MV drive


• Less losses compared to MV drive


• Squirrel Cage Induction Motor with VFD / MV Drive:
  Squirrel cage induction motors (SCIM) are simpler for maintenance than slip ring
  motors since they do not have slip rings which are prone for high maintenance.
  speed of SCIM can be controlled by changing voltage and frequency applied to
  stator. This is achieved using electronic circuits and power control devices which
  is known as VFD (variable frequency drives). The VFDs also replace conventional
  electrical starters since all features required to start and control SCIM are configured
  in these drives.

With development in electronics, based on micro controllers and power devices
like IGBT (Insulated Gate Bipolar Transistor) modern drives are available from
small capacity of motors to very large capacity of motors operating from normal
voltage to high voltage of 11 KV with time, it has become quite reliable.

While selecting either of the above systems following process parameters requirements
to be considered in fan and motor power design:

a. The design capacity of Process fan flow in case of SPRS drive should be equal
to 110% capacity of MV drive speed, which will have higher margin in preheater
fan and maximum flow can be achieved at 92% speed so that we can operate
the fan in efficient zone of SPRS to achieve maximum recovery. In ase of SPRS
with the high flow fan concept, initial capex value will be high.

2.2 Comparative statement of MV Drive and SPRS +GRR System

S. N. Parameter MV Drive SPRS with GRR

1 Type of Motor Squirrel Cage Induction Motor Slip Ring Induction

Motor

2 Speed Control

Band

10 % to 110% Speed Control band with

GRR 40% to 100% and

SPRS 60% to 95%

3 Line Harmonics Less than 4% THD for 36 Pulse

Drive Topology

Harmonics more than

5% for 6 pulse drive

topology

4 Power Factor Additional MV not required Additional MV capacitor

required

5 Overall Efficiency 92.67%(MV Drive ,Xmer, Motor) 90.77%(LV Drive, Xmer,

Motor)

6 Starting current Starting current will not be

more than motor rated current

Starting current will be

2-2.5 times the motor

rated current