Page 338 - Compression Machinery for Oil and Gas

Drivers Chapter 7 323

Breakdown
torque
Locked rotor torque
(pullout)
(starting torque) C
Percent of full-load torque 100 A B (pull-in) Unstable operating
200
Pull-up torque
point

D

Full-load torque
Load torque-speed curve
No-load speed
E
0
0 50 100
Percent of synchronous speed
FIG. 7.12 Unstable operating point.

characteristic for a given speed and given voltage, as it transitions from the start-
ing condition to the normal running condition. It is important to have the load
torque less than the motor torque for every speed and the corresponding voltage
during the motor start, so the motor can accelerate the load successfully. The
difference between the motor torque and the load torque is called the acceler-
ation torque. Fig. 7.13 shows the range of motor torque-speed characteristics
from starting voltage of 80% to the motor full-rated voltage.

200
% Full load torque 100 Torque at 100% voltage

Torque at 80% voltage

0
0 20 40 60 80 100
% Synch speed
FIG. 7.13 Motor torque-speed curve at 100% voltage and 80% voltage.