Page 337 - Compression Machinery for Oil and Gas

322 SECTION II Types of Equipment

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

Full-load torque
Stable operating
Load torque-speed curve
point
No-load speed
E
0
0 50 100
Percent of synchronous speed
FIG. 7.11 Stable operating point.

From Fig. 7.11, the motor would accelerate the load successfully since the
motor torque is greater than the load torque up to the operating point. If load
torque at the operating point increases, the motor will slow down. As it slows
down, at lower speed, the motor torque increases and catches up with the
increased load torque. The motor operation is stable.
From Fig. 7.12, the motor would also accelerate the load successfully since
the motor torque is greater than the load torque up to the operating point. If load
torque at the operating point increases, the motor will slow down. As it slows
down, at lower speed, the motor torque decreases so the motor slows down fur-
ther. Eventually, the motor would stall. The motor operation is unstable.
For a given speed, the motor torque varies at the square of voltage. This is
important during motor start. When motor starts direct on line, the starting cur-
rent is much larger than the full load current (e.g., 600% of the full load current)
and causes the voltage drop at the motor terminal. At the moment when the
motor starts from the locked rotor condition, the voltage at the motor terminal
is typically between 80% and 90% of the rated voltage. As the motor acceler-
ates, the motor current is reducing from the locked rotor current to the full load
current and the motor terminal voltage recovers from the initial drop to a value
approximately equal or close to the full-rated motor voltage. So if for example,
initially, the voltage drops to 80% of the rated voltage, the motor starting torque
would drop to 64% of the theoretical starting torque if starting voltage was equal
to 100% of the full voltage. As the voltage recovers from the initial drop, the
torque recovers too. Between the initial voltage drop at the motor terminals
and the full motor voltage, there is a family of torque-speed characteristics,
and the actual motor torque will be a point on a corresponding torque-speed