Page 357 - Compression Machinery for Oil and Gas

342 SECTION II Types of Equipment

insulation breakdown. The increase in frequency beyond rated frequency is pos-
sible and will produce higher speed but with voltage kept at rated voltage, and
consequently reducing V/Hz ratio, the flux density will reduce and the torque
will reduce.
The advantage of VFD supplied motors is that the motor can supply the same
maximum torque from zero speed to rated speed. This area of the motor torque-
speed characteristic is called “constant torque” area. Continuous operation at
peak torque is not done in practice because of the heat limitations. The upper
torque limit equal to motor rated torque is usually set in the controller.
With VFD supplied motors and with their availability of high torque at low
speeds, the starting problems common to fixed frequency operations (initial
high slip, high starting current, voltage drop, and torque reduction) are avoided.
The VFD-driven motor starts with low frequency, which is gradually increased.
The slip speed of the rotor is always small and the rotor continuously operates in
the optimum torque condition. Rated torque is available at low speeds and start-
ing current does not exceed the rated full load current. The motor can start from
a week power supply system without causing voltage disturbances in the supply
network.
As mentioned previously, the VFD-driven motor can develop any torque up
to rated torque at any speed up to rated speed. This area is called “constant tor-
que” area. Above rated speed, V/Hz will reduce because voltage is kept constant
at rated motor voltage, stator, and rotor current are also kept constant and speed
and frequency are increasing, so the flux density will reduce and the torque will
reduce inversely with the frequency. This area in the motor torque-speed char-
acteristic is called “constant power” area. Constant power area is up to approx-
imately twice the rated speed. Beyond constant power area is the high-speed
area where current limit coincides with the pullout torque limit, which reduces
inversely with the square of the frequency, so the constant power cannot be
maintained any further. Constant torque, constant power, and high-speed areas
are shown in Fig. 7.26.

Torque

Constant torque Constant
region power
region High-speed
region
Speed
FIG. 7.26 VFD supplied induction motor torque-speed curve in constant torque, constant power
and high-speed area.