Page 68 - Electric Drives and Electromechanical Systems

Chapter 2 Analysing a drive system 61

local generation (for example, on oil rigs and ships), considerable care needs to be
taken in the specification of the voltage limits. Since the peak speed of a motor is
dependent on the supply voltage, consideration needs to be given to what happens
during a period of low voltage. As a guideline, drives are normally sized so that they can
run at peak speed at eighty per cent of the nominal supply voltage. If a system is fed
from a vulnerable supply, considerable care will have to be taken to ensure that the
drive, its controller, and the load are all protected from damage; this problem is
particularly acute with the introduction of microprocessor systems, which may lock-up
or reset without warning if they are not properly configured, leading to a possibly
catastrophic situation.
In practice, the supply voltage can deviate from a perfect sinewave due to the
following disturbances.

Overvoltage. The voltage magnitude is substantially higher than its nominal value
for a significant number of cycles. This can be caused sudden decreases in the
system load, thus causing the supply to rise rapidly.
Undervoltage or brownout. The voltage is substantially lower than its nominal
value for a significant number of cycles. Undervoltages can be caused by a sudden
increase in load, for example a machine tool or induction motor starting.
Blackout or outage. The supply collapses to zero for a period of time that can
range from a few cycles to an extended period of time.
Voltage spikes. These are superimposed on the normal supply waveform and are
non-repetitive. A spike can be either differential-mode or a common-mode.
Occasional large voltage spikes can be caused by rapid switching of power factor
correction capacitors, power lines or motors in the vicinity.
Chopped voltage waveform. This refers to a repetitive chopping of the waveform
and associated ringing. Chopping of the voltage can be caused by ac-to-dc line fre-
quency thyristor converters, Fig. 2.14A.
Harmonics. A distorted voltage waveform contains harmonic voltage components
at harmonic frequencies (typically low order multiples of the line frequency). These
harmonics exist on a sustained basis. Harmonics can be produced by a variety of
sources including magnetic saturation of transformers or harmonic currents injec-
ted by power electronic loads, Fig. 2.14B.
Electromagnetic interference. This refers to high-frequency noise, which may be
conducted on the power line or radiated from its source, aspects of which are dis-
cussed in Section 2.7.3.

The effect of power line disturbances on drive systems depends on a number of
variables including the type and magnitude of the disturbance, the type of equipment
and how well it is designed and constructed, and finally the power conditioning
equipment fitted to the system or the individual drive.
Sustained under- and over- voltages will cause equipment to trip out, which is both
highly undesirable and with a high degree of risk in certain applications. Large voltage