MIL-HDBK-1003/6
8.11.3
Variable Speed Drives
8.11.3.1
Variable Frequency and Conventional Motor Drive. Normal
alternating current (AC) is rectified to supply direct current (DC) to an
electrical inverter. The inverter converts power from DC to controlled
frequency AC to supply a conventional motor drive. The speed of the motor
conforms to the synchronous frequency of the AC supply. The motor speed is
controlled by adjusting supply frequency. This type of speed control of AC
motors is accurate and relatively simple. The rectification and inverter
equipment is solid state. If problems arise in the frequency control system,
the motor can be run directly on the supply AC at a slightly reduced speed.
The initial starting surge of current (600 to 700 percent of full load power)
can be reduced with the variable speed drive to about 30 percent of full load
power. Cooling air must be provided with the frequency control device when
hot air is used and tempering heaters should be included when cold outside air
is used.
Steam Turbine Drives. A single stage steam turbine drive is
8.11.3.2
usually economical if exhaust steam can be used in the plant.
Internal Combustion Engine Drive. When this type of drive is used,
8.11.3.3
a heat recovery unit will increase the efficiency.
8.11.3.4
Conventional Motor Drive with Hydraulic Coupling or Magnetic
Coupling (Eddy Current). The motor runs at near synchronous speed and the
drive speed is controlled by changing the clutch slip. Hydraulic and magnetic
couplings have a wide range of speed reduction but normally have a slip-range
of 2 to 5 percent. The energy loss in the "slip" is referred to as speed loss
of the driven machine. The slip loss is included in the horsepower of the
driven unit. The slip expels heat which requires cooling in the larger units.
These methods are not energy efficient. Speed control for large motors is
generally accomplished more accurately, efficiently and economically with
other methods. For smaller motors, eddy-current efficiency is better for
controlling speed on driven equipment. Eddy-current drives are less efficient
than variable frequency but they cost less and are simpler to repair.
Direct Current Power Supply and Drive Motor. Normal alternating
8.11.3.5
current (AC) is rectified to supply direct current (DC) to a DC motor drive.
Speed control of DC motors is accurate and relatively simple. Equipment is
solid state or silicon control rectifier (SCR). Speed usually varies directly
with system input power factors. The DC motors are not recommended for
hazardous locations and may be difficult to maintain in dirty or corrosive
environments.
Mechanical Variable Speed Drives. These drives are generally below
8.11.3.6
50 horsepower. They control speed with V-belt drives, pulleys, gear reducers
or other variations of mechanical shifting of belts chains and clutches. They
are very high maintenance items and are suited to intermittent service.
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