4 IOMM Starter
Figure 2, Solid State Starter, Wall Mounted
Variable Frequency Drives
While known and specified for their ability to control compressor motor speed for
efficiency enhancement, VFDs also perform starting and motor protection functions. They
are only available for 3/60/460-480 service.
VFDs are available only from McQuay and when purchased as part of the original chiller
purchase. Installation and operation are covered in McQuay manual IOMM VFD.
Basic Electrical Terms
Locked rotor amps (LRA): The amount of current that a specific motor will draw at start-
up, when full voltage is applied across the line. The LRA may be 6 to 8 times FLA, or
possibly higher in some cases.
Inrush current
: The amount of current that a specific motor and starter combination will
draw during start-up. Normal inrush current will be substantially less than LRA for all
starter types, except for across-the-line starters.
Full load amps (FLA)
: The maximum amps the motor is designed for.
Rated load amps (RLA): Actual amperage that the motor draws for a specific application.
Centrifugal compressor motors operate at a RLA significantly below their maximum full
load amps. RLA is used to determine electrical component sizing such as wire size and
disconnect switches.
Starting torque
: Minimum torque required to begin the motor’s rotation.
Bypass contactor
: Contactors that bypass auto-transformers, reactors, or SCRs, and allow
full power to reach the motor directly.
Terminal Strip
Disconnect Switch
Primary Control Circuit
Fuses
Control Transformer
SCRs (Behind)
D3 Controller
Bypass Contactor
LED Display
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