DCS800 Firmware manual supplement for North America (rev. 2.60, 2.70) Rev. b 5/30/2008
11
Field heating
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Overview
Field heating (also referred to as “field warming and field economy”) is traditionally been
used in the US for a couple reasons. Previous generations of dc drives used voltage-
controlled field supplies, meaning that the only thing the field supply could directly control
was voltage. For DC motors, maintaining the field’s current
is what’s important to
maintaining optimal torque. Ohm’s law tells us that current equals voltage multiplied by
resistance, so as long as resistance remains constant, current will be proportional to
voltage. But field resistance changes with temperature. Therefore, a cold motor would
have a different field current than a warm motor, even though voltage remained
unchanged. To keep the resistance, and thus the current constant, the field was left on to
keep it warm. Then the voltage-controlled field supply would work just fine.
The new generation of drives, including all field supplies used with the DCS800, are current
controlled, meaning that the field supply directly regulates field current. This means that
field heating may no longer be necessary when this drive is employed.
The other reason field heating is used is to keep moisture out of the motor.
If field heating is still required, the drive does allow for this option.
Two parameters are used to turn on and control field heating:
• FldHeatSel (21.18)
• M1FldHeatRef (44.04)
Field heating modes of operation
There are two modes of operation, depending on the setting of FldHeatSel (21.18):
On and OnRun.
On Mode: In general, the field will be on unless the Emergency Off or
Emergency Stop inputs are clear.
OnRun Mode: In general, the OnOff input must also be set, in addition to the
Emergency Off and Emergency Stop inputs, in order for the field to
be on.
In both modes, the field current will at a reduced level, determined by M1FldHeatRef
(44.04), whenever the drive is in a non-run mode. See tables, below, for details.
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