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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

On and OnRun.

On Mode:

OnRun Mode: In general, the OnOff input must also be set, in addition to the

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.

DCS800 Firmware manual supplement for North America (rev. 2.60, 2.70) Rev. b

In general, the field will be on unless the Emergency Off or

Emergency Stop inputs are clear.

Emergency Off and Emergency Stop inputs, in order for the field to

be on.

FldHeatSel (21.18):

5/30/2008