Baldor UM Series, Installation & Operating Manual

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2-2 Product Overview MN1213
Functional Description
The servo control consists of a power amplifier, a velocity control amplifier, and
fault logic circuitry.
Power Amplifier
The power section of a PWM bridge normally acts as a low output impedance
voltage source. However, if an analog signal representing motor current is applied
to the input of the amplifier and a current command is present, the impedance of
the PWM bridge then becomes high. These high impedance bridges exhibit a
unique characteristic referred to as current sourcing. That is, motor current (and
therefore motor torque) becomes proportional to commanded input current.
Current Sense Coupler
The current sense coupler of the power amplifier serves as the current loop
feedback mechanism. The unit isolates the high voltage motor signal from the low
voltage control circuitry.
Switching Configuration
The power amplifier contains a transistor bridge output stage that conditions the
voltage and current that is applied to the motor.
Velocity Control Amplifier
The high gain front end velocity control amplifier accepts a speed command
(Signal) and a tachometer voltage (Tach). The VCA clamped error signal (C
adjustment) is applied to the power amplifier as a current command.
Fault Logic Circuitry
The Fault Logic Circuitry functions are described in the following paragraphs.
Excess Current Limit
A comparator checks motor current for overload conditions beyond rated output. If
the comparator is tripped, it disables the control, lights the EC indicator, and pulls
the interlock line ”low” (if P4 is in latch position). The control must be reset.
Surge Current Sensor
The surge current sensor pulls the interlock line low internally if the peak output
current is exceeded. The control must be reset.
Overheat Sensor
If the thermal switch on the heatsink indicates the temperature exceeded a preset
level, the switch sets the EC (Excess Current) latch, which operates the EC fault
indicator. The fault is cleared when the heat sink temperature decreases.
Left Limit, Right Limit
Limit switches are typically mounted on machinery to restrict the load from moving
into over–travel conditions. When a limit switch is activated, travel in that direction
is not allowed. However, the motor can be reversed to move the load.
Decoupled Current Sense (DCS)
The Decoupled Current Sense (DCS) pin is isolated from the high voltage motor
lines. Therefore, motor current is evaluated safely by connecting an oscilloscope
or voltmeter to the DCS pin of the input connector and common (TPl).
Tachometer Filter
The tachometer filter rejects tachometer commutator noise by reducing the tach
path bandwidth so that the high gain velocity control amplifier can be responsive
yet reject tachometer noise.