Trust Automation, Inc. TA333 High Power Linear Servo Amplifier
10-Apr-09 Page 13 of 38
1.13 Optional External 24VDC Supply
The TA333 internal logic may optionally be powered by an external 24VDC (±5%) source for
convenience when using serial monitoring or in extremely noise sensitive applications. The external
power connection is at J1. The internal 24V source provides power by default automatically but is
disabled if an external source is connected.
When internally powered, some low level noise may be generated by this supply that may have an
effect on the drive performance. While the noise level is small, it may be a factor depending on the
application. When externally powered, the electrical noise level is further reduced, providing the
quietest operation possible. When externally powered, there will be 50-100mV of random electrical
noise on the motor outputs. When internally powered, there will be about 150mV at 160 kHz riding on
the background 50-100mV of random noise.
1.14 Power Dissipation Calculations
Since the TA333 Power stage is linear, voltage not applied to the motor is converted directly to heat.
Heat generated by the drive is directly proportional to the voltage drop (across the amplifier) multiplied
by the motor current. (Think of a linear amplifier as a large variable resistor, current out = current in.)
Heat dissipation is a critical factor when the motor is in a stalled motion condition. (low voltage at the
motor, but high current output). The heatsink is limited to a maximum of 600W continuous dissipation.
Peak dissipation is limited to 1350W for a very short time period (<1ms). A practical design should limit
peak dissipation to 1000W or less. Actual dissipation limits depend on specific conditions including
temperature, load dynamics and event time. For most accurate peak dissipation allowable, see SOA
chart. (See section 2.4)
The TA333 features a microprocessor that constantly monitors the wattage across the drive to protect
the Class-AB power stage from damage. At any given moment in time there is one power device (upper
or lower) that is handling the majority of the drive wattage regardless of whether the load is a floating
brushless motor or a ground-referenced brushed-type load. Calculations are based on the highest
current and voltage across any phase with respect to the power supply ground.
When predicting SOA wattage limits with a brushless motor (or single brushed-type motor in bridge
mode), use half of the expected voltage across any pair of phase leads against the voltage of one of
the two supplies.
For a brushed-type load that is directly referenced to the power supply ground, use the full predicted
voltage across the motor against one of the two supplies.
Brushless example:
Assume a have a pair of 72V supplies and a motor that is expected to require 15A peak load at a phase
to voltage requirement of 20V according to calculations. Because a brushless motor voltage is specified
as phase-to-phase, we will divide the predicted voltage in half to give a ground referenced motor
voltage of 10V.
P
D
= I
motor
(V
supply
– V
motor
)
I
motor
= 15A [calculated based on required torque]
V
motor
= 10V [calculated based on velocity]
V
supply
= 72V [one of two 72V supplies]
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