Appendix A – DC Power Supply Design
05/2008
Danaher Motion
114
S200-VTS Product Manual
A.1.2
Main Supply Output Capacitance (J1-3 to J1-2)
NOTE
The location of the main output capacitor is not critical. Up to 10 ft
from the drive is an acceptable length, as long as the wire is sized so
resistive drops at peak current are low. Voltage clipping of the inverter
can cause the peak bus current to equal the motor current. A good
rule is to size the bus wiring for 18 ARMS x rt(2) = 25.4 ARMS peak per
drive. Use at least 16 AWG. The inductance of the bus and ground
wiring is not critical because the internal drive bus capacitance can
handle all the PWM current in most cases.
Servos put high peak power demands on the power supply. The easiest and best way to build a
power supply to deliver and absorb pulses of peak power is for the supply to have an
appropriately-sized output capacitor.
Capacitance Requirements
3/9 ARMS DC S200
6/18 ARMS DC S200
2,000 µf / drive at 75 V bus
4,000 µf / drive at 75 V bus
4,000 µf / drive at 48 V bus
8,000 µf / drive at 48 V bus
16,000 µf / drive at 24 V bus
32,000 µf / drive at 24 V bus
This can be the output capacitor of an unregulated power supply or a capacitor in parallel with
the output of a regulated supply. In most cases, this capacitor does not need to be close to the
drive, so a single capacitor can be shared by multiple drives. This capacitor does several jobs:
1. Bus capacitance absorbs net regenerated mechanical energy from the inertia when the
motor decelerates.
If the bus capacitance is sufficient, regeneration causes a controlled, limited rise in bus voltage
and the over-voltage fault is not tripped.
NOTE
If the regenerated mechanical energy is high, additional bus
capacitors can be added in parallel. The bus capacitance can be
increased almost without limit.
NOTE
The over-voltage fault is a non-latching fault that turns off the inverter
transistors when the bus voltage is above the over-voltage threshold.
An over-voltage fault trip interrupts the regeneration of mechanical
energy back to the bus. This limits the bus voltage rise and protects
the drive. However, it interrupts motor torque, so the machine cycle is
affected. In most cases it is undesirable to allow the bus voltage to
pump up to the over-voltage fault threshold.
In many applications, much or all of the rotational mechanical energy is dissipated as heat in
the motor windings when the motor decelerates. The maximum regenerated rotation energy
back to the bus occurs (counter-intuitively) during a low torque deceleration from high speed. In
this case, the resistive losses in the motor are low. If mechanical drag is low, much of the
stored rotational energy is regenerated to the bus.
2. Bus capacitance absorbs net regenerated inductive energy from the motor winding when
the drive is disabled or faulted.
The worse case, regenerated inductive bus voltage rise is a trip of the bus over-voltage fault
when decelerating the motor at full torque. Tripping the bus over-voltage fault, while stopping
the flow of regenerative mechanical energy back to the bus, causes a fraction of the inductive
energy stored in the windings to regenerate to the bus, causing the bus voltage to go higher
than the over-voltage threshold. If there is insufficient bus capacitance to absorb this energy,
the bus voltage rise is excessive and can damage the drive.
NOTE
Failure to provide adequate external capacitance on the main bus
can damage the drive. The regeneration of motor inductive energy
allows some pump up of the bus voltage above the bus over-voltage
threshold.
3. Bus capacitance improves motor dynamics by holding the bus voltage stable during
acceleration.
An adequately sized bus capacitor helps provide the high peak bus current needed for rapid
motor acceleration with minimum bus voltage sag. If the bus voltage sags excessively during
acceleration, inverter voltage saturation occurs with loss of motor torque.
4. Bus capacitance lowers peak current requirements in the silicon of the power supply.
Sizing the power supply for average power, rather than peak power, lowers power supply cost
and size.