Power Pack; Intermediate Circuit; Brake Path; Frequency Converter - VWR Mega Star 600R Service Manual

Power Pack

The power pack consists of 1 bridge rectifier, 1 transformer and 2 voltage regulators. It generates the
following low voltage levels:
5V VCC: By step-down voltage regulator LM2674
Supplies all processors and its electronic components on main board, keypad
and display. The reference potential is GND.
24V: Unregulated
Drives the lid lock stepper motor and the buzzer, supplies the LCD and the
relays K1, K2, K3 and K4.
The reference potential is GND.
15V: By voltage regulator LM317
For protection circuit. CAUTION: The reference potential is U-.

Intermediate Circuit

The DC intermediate circuit serves as an energy store between the AC power input and the
transmitted motor performance. The intermediate circuit consists of the line voltage selection via XT1,
a bridge rectifier (integrated in power module M1) and two reservoir capacitors which are charged
whenever the centrifuge lid is closed (K2 activated). The presence of the DC voltage is indicated by
LED V1. When switching the unit off, the DC voltage of the intermediate circuit is discharged across a
path of resistors.
Frequency inverter
The 3-phased motor is provided with chopped direct voltage blocks (chopped frequency approx.
5kHz, amplitude is height of DC intermediate voltage). These blocks are variable in frequency and
pulse-width modulation, 120° de-phased. These 3 phases are controlled during acceleration, running
at set speed and deceleration (e.g. for small speed: low frequency and small pulse-width length will
be affected). The frequency inverter is protected against over current, over temperature and over-
voltage.

Brake Path

Electrical power is fed back into the intermediate circuit during motor deceleration (motor acts as
generator). This braking power is transformed into heat by the connected resistor so that the
intermediate circuit voltage does not rise to an excessive level. The brake resistor is switched into the
intermediate circuit by a fast switching transistor (integrated in power module M1 - pulse-width
modulation, synchronised by the double mains frequency). This transistor is voltage dependent
controlled by a self-acting stage (closed loop).
Microcontroller (Central Processing Unit) Part
The controller block includes the central processor unit (CPU) LPC1764 programmed with software to
control the centrifuge and a data storage (NV-RAM). Current versions and revisions of software and
data can be seen in the user menu.
Software is loaded into the LPC1764 via a serial interface on the main board X5 and cannot be
upgraded in the field. The NV-RAM data can be updated by replacing the entire NV-RAM with one of
the latest revision.
NV-RAM data includes rotor parameters, such as max speed and g-force, acceleration and
deceleration profiles, imbalance behaviour and temperature control data.
FUNCTIONAL DESCRIPTION 3-4