LS SERIES SERVICE MANUAL
ISSUE 4.0 CROSSHEAD DRIVE 1
7. CROSSHEAD DRIVE SYSTEM
7.1 General
The machine is driven by a permanent magnet DC motor that is powered by a PWM Driver IC
on the Motor Drive PCB in the base of the machine. The encoder mounted on the end of the
motor shaft provides the motor speed feedback and the speed control is produced by the
microprocessor calculating the required crosshead speed then adjusting the PWM to achieve the
required speed.
The Motor Drive PCB contains the power rectifier, smoothing capacitor and drive electronics. This
is powered from the mains transformer which provides approx. 33V AC for the LS1 variant
machines and 50V AC for the LS2K5E and LS5S machines. This AC supply is rectified and
smoothed to give approximately +45V motor supply for the LS1 variant machines and 65V motor
supply for the LS2K5E and LS5S machines.
The PWM Driver IC (IC1) contains an H bridge together with the MOSFET driver circuits and
requires a +12V power supply for it's internal circuits and a higher voltage for the motor power
supply. The PWM IC is enabled when it's DISABLE input (pin 3) is Low and will drive the motor
when the PWM fed to pin 2 is not exactly a 1:1 squarewave. Note that the PWM Driver IC requires
a bias input on pin 4 and this is provided by the potential divider R5 and R7.
7.2 PWM Driver
See diagram 04/1458 in section 17
The PWM Driver requires 3 signals that are produced by the Main PCB.
The signals are: -
1 Reset Signal High = Normal Low = Reset
2 Drive Enable Signal High = Drive Low = Stop
3 PWM Signal 5V Pulses
The RESET signal is provided by IC26 pin 44 on the Main PCB and is fed to the Drive PCB
where it is inverted by Q5. When the machine is stopped, the RESET signal is Low, the "R"
inputs (pins 4 and 10) to the D Type latch IC3 are High, the latches are reset and their Q
outputs are Low. The /Q outputs of both latches are High so the output of the NAND gate IC5A
pin 3 is Low and the output of IC5C pin 10 is High. When the machine is to run, the RESET
signal will go High and the latches will be enabled.
The Drive ENABLE signal is provided by IC26 pin 43 on the Main PCB and is buffered by Q4
and Q6 on the Motor Drive PCB. When the machine is stopped, the ENABLE signal is Low, the
collector of Q4 is high, the collector of Q6 is Low and the output of the NAND gate IC5B pin 4 is
High. This is the DISABLE signal and is fed to pin 3 of the PWM driver IC (IC1) to inhibit the
motor drive.
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