The 80-2B centrifuge is a small 12-tube desktop centrifuge manufactured by Shanghai Anting Scientific Instrument Factory. The maximum capacity of this machine is 24Oml, and the maximum speed is 4000r/min. It can conveniently and quickly separate the serum of small batches of samples. Now we report a failure we encountered during use and its repair or emergency treatment methods as follows for reference by peers.
1. Fault phenomenon
Close the upper cover, turn on the timer, turn on the power switch, the power indicator light is on, and the centrifuge cannot be used because it does not rotate.
2. Failure analysis
(l) Working principle: After inspection, it is found that the power supply of the machine is composed of a half-controlled bridge rectifier circuit triggered by a unijunction transistor.
The motor is used as the load, which is a single-phase series-excited motor produced by Ningbo Micro Special Motor Factory; the conduction angle of the thyristor is changed by changing the variable resistor on the panel, thereby controlling its output voltage to change the motor speed.
(2) Cause of failure: Due to the long-term use of the centrifuge at a maximum speed of 4000r/min, the carbon brush of the motor is severely worn, and the contact resistance with the converter of the rotor coil is poor, which increases the contact resistance, thereby causing the starting current to overshoot Large and burnt thyristor.
3. Troubleshooting or emergency treatment
Disconnect the power supply of the centrifuge from the mains, open the upper cover, unscrew the 4 screws on both sides, pick up the upper case, and unplug the switch line interface of the upper cover, and take out the upper case. Remove a pair of severely worn carbon brushes from the motor, clean the converter of the rotor coil with absolute ethanol, and install a new carbon brush. Unscrew the 4 screws at the back of the case, take out the circuit board, remove the pair of thyristors that were burned out, and replace them with thyristors of the same model. If there is no thyristor of the same type, do not remove the burned thyristor, but short-circuit the anode and cathode of one of the burned thyristors for emergency repair. Reassemble the centrifuge in the reverse direction of the original disassembly. Connect the power supply of the centrifuge to the mains, close the top cover, turn on the timer, turn on the power, the power indicator light is on, the centrifuge rotates, and the fault is eliminated.
After the emergency treatment, the centrifuge can be used temporarily, but the speed of the centrifuge drops to 2000r/min, and it is uncontrollable, so long-term use is not recommended.
