Recently, our 5000 rpm centrifuge (Chinese manufacturer) started slowing down much slower than before. Typically it took 2-3 min to go from 5000 rpm to stop before, now it takes ca. 10 min.
The noise and vibrations of the centrifuge have not changed.
questions:
what could be wrong with it?
how can that be repaired easily?
Thanks a lot for your help.
I have never personally used one, but some centrifuges (and other rotating power equipment) use an electromagnetic brake to decelerate. If there is a circuit that converts the rotational kinetic energy into current (i.e. has a 'generator' function) and then shunts the current to the resistor, the process of electrical generation will impose a drag force on the rotating component. If the circuit becomes open, no electrical current will flow through the resistive (dissipative) element and the elevated drag of will no longer be generated.
Unplug the machine and let it sit for a few minutes to discharge any capacitance in the controls or motor drive circuitry. Then examine the internal circuitry for a large or high-power rating resistor(s) or 'shunt' physically isolated from other electronic components for elevated temperature. Use a volt meter to check for resistance across the shunt resistor and of that circuit in general. If resistance across the shunt is off-the-scale high, the resistor could be failed. Also, a physically-large resistor with burn marks on or around it is another clue of over-temp failure, as could be a burnt odor coming from that component. Replacing a shunt resistor must be done by specifying a power dissipation significantly greater than the expected shunt power demand, to avoid over-temp failure.
If the machine was designed with too low a power rating for that resistor, replacing it with one of the same power rating will likely result in repeat failure. This becomes a question of how well you trust the manufacturer or the supply vendor selling the product.
If any other parts look or smell burned, those would also be good hints as to what the failure could be.
Tinkering with the internals of lab gear is a good way to void any warranty if one exists, so be advised and don't go probing around with any metal or conducting objects while the thing is open, don't leave it open, and don't let anyone else near it while it is open. A centrifuge is a finely balanced instrument and should be treated with care, but many simple failures can be fixed fairly easily or simply after locating and diagnosing the trouble. Unfortunately a burned-out resistor or capacitor may no longer have any readable label or spec value remaining, and determining the spec for the replacement is important for proper and safe function going forward. Good luck and give us a report back. -TH