AKTA FPLC Mixer 925 Problem and Solution

Dr Engelbert Buxbaum via methods%40net.bio.net (by engelbert_buxbaum from hotmail.com)
Thu Nov 26 11:19:02 EST 2009

Am 17.11.2009, 08:00 Uhr, schrieb Chalmers <chalmers from nottingham.ac.uk>:

> The mixer is basically a magnetic stirrer.  I opened the unit and found  
> a little 12 V DC motor that turns the magnet.  I was confused at first  
> because it worked when I attached a 9V battery.  The final explanation  
> was that the motor had difficultly getting started, particularly at 4 C,  
> but not so bad at room temperature.  It worked intermittently for a  
> while then failed.  It had only 2 ohms resistance between the terminals  
> (I suspect it should be 12 ohms).
> The ID numbers on the motor specify a custom motor supplied to the AKTA  
> manufacturers, so you can not buy one.  However, it looked suspiciously  
> like a Premotec 990412018105.  You can buy one for £47 from RS  
> components (the name on the motor is different but the manufacturers  
> product code is identical).
> The Premotec motor is rated to run at 3600 rpm at 12 V.  However, it  
> receives only 1.89 V from the Mixer 925 unit.  It therefore runs close  
> to the specified 600 rpm when drawing power from the Mixer 925.

This is not unusual: the motor is operated with a lower than specified  
voltage to reduce the rpm. This is actually a design flaw, for  
unfortunately this also reduces torque. Thus the motor may not start,  
especially under condition that increase the torque required, like low  
temperatures (metal shrinkage in bearing, higher viscosity of lubricants).  
The solution is not to replace the motor (which is perfectly fine), but to  
build a puls width modulation power supply for it. PWM means that the  
motor is operated with impulses instead of DC, which have nominal voltage  
(giving full torque). The width of these impulses determines rpm. Any  
competent workshop can do that in a day or so, it's actually quite simple.

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