AKTA FPLC Mixer 925 Problem and Solution
Chalmers
We have an AKTA FPLC with the Mixer 925 unit. We don't have a service
contract so when the mixer unit stopped working and the system
provided a short-circuit warning message, we asked the price of a new
one. We were quoted �10,000, which is half the price of the whole
system. Presumably this is a joke, designed to force you to call in
the engineer and then get a service contract, perhaps.
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.
It takes about 5 minutes to change the motor using simple tools, and
we saved a significant amount of money.
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Han
news:mailman.308.12584...@net.bio.net:
Thanks. This is from home, and I don't know by heart whether we have
that system, or another. Neither do I know whether that motor would be
available in the US.
I do know that I could replace the power supply on the Compaq computer
that runs our HPLC after it quit. About US$38 on Ebay.
--
Best regards
Han
email address is invalid
Dr Engelbert Buxbaum
> 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 ￜ47 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.
