(ET) Re: varistor

[Paul and Sue Abendroth <pabendro gemair com>]

At 01:07 PM 4/8/03 , wombat wrote:

> Thats the one I am trying to get the spec on. Any ideas?

In response to my saying:

> I don't remember seeing one on the charger, but on some models there is 
> one effectively across the drive motor field winding - to protect the 
> switch/relay contacts.





The field has up to 48 volts steady state (less with field weakening) when 
the motor is running. The highest charging voltage should be around 60 
volts. Someone on the list can say what to expect. So that the varistor 
doesn't conduct under those conditions, the nominal voltage rating for the 
varistor can be around 70 volts. (Then it is safe if you are charging and 
run the motor) You can take measurements of this condition and maybe choose 
a lower voltage.

If the charger is on, but there is not a good connection to the batteries, 
and the field is actuated, the varistor can see full open circuit charger 
voltage, which may be 80 volts (guess) and burn out. Another reason for 
good batteries and good connections.

The current the varistor sees is probably a couple of Amps, but should be 
of short duration - limited by the energy stored in the magnetic field in 
the motor. One the size of a quarter or so seems to work for me.



Paul and Sue Abendroth
pabendro gemair com
our permanent address is pabendroth dma org