T-Pain Is On A Boat: Re: [Electric Boats] Relays to replace a controller

Wednesday, July 8, 2009

Re: [Electric Boats] Relays to replace a controller

dennislarson1939 wrote:
> What does the group think about replacing a controller with relays that reduce the voltage from 24 to 12 volts? What is the efficiency loss and what other problems would be expected? And could the same thing be done with a 48 volt system, reducing it to 36, 24 and 12?
It would be bad for the batteries. I assume you mean using relays to
tap off 12-24-36- etc off the battery string. Relays can't reduce
voltage on their own.

The batteries down the 'low' end of the string would go flat
significantly faster than those at the 'top' end of the string. You
could end up with the two 'low volts' batteries dead flat and unable to
continue but still have plenty of power in the 'high' end battery where
you can't get at it.

Even working around all that - the 'lower' end batteries would wear out
faster and have to be replaced earlier.

> There must be some reason why older golf carts used resistors to reduce speed rather than reducing the voltage.
>

They used resistors as silicon switching controllers we all know and
love hadn't been invented yet.

Electric trains had been built that used taps on their main transformer
to switch speed - akin to your idea - switching voltages - but that only
applied to systems fed with AC power. Most early railways used DC power
as it was easier to switch and control. And only DC motors had the
required torque. So if you had an AC supply you had to turn it into DC
at some point - and robust mobile rectifiers were difficult.

Many a tram (street car) has banks of resistors under them to provide a
'low speed' capability - but you didn't run in those 'notches' long or
the resistors over heated, they were provided only to get a smooth
start. (And if you have more than one motor, as these cars did, there
are other tricks you can use to get a number of 'efficient' speeds)