36 Volts, 96 Volts - Handling odd voltages

[domwild]

Hi,

There are some of us with ex-Telecom or army UPS systems running at odd voltages. Question: How does one shed the load?

Possibilities:

1. Instead of one relay to connect load, use several relays to dump the excess, e.g., three 12V relays to dump a 36VDC surplus.
   
2. Connect the UPS via a relay as the load and use a relay to shunt to a 240/110 AC load; rather inefficient.
   
   

Anyone out there with the same problem, who has managed to solve it?

[maker of toys]

in the specs for a relay (download the datasheet) is the current drawn by the coil.  

Knowing that, you can put an appropriate resistor in line (pay attention to power disipation in the resistor, not just resistance) to get the correct current through the coil of, say, a 24V relay for use in a 36V system.

of course, if it's a DC relay, you can just measure the coil resistance and calculate the approximate current required to operate it. . . and choose your resistor accordingly. if you're off by 10% one way or the other, most DC relays won't care.

If, for whatever reason, you're dealing with an AC relay, it's best to go to the data sheet, as DC resistance (what your $10 DVM measures) has very little to do with the AC impedance of the coil at 50 or 60 hz.

Regardless of the coil rating, the contacts don't care about voltage much so long as you don't exceed the current rating . . .  though you'll need to know what rating to use (DC, AC, inductive load) Usually that information is on the relay case somewhere; it's also usually listed in the catalog if you're buying new parts.

there's also the question of "if you're dumping the power, what does efficiency matter?"  (ignoring for the moment that you'd really like to pick where the heat ends up. . .)

the answer usually is:  Dumping power BEFORE the inverter is better for the mill and batteries from a reliablity standpoint, and easier on the inverter, because it's not worked as hard, as often. efficiency doesn't enter into it. . .

Hope that helps-

-Dan

[Nando]

If You have an "odd" battery voltage, like 36 volts, the only thing needed is to get the proper set up for the relay and just use a load capable to ballast the battery bank.

36 volts relays are not common any more.

Solution = use a 24 volts relay with a small resistor in series, which is equal to 1/2 half the coil resistance value and with enough watts to dissipate the voltage drop.

The contacts just insert the load to the 36 volts and the relay should have a rating at least 2 times the ballast current.

Do You know how to do these steps ?.

Nando

[domwild]

Nando and Maker of toys,

Thanks for that. I should have been more specific: There are pleny of 12 or 24VDC water heaters or other appliances to dump to excess to but certainly nothing along the 36VDC line.

Assuming I had three 12VDC water heaters, to balance the load evenly I would have to connect each one of the three 12VDC batteries in the 36V bank with a relay to switch in the three 12V water heaters.

This is the problem, no useful 36VDC sink along the lines of water pumps or heaters.

Thanks, Nando, for telling me how to use a 24V relay. There is Brucedownunder with his 96VDC UPS setup and most likey similar problems.

May just have to switch in the UPS as the load and a useful 240VAC appliance.

Regards,

 

[RP]

As long as the 3 12v heaters were the same, you could just connect all of them in series to create a 36v load/heater.

[domwild]

RP,

Thanks for that. Then I have to switch 36VDC and there are probably no 36DC relays, but as Nando explained, I could use a 24V relay with resistor.

[robl]

You could get an off-the-shelf 48V load and simply derate it proportionally. I am running 48VDC into a 120V electric hotwater tank. Don't remember the original rating (1500W?) but the tank draws about 400W at 48VDC.

Rob

[LeissKG]

Power is proportional to the square of the voltage. So 120V/48V is 2.5 and 2.5 squared is 6.25. This means your 400W load at 48V is 2500W at 120 Volts. An 48V load used with 36V has only 0.5625 of its power. A 120V load at 96V has only 0.64 of its power.

Klaus Leiss

 

[Seaspray0]

The relay is rated for 3 items... maximum current capacity of the contacts, voltage maximum between the contacts, and specific voltage required to activate the coil.  As long as you use the specified voltage to activate the coil, you can apply any voltage or current to the contacts provided you do not exceed the limits.  Most relays are rated between 240 to 600 volts between the contacts.

[jonas302]

Hi check out this company they have plenty of 36v relays and some military surplus generators http://www.surpluscenter.com/home.asp?UID=2006122121112130

[domwild]

Thanks for all the answers.

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