<html><head></head><body><div style="color:#000; background-color:#fff; font-family:Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif;font-size:13px"><div id="yui_3_16_0_ym19_1_1497960237147_3142" dir="ltr"><span id="yui_3_16_0_ym19_1_1497960237147_3141">I added a wire wound rheostat. I would have to check to see the rating. I think it was 25 watts. What I did is put a pushbutton that turns on the field weakening circuit with a self keeping relay circuit. Any time I stop the holding circuit drops out so I avoid starting up with the field weak. I do not use a card at all. It gives me infinite adjustment. I keep an eye on the power meter and back off on the filed adjustment if the amps climb to high and you can feel it when the motor is struggling and FW needs to be reduced. I have used this for 12+ years. Before that I used a 3 position switch and two power resistors.</span></div><div id="yui_3_16_0_ym19_1_1497960237147_3255" dir="ltr"><span><br></span></div><div id="yui_3_16_0_ym19_1_1497960237147_3256" dir="ltr"><span id="yui_3_16_0_ym19_1_1497960237147_3257">Assuming the field coil is about 19 ohms you get about 1.9 amps at 36 volts nominal. Example target current limiting resistors would be 17 ohms for 1 amp and 50 ohms for .5 amps.</span></div><div id="yui_3_16_0_ym19_1_1497960237147_3286" dir="ltr"><span>Rob Brockway</span></div><div class="qtdSeparateBR" id="yui_3_16_0_ym19_1_1497960237147_3137"><br><br></div><div class="yahoo_quoted" id="yui_3_16_0_ym19_1_1497960237147_3136" style="display: block;"> <div id="yui_3_16_0_ym19_1_1497960237147_3135" style="font-family: Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; font-size: 13px;"> <div id="yui_3_16_0_ym19_1_1497960237147_3134" style="font-family: HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; font-size: 16px;"> <div id="yui_3_16_0_ym19_1_1497960237147_3133" dir="ltr"> <font id="yui_3_16_0_ym19_1_1497960237147_3163" face="Arial" size="2"> <hr size="1" id="yui_3_16_0_ym19_1_1497960237147_3162"> <b><span style="font-weight: bold;">From:</span></b> Robert Laird <rsl360@gmail.com><br> <b><span style="font-weight: bold;">To:</span></b> Tim Maxwell <timmaxwell@chartertn.net> <br><b><span style="font-weight: bold;">Cc:</span></b> Elec-Trak Tractor <elec-trak@cosmos.phy.tufts.edu><br> <b><span style="font-weight: bold;">Sent:</span></b> Tuesday, June 20, 2017 12:54 AM<br> <b><span style="font-weight: bold;">Subject:</span></b> Re: (ET) field weakening potentiometer with a Curtis 1204<br> </font> </div> <div class="y_msg_container" id="yui_3_16_0_ym19_1_1497960237147_3294"><br><div id="yiv1856505904"><div id="yui_3_16_0_ym19_1_1497960237147_3293"><div id="yui_3_16_0_ym19_1_1497960237147_3292" dir="ltr"><div id="yui_3_16_0_ym19_1_1497960237147_3291"><div id="yui_3_16_0_ym19_1_1497960237147_3290"><div id="yui_3_16_0_ym19_1_1497960237147_3289"><div id="yui_3_16_0_ym19_1_1497960237147_3288"><div id="yui_3_16_0_ym19_1_1497960237147_3287">A few notes on using a rheostat. (a high wattage potentiometer)<br clear="none"></div>-A rheostat's wattage is proportional to its setting. So a 100 watt, 100 ohm rheostat is only a 50 watt rheostat when set to 50 ohms.<br clear="none"></div>-A trick that can be used in rating rheostats, (well, any resistor actually), is to rate them in amps, not watts. <br clear="none"><br clear="none"></div>For instance, a 100 ohm rheostat set to 100 ohms, with 100 volts across it will dissipate 100 watts and will have 1 amp flowing in it. If you set the rheostat to 50 ohms, in order not burn up your rheostat, you will have to lower the voltage to 50 volts, which will give you a dissipation of 50 watts. This will again be 1 amp. And so on.<br clear="none"><br clear="none"></div>So you need a rheostat that can take the field current. 2 amps? (I'm too lazy to look it up right now.) But now we run into a problem. When you first put the rheostat inline, it has to take just about the entire field current. (not much limiting is taking place yet.) A 50 ohm rheostat for 2 amps is 200 watts. Kind of big. But, if you simply switch in a 20 ohm resistor, the current (about) gets cut in half, and you only need a 20-ish watt resistor. Play with some numbers and you will see how this works. It's not totally obvious.<br clear="none"><br clear="none"></div>Robbie Laird<br clear="none"><div><br clear="none"><br clear="none"><br clear="none"><br clear="none"><br clear="none"><br clear="none"></div></div><div class="yiv1856505904yqt5157863312" id="yiv1856505904yqt98578"><div class="yiv1856505904gmail_extra"><br clear="none"><div class="yiv1856505904gmail_quote">On Mon, Jun 19, 2017 at 1:59 PM, Tim Maxwell <span dir="ltr"><<a href="mailto:timmaxwell@chartertn.net" target="_blank" rel="nofollow" shape="rect" ymailto="mailto:timmaxwell@chartertn.net">timmaxwell@chartertn.net</a>></span> wrote:<br clear="none"><blockquote class="yiv1856505904gmail_quote" style="margin: 0px 0px 0px 0.8ex; padding-left: 1ex; border-left-color: rgb(204, 204, 204); border-left-width: 1px; border-left-style: solid;">I am considering putting a Curtis 1204 (sitting on the shelf collecting dust) in an E15 (most of card 1 has been bypassed by the PO due to failures).<br clear="none"> <br clear="none"> I vaguely remember reading somewhere about a user putting a potentiometer in the field to provide a variable "turbo mode."<br clear="none"> The resistors on card 3 are 20 ohm 10%, but what wattage are they?<br clear="none"> IE what watt rating would the potentiometer need?<br clear="none"> <br clear="none"> Tim M<br clear="none"> <br clear="none"> <br clear="none"> <br clear="none"> ______________________________ _________________<br clear="none"> Elec-trak mailing list<br clear="none"> <a href="mailto:Elec-trak@cosmos.phy.tufts.edu" target="_blank" rel="nofollow" shape="rect" ymailto="mailto:Elec-trak@cosmos.phy.tufts.edu">Elec-trak@cosmos.phy.tufts.edu</a><br clear="none"> <a href="https://cosmos.phy.tufts.edu/mailman/listinfo/elec-trak" target="_blank" rel="nofollow" shape="rect">https://cosmos.phy.tufts.edu/m ailman/listinfo/elec-trak</a><br clear="none"> </blockquote></div><br clear="none"></div></div></div></div><div class="yqt5157863312" id="yqt55235">_______________________________________________<br clear="none">Elec-trak mailing list<br clear="none"><a href="mailto:Elec-trak@cosmos.phy.tufts.edu" shape="rect" ymailto="mailto:Elec-trak@cosmos.phy.tufts.edu">Elec-trak@cosmos.phy.tufts.edu</a><br clear="none"><a href="https://cosmos.phy.tufts.edu/mailman/listinfo/elec-trak" target="_blank" shape="rect">https://cosmos.phy.tufts.edu/mailman/listinfo/elec-trak</a><br clear="none"></div><br><br></div> </div> </div> </div></div></body></html>