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Thanks Rob, that will be helpful.
Mike
Michael S. Briggs, PhD
UNH Physics Department
(603) 862-2828
From: Rob Brockway <getractor yahoo com>
Sent: Saturday, April 30, 2016 3:25 PM To: Briggs, Michael; elec-trak cosmos phy tufts edu; noaddress drmm net Subject: Re: (ET) Curtis controller mod - Field - Buck converter maybe Mike,
To get an idea of the actual motor performance and calculate the efficiency you can use one of the GE performance curves such as:
I have found it interesting to calculate actual motor Hp and efficiency at different torque loads. If you limit the HP level the efficiency can be quite
good at different FW. The key is the HP essentially fixed and as field weakening is used the magnetic field becomes more easily demagnetized by large amps/ flux in the armature
Rob
From: "Briggs, Michael" <Michael Briggs unh edu> To: "elec-trak cosmos phy tufts edu" <elec-trak cosmos phy tufts edu>; "noaddress drmm net" <noaddress drmm net> Sent: Saturday, April 30, 2016 6:27 AM Subject: Re: (ET) Curtis controller mod - Field - Buck converter maybe David, I'm just lumping all DC-DC converters together. Yes, a buck converter would be a good approach, I just figured many people may not know what that is, so I was just calling it a DC-DC converter (since it is a type of DC-DC converter). The field resistors are only rated at 10 Watts? Ahhh, I didn't realize there was so little power being dissipated across them. If that's the case, then you're right, weakening the field with an inline resistor isn't all that wasteful. In general, I just don't like the practice of using a resistive voltage divider to reduce the amount of voltage your load gets, since it can be an inefficient practice (such as the toaster resistors). But, I didn't think about the fact that the current through the armature is significantly greater than the current through the field coils, so a voltage divider on the field coils isn't going to be as costly overall. Ok, I'm back to thinking that it would be fine to just use a "turbo" switch that lets you put a resistor (or variable resistor) into the field circuit for weakening. That would be simple to do, and apparently not that inefficient. Off-hand, I can't think of any other losses that would be introduced due to weakening the field. The main downside is that you have less torque. But if you only use it when you're driving around (not doing anything active like tilling), that shouldn't be a big problem. IMO at least. Mike ________________________________ Michael S. Briggs, PhD UNH Physics Department (603) 862-2828 ________________________________ ________________________________________ From: David Roden <etpost drmm net> Sent: Saturday, April 30, 2016 12:55 AM To: elec-trak cosmos phy tufts edu Subject: Re: (ET) Curtis controller mod - Field - Buck converter maybe On 29 Apr 2016 at 17:46, Briggs, Michael wrote: > using a DC-DC converter to vary the field strength would be preferable > over the load resistor approach ... I may be missing something, but I don't understand why you'd want to use a DC:DC converter. Isn't a DC:DC converter usually used to produce a higher or lower fixed DC voltage from a given DC input? It seems to me that we don't really want that here, since the GE motor's field requires a variable voltage <= the battery voltage, 36 volts. Maybe I'm just using a different term for the same thing, but it seems to me that what you want for more efficient FW on the ET is not a DC:DC converter, but - as the subject here suggests - a buck converter. As it turns out, that's exactly what the Alltrax DCX300-ET controller uses. That said, from what I can see, the amount of energy lost in the GE FW resistors is trivial compared to what you're losing in the armature resistors. Look at the relative size of the resistors. IIRC the field resistors are rated at 10 watts, so the average power dissipated in them can't exceed that amount. I'd guess that the typical running power of the motor is about 1500 watts, so unless I'm missing something, we're talking about - at most - a 0.7% loss in the FW resistors. I'm not a motor expert, but my understanding has for some years been that the reduced efficiency during field weakening is caused more by losses in the motor than by losses in the field control method. But I can't find any reference for that online right now, so maybe I'm remembering it wrong. In any case, if you're going to use a PWM controller or buck converter or whatever for only one part of the motor, it makes more sense to use it for armature control. > Back when the Elec-Trak was designed, converting low voltage DC to high > voltage DC was not an easy task. I guess that depends on what you mean by "easy task." Switchmode power supplies were, if I'm not mistaken, developed in the mid- or late 1960s, mostly thanks to the US space program. So semiconductor based DC:DC converters certainly existed by the time GE was making the ET. However, I suspect that the price of high power semiconductors at the time made them too costly for a more-or-less mass market product like our ETs. BTW, if they'd been willing to spend the dough, GE absolutely could have used a solid state speed controller in the ET. The earliest General Electric SCR speed controller manuals I've seen bear a copyright date of 1967. So GE should have had plenty of solid state controller expertise in house when they designed the ET. I suspect again that they went with the cruder resistive design to keep the cost of the already-expensive ET somewhat closer to competitive. In fact GE could have used a transistorized speed controller design, though again the cost might have been prohibitive. In 1968 Motorola released an app note for their MP506 germanium (!) transistor, showing a practical 36v 300a controller design. David Roden - Akron, Ohio, USA = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Note: mail sent to the "etpost" address will not reach me. To send me a private message, please use the address shown at the bottom of this page : https://urldefense.proofpoint.com/v2/url?u=http-3A__www.evdl.org_help_&d=CwICAg&c=c6MrceVCY5m5A_KAUkrdoA&r=kRHnlt3cKuWr4wuU8rx6ayYoAVctaoS9Z3cUPIkQn-k&m=ibrc2yXC6la45o3zRaL6Cmb1HrquBzszDvTMbRbvU_4&s=AyBu7F3HW5eq8EW-7SVbRbeFl21fe4IZW2DXoticrO0&e= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = _______________________________________________ Elec-trak mailing list Elec-trak cosmos phy tufts edu https://urldefense.proofpoint.com/v2/url?u=https-3A__cosmos.phy.tufts.edu_mailman_listinfo_elec-2Dtrak&d=CwICAg&c=c6MrceVCY5m5A_KAUkrdoA&r=kRHnlt3cKuWr4wuU8rx6ayYoAVctaoS9Z3cUPIkQn-k&m=ibrc2yXC6la45o3zRaL6Cmb1HrquBzszDvTMbRbvU_4&s=gzlONAlWMcwMIPuGB2F7ue1KtvK0zc8_Hm6z2-RuWvo&e= _______________________________________________ Elec-trak mailing list Elec-trak cosmos phy tufts edu https://cosmos.phy.tufts.edu/mailman/listinfo/elec-trak |
