I've been away and have just now caught up with the responses to my query. A hugh thanks to Ed, Just plain Mike, RayO, and Mr Bullfrog for your thoughtful and useful information! @ J.W. Bullfrog: 175V!, that is good news. Approximately how many roasts does your P1 have on it at this voltage? Mike reports no problem with several hundred roasts in the 138-140V range. @RayO: from what I know about series wound motors, your suggestion should work. I haven't tried it yet but for those that wish to, be aware that for the Pumper I motor and likely too the P1 motor the field is wound in two segments with the armature in between. It might be best to use an equal value resistor on each though in theory you should be able to get some reduction in field strength by shunting some of the current in only one of the two windings. @Ed: I previously had read the achieves concerning increasing slot width but did not appreciate the degree of benefit obtained. Your post has caused me to revisit this method. In working with the Wearever, I stumbled onto a "feature" that may have some bearing on the choice of which slots to widen. The temperature of the air existing the 16 slots in the Wearever is not equal but varies in a predictable pattern based on the design of the popper. All three of the pumpers I checked showed the same pattern. Temperatures from the slots vary through a range of about 250F degrees. Widening the highest temperature slots should serve to lower their temperature and thus achieve better uniformity among the slot temperatures.
Douglas Strait wrote: <Snip> several hundred? <Snip> -- Life in the fast lane ...... It is by will alone I set my mind in motion. It is by the juice of aribica that thoughts acquire speed, the lips acquire stains. The stains become a warning. It is by will alone I set my mind in motion.
Hi Douglas, I assume your suggestion of partially shunting the field windings is to = get some control of motor speed. Let me offer my thoughts on this. The motor in the Poppery 1 is a series wound universal motor. These = motors are basically series wound DC motors which have been tweaked = (laminated field core) to operate on AC about as well as they operate on = DC. One of the things which mitigates the fact that they are operating = on AC is that identically the same current flows thru the field as thru = the armature. By definition the phase is identical. If you bypass the = somewhat reactive field with a resistor the armature current will be the = vector sum of the resistive current and the field current. The phase = relationship between the field and the armature will be altered. If you = were to run the motor on DC and partially bypass the field with a = resistor the speed will increase. Field control of DC motor is done, = however usually on shunt wound motors. If you want to do this my = suggestion is to first rectify the line voltage with a bridge rectifier = then filter it with a capacitor. Phil
At 12:17 PM -0400 9/14/05, Douglas Strait wrote: <Snip> Nice work. That's good to know. --E
Philip you are quite right that the current shunted around the field winding will affect the phase relationship between the field and armature windings. I had overlooked this. I retract my statement that RayO's suggestion should work. I consider it an unknown that is best tested empirically by someone who is interested. You are also correct that these are universal motors. In the case of the Wearever 1400W model motor, performance on DC is actually better than AC for an equal applied voltage. The design approach that I am taking on my current mod is to rectify and filter the input AC and then pulse width modulate DC for speed control. A diode is placed antiparallel to the motor to utilize the energy stored in the windings inductance during the "off" period of the applied PWM DC. Using a full wave bridge and 220uF of filter capacitance I can deliver an average of 155VDC to the motor from 120VAC line input. This produces considerably better bean loft than the formerly tested 160VAC input. For thread continuity, Ray's post copied below: Ray wrote: If your goal is to speed up a series wound (AC or DC) motor, you could just shunt the field with a low value wirewound resistor, or even a variable low power rheostat for variable speed. When the motor is running at speed, about half of the applied EMF will be across the armature and half across the field. If you shunt power away from the field, the field magnetism will decrease, and the armature will have to accelerate to a higher speed to generate the back EMF equal to that applied. If you really have a 120 volt series wound motor, try a 50-60 ohm field shunt resistor for starters. The few poppers and hair dryers I've seen recently all had low voltage, PM Field motors, with AC tapped off the heating element feeding a diode bridge soldered to the motor terminals I haven't seen a P I, but my P II has the cheaper PM Field Lo Voltage motor, so no field winding to shunt, and it uses diodes to rectify the AC power. A series wound motor runs fine on AC or DC so would not have the 4 diodes. Cheers -RayO, aka Opa!
Hi Douglas, I should have said universal motors are series wound DC motors tweeked = to work nearly as well on AC as on DC. PWM control on a series motor works very well. I did just what you have = described however at about 12 volts for the AC blower in my '65 Mustang. = In those days such blowers were typically series wound. My PWM = controller used a unijunction transistor VCO, a descrete transistor one = shot (2N2222 if I remember), some big ugly germanium power transistors = for the output, and a catch diode like you mentioned. More precisely it = was not PWM as the pulse width was fixed and the rep rate was varied, so = it was PFM. Phil