HomeRoast Digest


Topic: 1400W Wearever motor overvoltage tolerance (6 msgs / 389 lines)
1) From: Douglas Strait
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.

2) From: J.W.Bullfrog
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.

3) From: Philip Keleshian
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

4) From: Edward Spiegel
At 12:17 PM -0400 9/14/05, Douglas Strait wrote:
<Snip>
Nice work. That's good to know.
--E

5) From: Douglas Strait
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!

6) From: Philip Keleshian
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


HomeRoast Digest