I too was a bit confused when I first read that. Variable speed blowers are ECM
(electronically commutated motor). The motor has 3 windings, and a rotor with
permanent magnets glued to it. Speed is determined by the rate that the
electronics pulse each winding sequentially, the rotor then chases the
fields. The true variable speed units actually measure RPM and amps to
determine CFM, and will adjust to maintain whatever CFM setpoint is programmed.
As conditions change (dirty filter, wet evap coil, etc) the motor will
compensate. Older versions have a programmed module built into the motor's end
bell to determine preset CFM settings based on jumper or dip switch settings.
They also have programmed ramp-up and ramp-down rates and profiles based on the
manufacturer's programming. They can monster things like heat vs cool mode on a
heat pump, or fan "on" mode and adjust accordingly. Newer VS furnaces and AHUs
use pulse width modulation to control fan speed, and the programming is taken
out of the motor itself and put into the unit's board. A pulsing signal is sent
to the motor, the faster the pulse the faster the motor. Older programmed
motors could be wired up to operate this way.
Both motors are more efficient than their predecessors, the old induction PSC
motors. There were some reliability issues with the ECM motors in certain parts
of the country several years ago, but I haven't heard many complaints lately.
They are both more expensive to replace than a PSC motor, but the technology is
solid and for the most part reliable - these have been around for over 20
years.
Both make for a good system, but the true variable speed will give you tighter
control, especially with two-stage equipment as they can slow down for
low-fire. In many cases the equipment manufacturer will also give you an option
for dehumidification control in cooling mode. My system is a two-stage 95%+
furnace and a two-stage 20 SEER heat pump, which, when operating properly will
give me four stages of heat. Pulse width modulation is used and works
magnificently. Bill W5LO
From: Roger (K8RI) <k8ri@rogerhalstead.com>
To: rfi@contesting.com
Sent: Sunday, July 9, 2017 2:27 AM
Subject: Re: [RFI] Any RFI issues with Amana AMVC96 Furnace?
I agree with Jim and Dave. However I'm a bit confused in the variable 2
speed. Is it variable or 2 speed? Normally all the H & V guys I talked
to said, 2 speed, OR variable. Two speed was just that, one speed, or
the other. All the variable speed used Pulse Width Modulation (PWM) for
speed control. They readily agreed (in a written contract), that they
would replace the unit if it created noise heard by the rig (paraphrased)
The unit is a RUUD rather than Amana. It is a high efficency and
qualified for a good rebate.
I hear nary a sound from the H&V unit on the receiver
We ended up replacing our furnace and central air in the middle of this
past winter. When we fired up the air conditioner late this spring it
ran two days and froze up. They replaced the entire exterior unit.
Both jobs were miserable for the techs. Cold and windy in the winter and
bright sun and middle 90s (with no breeze) in the summer
73, Roger (K8RI)
On 7/7/2017 Friday 11:22 AM, George VE3YV wrote:
> Time for a new furnace. Does anyone have any RFI experience, positive or
> negative, with an Amana variable 2-speed gas furnace?
>
> Thanks!
>
> 73, George VE3YV / K8HI
> _______________________________________________
> RFI mailing list
> RFI@contesting.com
> http://lists.contesting.com/mailman/listinfo/rfi
>
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