Taylor 337 Service Manual - Pulse Width Modulation
13
Model 337
Controls and Systems
070321
Pulse Width Modulation
The central feature of the Model 337 is the precise
control of the refrigerant flow. The flow of the
refrigerant is distributed between the freezing
cylinders on the basis of cylinder cooling load. This
precise control is achieved by using Pulse Width
Modulated (PWM) valves as the refrigerant control
devices.
The universal control regulates the amount of time
each PWM valve is open, thereby controlling the
amount of refrigerant that flows through each freezing
cylinder. Taylor uses the term “split” to refer to the
amount of time each PWM valve is open in relation to
the amount of time the other PWM valve is open.
There are three types of splits: high, low, and even.
For example, if the left PWM valve is described as
operating at the HIGH split, it is passing more
refrigerant (open longer) than the right PWM valve. In
this case, the right PWM valve is operating at the LOW
split. If the left PWM valve is described as operating at
the EVEN split, it is passing as much refrigerant (open
for the same amount of time) as the right PWM valve.
Note: Whenever the refrigeration system is active,
both PWM valves are active.
The valve of the PWM duty cycle low percent is the
equivalent of the low split. The valve of the PWM high
split is determined by subtracting the PWM duty cycle
low percent from the PWM duty cycle. When an even
split occurs, the value of the PWM duty cycle is evenly
divided between the freezing cylinders.
PWM Valve Split During Freeze Down
When the AUTO mode is selected, the refrigeration
system is activated and the PWM valve split is
determined by the temperature of the freezing
cylinders and the difference between the freezing
cylinder temperatures at the time the AUTO mode was
selected. (See Figure 3 on page 14.)
Example 1
If only the left freezing cylinder temperature is below
25_F(--4_C), and the difference between the right and
left freezing cylinder temperatures is more than 10_,
the left PWM valve operates at the low split and the
right PWM valve operates at the high split. (If the
difference between the left and right freezing cylinder
temperatures is less than 10_, the left and right PWM
valves will operate at an even split.)
Example 2
If only the right freezing cylinder is below 25_F (--4_C)
and the difference between the right and left freezing
cylinder temperatures is more than 10_, the right PWM
valve operates at the low split and the left PWM valve
operates at the high split. (If the difference between the
right and left freezing cylinder temperatures is less
than 10_, the left and right PWM valves will operate at
an even split.)
Example 3
If both freezing cylinders are at or above 25_F (--4_C)
or both freezing cylinders are below 25_F (--4_C), the
right and left PWM valves operate at an even split.
If, during the AUTO mode one barrel temperature is
more than 10_F (12_C) above the other barrel
temperature, the valve for the warmer barrel will be
modulated at the high split, and the valve for the colder
barrel will be modulated at the low split.