Honeywell W7750A Manual

A SERVICE OF

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EXCEL 10 W7750A,B,C CONSTANT VOLUME AHU CONTROLLER

61 74-2958—1

coolin

g

control al

g

orithm compares the current space

temperature to the EffectiveHeatSetPt, and calculates a PID

error si

g

nal. This error si

g

nal causes the heatin

g

sta

g

e

outputs to be c

y

cled, as re

q

uired, to drive the space

temperature back to the Setpoint. Fi

g

. 50 illustrates the

relationship between PID error and sta

g

ed output activit

y

.

As the error si

g

nal increases, the space temperature

g

ets

further awa

y

from the setpoint, or is remainin

g

below the

setpoint as time elapses, additional sta

g

es of heatin

g

are

ener

g

ized until, if PID error reaches 100 percent, all

confi

g

ured sta

g

es are on.

The PID control al

g

orithm used to control sta

g

ed heatin

g

is

anticipator-driven, and is similar to the al

g

orithm used in the

T7300 commercial thermostat. All sta

g

in

g

events are sub

j

ect

to a minimum intersta

g

e time dela

y

, that is based on the

c

y

cles per hour user settin

g

(

HeatC

y

cHr

)

. The minimum

intersta

g

e time dela

y

ran

g

es from 90 seconds

(

at 12 c

y

cles

per hour

)

to ei

g

ht minutes

(

at two c

y

cles per hour

)

. See Table

17. The user has the option to disable the minimum run timer

for heatin

g

(

DisMinHtTimer

)

. If the minimum run timer is

disabled, the intersta

g

e time dela

y

is fixed at 20 seconds. The

c

y

clin

g

rate is separatel

y

selectable for heatin

g

and coolin

g

between two and 12 c

y

cles per hour

(

cph

)

.

Setpoints for the PID

g

ains allow for unit-b

y

-unit ad

j

ustment of

the control loop, if re

q

uired; however, an

y

chan

g

e from the

default values should be minimal.

CASCADE CONTROL OF MODULATING COOLING/HEATING

The Excel 10 W7750 Controller supports modulatin

g

coolin

g

and heatin

g

valves. These valves can be controlled directl

y

from the space temperature

(

like the sta

g

ed control

)

or, if the

CascCtrl fla

g

is set, the

y

are modulated to maintain the

dischar

g

e air temperature at its setpoint. The dischar

g

e air

setpoint is calculated based on the space temperature

deviation from the space setpoint. This is commonl

y

called

cascade control. In the W7750 Controller, cascade control is

available for use with PWM

(

W7750B,C onl

y)

and Series 60

modulatin

g

heatin

g

and coolin

g

, but not for use with sta

g

ed

heatin

g

/coolin

g

.

Setpoints for the PID

g

ains and for the control band allow for

unit-b

y

-unit ad

j

ustment of the control loops, if re

q

uired;

however, an

y

chan

g

e from the default values should be

minimal. Also, the W7750 Controller uses an adaptive

al

g

orithm

(

patent pendin

g)

to continuousl

y

assess the validit

y

of the calculated dischar

g

e setpoint, and ad

j

ust it, as needed,

to ensure precise, accurate control.

SERIES 60 MODULATING CONTROL

Series 60 Control is also commonl

y

referred to as Floatin

g

Control. The Excel 10 W7750A,B,C Controllers can drive

Series 60 t

y

pe actuators to control a modulatin

g

coolin

g

valve,

a heatin

g

valve, and economizer dampers. When floatin

g

control is used, the full-stroke motor drive time of the actuator

must be entered into the confi

g

uration parameter CoolMtrSpd

(

for coolin

g)

, HeatMtrSpd

(

for heatin

g)

, or EconMtrSpd

(

for the

economizer dampers

)

.

PULSE WIDTH MODULATING (PWM) CONTROL

The Excel 10 W7750B,C Controllers can drive a PWM-t

y

pe

actuator to control a modulatin

g

coolin

g

valve, a heatin

g

valve, and economizer dampers. PWM control positions the

actuator based on the len

g

th, in seconds, of the pulse from

the di

g

ital output. The controller outputs a pulse whose len

g

th

consists of two parts, a minimum and a maximum. The

minimum pulse time represents the analo

g

value of zero

percent

(

also indicates a si

g

nal presence

)

and the maximum

pulse len

g

th that represents an analo

g

value of 100 percent. If

the analo

g

value is

g

reater than zero percent, an additional

time is added to the minimum pulse time. The len

g

th of time

added is directl

y

proportional to the ma

g

nitude of the analo

g

value. If PWM control is used, the confi

g

uration parameters

for the PWM operation must be specified. These are

PwmPeriod, PwmZeroScale, and PwmFullScale. These three

parameters are shared b

y

all confi

g

ured PWM outputs; this

means the heatin

g

, coolin

g

, and economizer actuators must

be confi

g

ured to accept the same st

y

le of PWM si

g

nal.

Example

: To find the pulse width of a valve actuator

(

for

example stroke mid position - 50 percent

)

with the

PwmZeroScale = 0.1 seconds, PwmFullScale = 25.5

seconds, and the PwmPeriod = 25.6 seconds. There are 256

increments available, so the number of increments re

q

uired

for 50 percent would be

(

0.5 X 256

)

or 128. The time for each

increment for this industr

y

standard pulse time is 0.1 seconds.

The pulse width is the minimum time

(

0.1 second

)

+ the

number of increments

(

128 times the

(

0.1 second

)

plus 0. 1

)

=

12.9 seconds. The W7750B,C Controllers would command

the valve output on for 12.9 seconds for the PwmPeriod of

25.6 seconds to maintain the valve position at 50 percent.

OUTDOOR AIR LOCKOUT OF HEATING/COOLING

A mechanism is provided in the W7750 to disable the heatin

g

e

q

uipment if the outdoor air temperature rises above the

OaTempHtLkOut setpoint. Similarl

y

, the coolin

g

e

q

uipment is

disabled if the outdoor air temperature falls below the

OaTempClLkOut setpoint. The al

g

orithm supplies a fixed 2

°

F

(

1.1

°

C

)

h

y

steresis with the lock-out control to prevent short

c

y

clin

g

of the e

q

uipment.

ECONOMIZER DAMPER CONTROL

A mixed-air economizer damper packa

g

e can be controlled to

assist mechanical coolin

g

in maintainin

g

the dischar

g

e air at

setpoint. Therefore, if modulatin

g

economizer damper control

is desired, a dischar

g

e air temperature sensor is re

q

uired. If

the outdoor air is not currentl

y

suitable for coolin

g

use

(

see the

Economizer Enable/Disable Control section

)

, the outdoor air

damper is held at the user-settable minimum position

(

EconMinPos

)

for ventilation purposes.

Because the outdoor air can be used to supplement

mechanical coolin

g

, the economizer operates as if it were the

first sta

g

e of coolin

g

. So, if the outdoor air is suitable for

coolin

g

use, the mechanical coolin

g

(

either sta

g

ed or

modulatin

g)

is held off until the economizer has reached its

full

y

open position. Then, if the dischar

g

e temperature

continues to be above setpoint, the mechanical coolin

g

is

allowed to come on. If the outdoor air is

not

suitable for

coolin

g

use, the economizer is set to its minimum position,

and mechanical coolin

g

is allowed to come on immediatel

y

.

When the controller is in the Heat mode, the economizer is

held at the minimum position settin

g

(

EconMinPos

)

. The

minimum position settin

g

is onl

y

used durin

g

Occupied mode

operation. When in Standb

y

or Unoccupied modes, the

outdoor air dampers are allowed to full

y

close if there is no call

for coolin

g

, or if the outside air is not suitable for coolin

g

use.