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VLT® HVAC Basic Drive Casca...

1.1 Cascade Controller

Constant Speed

Pumps (2)

Variable Speed

Pumps (1)

Motor starter

Illustration 1.1 Cascade Controller

The Cascade Controller is used for pump applications

where a certain pressure ("head") or level needs to be

maintained over a wide dynamic range. Running a large

pump at variable speed over a wide for range is not an

ideal solution because of low pump efficiency and because

there is a practical limit of about 25% rated full load speed

for running a pump.

In the Cascade Controller the frequency converter controls

a variable speed motor as the variable speed pump (lead)

and can stage up to two additional constant speed pumps

on and off. By varying the speed of the initial pump,

variable speed control of the entire system is provided.

This maintains constant pressure while eliminating

pressure surges, resulting in reduced system stress and

quieter operation in pumping systems.

Fixed Lead Pump

The motors must be of equal size. The Cascade Controller

allows the frequency converter to control up to 5 equal

size pumps using the drives two built-in relays and

terminal 27, 29 (digital input/digital output). When the

variable pump (lead) is connected directly to the frequency

converter, the other 4 pumps are controlled by the two

built-in relays and terminal 27, 29 (digital input/digital

output). Lead pump alternation can not be chosen when

lead pump is fixed.

Lead Pump Alternation

The motors must be of equal size. This function makes it

possible to cycle the frequency converter between the

pumps in the system (when 25-57 Relays per Pump =1,

maximum pump is 4. When 25-57 Relays per Pump =2,

maximum pump is 3). In this operation the run time

between pumps is equalized reducing the required pump

maintenance and increasing reliability and lifetime of the

system. The alternation of the lead pump can take place at

a command signal or at staging (adding lag pump).

VLT

HVAC Basic Drive Cascade Controller

Pressure Sensor

Frequency Converter

with

Cascade Controller

MI18I302 - VLT

is a registered Danfoss trademark

The command can be a manual alternation or an

alternation event signal. If the alternation event is selected,

the lead pump alternation takes place every time the

event occurs. Selections include whenever an alternation

timer expires, when the lead pump goes into sleep mode.

Staging is determined by the actual system load.

⇐ 50%=1, if load >50% Alternation

25-55 Alternate if Load

will not happen. If load ⇐50% Alternation will happen.

⇐ 50% =0, Alternation will

When 25-55 Alternate if Load

happen no matter with Load. Total pump capacity is

determined as lead pump plus lag speed pumps capacities.

Bandwidth Management

In cascade control systems, to avoid frequent switching of

fixed speed pumps, the desired system pressure is kept

within a bandwidth rather than at a constant level. The

Staging Bandwidth provides the required bandwidth for

operation. When a large and quick change in system

pressure occurs, the Override Bandwidth overrides the

Staging Bandwidth to prevent immediate response to a

short duration pressure change. An Override Bandwidth

Timer can be programmed to prevent staging until the

system pressure has stabilized and normal control

established.

When the Cascade Controller is enabled and running

normally and the frequency converter issues a trip alarm,

the system head is maintained by staging and destaging

fixed speed pumps. To prevent frequent staging and

destaging and minimize pressure fluxuations, a wider Fixed

Speed Bandwidth is used instead of the Staging

bandwidth.

Illustration 1.2 Bandwidth