Afd Control

Drives 101
Lesson 2
Control Arrangements for an AFD

1) Local Control
Local control means displaying and controlling the
system through the keypad on the AFD.

This training covers the major control arrangements for an AFD. The
control arrangement shown in the previous lesson (Lesson 1: Functions of
an AFD) were all Local Control. There can be other arrangements such as
Remote control, Multi-motor, Master/Slave, Closed Loop, PLC signals and
Serial communications, which are shown in this lesson. This lesson covers
the simplest arrangement first then moves to the more complex.

1. Local Control
In the picture above, an AFD, motor and conveyor are operated from
the keypad on the front of the drive. Local control of the AFD means that
operation of the AFD is completed strictly through the keypad on the front of
the drive, or Local Control Panel (LCP). An operator monitors the readings
and controls the AFD by using this keypad. Even if the keypad, LCP is
remotely mounted away from the drive, maximum of 3m (10’), the control
arrangement inside the program of the AFD is still considered as LOCAL.
Anytime LOCAL is seen in the program, please think KEYPAD.

1) Local Control

Start key starts the drive
Stop/Reset key stops the drive and also resets the
drives after an alarm.
FWD/REV changes direction
Jog is a pushbutton that runs at a low frequency,
say at 10Hz, usually used during setup.

In the picture above there are 2 AFDs, each being controlled by its
own LCP or keypad. The one on the right uses a remote keypad kit to place
the keypad in a convenient location. The operational site on both AFDs is
considered as LOCAL. Anytime the word Local is used in the programming,
example: Parameter 002 (Quick Menu 12 of 13), it means the local keypad
operates the drive.
The bottom keys on the keypad can be used to start, change direction
FWD/REV (Forward/Reverse), jog, stop and reset the drive. The Jog key
acts like a push button which sends a slow (10Hz) signal to the motor usually
used during setup. When the drive is in Local the drive only responds to the
keypad; all other commands coming to the drive are ignored.

2) Remote Control

Remote signals are wired into the control section of the
AFD. They come in 4 types:
Digital Inputs, Analog Inputs,
Analog Outputs, Digital/Relay Outputs.

2. Remote Control
Other arrangements are possible including remote signals. If there is
a problem and the conveyor must be stopped immediately, it would be
difficult to run back to the AFD to stop it. Stop switches can be placed at key
positions to stop the AFD, AC motor and conveyor. It is important that the
AFD accept these stop signals as well as other remote signals. These
remote control signals come in Four Types:
1) Digital Inputs (DI) are 2-position (ON/OFF) signals sent into the AFD.
These commands tell the AFD to Start/Stop, go Forward/Reverse, Jog or No
Jog, etc. A DI requires 24Vdc which is supplied by a terminal on the drive.
2) Analog Inputs (AI) are proportional or modulating signals sent into the
AFD. These commands tell the AFD what the reference speed should be or
tell the AFD what a feedback signal is doing such as pressure.
3) Analog Outputs (AO) are modulating signals sent by the AFD to a device
such as a meter which could display speed or current.
4) Digital/Relay Outputs (DO/RO) are 2-position (ON/OFF) signals sent by
the AFD to a device such as a light to indicate an Alarm, or when the AFD
has reached a certain speed or to a relay to remove the mechanical brake.
Digital Outputs have power 24Vdc already attached and Relay Outputs do
not have power also known as “dry” contacts.

2) Remote Control

In the picture above, 2 normally-closed push-button stop
switches have been wired to 2 Digital Input (DI)
connections of the control section of the AFD. Notice
that 24Vdc coming from the AFD is also connected to
the switches.

Besides stop switches, other signals can be sent into the AFD. These
could be a reference pot to change the speed, increase and decrease
buttons which would also change the speed, remote Start and Stop switches,
a Jog switch or other signals. All the different options for remote signals are
considered as control wiring. In the program of the drive, if anything other
than the keypad is used to control the drive the operational site, Parameter
002 (Quick Menu 12 of 13), is considered as REMOTE. On the AFD shown
above, all the control wiring terminals are shown on the black plastic cover
just under the keypad (LCP).
The amount of connections for the control wiring is a means of
comparison between manufacturers of AFDs. As an example, the Danfoss
drive shown above has the ability to accept the following signals:
8 – Digital Inputs (2 inputs can accept pulsed signals from an encoder)
3 – Analog Inputs (2 are setup for 0-10Vdc; 1 for 0-20mA)
2 – Analog/Digital Outputs (0-20mA or 4-20mA signals or these can be
programmed as Digital Outputs with 24Vdc attached)
2 – Relay Outputs (dry contacts)

3) Multi-Motor Operation

One AFD is used to operate 3 separate AC motors.
The AFD must be able to handle the maximum
current for the 3 motors. Some features of the AFD
in this arrangement are restricted.

3. Multi-Motor Operation
The multi-motor arrangement is usually done because of strict cost
considerations. Not only must the AFD have the current capacity for all the
motors, but each individual motor must have overload protection. In the
picture above, one AFD operates 3 AC motors, which in turn operates the 3
conveyors together. They operate at the same speed or close to the same
speed.
When this arrangement is used some AFD features are restricted.
First the motors and conveyors must all run at the same speed. Another
restriction is that the AFD can not be tuned to a individual motor. Motor
Tuning is where the AFD is tuned or matched to an individual motor for
better performance and energy saving. The last restriction given here is that
the slip compensation should be set in the OFF position.

3) Multi-Motor Operation

In the example above each of the 3 AC motors is the
same size and has a maximum current rating, Full
Load Amps (FLA) of 5 amps. The AFD must be
sized for 15 amps. Note individual overload
protection on each motor.

If the conveyors were operating with a very light load but motor 3 got
jammed, the amp draw on motors 1 and 2, might be low say 2 amps each,
but the draw on motor 3 would need to surpass 11 amps before the drive
saw any problem. As shown in this example, individual overload protection,
such as thermal overloads are needed to protect each motor. Motors in this
arrangement operate close to the same speed.

4) Master/Slave (Leader/Follower)

In the Master/Slave arrangement a Master or Lead
AFD is used to operate a single motor and send its
corresponding signal to the other 2 following motors.
Features of the AFD which were restricted in the
multi-motor can be used in this arrangement.

4. Master/Slave (Leader/Follower)
This arrangement allows motors to operate closer together than the
multi- motor application. One AFD can be selected as a Master or Leader
drive. It is setup to send a reference signal and ON/OFF commands to the
Slave or Follower drives. In this arrangement the special features of the
drive can be used such as automatic motor tuning and slip compensation.
This arrangement is used when the motors are generally operating at the
same speed with a variance between the drives of 0.3Hz or more.
Different gear ratios (say 2 to 1) can also be programmed
electronically so that when one drive is at 30Hz the other drive is at 15Hz.
The Master/Slave arrangement is not used where precision of the conveyors
and their speeds must be exact.

4) Master/Slave (Leader/Follower)

Motor 1, the Master/Leader, sends a reference using
a 4-20mA signal from its AO to the AI of motor 2. It
is also possible to send a pulsed signal as a
reference.

In the picture above, motor 1 is the master and it sends a reference
signal using one of its analog outputs (AO). Slave/Follower 1, motor 2, then
monitors this 4-20mA signal from the master using its analog input (AI). This
same arrangement is copied between motors 2 and 3. The greatest variance
between drives is around 0.3Hz along the entire range of frequencies, say
from 2 to 60Hz when using the analog 4-20mA signal.
It is possible with Danfoss drives to use a Digital Output (DO) as the
reference simulating a pulsed output similar to the signal coming from an
encoder. The follower uses a Digital Input (DI) to follow this pulsed
reference signal. The greatest variance between drives using the pulsed
signal is around 0.2Hz along the entire range of frequencies (2 to 60Hz).
There is an additional time delay in this arrangement. The master drive
starts going to its reference before it sends the signal to the slave drives.
Usually this delay is very small, in milliseconds, but it can have a very
harmful effect on certain exact processes. This delay is referred to as a
“propagation delay”.

5) Closed Loop/PID Control

In this arrangement, a master speed command is
sent as a reference to each AFD. An encoder at
each motor verifies the speed. This is referred to
as Closed Loop, because the AFD has a direct
feedback from the motor.

5. Closed Loop/PID Control for Speed Regulation
Up to this point all the previous control arrangements have been open
loop, which means that there is no feedback going directly to the AFD. Closed
Loop is used for more accurate control. In the example above, each drive is
monitoring and controlling at a certain rpm (revolutions per minute), which may
vary from say 100 to 1800rpm (1500rpm for 50Hz motors) Danfoss drives have
the ability to accept a signal from an encoder (one for speed, A; the other for
direction, B) into the AFD without any additional cards or modules.
In all closed loop applications, additional parameters must be
programmed. These include a speed reference (setpoint), and PID settings. In
this application, the AFD is constantly comparing the reference or setpoint with
the actual feedback value coming from the encoder. If 900 rpm must be
maintained, the encoder should send enough pulses to match it. There is
always a difference between the reference, 900rpm and the actual speed say
893rpm, 7rpm to be exact. This is referred to as Offset, “off the setpoint” or
error.
PID settings attempt to reduce this error. “P” stands for Proportional
Gain which can be considered as a multiplier of the error. The higher the gain
the more accurate, but the control can be unstable, jittery. “I” stands for Integral
which looks at the error over a certain amount of time. The lower the number
the more frequently it checks the error. If the I setting is too low, the motor
again appears to be jittery. “D” stands for derivative which, if used,
compensates for momentary changes in the load.

5) Closed Loop/PID Control

In the picture above, the master speed control is sent
through ratio control pots. Each AFD has its own
ratio pot to trim the master reference signal, either
being at full speed or a fixed ration of the master
speed.

The application above is an example of how conveyors are used
to mix product together in a certain ratio. In a frozen food plant when
packages of mixed vegetables are put together, there is a certain ratio of
carrots, of green beans, of corn, etc. Each ratio control pot can be
placed so that carrots run at 1000rpm, green beans at 600rpm and corn
at 1200rpm. Even though this arrangement is still used, most factories
use PLCs which are covered later in this section.
The arrangement without the feedback, known as Open loop is
accurate to 0.5% of actual speed. At 30Hz, 900rpm, open loop is within
+ or – 4.5rpm. With feedback from the encoder, using Closed loop the
accuracy is 0.1% of actual speed. At 30Hz, 900rpm, closed loop is within
+ or – 0.9rpm. For the best precision, a separate card called the Sync-
Pos card is installed inside the drive. Not only does it have as good if not
better accuracy than the closed loop, it also gathers additional
information from the encoder to correct the speed if it lags behind. The
Sync-Pos is covered next.
Before going on to the Sync-Pos arrangement, another example
of closed loop must be mentioned. Besides Speed Closed Loop, there
can also be Process Closed Loop. Instead of an encoder sending a
feedback signal to the drive, another feedback device such as a 4-20mA
pressure transmitter sends a signal back to the drive. As with any closed
loop application, this application requires proper feedback setup, a
setpoint, controller action and PID settings.

6) Multi-motor with Sync-Pos card

For precise applications where the drives must be
timed perfectly together, less than 0.1Hz. This
arrangement requires the use of encoders.

6. Multi-motor operation using Sync-Pos Card
On other applications it is important that AFDs and motors work
preciously together. In the example above there is a master or leader AFD
and slave or follower AFDs. The special Sync-Pos
(Synchronizing/Positioning) card is placed in all 3 drives. This card fits on the
back of the control card so that no additional space is required. The master
Sync-Pos card sends a virtual master signal to all 3 drives. All drives follow
this signal even the master at precisely the same time. Each drive uses an
encoder on the motor to insure that position is maintained exactly. Although
in the example above, matching motors is not essential; there are
applications where timing between processes, such as a bottling line, are
critical.

6) Multi-motor with Sync-Pos card

Here the master drive sends a virtual signal to all drives.
Encoders [example:4096 ppr (pulses per one
revolution)] on each motor check for accuracy, which
can be within 1/2 of a pulse between drives.

The synchronization example has been used here, but the Sync-Pos
Card has other functions. It can be used to enter a special program for
positioning. Perhaps the motor needs to run 10 turns forward then waits 5
seconds and returns 10 turns, waits 30 seconds then starts the process all
over again. The Sync-Pos card has programmable capabilities to operate an
AFD in such a manner. A computer or laptop with the proper software is
used to enter this type of program into the Sync-Pos card.
Danfoss has preprogrammed different applications into the Sync-Pos
card, one for Synchronization and the other for Positioning. These “canned”
applications add a new parameter group into the drive, which allows the
programmer to enter information through the keypad of the drive without a
computer or laptop. Anytime an application is “canned” there are restrictions
on inputs, outputs as well as some programming features.

7) PLC Signals

Programmed Logic Controllers (PLC) coordinate the
use of the AFD with numerous other schedules and
commands. This can be used in very sophisticated
control arrangements.

7. PLC signals
On numerous applications an electronic controller known as
Programmable Logic Controller or PLC is used to add other sensing devices,
scheduling and other coordinated signals. Danfoss does not make PLCs but
it is important that the AFD has the capability to interact with these devices.
The PLC sends a start/stop command (digital output) and the reference
(analog output) to directly control an AFD. These different types of
connections can be referred to as “points”.
It is also important that the AFD send information back to the PLC,
such as an alarm using a digital/relay output. Using an analog output from
the AFD a true indication of the speed might also be sent back to the PLC.

7) PLC Signals

Start/Stop command from a DO of the PLC goes to a DI
of the AFD. A Reference command from an AO of
the PLC goes to an AI of the AFD.

An example of a PLC is shown above. Connection wires are run from
the PLC to terminals in the AFD to start/stop and give a reference point.
Other wires can be run from the outputs of the AFD back to PLC to indicate
the status of the drive. With each request for a command or status
information, additional wire connections must be used. Obviously there is an
added expense with each set of wires.

8) Serial Communications

Serial Communications is accomplished by the wiring
of 2 wires between each device, which is also
known as an RS-485 connection. All the +
terminals are attached together and all the – are
attached together.

8. Serial Communications
With complex PLC applications there can be numerous wires,
perhaps 5 to 10 pairs of wire, between a PLC and the AFD. To reduce this
group of wires the industry is embracing the idea of serial communications
between PLCs and other devices including the AFDs. This allows the
sharing of all sorts of signals including frequency, power, voltage,
temperature, warnings and alarms with the use of an RS-485 (2-wire, + -)
connection.

8) Serial Communications

In the example shown, the PLC controls (start/stop
direction change, reference speed, etc) the AFD
using the RS-485 connection. The AFD also
sends back status information to the PLC.

But with serial communications there are some challenges. Different
manufacturers of PLCs communicate in different computer languages.
Siemens which is very popular in Europe communicates using a language
called Profibus. North America uses a great deal of Allen Bradley which
communicates with DeviceNet. These 2 languages do NOT communicate
with each other directly. There are more than just 2 languages, as there are
more than a few PLC manufacturers. A separate communication card must
be installed into the AFD to communicate with each different language.
Presently, Danfoss has a number of different types of serial
communications cards that slide into the AFD. There is a DeviceNet card
used with Allen Bradley PLCs, Profibus used with Siemens PLCs, Modbus
Plus used with Modicon PLCs/ Square D, an Echelon Lonworks card, which
is used mainly in building automation, HVAC (Heating Ventilating and Air-
Conditioning) and an Interbus card, which is used in the automotive industry.

9) Control Combinations

It is very possible to have combinations of different
control arrangements, such as pictured here where
a PLC, using serial communications, sends signals
to 3 AFDs that have Sync-Pos cards.

9. Combinations
There can be any number of combinations. Perhaps an operator
wants to flip a switch and shift between Local and Remote control, or shift
between serial communications and remote. In the Danfoss drive there are
4 different setups which allow for this shifting between control arrangements
if needed.

Summary

1) Local Control

2) Remote Control

3) Multi-motor Control

Summary
Local Parameter 002 (Quick Menu 12 0f 13) = Local
Start/Stop commands are made at the keypad
Local Reference changes are made at keypad-
Parameter 003 (Quick Menu 13 of 13)

Remote Parameter 002 (Quick Menu 12 0f 13) = Remote
Digital Inputs give Start/Stop commands
Analog Input - Pot changes speed – reference
A Drive always needs a Start command and a reference
other than 0 to operate.

Multi-motor FLA of all motors = Amps of Drive
Individual protection for each motor
Do not use AMA (Motor Tuning)
Do not use Slip comp

Summary

4) Master/Slave

5) Closed Loop/PID

6) Sync-Pos

Summary
Master/Slave Leader/Follower
AO of Master goes to AI of Slave (4-20mA)
DO of Master goes to DI of Slave (Pulse)
Synchronization – close but not precise

Closed Loop Feedback using an encoder for speed
PID (Proportional Gain, Integral, Derivative)

Sync-Pos Precise synchronization is critical
Sync-Pos Card generates Virtual master
Encoder required monitoring each motor
A special Sync-Pos card is available just for
Synchronizing and another special card just for
Positioning. The original Sync-Pos card can be
programmed for either of these or many more
applications.

Summary

7) PLC Signals

8) PLC Serial Communications

9) Combinations

Summary
PLC Signals Digital Outputs from PLC to Digital Input on AFD
Analog Outputs from PLC to Analog Inputs on AFD
“Points” are connections to PLC
AFD Status uses AO and DO back to PLC

PLC Serial RS-485 (2 wire, + and -) connection
DeviceNet – Allen Bradley
Profibus – Siemens
Modbus Plus – Modicon Square D
Lonworks – Echelon -HVAC
Interbus - Automotive

Combinations Numerous combinations of control arrangements are
possible

Afd Control

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