Subject to change without notification
Subject to change without notification
20
3.5.4 Output Voltage and Back-EMF Compensation
Whereas a DC motor is normally current-controlled, this controller works by voltage-control.
The command
ov
defines the voltage; the argument is the effective voltage. In a three-phase
system this is 1 V
eff
≈
1,63 V
SS
(where V
SS
is the peak-to-peak voltage of one phase).
At high rotational speeds the drive generates a self-induced voltage, which opposes the
applied voltage, the back-EMF (EMF = electromotive force). This means that less voltage and
current is used: at constant voltage and high speed the drive will have a very low torque.
In order to counteract this, a compensator is built into the controller, which raises the voltage
as the speed increases so that it produces a current that is virtually independent of the speed
of rotation. This has the distinct advantage that the power generated is also independent of
speed.
The amplitude of the compensating voltage is dependent on the speed. It is added to the
voltage created in response to the command
ov
. It is determined empirically and calculated
according to the following:
where n is the speed and V
c
is the compensation voltage preset by the user with the command
comp
. The standard setup is V
c
= 600 mV
eff
. However, for each motor the optimum setting may
be marginally different from this value. Set up the value so that the current consumption over
the desired speed range is as constant as possible.
Example:
The requested value of output voltage is set by the command
ov
to be V
out
= 1000
mV
eff
, and then the compensation voltage is set by the command
comp
to be V
c
= 300mV
eff
.
If a speed of n = 110000 1/
min
is requested by the command
v 110 000
, then the actual applied
voltage will be V
out
= 1000 + (110 000/120 000)
2
• 300 = 1252 mV
eff
.
Important commands:
Command Function Description
ov
Output Voltage Output voltage [mV
eff
]
gov
Get Output Voltage Displays the value of the output voltage [mV
eff
]
comp
Compensation Value of the compensation voltage at 120 000 rpm [mV
eff
]
gcomp
Get Compensation Displays the compensation voltage at 120 000 rpm [mV
eff
]
gcov
Get Current Displays the true output voltage, i.e. the requested
Output Voltage value plus the compensation value [mV
eff
]
gov
Get Output Voltage Displays the effective value of the output voltage [mV
eff
]
n
120 000
1
––––
min
.
V
c
2
