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Unidrive SPM User Guide
137
Issue Number: 3 www.controltechniques.com
Pr 0.07 {5.14} Voltage mode
There are six voltage modes available which fall into two categories, vector control and fixed boost.
Vector control
Vector control mode provides the motor with a linear voltage characteristic from 0Hz to motor rated frequency (Pr
0.47
), and then a constant voltage
above motor rated frequency. When the drive operates between motor rated frequency/50 and motor rated frequency/4, full vector based stator
resistance compensation is applied. When the drive operates between motor rated frequency/4 and motor rated frequency/2 the stator resistance
compensation is gradually reduced to zero as the frequency increases. For the vector modes to operate correctly the motor rated power factor (Pr
0.43
), stator resistance (Pr
5.17
) and voltage offset (Pr
5.23
) are all required to be set up accurately. The drive can be made to measure these by
performing an autotune (see Pr
0.40
Autotune). The drive can also be made to measure the stator resistance and voltage offset automatically every
time the drive is enabled or the first time the drive is enabled after it is powered up, by selecting one of the vector control voltage modes.
(0)
Ur_S
= The stator resistance and the voltage offset are measured and the parameters for the selected motor map are over-written each
time the drive is made to run. This test can only be done with a stationary motor where the flux has decayed to zero. Therefore this mode should
only be used if the motor is guaranteed to be stationary each time the drive is made to run. To prevent the test from being done before the flux
has decayed there is a period of 1 second after the drive has been in the ready state during which the test is not done if the drive is made to run
again. In this case, previously measured values are used. Ur_s mode ensures that the drive compensates for any change in motor parameters
due to changes in temperature. The new values of stator resistance and voltage offset are not automatically saved to the drive's EEPROM.
(4)
Ur_I
= The stator resistance and voltage offset are measured when the drive is first made to run after each power-up. This test can only be
done with a stationary motor. Therefore this mode should only be used if the motor is guaranteed to be stationary the first time the drive is made
to run after each power-up. The new values of stator resistance and voltage offset are not automatically saved to the drive's EEPROM.
(1)
Ur
= The stator resistance and voltage offset are not measured. The user can enter the motor and cabling resistance into the stator
resistance parameter (Pr
5.17
). However this will not include resistance effects within the drive inverter. Therefore if this mode is to be used, it is
best to use an autotune test initially to measure the stator resistance and voltage offset.
(3)
Ur_Auto
= The stator resistance and voltage offset are measured once, the first time the drive is made to run. After the test has been
completed successfully the voltage mode (Pr
0.07
) is changed to Ur mode. The stator resistance (Pr
5.17
) and voltage offset (Pr
5.23
)
parameters are written to, and along with the voltage mode (Pr
0.07
), are saved in the drive's EEPROM. If the test fails, the voltage mode will
stay set to Ur_Auto and the test will be repeated next time the drive is made to run.
Fixed boost
Neither the stator resistance nor the voltage offset are used in the control of the motor, instead a fixed characteristic with low frequency voltage
boost as defined by parameter Pr
0.08
, is used. Fixed boost mode should be used when the drive is controlling multiple motors. There are two
settings of fixed boost available:
(2)
Fd
= This mode provides the motor with a linear voltage characteristic from 0Hz to rated frequency (Pr
0.47
), and then a constant voltage
above rated frequency.
(5)
SrE
= This mode provides the motor with a square law voltage characteristic from 0Hz to rated frequency (Pr
0.47
), and then a constant
voltage above rated frequency. This mode is suitable for variable torque applications like fans and pumps where the load is proportional to the
square of the speed of the motor shaft. This mode should not be used if a high starting torque is required.
For both these modes, at low frequencies (from 0Hz to ½ x Pr
0.47
) a voltage boost is applied defined by Pr
0.08
as shown below:
Pr 5.27 Slip compensation
When a motor, being controlled in open loop mode, has load applied a characteristic of the motor is that the output speed droops in proportion to the
load applied as shown:
In order to prevent the speed droop shown above slip compensation should be enabled.
To enable slip compensation Pr
5.27
must be set to a 1 (this is the default setting), and the motor rated speed must be entered in Pr
0.45
(Pr
5.08
).
The motor rated speed parameter should be set to the synchronous speed of the motor minus the slip speed. This is normally displayed on the
motor nameplate, i.e. for a typical 18.5kW, 50Hz, 4 pole motor, the motor rated speed would be approximately 1465rpm. The synchronous speed for
a 50Hz, 4 pole motor is 1500rpm, so therefore the slip speed would be 35rpm.
If the synchronous speed is entered in Pr
0.45
, slip compensation will be disabled. If too small a value is entered in Pr
0.45
, the motor will run faster
than the demanded frequency.
The synchronous speeds for 50Hz motors with different numbers of poles are as follows:
2 pole = 3000rpm, 4 pole = 1500rpm, 6pole =1000rpm, 8 pole = 750rpm
Output
voltage
Pr
0.44
Pr /
2
0.47
Pr
0.47
Output
frequency
Output voltage characteristic
(SrE)
Voltage boost
Pr
0.08
Output
voltage
Pr
/
2
0.44
Pr
0.44
Pr
/ 2
0.47
Pr
0.47
Output
frequency
Output voltage characteristic
(Fd)
Voltage boost
Pr
0.08
Shaft speed
Demanded speed
Load
