Emerson Unidrive M702 User Manual Download Page 128

Page: 128 / 284

128

Unidrive M702 User Guide

Issue Number: 3

8.5 Switching frequency

The default switching frequency is 3 kHz (6 kHz in RFC-S mode),
however this can be increased up to a maximum of 16 kHz by Pr

05.018

(dependent on drive size). The available switching frequencies are
shown below.

Table 8-1 Available switching frequencies

* Size 5 - 575 V variant does not support 16 kHz switching frequency.
If switching frequency is increased from 3 kHz the following apply:
1. Increased heat loss in the drive, which means that derating to the

output current must be applied.
See the derating tables for switching frequency and ambient
temperature in

section 12.1.1 Power and current ratings (Derating

for switching frequency and temperature)

on page 227.

2. Reduced heating of the motor - due to improved output waveform

quality.

3. Reduced acoustic noise generated by the motor.
4. Increased sample rate on the speed and current controllers. A trade

off must be made between motor heating, drive heating and the
demands of the application with respect to the sample time required.

Table 8-2 Sample rates for various control tasks at each

switching frequency

8.6 High speed operation

8.6.1 Encoder feedback limits

The maximum encoder frequency should be prevented from exceeding
500 kHz. In RFC-A and RFC-S modes the maximum speed that can be
entered in to the speed reference clamps (Pr

01.006

and Pr

01.007

) can

be limited by the drive. This is defined by the following (subject to an
absolute maximum of 40,000 rpm):

Where:

ELPR is the equivalent encoder lines per revolution and is the
number of lines that would be produced by a quadrature encoder.
•

Quadrature encoder ELPR = number of lines per revolution

•

F and D encoder ELPR = number of lines per revolution / 2

•

SINCOS encoder ELPR = number of sine waves per revolution

This maximum speed limit is defined by the device selected with the
speed feedback selector (Pr

03.026

), and the ELPR set for the position

feedback device. In RFC-A mode it is possible to disable this limit via
Pr

03.024

, so that the drive can be switched between operation with and

without feedback when the speed becomes too high for the feedback
device. The maximum speed limit is defined as above when Pr

03.024

0 and is 36,000 rpm when Pr

03.024

= 1,2, 3 or 4.

8.6.2 Field weakening (constant power) operation

(Open loop and RFC-A mode only)
The drive can be used to run an induction machine above synchronous
speed into the constant power region. The speed continues to increase
and the available shaft torque reduces. The characteristics below show
the torque and output voltage characteristics as the speed is increased
above the rated value.

Figure 8-3 Torque and rated voltage against speed

Care must be taken to ensure the torque available above base speed is
sufficient for the application to run satisfactorily.
The saturation breakpoint parameters (Pr

05.029

, Pr

05. 030

, Pr

05.062

and Pr

05.063

) found during the autotune in RFC-A mode ensure the

magnetizing current is reduced in the correct proportion for the specific
motor. (In open loop mode the magnetizing current is not actively
controlled).

8.6.3 Permanent magnet motor high speed operation

High speed servo mode is enabled by setting Pr

05.022

=1. Care must

be taken when using this mode with permanent magnet motor to avoid
damaging the drive. The voltage produced by the permanent magnet
motor magnets is proportional to speed. For high speed operation the
drive must apply currents to the motor to counter-act the flux produced
by the magnets. It is possible to operate the motor at very high speeds
that would give a very high motor terminal voltage, but this voltage is
prevented by the action of the drive.
If however, the drive is disabled (or tripped) when the motor voltages
would be higher than the rating of the drive without the currents to
counter-act the flux from the magnets, it is possible to damage the drive.
If high speed mode is enabled the motor speed must be limited to the
levels given in the table below unless an additional hardware protection
system is used to limit the voltages applied to the drive output terminals
to a safe level.

Drive

size

Model

kHz