Emerson Unidrive M702 User Manual Download Page 14

Page: 14 / 284

Unidrive M702 User Guide

Issue Number: 3

2.3.1 Typical short term overload limits

The maximum percentage overload limit changes depending on the selected motor. Variations in motor rated current, motor power factor and motor
leakage inductance all result in changes in the maximum possible overload. The exact value for a specific motor can be calculated using the
equations detailed in Menu 4 in the

Parameter Reference Guide

Typical values are shown in the table below for RFC (RFC-A or RFC-S) and open loop (OL) modes:

Table 2-5 Typical overload limits

Generally the drive rated current is higher than the matching motor rated current allowing a higher level of overload than the default setting.
The time allowed in the overload region is proportionally reduced at very low output frequency on some drive ratings.

The maximum overload level which can be attained is independent of the speed.

2.4 Operating modes

The drive is designed to operate in any of the following modes:

Open loop mode

Open loop vector mode
Fixed V/F mode (V/Hz)
Quadratic V/F mode (V/Hz)

2. RFC - A

With position feedback sensor

3. RFC - S

With position feedback sensor

2.4.1 Open loop mode

The drive applies power to the motor at frequencies varied by the user. The motor speed is a result of the output frequency of the drive and slip due
to the mechanical load. The drive can improve the speed control of the motor by applying slip compensation. The performance at low speed depends
on whether V/F mode or open loop vector mode is selected.

Open loop vector mode

The voltage applied to the motor is directly proportional to the frequency except at low speed where the drive uses motor parameters to apply the
correct voltage to keep the flux constant under varying load conditions.
Typically 100 % torque is available down to 1 Hz for a 50 Hz motor.

Fixed V/F mode

The voltage applied to the motor is directly proportional to the frequency except at low speed where a voltage boost is provided which is set by the
user. This mode can be used for multi-motor applications.
Typically 100 % torque is available down to 4 Hz for a 50 Hz motor.

Quadratic V/F mode

The voltage applied to the motor is directly proportional to the square of the frequency except at low speed where a voltage boost is provided which is
set by the user. This mode can be used for running fan or pump applications with quadratic load characteristics or for multi-motor applications. This
mode is not suitable for applications requiring a high starting torque.

2.4.2 RFC-A mode

otor

lux

ontrol for Asynchronous (induction) motors (

RFC-A

) encompasses closed loop vector control with a position feedback device

With position feedback sensor

For use with induction motors with a feedback device installed. The drive directly controls the speed of the motor using the feedback device to ensure
the rotor speed exactly as demanded. Motor flux is accurately controlled at all times to provide full torque all the way down to zero speed.

With position feedback sensor (Sensorless)

Sensorless mode provides closed loop control without the need for position feedback by using current, voltages and key motor parameters to
estimate the motor speed. It can eliminate instability traditionally associated with open loop control such as operating large motors with light loads at
low frequencies.

2.4.3 RFC- S

otor

lux

ontrol for Synchronous (permanent magnet brushless) motors (

RFC-S

) provides closed loop control with position feedback device.

With position feedback

For use with permanent magnet brushless motors with a feedback device installed.
The drive directly controls the speed of the motor using the feedback device to ensure the rotor speed is exactly as demanded. Flux control is not
required because the motor is self excited by the permanent magnets which form part of the rotor.
Absolute position information is required from the feedback device to ensure the output voltage is accurately matched to the back EMF of the motor.
Full torque is available all the way down to zero speed.

Operating mode

RFC from cold

RFC from 100 %

Open loop from cold Open loop from 100 %

Normal Duty overload with motor rated current = drive rated current

110 % for 165 s

110 % for 9 s

110 % for 165 s

110 % for 9 s

Heavy Duty overload with motor rated current = drive rated current

200 % for 28 s

200 % for 3 s