Velocity Loop Signal Attributes

These are the velocity control signal related attributes associated with a Motion Control Axis.

Velocity Command

The Velocity Command attribute is the command velocity output from the Fine Command Generator (if active) into the velocity loop when configured for the Velocity Loop control or frequency controller when configured for Frequency Control operation.

Velocity Trim

Additional velocity command added to the velocity loop summing junction.

Acceleration Feedforward Command

The Acceleration Feedforward Command attribute is an Acceleration feedforward command signal that represents a scaled version of the command acceleration profile. This signal is the Acceleration Fine Command signal scaled by Kaff and applied to the output of the velocity loop.

Velocity Reference

Command velocity reference into the velocity loop summing junction, or in the case of Frequency Control, the signal that is scaled to become the Frequency Reference.

Velocity Feedback

Actual velocity of the axis that is applied to the velocity summing junction, if applicable, based on Control Mode selection. In most cases, the Velocity Feedback signal is derived directly from the feedback device specified by the Feedback Mode selection. If the axis is configured for Feedback Only mode, Velocity Feedback represents the actual velocity of the feedback device specified by the Feedback Master Select. If the axis is configured for Frequency Control, the Velocity Feedback signal is derived from the Velocity Reference signal. If configured for Sensorless Velocity Loop operation, that is, Feedback Mode set to No Feedback, the sensorless control algorithm estimates the Velocity Feedback.

Velocity Error

Error between the velocity reference and velocity feedback value that is the output of the velocity loop summing junction.

Velocity Integrator Output

Output of velocity integrator representing the contribution of the velocity integrator to Velocity Loop Output.

Velocity Loop Output

Output of velocity forward path representing the total control effort of the velocity loop.

Velocity Limit Source

The Velocity Limit Source attribute is an enumerated value that specifies the source of the operative velocity limit.

Velocity Limiter Extensions

Permanent magnet (PM) motor applications sometimes require drives to provide extensions to the velocity limiter function to protect the drive electronics and motor from potentially destructive overspeed conditions when operating at speeds above the motor's rating. The Velocity Limiter serves to protect the drive and motor when applied in these high-speed applications.

PM Motor Torque-Speed Curve

A generalized Torque-Speed curve for a PM motor is shown in the following graph. The two curves shown define the continuous (T

c

) and peak torque (T

p

) capabilities of the motor. PM motors typically specify a 'rated speed' (S

r

) based on rated voltage and continuous torque and a 'max speed' (S

m

) based on the maximum operating speed. Often the specified rated speed and max speed for the motor are set to the same value.

Field Weakening

PM motors can also be operated using a technique called "field weakening" to extend the top speed capability of the motor. Field weakening uses active current vector control to reduce the effective magnetic field strength from the permanent magnets, enabling higher speeds at the expense of lower torque production. While the use of field weakening to extend motor speed range is more common for Interior PM (IPM) motors, the speed range of Surface-mount PM (SPM) motors can be extended as well. The following graph illustrates the PM Motor Torque-Speed Curve with Field Weakening.

When the drive applies field weakening to a PM motor to reach higher than rated speed, the drive's motor current vector control algorithm effectively decreases the motor Ke. This reduces the resulting Counter EMF (CEMF) voltage to be less than the DC bus voltage. However, if this active current vector control is suddenly removed, the Ke value would revert to the nominal value and the CEMF voltage would increase rapidly. Active control of motor current is lost whenever the power structure is disabled. So the concern is when the power structure is disabled with the motor still spinning. This can be the case when the drive executes a Category 0 Stop due to a control initiated Disable Request, a Major Fault action, or a Safe Torque Off activation.

Three speed values are shown in the preceding figure. As defined in the first figure, the rated speed (S

r

) corresponds to operation at rated voltage and continuous torque. This is the maximum continuous torque that can be achieved without field weakening. S

OV

is the speed at which the CEMF voltage from the nominal Ke would be equivalent to the maximum DC Bus Voltage rating of the drive, or the DC Bus Overvoltage Limit.

If active motor current control is removed while the motor is operating at speeds above S

OV

, the CEMF voltage results in a DC Bus overvoltage condition that can damage the drive and, possibly, other drives sharing the DC Bus.

S

m

is the absolute maximum operating speed of the motor based on mechanical constraints. For a rotary motor, the Rotary Motor Max Speed attribute gives the S

m

.

Major damage to devices connected to the DC Bus can occur when the PM motor is allowed to run between S

OV

and S

m

and the drive's power structure is disabled.

Velocity Limiter Behavior Diagram

The following diagram shows the extensions that have been added to the Velocity Limiter to help manage the inherent risks of high-speed PM motor operation. In addition to the existing Velocity Limit - Positive/Negative attributes that can be used to limit the Velocity Reference signal, two new limits have been defined based on S

OV

and S

m

defined above. Specifically, the PM Motor Rotary - Bus Overvoltage Speed and PM Motor Linear Bus Overvoltage Speed attributes establish an absolute limit on the Velocity Reference signal that corresponds to S

OV

. This speed limit can only be exceeded if the PM Motor Extended Speed Permissive attribute is set to True. The PM Motor Rotary Max Extended Speed and PM Motor Linear Max Extended Speed attributes establish an absolute limit on the Velocity Reference signal that corresponds to S

m

. The Velocity Limit function limits the Velocity Reference signal to the minimum of these attribute values. The Velocity Limit Source attribute indicates the source of the velocity limit.

Through these extensions to the Velocity Limiter function, a drive that supports field weakening can be configured to manage extended speed operation, only allowing operation above S

OV

by setting the PM Motor Extended Speed Permissive attribute. Systems that can run safely above S

OV

are equipped with a DC Bus Regulator or a Resistive Brake Module.