Installation manual

GEO Direct PWM Amplifiers – Preliminary Documentation
Overview 3
Some basic equations can help guide an applications engineer to mate a proper drive with a motor. A
typical application accelerates a load to a speed, running the speed for a while and then decelerating the
load back into position.
Feedback Devices
Many motors incorporate a position feedback device. Devices are incremental encoders, resolvers, and
sine encoder systems. The direct PWM version of the GEO drive does not require feedback to it from the
motor. The macro version of the GEO drive accepts feedback. In its standard form, it is set up to accept
incremental encoder feedback. With the E option, it is possible to use either resolver or sinusoidal encoder
feedback. Historically, the choice of a feedback device has been guided largely by cost and robustness.
Today, feedbacks are relatively constant for the cost and picked for features such as size and feedback
data. More feedback data or resolution provides the opportunity to have higher gains in a servo system.
Torque
The torque required for the application can be viewed as both instantaneous and average. Typically, the
instantaneous or peak torque is calculated as a sum of machining forces or frictional forces plus the forces
required to accelerate the load inertia. The machining or frictional forces on a machine must be
determined by the actual application. The energy required to accelerate the inertia follows the equation: t
= JA, where t is the torque in pound-feet required for the acceleration, J is the inertia in pound-feet-
second squared, and A is in radians per second per second. When the desired acceleration rate and the
load inertia reflected back to the motor are known, the required torque can be calculated. The t-JA
equation requires that the motor's inertia be considered as part of the inertia-requiring torque to accelerate.
Once the torque is determined, the motor's specification sheet can be reviewed for its torque constant
parameter (Kt). The torque required at the application divided by the Kt of the motor provides the peak
current required by the amplifier. Again, a little extra room should be given to this parameter to allow for
good servo control.
Most applications have a duty cycle in which the acceleration profile occurs repetitively over time.
Calculating the average value of this profile gives the continuous rating required by the amplifier.
Applications also concern themselves with the ability to achieve a speed. The requirements can be
reviewed by either defining what the input voltage is to the drive, or defining what the voltage
requirements are at the motor. Typically, a system is designed at a 230- or 480V input line. The motor
must be able to achieve the desired speed with this voltage limitation. This can be determined by using
the voltage constant of the motor (Kb), usually specified in volts-per-thousand rpm. The application speed
is divided by 1000 and multiplied by the motor's Kb. This is the required voltage to drive the motor to the
desired velocity. Again, 20% headroom is suggested to allow for good servo control.
Peak Torque
The peak torque rating of a motor is the maximum achievable output torque. It requires that the amplifier
driving it be able to output enough current to achieve this. Many drive systems offer a 3:1 peak-to-
continuous rating on the motor, while the amplifier has a 2:1 rating. To achieve the peak torque, the drive
must be sized to be able to deliver the current to the motor. The required current is often stated on the
datasheet as the peak current through the motor. In some sense, it can also be determined by dividing the
peak amplifier's output rating by the motor's torque constant (Kt).
Continuous Torque
The continuous torque rating of the motor is defined by a thermal limit. If more torque is consumed from
the motor than this on average, the motor overheats. Again, the continuous torque output of the motor is
subject to the drive amplifier's ability to deliver that current. The current is determined by the
manufacturer's datasheets stating the continuous RMS current rating of the motor and can also be
determined by using the motor's Kt parameter, usually specified in torque output per amp of input current.