Delta Tau GEO PMAC User Manual download pdf (Page 14)

Geo PMAC Drive User Manual

4 Introduction

also limits the maximum achievable speed. In addition, some manufacturers will provide motor data with

their drive controller, which is tweaked to extend the operation range that other controllers may be able to

provide. In general, the maximum speed can be determined by input voltage line-to-line divided by Kb

(the motor’s back EMF constant). It is wise to de-rate this a little for proper servo applications.

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 Newton-meters or pound-feet required for the acceleration, J is the inertia

in kilogram-meters-squared or pound-feet-second squared, and A is in radians per second per second.

The required torque can be calculated if the desired acceleration rate and the load inertia reflected back to

the motor are known. 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. 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. Headroom of 20% 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.

Motor Poles

Usually, the number of poles in the motor is not a concern to the actual application. However, it should

be noted that each pole-pair of the motor requires an electrical cycle. High-speed motors with high motor

pole counts can require high fundamental drive frequencies that a drive amplifier may or may not be able

to output. In general, drive manufacturers with PWM switching frequencies (16kHz or below) would like

to see commutation frequencies less than 400 Hz. The commutation frequency is directly related to the

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Delta Tau GEO PMAC User Manual Page 14

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