Delta Tau GEO MACRO DRIVE User Manual download pdf (Page 16)

Geo MACRO Drive User Manual

4 Introduction

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

number of poles in the motor.

Motor Inductance

PWM outputs require significant motor inductance to turn the on-off voltage signals into relatively

smooth current flow with small ripple. Typically, motor inductance of servomotors is 1 to 15 mH. The

Geo drive product series can drive this range easily. On lower-inductance motors (below 1mH), problems

occur due to PWM switching where large ripple currents flow through the motor, causing excessive

energy waste and heating. If an application requires a motor of less than 1mH, external inductors are

recommended to increase that inductance. Motors with inductance in excess of 15mH can still be driven,

but are slow to react and typically are out of the range of high performance servomotors.

Motor Resistance

Motor resistance is not really a factor in determining the drive performance, but rather, comes into play

more with the achievable torque or output horsepower from the motor. The basic resistance shows up in

the manufacturer's motor horsepower curve.

Motor Back EMF

The back EMF of the motor is the voltage that it generates as it rotates. This voltage subtracts from the

bus voltage of the drive and reduces the ability to push current through the motor. Typical back EMF

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Delta Tau GEO MACRO DRIVE User Manual Page 16

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