These values are considered minimums and assume that the load is well characterized and the control algorithm does a loop-ahead calculation on actual output based on an output model (smart controller). Naturally, the question may arise: If smart controllers help brushlessdc motors accurately track control inputs, can they do the same for ac-induction motors? Yes and no. Smart controllers can make ac-induction motors respond faster to control inputs, but they can't adequately overcome high rotor inertia for them to work in this application.

Another measure of drive performance is the rate at which the drive updates analog inputs. Many drives take 1 msec or longer to update analog inputs, excessively long to meet the bandwidth needs in this case. Analog inputs are ±10-Vdc control signals that direct the drive to move a motor in a specified way. The signals may come from a signal generator, a computer program, or from the drive itself if it is programmed internally. Position and velocity feedback typically comes from a digital encoder or resolver.

Analog-to-digital converters should have at least 12-bit resolution to accurately define analog inputs over a broad speed range. A sinusoidal velocity signal processed by an 8-bit converter, in contrast, would pass to the controller as a trapezoid or other distorted shape because the resolution is too coarse.

PUTTING MOTORS TO THE TEST

A torsional test machine inspects automotive superchargers during production at speeds to 6,000 rpm. Torsional-vibration tests take place at 2,000 rpm and vary speed ±5% at 60 Hz. Total inertia is about to 1 lb-ft2. A brushlessdc motor rated at 60 hp at 6,000 rpm can do the job.

Another rig tests IC-engine counterbalancers. Such devices install on the crankshaft to counminimums teract primary and secondary torsional vibration from piston forces. This application varies rotation velocity ±5% about 1,200 rpm at 100 Hz. This application is impossible with motors having a torque-to-inertia ratio less than 60 lb-ft / lb-ft2.

Considering that the load itself has some inertia, motor inertia must be as small as possible, to a point. It is generally agreed for stability reasons that motor inertia should not be less than 20 to 25% of load inertia. Systems that match motor and load inertia consume the least amount of power and best track input signals (highest system bandwidth).

Plot of command and actual motorshaft velocity. The motor accurately follows an analog velocity command variation input. The controller accepts an analog input voltage and produces a smooth sinusoidal output with effectively no phase delay.

MAKE CONTACT Powertec Industrial Motors Inc., www.powertecmotors.com