Conveying is a prime applications area for inverters, which bring greater
control and efficiencies to the process. In this article, Helmut Greiner
compares the use of pole-changing braked motors with that of combined
inverter/geared motor drives, and provides some design pointers for those
with special materials handling needs
Hitherto, all those desirable aspects of a materials conveying system -
soft starting, controlled deceleration, precise positioning (fixed cycle
mode) and high efficiency - have been the province of the pole changing,
three phase cage induction motor. Today, even more demands are being
placed on conveying systems, not least being the need for higher speed
and - most topical of all - the need to reduce energy consumption. These
demands are now being met by inverter-fed steplessly adjustable drives -
a variable speed drive installed remotely within the conveyor motor
control system or an ac induction motor, gearbox and inverter module
combination, providing everything you need at the point of power
transmission.
Figure 1
Conventional starting methods usually produce reduced acceleration torque
and thus longer run-up times. Figure 1 illustrates this for direct online
starting (Curve 1) and when conventional soft starting with reduced
starting torque is used (Curve 2). Notice that the acceleration (angle
'a' in Figure 1) changes suddenly at the beginning and at the end of the
process. This change in acceleration can be regarded as a 'jolt'
represented by the derivative da/dt. Jolts are obviously undesirable when
conveying unstable or sensitive products, or those that must remain
precisely positioned on the conveyor belt.
Curve 3 shows inverter controlled starting along a so-called 'sine ramp'.
Here the inverter frequency is ramped up to a predetermined upper value
over an adjustable time span. The rotor speed increases according to the
rise in frequency, with the degree of slip needed for the particular
torque requirement. Typical applications include any conveyor system for
delicate products, such as rack drives, turntables and mobile welding
robots.
Figure 2
Conveying systems running in start/stop mode are served very well by
pole-changing brake motors, typically those offering a 1:4 speed ratio.
Some of these motors are available with speed ratios up to 1:10, but they
cost nearly twice as much as the 1:4 ratio motor. Generally, the higher
the running speed and the lower the permissible stopping tolerance
(Stol), the greater is the speed ratio (R) required. Figure 2 provides a
useful guide to the limits of such drive systems (note that
PU=pole-changing motor drive and VF=inverter drive in the diagram).
Showing the relationship between speed ratio and stopping tolerances for
positioning conveyor drive systems, it suggests a limit for the
application of pole-changing motors (R being 4 or less), and a point at
which it is more appropriate to use inverter control (when R is greater
than 10). Note also a fairly wide transitional range between R=4 and
R=10, where both pole-changing motor and inverter drives are feasible
solutions.
This is largely down to cost considerations. Modern combined
inverter/geared motor assemblies offer a very economic inverter based
solution for conveyor drives up to 7.5kW. Remember, a pole-changing motor
will also require a contactor (not so with inverters), some form of
thermal protection (a circuit breaker comes as standard with an inverter)
and additional wiring requirements. Depending upon the application, a
careful cost analysis may also suggest the inverter/motor solution for
applications requiring a speed ratio of less than 1:4.
Controlled run-up versus DOL start
When a motor is started on direct connection to the mains supply (direct
on-line or DOL starting), the rotating