Energy-saving drive cuts demand 48% to 74% on longer cycles.

Energy-Saving Drive Cuts Demand 48% to 74% on Longer Cycles

Real savings potential of thousands of dollars per machine per year appears to be available to injection molders with a new hydraulic pump motor and drive system just announced by Cincinnati Milacron's U.S. Plastics Machinery Div., Batavia, Ohio. Milacron has applied for patents on the use of relatively new variable-speed, brushless d-c motors to power a hydraulic pump on an injection machine. Milacron has tested these motors for two years on three of its latest Vista machines--a 165-ton toggle, and 250- and 500-ton hydraulic models--and released the first data last month comparing their performance against machines using conventional fixed-speed a-c motors.

Because the variable-speed motor draws much less power when it's not needed at certain stages of the molding cycle, Milacron's data show that power demand by the pump motor can easily be 48% to 58% lower than for Milacron's standard Vista machines. Since the latter are already 30% more energy-efficient than older machines, says Milacron, thanks largely to their use of a variable-volume main pump, a Vista machine with the new brushless d-c drive could typically use 74% less power than an older machine with fixed-volume pumps driven by a-c motors.

These figures are for pump-motor power only, which Milacron says accounts for 87% of total electrical energy consumed by an injection machine, the rest going to power the heaters and controls. Taking that into account, the net energy saving for the machine as a whole might typically be 42-50% with the new pump compared with a standard Vista machine, and 65% compared with an older-style machine.

Company sources also note that the new pump motor adds about 7-10% to machine price, and that energy savings will be significant mainly in longer cycles where the motor is essentially idling or running at low load much of the time. These would typically be cycles where cooling time significantly exceeds plasticating time, and/or with long packing and holding stages. Molding any part with wall thickness of 50 mils or greater could produce a perceptible saving, say company engineers. And, of course, plants in areas with higher electric power costs will reap fastest payback.

WHY PAY TO SAVE ENERGY?

Milacron officials feel that American manufacturers have been lulled into a false sense of security about energy costs in recent years. They are not alone in predicting that the public attitudes and environmental restraints that have inhibited construction of new energy plants in recent years may produce at least a temporary energy crunch in some regions during this decade.

In an effort to go beyond the level of energy efficiency that was achieved with the new Vista series four years ago, Milacron tested several possible motor and drive combinations. Although the variable-volume pump reduced the amount of oil pumped when it was not needed, the pump was still driven by an a-c motor that ran at the same speed all the time. As an alternative, Milacron first tested various adjustable-frequency drives to vary the rpm of a standard a-c pump motor. Adjustable-frequency a-c drives have been used on injection machines in a limited number of cases, but Milacron found that they had unacceptably long response times and that torque dropped off at higher speeds.

Milacron next began testing brushless d-c drives that were introduced to the market only two and a half years ago by a new firm, Wertec Corp., Pineville, N.C., which changed its name in January to Powertec Corp. This firm reportedly is the only one producing brushless d-c motors in large sizes.

These motors have already proved themselves reliable in extrusion applications, where some machine builders--including Cincinnati Milacron--have standardized on them as alternatives to conventional brush-type d-c drives. Brushless drives are said to have the advantages of a 10-times-greater speed range, much more precise speed regulation, full-torque capability throughout the speed range, higher efficiency, and much smaller size for comparable horsepower. Also, because there are no brushes to wear out, maintenance is lower--especially in corrosive environments, such as shops processing PVC (see PT, Nov. '89, p. 120).

These brushless d-c motors are said to be about 95% efficient--i.e., 95% of the energy input is converted to usable horsepower--vs. 88% at best for standard a-c induction motors. What's more, the power factor (actual delivered power divided by the apparent power consumed) is 0.92 to 0.96, vs. 0.40-0.80 for a-c induction motors. Power companies sometimes penalize customers for low power factors, forcing them to purchase capacitor banks to compensate.

Milacron has programmed its CA-MAC controller for Vista machines to regulate the speed of the brushless d-c motor according to the machine's requirements for oil flow and pressure. The new motor can accelerate or decelerate between 0 and 1800 rpm in 0.70-0.80 sec. (In practice, a minimum 100 rpm is maintained to provide sufficient flow and pressure for safeties.) This is said to be considerably faster response than is provided by either variable-frequency a-c or brush-type d-c drives.

Milacron compensates for the brushless d-c motor's response time with the same small accumulator and injection servovalve used on standard Vista machines to compensate for the 0.10-0.20 sec response time of the variable-volume pump. Thus, overall response time remains at 10-20 millisec, according to the company.

Besides the accumulator, Milacron notes that the hydraulics on the Vista machines are suited to take advantage of the energy-saving brushless d-c drives for another reason: clamp pressure is "locked in," so that no energy is required to maintain clamp tonnage.

LARGE SAVINGS POSSIBLE

Energy savings with new drives are illustrated in the accompanying graphs. Figure 1 shows significant savings on a 250-ton Vista hydraulic machine compared with the same machine and a conventional drive, or even with a 150-ton pre-Vista hydraulic press. In both comparisons, the savings are greater at longer cycles. Note also that the variable-frequency a-c drive produces an intermediate level of energy savings, despite its other disadvantages.

Figures 2 & 3 compare new and conventional drives on Vista toggle and hydraulic presses. In both cases, the pump-motor energy savings range from 48% on short cycles to 58% on long ones. Figures 4 & 5 make clear exactly where in the molding cycle the major energy savings occur. For both toggle and hydraulic presses, the only stages of the cycle where little energy is saved are clamp opening and closing and building tonnage.

Further energy savings are not shown here: less oil cooling and plant air conditioning are required to remove the excess energy consumed by a conventional pump. And there's one more advantage of the new drive: because it converts a-c to d-c, it is not affected by line-voltage fluctuations.

Milacron is offering the new "VP" (Variable Power) drive option on its full line of Vista hydraulic and toggle machines up to 850 tons; larger machines (where it could prove especially useful) will come later. Milacron's Contract Services Div. also offers to retrofit the new drives to Vista or non-Vista (even non-Milacron) machines. Company sources speculate that some degree of economic benefit--though smaller--could even be achieved with the new drive on machines with a single fixed-volume pump.