Accuracy

A good place to start a discussion about accuracy is probably to determine how much accuracy is required for the process and then look at ways it can be achieved.

A number of years ago, a 7 motor Synphase system was installed on a 10 section triple Narrow Neck Press & Blow IS machine running 172 cuts per minute. In an experiment during production, the induction motor running a Maul mechanical gob distributor was deliberately differentialed out of position to determine accuracy requirements. A 0.070 second movement brought the scoop alignment close to the edge of its window for proper load. It was felt that a 0.090 second movement would have cause the gob to string instead of load the trough. The gob distributor had a 3.488 second cycle time. The 0.070-second interval is 2.1% of the 3.488 seconds cycle time.

The Synphase control has been programmed for a 0.010 second deadband and variations of 0.012 seconds were measured. Pack was 98% or better for days.

Controlling the process at 5-10 times better than required is standard practice. An accuracy of 0.2% should be sufficient.

No chain is stronger than its weakest link. Likewise no drive system is more accurate than its least accurate part.

Applying this axiom to a drive on an IS machine mechanism forces us to look at the mechanical connection between the motor and the final driven part (shear blade, scoop, mold, etc.) Gearboxes, timing pulleys, couplings are all commercial grade components. None of them are selected as special, high accuracy, anti-backlash parts. Most machining tolerances are within 0.005 inch (0.13 mm). A gear tooth misplaced on a 5-inch diameter gear by .005 inch is 0.03% out of position. As more than one part is used in the total drive train, it is safe to conclude that the mechanical connection on average cause at least .1% variation from theoretical instantaneous positions. This is close to the measured requirement of 0.2%, but is still acceptable.

A Permanent Magnet Synchronous motor’s rotor will lag 90 degrees behind electrical phase at full load and will lead 90 degrees ahead of electrical phase at full overhaul. In the experiment above, the inverter would have been supplying 71 Hz. A half cycle of 71 Hz is approximately 0.007 seconds or 0.7% out of position. Again, the accuracy is fine for the process.

Induction motors are the workhorses of industry. They are inexpensive, readily available, and familiar. A Synphase controller attached to an induction motor will maintain approximately 1 times (at high speed) to 3 times (at low speed) better accuracy than its uncontrolled P.M. Synchronous counterpart. This is better than required by the process.

In summary, because of the combination of low price and local replacement supply, TCD Systems recommends an inverter driving an induction motor if your IS machine will not exceed 550 bottles per minute.

However, if you are still not convinced, the Synphase system can be supplied with synchronous motors.

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Accuracy.pdf

Motor Type Selection

By design, the Synphase system phases the differential and synchronizes the speed of the various parts of the production line by adjusting the 0-10 volt control interface to the motor controllers. More specifically, one Synphase card controls one commercial electronic motor drive, which in turn controls one motor that is mechanically attached to one mechanism (feeder, gob distributor, etc.).

The Synphase drive can work with any combination of motor/drive package on the market because the only connection between the Synphase controller and the motor drive is a standard 0-10 volt interface. In the past, Synphase has driven S.C.R. controllers with DC motors, inverters with Permanent Magnet Synchronous motors, inverters with 3% slip induction motors (NEMA-B), and inverters with 5-13% slip induction motors (NEMA-D). It is capable of controlling Servo amplifiers with Servomotors.

  • DC Motor:
    When the Synphase system was first developed, the combined price of a DC motor and an SCR controller was significantly less than the equivalent AC induction motor and inverter combination. The advance of electronics in the late 1980s reversed the pricing structure making the AC induction motor the better deal. Combined with the brush wear associated with DC motors, TCD Systems now recommends some form of AC motor be used. The induction motor is the most cost effective and its accuracy is more than adequate for speeds below 550 bottles per minute.
  • Permanent Magnet Synchronous Motor:
    Also known as a Brushless DC Motor is more expensive than an induction motor. It usually has a longer backorder time when replacements are necessary. The higher currents associated with synchronous motors require larger inverters than their induction counterparts. It has been used with Synphase controllers, but offers no advantage in accuracy. In spite of these disadvantages, the synchronous motor does not provide any greater accuracy than a controlled induction motor.
  • Induction Motor:
    Induction motors are the workhorses of industry. They are inexpensive, readily available, and familiar. A Synphase controller attached to an induction motor will maintain approximately 1-3 times better accuracy than its uncontrolled P.M. Synchronous counterpart.

In summary, because of the combination of low price and local replacement supply, TCD Systems recommends an inverter driving an induction motor if your IS machine will not exceed 550 bottles per minute.

However, if you are still not convinced, the Synphase system can be supplied with synchronous motors.

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Motor Type Selection.pdf

Motor Specifications

TCD Systems recommends using common 4 pole induction motors having approximately 3% slip at full load. No mater which motor you choose, use these specifications as a general guide.

  • The motors may have either 50 Hz or 60 Hz nameplate value.
    Although a 50 Hz motor has approximately 20% more torque than the same motor rated at 60Hz, both have more than enough torque to accomplish the task. If a replacement motor is different from the old one removed, make sure the settings in the inverter match the requirements of the motor. Check with the inverter instruction manual if in doubt.
  • Select either “inverter duty” or “high temperature” motors (NEMA class “H”.)
    All inverters now use fast switching output drivers (IGBTs) to generate the PWM waveform. The fast switching times generate harmonics (1-10 MHz typical) and thus require the motor leads to be considered as improperly terminated antennas. Inverter manufacturers tend to recommend maximum wire pulls from inverter to motor of 250 feet (90 meters) because of this problem. The improper termination also tends to set up reflected waves at the motor. This means that the motor windings may see 1600 to 2000 volts if wired to run high voltage (460). The high frequency components of the motor current can capacitively couple through the microscopic voids in the varnish insulation of normal motor windings. This phenomenon will cause the inverter to trip on “over current” even though the windings are not shorted when tested with a DC Meggar. To avoid this over current trip, 'Inverter duty' motors basically have 2 applications of varnish (each baked) and paper insulation. High temperature motors also have paper insulation and are double baked but usually cost less than ‘inverter duty’ motors.
  • Select the proper motor frame.
    A DIN motor will not directly replace a NEMA motor because of different dimensioning. If you are willing to modify the tachometer mounting ring and possibly the base plate for foot mounting, a DIN or NEMA motor is interchangeable from a Synphase system standpoint and can be mixed throughout the system. These pictures show one customer's solution.
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Motor Specifications.pdf
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