Arc-spray metallizing: where dc motors & SCR drives still win

Those trade-offs are needed in commercial electric-arc coating equipment. The equipment, Figure 1, sprays molten metal to give conventional materials special surface properties. Dual metallizing wires are advanced into the arc before the metal is sprayed onto the workpiece. Depositing the high-quality metal coating that is practical with arc-spray technology takes a consistent, controllable wire-feed rate.

However, for Metco Div., Perkin- Elmer, Westbury, N.Y., the wire-feed drive and the rest of Metco’s 5R arc spray gun system must be competitive in a market where low cost is crucial. The maker holds cost down by choices such as a simple SCR drive by Lenze Power Transmission for the small, high-torque dc motor that runs the wire feed. Small-motor dcdrive technology shines here, despite the trend to ac-motor, adjustable-frequency drives.

According to the drive’s engineers, small permanent-magnet (PM) dc motors are more compact and efficient than ac motors of equivalent ratings. Permanent magnets instead of an excitation winding explains the difference.

In comparing efficiencies, small SCR dc drives for PM motors also come out ahead, says the drive maker — about 98% vs. 94% for adjustable-frequency drives and dc choppers.

Arc-spray equipment

The 5R gun, Figure 2, is a high-rate-deposition unit that sprays at arc currents to 500 A, what Metco feels is the effective limit. In a typical installation, instead of being hand-held, the gun mounts on the tool post of a metal-turning machine. Mechanical control of gun traverse and rotating workpiece surface helps ensure coating uniformity. Two coils of wire feed the gun. The motorized drive inside the gun pulls wire from the coils through the gun and through electrical contact blocks where the arc power supply connects.

The two wires establish the arc where they intersect in front of the gun, and they are continuously fed into the arc. One or more air jets from the gun, directed into the arc, create the spray and propel the molten-metal particles.

A remote 5R controller, Figure 3, supplies air and motor-drive power to the gun. The controller monitors and sets arc current and voltage, primary and secondary air pressures, and operating mode. Once the arc is established, the arc-current setting adjusts the drive speed to advance the wires into the arc. Speed and wire type determine arc-current magnitude.

Motor control helps quality

Assuming homogeneous wires, a constant- velocity wire feed into an arc as high as 500 A and a high-velocity air stream is important for coating quality.

For wire feed, a one-quadrant SCR motor controller powers a high-torque PM motor on the gun. From an input voltage of 110 Vac, 50/60 Hz, the controller delivers 90-Vdc, ½-hp output. Motor speed can adjust from 840 to 6,100 rpm to cover the feed-rate range. Gearing inside the gun is a two-stage system of worm and spur gears of steel and phenolic materials. It reduces the motor speed to drive twin rollers which pull the wire, feeding it into the arc.

With the standard 42:1 reduction, wire feed can be adjusted between 65 and 470 ipm from the controller front panel. This range corresponds to an arc-current range of 70 to 500 A for 11-AWG zinc wire, for example. Low-speed (81:1) and high-speed (32:1) change gears are also available.

The small gun-mounted motor keeps the gun compact. But high torque is needed, because the motor pulls wire at a variety of load conditions in the field. Sometimes the wire must be pulled over a long distance because of the productionfacility layout. For that, the spray gun needs a continuous-duty motor capable of substantial torque at controlled speed.

In this application, rather than tachometer or encoder feedback for motor speed control, Metco uses only armature- voltage feedback with IR compensation. It gives the required results and keeps the equipment cost-competitive.

Tough environment

Arc spray gun work is not for sissy equipment. Unlike conventional PM motors, the samarium-cobalt magnet motor normally runs hot and mounts to a forced-air-cooled heat sink. Often, guns are within enclosures to protect personnel from fumes and overspray. Also, parts being coated may be large, generating much heat inside the enclosure. Air temperature can be 120 to 150 F. Occasionally, a gun operates in such confinement that Metco recommends additional air jets to cool gun components.

The basically off-the-shelf motor controller has built-in interference immunity from line spikes, surges, and similar line disturbances. More interference filters are in the arc power supply.

The waveform an SCR controller delivers to a dc motor is usually far from smooth with respect to time and that brings four disadvantages:

• Armature heating.
• Torque pulsation.
• Brush-life limitations.
• Noise from lamination vibrations.

Armature heating was of special concern to Metco, so the drive maker added an optional armature choke to the motor controller. It provides an output waveform that is much more nearly constant. The choke, in series with the armature, improves the form factor, which reduces heating and also overcomes the other disadvantages.

Boosting reliability

Metco advanced engineering manager Richard Trapani reports that simplicity and resulting reliability figured in the decision to choose a packaged SCR drive.

PM motors are subject to irreversible demagnetization, a selection consideration. But the rare-earth magnets in this motor withstand all field conditions and remain unaffected despite continuous operation, Mr. Trapani says.

Set up, then ship

The drive maker does final adjustments to the motor controllers before delivery to Metco, using a Metco test fixture. The fixture, incorporating a spray-gun motor and speed reducer, has an adjustable friction brake to load wire-drive rollers. Motor voltage, current, IR compensation, and acceleration rate are set for minimum and maximum speed, and no-load and full-load conditions. Thus, the controllers are ready for use when delivered.

Versatility

Metco and its drive supplier market nationally and abroad. All arc-spray-system electronics including the motor controller work on 110-Vac, 50 or 60-Hz power. Metco provides a step-down transformer for linevoltage conversion in overseas applications. No other change is needed.

Metco introduced the interchangeable spray air cap so one gun can deliver several spray patterns. Caps delivering fanshaped sprays cover large, flat surfaces such as panels. Fine-spray caps make smooth, low-porosity coatings with high bond strength. New high-velocity air caps give a very high velocity to molten particles, providing a more dense coating. A fourth cap design produces the antiskid coating.

The dc PM motor with SCR controller remains a highly competitive approach for demanding service. The combination offers simplicity, reliability, and constanttorque speed ranges exceeding 20:1.