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Copyright 2005
Smith Electric Motorworks

Static Converter

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Producing the "Third Leg:"
The Static, or Capacitor Method

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 Figure 3 - A capacitor may be viewed as an "electrical trampoline." AC power pauses as it bounces through, producing a distinct voltage.

Capacitors have been used to operate 3-phase motors on single-phase power for decades. In this method, the two single-phase wires are connected to two of the inputs on a 3-phase motor. A capacitor is then connected to one of the single-phase inputs and the third leg of the motor (Fig. 3).

A phase shift through the capacitor allows the voltage to be displaced in time from its parent voltage. The result is a voltage distinct from the 2 single-phase lines. If the capacitor's value--its ability to process electrical current--is sufficient, the motor will operate.

A motor requires about 6 times as much current to start as it does to run, so a static-capacitor phase converter must have some means of switching a large group of capacitors in and out during motor starting (Fig. 4).

  u&sfig4
 Figure 4 - A typical static-capacitor type converter uses a single-phase motor starting switch to run a 3-phase motor.

Here, a typical unit uses a potential-type motor starting relay (pirated from a single-phase motor) to regulate the larger start capacitor, while a smaller (in value) capacitor provides continuous power to run the motor. As the motor speed increases, the potential relay removes the start capacitor from the circuit, and the motor operates.

 


Applying the Fixed-Capacitor Method:
Some Limitations

static
The Autogen CD-Series static phase converter is an inexpensive solution to powering anything from drill presses to air compressors.

The capacitor method alone is not as good as generated 3 phase power. With a capacitor-start, capacitor-run operation, you can run a 3-phase motor at 70% load all day; or make short bursts of power at around 90-95% load for 15 minutes at a time. Also, capacitors alone will not operate 3-phase welders, transformers or rectifiers. The electrical "fuel" provided by capacitors alone is simply of poor quality and distribution for loads that require precise power.

Some manufacturers build a device that starts a 3-phase motor on capacitors but allows the motor to run on single-phase. This can produce dangerous motor overheating on loads above 45-50% of the motor's Hp rating. The motor will usually stall when loaded past 65-70%, since only one-third, or one phase of the 3-phase motor is actually receiving power. The applications below will not apply to these type converters, but only to GWM's Autogen or similar models with full-time three-wire power.

Back in Figure 1, generated AC looked like a smooth wave of power. Well, the capacitor charge looks like a bucket of power being thrown. Capacitors can start almost any 3-phase load--if you pile enough of them on. But a motor with a heavy running load will begin to slow down as soon as the starter kicks off. many static converters--our Autogen is included--have a red light which should only come on if a motor is starting. If the red light comes on while the motor is running, you must lighten the load to keep from ruining your converter.


The new CD series static phase converters can be installed in minutes with no control modifications. They incorporate the industry's first true 3-phase electronic motor starting relay (patented). This is superior to the modified single-phase motor starting switches or timers used by other manufacturers.


Here is a good rule of thumb to follow: Motor-driven tools are generally OK for the static converter. Most mills, drills, saws, grinders, etc. will work well. Even mechanical shears, brakes, and punch presses work well if they have a flywheel.

Back to machine tools. Lathes with a clutch are an easy load. Lathes with no clutch and a high speed spindle (over 1500 rpm) are a different story. High-speed lathes that can start under full load require more starting torque then a standard static phase converter can produce. Usually, a rotary converter 2 to 3 times the HP rating of the lathe's spindle motor is required. However at GWM, we have developed a unique and patented 3-phase motor starting relay for our static converters that permits operation of many of these demanding lathe motors on our Autogen Frankenphase™ converter -- so named for the "Frankenstein" starting torque it produces. For use in a production machine shop, however, choose the rotary converter. You'll need to run the other 3-phase machines you'll buy as you grow, and it's a good investment

A good static converter will connect at the incoming power to the machine so no motor control modification is necessary, and will permit instant-reversing of motors at full-speed. All Autogen models will do this, and not require fiddling with start circuit timers. Most static converters will not cause any disturbance to electronic or computer equipment. You'll find a Comparison Chart of many brands of popular static-type converters in the GWM Catalog and information as to which manufacturers include full time 3-wire power in their converters.

You can run many brands of belt-drive air compressors on the Autogen. Air compressors produce a heady motor load, but it doesn't last long, and the motor gets to cool off between cycles. Heavy loads that last 15 minutes or less are okay for the Autogen. But full-time loads should be limited to 65 or 70 percent of the motor's HP rating.

Here are some machines that generally won't work on a capacitor-type static converter: Refrigeration pumps are out. Hydraulic pumps are usually out. Most hydraulic machines push the motor too hard for any static converter to keep up. Printing presses, wide-belt sanders, and almost all water pumps are out. Dust collectors and other high-speed, high-volume fans are out. Vacuum pumps -- no. Any load that is not a 3-phase motor is out. Transformers, CNC machine tools and DC drives will not run on any static converter. There is a method though, that may be used to coax some loads to run on the Autogen that otherwise might not operate. We regularly encounter customers who need a rotary phase converter but can't afford it... If you find yourself in their shoes, we may have a workable alternative.

Getting More Performance Out of a Static/Capacitor Converter

We regularly encounter customers who have machines that require more power than a low-priced static converter will produce. However, the application -- or their finances -- often will not permit the purchase of a rotary converter. This may be a workable solution: the nature of the Autogen is to perform better when more than one motor is operated if one of the motors is not connected to a physical load. Functioning like an "electrical flywheel", the unloaded, or "idle" motor will carry a loaded motor along. This is also a useful method for operating one or more small motors on a large static converter. Start a motor that is in the converter's range and let it run idle, then operate the small motors. This method can also be used to adapt the Autogen's full-time capacitor motor to a 3-phase transformer load, such as wire feed welders or battery chargers.

This is not a cure-all. If it was, we would seldom sell a rotary phase converter. There are a great many loads that will not operate on this method, such as many multi-speed and low rpm motors, deep well submersable pumps, some SCR (DC) drives and computer-controlled (CNC) machines. GWM's Autogen CD-Series' instructions, as do several other manufacturers', include a diagram for hooking up a 3-phase DC welder or 3-phase motors using a static converter plus an idle motor. The good news is, the idle motor shouldn't add over 10 cents to 25 cents per hour to the operating cost of your equipment. You only need to have a source of used 3-phase motors, and an aptitude for electrical tinkering. Just fire up the idle motor first, then run your machines.

Summary of the Static-Capacitor Method

The fixed, static-capacitor method of phase conversion, then, is useful in operating moderately loaded motors. A motor improperly applied to a static converter may suffer from too little power. If a motor-run capacitor is not included in the converter, overheating and premature motor failure may result from dangerous single-phasing (under-powering) of the motor. If too large a static converter is used, the capacitors may force too much current into a motor, also producing overheating and failure. An "idle" motor of equal or greater HP than a heavily-loaded motor may be used with an Autogen or similar static converter to enhance the converter's output.

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