How Phase Converters Help Apply Motors Pt1.Pub

Reprinted from POWER TRANSMISSION DESIGN magazine March 1989 HOW PHASE CONVERTERS HELP APPLY MOTORS: PART 1 Many prospective users of 3-phase machines face a lack of 3-phase utility service. A phase converter can solve the problem by letting a 3-phase load operate from a single-phase supply. The first of two parts defines converter types. LARRY H. KATZ Chief Engineer, Kay Industries Inc., South Bend, Ind. rue enough, electric utility companies distribute 3-phase power to large industrial and T commercial customers, as well as areas of high load density like central business districts. Yet, utilities have never found it economical to run 3- phase service to residential areas or to certain small businesses. Thus, many facilities have no 3- phase service. Commonly affected are small businesses that depend on equipment with 3- phase motors, such as bakeries, woodworking shops, laundries, printers, service stations, car washes, bowling alleys, and small machine shops. Phase converters were first developed in the 1950s to circumvent the problem on agricultural equipment. In recent Typical rotary phase converter, with ratings to 100 hp, suitable for years, industrial and commercial equipment such as compressors, pumps, machine tools, and application has expanded woodworking machines. dramatically. The trend is driven by economic factors including utility charges for extending 3- pressure from regulatory agencies fast enough through energy phase lines, construction delays in to justify capital investments revenues, the utility will charge obtaining new service, and limited before granting rate increases. the customer for the new service. availability of single-phase They respond by scrutinizing Charges vary according to terrain, equipment. requests for service extensions. If but it is widely accepted that line Utilities now come under the investment in extending a extension costs range from great customer line can’t be paid back $30,000 to $90,000 per mile, 3/89 POWER TRANSMISSION DESIGN depending on the complexity of the until the mid 1950s. The converter manufactured phase voltage. This change. A common average is truly came of age in the 1960s as voltage is shifted 90 electrical $50,000. The charge to extend a line rotary types appeared. Predictably, degrees relative to incoming line even a short distance can be increased converter use spawned a voltage. When this “out-of-phase” prohibitive for a small business. greater incidence of misapplication – voltage is applied to one coil group Then there is the additional expense most of it avoidable. Successful of an induction motor, its resulting of installing phase distribution panels application of a phase converter magnetic field interacts with the field in the building. Even when the utility depends not only on an produced by the utility supply in an agrees to bring in new service, the understanding of its capabilities and adjacent coil. The outcome: motor time lapse before a line is installed limits, but also on starting starting and running torque. can cost the business substantial characteristics and operating A motor usually draws five to eight revenue. demands of the load. Nearly all times full-load currant during start. Three-phase motors are well- application problems come from Thus, to get the motor up to speed, it established in industrial machinery. ignoring this reality. takes about five to eight times as They are generally more efficient, Understanding how converters work, much converter capacitance as it less expensive, more readily starts with knowing how they are takes to keep it running. A static available, and more reliable than connected. Every converter requires converter does it with two sets of equivalent single-phase motors. the two single-phase utility lines, L1 capacitors: one for starting; another Therefore, many manufacturers and L2 in Figure 1, to connect to both for running. Starting capacitors are design 3-phase motors into their converter and load. In other words, electrolytic because of their equipment, often without considering converter and load are parallel on the compactness. Run capacitors are oil- 3-phase service availability. Many single-phase line. The third wire or filled. owners of such machines, lacking 3- manufactured phase (T3 on all By far the most common and least phase service, try to substitute a figures in this article) is the only lead costly version of static converters is single-phase for a 3-phase motor. directly from the converter. The the capacitor phase-shift or simply Unfortunately, they usually find the converter’s job is to produce enough capacitor type. It is characterized by availability of integral-hp single- voltage on the manufactured phase to low cost, small size, and limited load phase motors to be limited, especially create sufficient motor starting and capability. As mentioned, the above 5 hp. Special motor designs running torque. It must also produce capacitor-type converter uses a such as multispeed types or custom a set of reasonably balanced currents combination of start and run flange arrangements, can make to each load phase. Reaching these capacitors to accelerate the load and finding a single-phase replacement performance goals depends on many maintain running current balance, impractical if not impossible. In this variables, including converter type, Figure 2. context, a converter can adapt any 3- load magnitude, power factor, and Starting capacitors are switched phase machine to single-phase power single-phase supply stability. service. The potential savings a phase Static converters converter can bring can be Construction, operation. A static breathtaking compared with the converter (in industry parlance, a alternative utility costs. Documented “static”) has no moving parts. It savings of $20,000 and more are occupies the lower end of the commonplace. Still, the use of phase converter capability spectrum, but it converters remains a mystery to usually has a price advantage over a many who could benefit most. Chief “rotary”. All statics use capacitors to Figure 2—Simplified equivalent circuit for a capacitor phase-shift reasons are widespread anecdotal create the accounts of field problems and other static converter misunderstandings of application. Nonetheless, the connected capacity of motors operating on phase converters of all types in the U.S. exceeds 3 million hp. Some service records exceed 45 years. Coming of age The two types of phase converters are static and rotary. The principals of both are documented in ac theory dating to the early 20th century. But the modern static phase converter did not become commercially popular Figure 1—Typical phase converter system 3/89 POWER TRANSMISSION DESIGN in and out of the circuit by a timer or capacitor-type converter can operate a specific load motor. relay, which senses voltage on the this type of motor up to full Ratings, dimensions, prices. Static manufactured phase during start. As nameplate power. Exceptions aside, converters are widely available in the motor accelerates to full speed, inability to provide balanced current ratings from under 1 to more than 50 inrush current falls off and at full output is why capacitor-type hp. They are compact and usually manufactured phase voltage rises. converters serve mostly on light-duty suitable for wall mounting. Some The relay responds to this back loads such as machine tools --- capacitor statics are small enough to voltage from the motor by opening a especially true of the start-only type. mount on the driven equipment. set of contacts, switching off the The most versatile static is the Autotransformer types are larger and starting capacitors. Similarly, if the autotransformer type, Figure 3. It heavier. load causes the motor to slow, the consists of an autotransformer A 5-hp capacitor static weighs less resulting fall-off in manufactured besides start and run capacitors. It than 25 lb and sells for $150 to $350. phase voltage makes the contacts steps up incoming line voltage and The lower pricing applies to the start- close again and bring the starting applies it to the load through the only version. An autotransformer capacitors back into the circuit to re- capacitor. The autotransformer static converter of the same rating weighs accelerate the motor. Thus, the uses the same start-run about 150 lbs and costs around $800. converter tries to hold constant load The autotransformer brings the output. higher weight and price --- clearly a A popular, inexpensive home substantial premium. workshop version of the capacitor At 25 hp, differences are more phase shift converter is the start-only pronounced. The capacitor type variety. It has a set of starting weighs under 90 lbs and costs $650 capacitors, but none for running. to $1,300. The autotransformer type They are in the circuit long enough to weighs over 400 lbs and sells for get the motor to speed; then they $2,750. Weight and price switch out. In effect, the start-only relationships for other ratings are converter is little more than an Figure 3—Simplified equivalent roughly linear to these. electric “rope starter” for a 3-phase Circuit for an autotransformer motor. After start, all capacitors are Static converter. Rotary converters switched off and the motor is simply Construction, operation. A rotary allowed to single-phase. This phase converter is an induction condition is tolerable if the motor is switching mechanism as the capacitor machine that operates on a single- loaded to no more than two thirds of type. But the added advantage of phase supply and produces a true 3- nameplate power. However, single- taps on the autotransformer allows phase output. It can supply the full phasing is a common cause of motor adjustment of the manufactured rated input requirement of any 3- failure, so this converter is not a phase voltage to produce optimal phase motor (including service serious candidate for most current and voltage balance. This factor), resistance, or rectifier load.

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