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Static Phase Converter FAQs

General | Technical


What is the ADD-A-PHASE® power converter?

It is a static converter capable of producing symmetrical three-phase currents from a single-phase source. The name ADD-A-PHASE implies that a phase is added to the already present single phase, accomplished by an autotransformer and a capacitor bank along with the motor’s impedance. Through this combination, a third phase is introduced whose relation to the other two comprises symmetrical three-phase. The motor can then operate as if it were connected to a three-phase power line.

What horsepower ratings are available with ADD-A-PHASE?

Standard types are available from HP1 through 100 HP. Units up to 150 HP have been constructed. Where high horsepower loads are to be operated, consult the power supplier to determine whether the single-phase line capacity is sufficient to carry the load.

What are typical applications?

ADD-A-PHASE has been successfully applied to operate motors on many different kinds of equipment. Some of the principle applications are pumps, fans, valve actuators, oil pump jacks, and piston type compressors.

What motor speeds can be satisfactorily operated with the standard ADD-A-PHASE, Type 2S?

Speeds of 3,600 RPM and 1,800 RPM can be operated from the standard units. For speeds below 1,200 RPM, the ECONO-PHASE® Shifter may be a better choice, depending on the motor’s power factor.

Are special ADD-A-PHASE models available?

Yes. Consult the factory for details. Our engineers are always willing to work on problems arising from specialized applications.

Is ADD-A-PHASE available for 120 V operation?

No. 120 V input unit is not available.

Is ADD-A-PHASE available for 50 cycle, as well as 60 cycle?

Yes. The basic ADD-A-PHASE types are designed for 60 cycle, but 50 cycle units are available. Again, consult the factory for prices and delivery.

Is ADD-A-PHASE available for 480 V or 575 V operation?

Yes. The units for 480 V are catalogued, but the factory should be consulted for 575 V operation.

Can ADD-A-PHASE operate an air conditioner?

Yes. Most air conditioners can be operated with the Type 2SAC. Note that any additional three-phase motors, like evaporator fan motors, units require Type K-Duo for multiple-motor operation.

Can ADD-A-PHASE be applied to submersible pumps?

Yes. It is necessary to check the full load amp rating of the motor. If the FLA of the submersible’s motor is higher than a standard NEMA motor’s FLA of the equivalent horsepower, Type SUB should be used.

Can 480 V three-phase motors be operated from a 240 V single-phase line?

Because of voltage drop, wiring size, starter size, etc., it may be desirable to operate large motors, submersible pumps, etc., at 480 V when only 240 V single-phase is available; the Type “3” ADD-A-PHASE is designed for these applications. Other special voltages are also available such as 240 in/208 out. Consult the factory for pricing and specifics.

How does ADD-A-PHASE benefit the utility?

The ADD-A-PHASE draws close to unity power factor while the motor is starting and running, resulting in lower line current and lower losses in line voltage. The inrush current of the ADD-A-PHASE and three-phase motor is only 3 to 4 times the running current.

Will a three-phase motor produce 100% rated power when operated through the ADD-A-PHASE?

Yes. ADD-A-PHASE is the only static converter for which this is possible, due to its autotransformer design. In fact, continuous overload (service factor) operation is available that equals the permissible overload of the average three-phase motor (1.15).

Note: while this operation is possible, some motor manufacturers (notably submersible pump motor manufacturers) may not warranty their motors if it is operated into its service factor range while powered by a phase converter. In practice, it is best to avoid service factor operation if possible to assure normal life expectancy of equipment.

Can ADD-A-PHASE operate a motor larger than the rating of the converter?

It is not recommended; however, there may be applications where the load of the motor is reduced and within the rating of the converter. Extreme caution should be observed, and it is advisable to contact the factory before attempting such an operation.

Can ADD-A-PHASE operate motors smaller than the rating of the converter?

Yes. The design of the autotransformer converter permits a smaller motor to be operated with capacitor adjustments.

Can more than one motor be operated from a single ADD-A-PHASE?

Type K-Duo ADD-A-PHASE is designed specifically for independent or simultaneous operation of two motors. Special models are also available for other multiple motor applications. Consult the factory for recommendations.

Can two speed motors be operated satisfactorily?

Ronk offers converters to operate two-speed applications. Type K-Duo or Type AA ADD-A-PHASE can be used for some two-speed motors. Consult the factory for recommendations.

Is ADD-A-PHASE also suitable for outdoor installations?

Although ADD-A-PHASE is primarily designed for indoor installation, the louvered enclosure is drip proof and may be mounted outdoors.

Can ADD-A-PHASE be installed in damp underground locations?

Yes. An example would be the many satisfactory applications in underground sewage lift stations.

Is ADD-A-PHASE available in an explosion-proof enclosure?

No. Normal heat rise of components requires sufficient cooling, one of the factors which has made the explosion-proof case impractical.

What is the life expectancy of ADD-A-PHASE?

The components selected and manufactured for the ADD-A-PHASE have a lifespan comparable to that of the average three-phase motor. Long life and reduced maintenance are among the many advantages of the ADD-A-PHASE over the conventional single-phase motor.

What is the availability of ADD-A-PHASE?

Distributors are located throughout the United States and Canada. Most ADD-A-PHASE types are available from these distributors within a week (plus shipping time), except special application types, which may require slightly longer.

What is the need and availability of replacement parts?

The ADD-A-PHASE, being a static device with only a few moving parts, is relatively free of maintenance. Should replacement parts be necessary, immediate shipment can be made from factory stocks, or they can be readily acquired from local sources.

What is the ADD-A-PHASE warranty?

ADD-A-PHASE is warranted to be free from defects in material and workmanship under normal use and service for a period of one year from date of installation or 18 months from original date of shipment. A postpaid warranty validation card is supplied with each unit, which must be filled out and returned to the factory within 15 days of installation to validate warranty.


What is the power factor during starting?

The power factor will be very high during starting, near unity. The ADD-A-PHASE draws a reduced starting current due to the capacitance built into the converter. Starting KVA is approximately that of three-phase across-the-line starting.

What is the power factor when the three-phase motor is supplied with energy from ADD-A-PHASE?

Independent laboratory studies rate the power factor of the converter at almost unity on the single-phase line. This feature is of extreme importance and advantage to both the power supplier and consumer. On the other hand, repulsion-induction, single-phase motors have a power factor range of only 60% to 80%.

How close can the currents be balanced?

On a constant load application, balance close to that of a three-phase motor operating from a three-phase line can be achieved.

Are currents in balance under all conditions, particularly where a swinging load is encountered, as with oil pumping?

No. Swinging load conditions necessitate the phases' being balanced on installation for the maximum load. Where the load is reduced due to swinging, the unbalance should be disregarded insofar as none of the phases carries a current exceeding the nameplate rating. Minimal loss of efficiency is normally encountered.

Do the balanced current and voltage relationships exist only when both motor and converter are in normal operation under load?

Yes. The voltage that exists, with the converter energized and the motor off, will reflect the autotransformer’s settings. They are useful only for identifying the phases and the tap setting. The motor must be connected, with its normal load applied, to take readings which verify, or can be used to achieve, balanced currents and voltages.

Can ADD-A-PHASE be balanced by checking output voltages?

Yes, but it is not recommended. For best results and accuracy, readings should be taken with an ammeter of the three leads to the motor. When the currents are balanced, the voltages should also be balanced.

What is the single-phase inrush current during starting?

When starting the three-phase motor through ADD-A-PHASE, the inrush current ranges from 3 to 4 times the rated single-phase running current, depending on the motor’s code letter.

Will ADD-A-PHASE operate both “delta” and “wye” wound motors?

Yes. The type of winding in the motors is immaterial as far as ADD-A-PHASE is concerned. Unlike some other phase converters, ADD-A-PHASE will operate any configuration of three-phase induction motor.

What starting torque can the standard ADD-A-PHASE develop?

The standard Type 2S will develop a minimum of 150% starting torque, sufficient for most general applications.

Are models available that have a higher starting torque than the standard ADD-A-PHASE?

Yes. Consult the factory if special starting conditions exist.

What breakdown torques can I expect?

ADD-A-PHASE generally provides minimum breakdown torque of 150%. This greatly exceeds most motor load requirements. Higher breakdown torques are possible with some motors or with use of ADD-A-PHASE Type AA (auto-adjusting).

Does the three-phase motor operating through ADD-A-PHASE lose its normal efficiency?

No. The overall efficiency, when properly balanced, is the same as the motor’s efficiency on a balanced three-phase line.

What is the overall efficiency of ADD-A-PHASE?

The overall efficiency is approximately 97%.

What about efficiency with other type converters?

Efficiencies with other types of converters, exclusive of ADD-A-PHASE, are limited to 80% or less, since the phase currents are not normally balanced, or a rotating element must be maintained.

What is the voltage input to ADD-A-PHASE?

The voltage input should be within 10% of rated voltage.

How is ADD-A-PHASE affected by low voltage?

As with a motor, too low a voltage can create a malfunction of the converter, particularly during starting. If low voltage is encountered, the cause should be determined and corrected.

Is there a limitation to the number of starts per hour?

The general-purpose ADD-A-PHASE incorporates electrolytic capacitors, which are designed for 20 starts of three-second duration, or 10 six-second starts per hour. If more starts per hour are required, Type HD is recommended.

Is there an ADD-A-PHASE type with no limitation to the number of starts per hour?

Yes. Type HD does not incorporate electrolytic capacitors.

Can an across-the-line magnetic starter be installed?

Yes, and it is recommended with all installations. Consult the factory if anything other than an across-the-line starter is supplied with the equipment. Three-phase electronic “soft” starters cannot be used with ADD-A-PHASE.

Where should the magnetic starter be installed?

Between the motor and the ADD-A-PHASE, in accordance with the instructions that accompany each unit.

Can an ADD-A-PHASE/three-phase motor be automatically controlled?

Yes. Power for the control circuits should be taken from the single-phase or the B-C phase of the converter.

May three-phase unbalance protection equipment be used with ADD-A-PHASE/three-phase motor installations?

This is not recommended because of the voltages that exist when the motor is off. If over/under voltage protection is necessary, it should be used across the two single-phase power leads only. Current monitors that do not look at voltage can be used, however.

What auxiliary equipment is required and recommended?

A single-phase fused disconnect switch and an across-the-line magnetic starter with overload protection is required. A three-phase disconnect may also be required where the “in-sight” rule in Section 430 of the NEC applies.

What size single-phase switch is required?

The installation should include a single-phase switch sufficient to carry the single-phase FLA of ADD-A-PHASE, plus other auxiliary equipment, if any, on the single-phase line. These fuses must be lag type to allow starting of the motor.

Within what temperature range can ADD-A-PHASE be operated?

For best operating results, ADD-A-PHASE should be operated within a temperature range of -25 °F. to 110 °F.

Must ADD-A-PHASE be installed near the motor?

Not necessarily. ADD-A-PHASE can be located at any reasonable distance from the motor. When the unit is to be located at a considerable distance it may be necessary to increase the wire size to compensate for loss of voltage. Also, “in-sight” rules of Section 430 of the NEC may apply. Consult the converter wiring diagram and NEC before installation.

Can single-phase be applied to ADD-A-PHASE at all times, and the motor controlled from the three-phase side?

Yes, idle losses are very low. These are due to eddy current losses in the transformer and range from 5 watts to 200 watts, depending on the size and type of the ADD-A-PHASE converter.

What KVA transformer is required per horsepower of motor load?

Generally, with the induction motor, 1 KVA is recommended per HP. Since losses in ADD-A-PHASE are very low, this same guideline is recommended for ADD-A-PHASE applications; 1 KVA per connected HP.

Why does Ronk manufacture both rotary and static converters?

Satisfactory converter operation requires application of the best converter for the job. Only Ronk provides the broad product line and experience necessary to provide a converter for any application providing the best performance at the lowest cost.

How does ADD-A-PHASE differ from ROTOVERTER® and Rotocon®?

ADD-A-PHASE is a static phase converter, whereas ROTOVERTER and Rotocon are rotary types. Rotary converters are best applied to applications where several motors operate independently or for non-motor loads.

What are the determining factors in selecting a static or rotary phase converter?

ADD-A-PHASE is usually recommended for single motor applications that operate at constant loads, though there are some exceptions such as multi-speed motors. In these applications, where automated operation is required, ADD-A-PHASE is generally recommended. A rotary converter is usually recommended for the operation of a group of motors or motors with varying loads. The rotary converter is also recommended for three-phase resistive, rectifier, or electronic loads.

Does ADD-A-PHASE cost more than rotary converters?

Rotary converters may be significantly less expensive for multiple-motor applications where load characteristics allow their use. However, additional equipment is often required when rotary converters are used in automated applications, which raises the installation and maintenance costs. ADD-A-PHASE’s higher efficiency offers significant savings in operating costs. These savings can offset its higher initial cost.

How do the maintenance costs of ADD-A-PHASE compare to that of a rotary converter?

The ADD-A-PHASE has no moving parts and will generally require less service.

Is ADD-A-PHASE more efficient than ROTOVERTER or Rotocon?

Yes. The losses will be greater in a rotary converter. The losses are much greater when the rotary converter is running, but has no load applied.

Can the phase currents be more closely balanced using ADD-A-PHASE than a rotary converter?

Generally they can be; however, adjustments are available on both ADD-A-PHASE and ROTOVERTER for balancing all three phase currents. The ROTOVERTER is the only rotary converter that provides this capability.