Rotary Design Considerations

Rotary Vs. Solid State Design

In general, the advantages of rotary design over solid state design are:

1. Price-- In sizes over 12KVA, 3ph., rotary is about 50% of the price compared to the same KVA size in a solid state design. Rotary has the ability to start more then twice the horsepower motor vs. a solid state design of the same KVA rating which often means that you are comparing a 12KVA rotary with a 25KVA or larger solid state unit because you need the motor starting capability of the rotary design--which further increases the rotary price advantage.

2. Longevity and Repairability-- our rotary units are designed (class H insulation, brushless) to operate for 30 years of continuous (24hr/day) operation with minimum maintenance (50,000 hour permanently lubed bearings are typical). Bearing changeover can be done in any motor repair shop by two men in 6 hours using worldwide standard available bearings. The weakest link is the solid state voltage regulator which, like most solid state power devices, is subject to failure after 8 or 9 years. Spare regulators are relatively inexpensive and always readily available as a package device from our factory stock or Basler Electric, Highland, IL, the worldwide leader in such devices (components change but device function remains constant). Downtime on rotary units rarely exceeds two days and any downtime is uncommon (once in 10 years, maybe). Our solid state units, like all other manufacturers, are subject to component failure (often due to repeated spikes on the input utility line) after 8 or 9 years (and sometimes much sooner). Replacement boards will have to be ordered from us and a technician will have to trouble shoot the unit over the phone using the schematic(often a language problem overseas)--or the unit could be shipped back to us for service. Either way, there will be more time down. Another factor is obsolescence--solid state power design and the components used change very rapidly. Components commonly used today are phased out in a few years and start becoming hard to find. We try to ensure that we maintain a stock of any component used within the last 10 years sufficient to cover any repair needs but this is a constant problem. We can always repair and replace parts, if obsolete, for our solid state units if returned to the factory but that takes time.

For those rare instances where in shop repair is required, we have a worldwide list of qualified repair shops. For warranty repairs only, we pay freight to and from such a shop.

Quite simply, which do you expect to last longer -- your VCR or your refrigerator?

3. Operating Noise Level-- The main advantage of solid state design in a 3phase unit is low noise level. The 3 phase solid state units are not totally quiet (which I define as below 60 dBa@3ft.) but they are relatively quiet (all are under 80dBa and most are under 73dBa). All our data sheets on rotary units show the operating noise level. Obtain the data sheet for the unit you are considering and click here for a comparison of dBa level with commonly heard sounds. Generally, sound levels under 80 dBa@3ft are not a problem in an industrial plant. Levels over 83 dBa are an OSHA problem for 8 hour exposure.

There are three approaches to controlling the sound level of rotary units:

1. Put the rotating component skid in another room with other equipment such as compressors etc. The totally noiseless control panel would be the only thing the operator would see and this allows him access to the Stop-Start pushbuttons, meters, output voltage adjust, output circuit breaker, etc. There is no charge for this but it does add a little to installation wiring cost.
2. Enclose the rotary equipment skid in a ventilated, enameled steel outdoor enclosure and place the skid outside on a pad with the control panel on the inside of the wall. All outdoor units are furnished with anti-condensation heaters in the motor and generator coils.
3. Specify an optional indoor, quietized enclosure which is lined with an acoustic foam that will drop the sound level by 10dBa. These come in horizontal and vertical configurations. The vertical adds cost but cuts the footprint by almost 50%.
   To see photo click here.

5. Consider our Solid State 3phase IGBT Design when:

1. Any electric motor in the load constitutes less than 25% of the full load.
2. SCRs, triacs, welders etc. that feed distortion back into the supply line (which will be the frequency converter in this case) constitute less than 20% of the load or have input filtering or isolation built-in.
3. A 50% or more cost adder is acceptable (vs. rotary design) primarily to achieve the desired operating sound level after considering the three alternatives for rotary and finding them unacceptable under the circumstances.
4. An English speaking electrical technician capable of using an oscilloscope is available for out of warrantee repairs or:
   1. The unit can easily be shipped back to the factory (it is in the U.S., Canada or Mexico).
   2. For out of warrantee repairs, the owner is willing to pay our travel, labor, parts and expenses.

Sizing The Converter-- In general, the converter must be rated at the higher of:

1. The starting surge of the largest motor (or motors that start simultaneously). This involves the KVA rating of the generator portion of the set only- i.e. starting surge is at very low power factors and is "seen" by the generator only, not the converter motor or other converter components. It is important to know if the motor load is started "across the line&quot (as is usually the case with motors that cycle on and off line) or uses a "soft start&quot or reduced voltage starter which minimizes the starting surge. In this regard, it helps to know the LRA or locked rotor amps. of the motor (or the design code-not the insulation code). If the LRA is not easily determined we will assume a figure of 6X the full load current of the motor.
2. The total amperage of all the devices in the load.

¹Specifications subject to change without notice.