Design Help Q&A

When determining your needs, you may be asking yourself:

  • What size unit do I need?
  • Which design is best for my application?
  • Do I need Rotary Design?
  • Does my application require Solid State Design?

Question and Answer

Q: Which design is best for my application, rotary or solid state?

A: This question involves too many factors to consider on this website so we will try to bring up enough questions for you to do some fact-finding and get a feel of what is involved. We would remind you though that the “Job Satisfaction” clause in our warrantee means that once you have discussed the application with us, any unit furnished subsequent to our discussion is guaranteed to do the job. No reservations, no questions asked, no-fault assigned for things overlooked–just ask for a full refund or credit on another unit to do the job (completely your choice). If you choose to take another unit, as always, we will provide a loaner (in this case at no cost to you) until we build your replacement unit if we do not have one in stock. Make us fully responsible for your decision and call us at 1-866-221-3131 or complete our Contact Us form and someone will follow up with you shortly.

If an electric motor is started on the utility (cycling across the line start), it commonly draws about 6 times its full load amperage (ever notice the lights dim when a large motor starts). The starting surge varies with the starting method. This can be a problem since a solid state frequency converter is commonly currently limited at 1.5 times its full load amperage. A rotary converter on the other hand commonly can supply 4 times its full load rating (for motor starting) and often more.

In general, if no large motors or electrical distortion causing devices (large SCR’s, thyristors, etc.) are involved as part of the load, and a modest cost premium (an advantage in the case of smaller units) may be acceptable as a trade-off for operating noise level in an office or lab environment, a solid state unit may be the preferred approach. Given the presence of these “problem” loads, a rotary design may be best. There are many factors involved in both design choice and sizing. Again, we will be glad to discuss this with you and assume responsibility for the final product choice and performance, just call 1-866-221-3131 or complete our Contact Us form and someone will follow up with you shortly.

Q: What size unit do I need?

A: The question of what size usually breaks down into two basic questions:
What is the total full load amperage (all equipment running worst case) at what voltage and frequency? If you do not know the amperage at the new frequency, what was its designed frequency? Try to avoid just buying a converter rated at the amperage of the circuit breaker (and KVA) powering the load at the designed for frequency–that commonly results in a grossly oversized and needlessly expensive converter.
Are there any electric motors in the load? If so, what horsepower and what is the design code or locked rotor amperage (should be on the nameplate). What is the type of starter used–reduced voltage or is it across the line and cycling? Does the nameplate have any LRA (locked rotor amps) figures? Design Code letters?

Does the load have resistance-type electric heating elements (increases load power factor)? Any SCR’s etc.(variable speed motor drives)?

Again, make us fully responsible for your decision under our warrantee and call us at 1-866-221-3131 or complete our Contact Us form and someone will follow up with you shortly.