DON’T PANIC – Tighten Up Equipment Orders

Equipment procurement spans global markets, with manufacturing frequently relocated to lower-cost regions across numerous countries. Even large domestic companies may relocate their production facilities or face acquisition by foreign entities that subsequently move manufacturing operations overseas.

Over the last 50 years, plants have shifted so that a much higher percentage of small-to-medium power drivers are electric motors instead of turbines. As motors became cheaper and electric supply more reliable, the benefits of turbines dropped. Except for unusual cases, the modern plant uses electric motors for nearly all services with power demands less than 1,000 HP.

Why Frequency Matters in Motor Sizing

Internationally, nearly all plant electrical systems are either 50-cycle or 60-cycle alternating current. North America, most of Central America, much of South America and some other regions such as Saudi Arabia, the Philippines and South Korea use 60-cycle alternating current. The rest of the world uses 50-cycle current.

The speed of a standard squirrel-cage motor is a multiple of the cycle frequency of the electricity feeding it. The construction of the motor determines the multiplier. The most common motor speeds are approximately 30 times the frequency and 60 times the frequency. The standard speeds of motors in North America are 1,800 rpm or 3,600 rpm. The same motors would operate at 1,500 rpm or 3,000 rpm in Europe.

Once built, most motors can operate at either cycle rate. However, the power demand on the motor will change with the equipment's operating speed. If we take the centrifugal pump, we can apply affinity laws to see how pump-motor performance changes with speed. For an unconstrained pump, flow rate varies with speed (Q ~ N), dynamic head varies with speed squared (H ~ N²) and pump power varies with speed cubed (P ~ N³). Hence, the change from 50-cycles in Europe to 60-cycles in North America would result in a pump power increase of (60/50)³ or 1.73 times as much at the higher speed.

Positive displacement pumps, by contrast, have the capacity increase, but the head typically remains constant, so power demand ends up being linear with speed as well (P ~ N).

These changes should be well understood by all involved in international procurement or supply of electrical motors.

A Case Study in Frequency Mismatch

A recent case illustrates a near-miss on delivery of an undersized pump for a positive displacement vacuum pump. The vacuum pump was a Roots blower. A Roots blower is a positive displacement compressor using multiple lobes to move a volume from the suction to the discharge. While they were initially used as a type of compressor for air and other services, many are used today as vacuum pumps. Because it’s a positive displacement machine, the capacity, and hence the power requirement, is linear with operating speed.

The pump was purchased from a European supplier and used in a newly constructed U.S. unit. The unit was having problems maintaining vacuum. Hence, the vacuum pump was checked. The pump came with paperwork labeled ‘CERTIFICATE OF CONFORMITY’ in large, friendly letters (reminiscent of DON’T PANIC from the “Hitchhiker’s Guide to the Galaxy”). What was fascinating was that the ‘Certificate of Conformity’ had an operating speed of 3,600 rpm (expected for a pump that would operate in the United States), but checking the capacity figures showed that the suction flow was the expected suction flow in the catalog for European operation at 3,000 rpm. This certainly raises questions about the performance test supposedly documented by the ‘Certificate of Conformity’. Was it done correctly, or even done at all?

However, the delivered motor's power capability was quoted as being the right size for 3,600 rpm operation. A field check confirmed that the motor's nameplate also showed the correct power rating for 3,600 rpm. Evidently, whoever was making sure the right motor came with the pump had correctly accounted for the U.S. electric frequency.

However, the documentation certainly needed fixing, and legitimate questions needed to be asked if the performance tests that had been paid for had been run.

Lessons Learned from the Field

Machinery operating speed for internationally procured equipment should always be checked when purchased, along with the required power of motors. Not getting the power right would have led to serious performance problems. The good news for this pump was that it was correct for the service and was operating properly. The vacuum system problems were coming from other areas. I would call this case at least an indication that the company's internal checking of equipment orders should be tightened up.