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4:24 pm
March 14, 2011
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Dual Voltage: Twice As Much To Go Wrong?

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The issue is not as simple as you might have thought. Here’s what the experts have to say about dealing with voltage ratios and wye/delta connections.

In the world of three-phase electric motors, the use of motors with more than one voltage rating seems to cause great confusion. Especially today, with so much international commerce, misunderstanding of the simple term “dual voltage” often leads to premature motor failures.

Maintenance professionals in the U.S. are accustomed to “dual-voltage,” 230/460-volt ratings. This 1:2 ratio lends itself to 9-lead windings that are connected externally as either 1 & 2-Y or 1 & 2-delta. The high-voltage (460v) connections are the same for both; the 230v connections are not. Internally, the winding connection can vary, but always with that 1:2 ratio between circuits. The common factor is that the circuits and possible operating voltages have the same 1:2 ratio.

Using two circuits at 230 volts produces the same magnetic flux density—and therefore the same torque—as using one circuit at 460 volts:

230v x 2 circuits = 460v x 1 circuit

That makes off-the-shelf electric motors more versatile. (We use 60 Hz power in North America, Brazil and parts of Japan—albeit not all use the same voltage systems.)

The rest of the world uses 50 Hz power, and is accustomed to the Y/delta dual-voltage system, which is wye-delta for a ratio of 1:1.732. Common voltage ratings for these machines are 220/380 and 230/400. The same ratio applies to the medium-voltage (2300v delta/4000v wye) machines in North America. The ratio is derived from the square root of three (√3) and results in the Y-connected motor producing the same torque at 400v as it would when connected delta for 230v.

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It is understandable that a person accustomed to one system might not realize the differences in the other. Based on reported winding failures, that is indeed the case. Let’s look at the reason for the confusion, and why many motors successfully operate on the other system­—the one they were not designed for.

North America generates and uses 60 Hz (Hertz or cycle) power, while most of the rest of the world uses 50 Hz power. The history behind this is fascinating, but that’s for another article. Let’s just say there used to be a lot of competing frequencies, and like Apple versus PC, both have advantages, disadvantages and loyal followers.

The relationship between voltage and frequency is straightforward: The volts/Hertz ratio must be constant to produce the same torque. The speed of the motor (rpm) is proportional to the frequency, so a motor operating at 60 Hz turns 1.2 times as fast at it would on 50 Hz power (50 x 1.2 = 60):

60/50 x 50 Hz voltage =
the 60 Hz voltage that will produce the same torque

For example, 400v x 60/50 = 480v—meaning that a 400v 50 Hz winding is equivalent to a 480v 60 Hz winding. In other words, a 400v 50 Hz motor, applied to a 480v 60 Hz power supply will operate at the same magnetic flux density and, thus, produce the same torque. Since it is rotating 20% faster (60/50), however, the output power (measured as Hp or kW) also increases by 20%.

Understandably, someone might look at the 400v 50 Hz/480v 60 Hz issue and feel safe in using a 230/400v 50 Hz motor on a 60 Hz system. After all, when the motor is connected wye, and operated at 460v 60 Hz, it produces the correct torque. So what’s the problem?

0311easa1Fig. 1. A motor nameplate like the one shown here might imply that a unit can operate at all listed voltages and frequencies.

Consider what happens when a supplier produces a nameplate with an array of voltages and frequencies like those shown in Fig. 1. Here, the implication is that the motor can operate at 380-410v 50 Hz, 460-480v 60 Hz, and 230v 50 or 60 Hz. There is a logical ratio between the 50 Hz and 60 Hz “high voltage” rating, but what about that 230v rating for both frequencies?

When that original 6-lead, 230v 50 Hz motor is operated at 230v 60 Hz, it is likely to fail prematurely. That’s because the equivalent 60 Hz voltage would actually be 276v. Using the same volts/Hz ratio as before:

230v x 60/50 = 276v

That means the 230v 50 Hz delta connection can operate successfully at 276v 60 Hz—not at 230v. In too many cases, somewhere between the manufacturer and the motor user, someone decided that 230 volts is 230 volts, regardless of frequency. They assumed that a 230v 50 Hz motor would work just as well on a 230v 60 Hz system.

But a motor operating at lower-than-rated voltage experiences a reduction in torque proportional to the square of the ratio of the applied voltage to the rated voltage. When delta-connected for 230v 50 Hz, but operating at 230v 60 Hz, the motor produces only 70% of rated torque.

Since the motor is operating at 60 Hz instead of 50 Hz, it rotates 1.2 times as fast, offsetting some of the lost power. But 1.2 x 0.7 is still only 0.83, so the motor only delivers 83% of rated Hp.

Under the radar
In North American industrial applications, more dual-voltage motors operate on 460v than 230v. We’ve seen that 400v 50 Hz is equivalent to 480v 60 Hz, so a motor will operate on either system and produce the same torque. It is only when the motor is connected for low voltage that it produces less torque.

Because many motors operate below their nameplate rating (Hp/kW), the 230v 50 Hz motor operating on 230v 60 Hz power may not be so drastically overloaded as to fail immediately. That reinforces the notion that an IEC 230/400v 50 Hz motor can be operated successfully on 230/460v 60 Hz systems.

When only the motors operated at 230v 60 Hz lose torque, and only the ones sized for the full load torque requirement fail, it is easy to understand why those responsible for application considerations do not recognize the problem.

The lesson here is to consider the operating voltage any time a 6-lead, dual-voltage motor is labeled for 230/460v use. If the motor is being used on a 230v 60 Hz system, it should be connected with 9 leads for 230/460v. For example, if a 6-lead motor is labeled for 230/400v 50 Hz operation, the connection is a Y/delta. To operate successfully on 230/460-volt systems, the winding connection must be changed to a 9-lead, dual-voltage connection. An EASA service center can raise the winding connection and reconnect the motor for this purpose.

If the nameplate lists multiple voltages, it gets even more confusing. People often assume that motor manufacturers know everything about their products, so they accept nameplate data at face value. But the nameplate markings may have been dictated by an equipment manufacturer who is buying the motor for international use—with little or no input from motor design engineers. Manufacturing groups like the National Electrical Manufacturers Association (NEMA) have done a tremendous job of standardizing frame sizes and ratings, but the internationalization of voltage and frequency ratings has added confusion to the mix.

Summing it up
If you remember these basics, you can head off a lot of application problems and make your life easier.

  • Volts/Hz ratio must be constant to produce same torque.
  • Torque is proportional to the square of the magnetic flux density.
  • At constant frequency, the effect of voltage on torque is squared.
  • There is a linear relationship between torque and Hp (kW).

If a motor is labeled as a 6-lead, wye-delta connection, and is marked for dual voltages, the ratio of the voltages must follow that square root of three ratio.

Contact a qualified service center when you recognize a conflict between the voltage rating and connection method. Sharing this knowledge with your staff is a force multiplier that’s likely to head off other potential problems—the type that could lead to late-night emergency phone calls. MT


 

Chuck Yung is a senior technical support specialist with the Electrical Apparatus Service Association (EASA), in St. Louis, MO. Telephone: (314) 993-2220. EASA is an international trade association of more than 1900 firms in 56 countries that sell and service electrical, electronic and mechanical apparatus. Web: www.easa.com

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