Chuck Yung 
EASA Technical Support Specialist 

In the world of three-phase electric motors, one area which seems to cause great confusion is the use of electric motors which are rated for more than one voltage. Especially today, with so much international commerce, it is understandable that different meanings might be assumed for this simple term. 

Those readers in the U.S. are ac­customed to “dual-voltage” 230/460v ratings. The 1:2 ratio lends itself to 9-lead windings, with connection combinations such as 1- and 2-circuit wye, 2 and 4-delta, 3 and 6-wye, etc. The common factor is that the circuits and the possible operating voltages have the same 1:2 ratio.

The rest of the world is accus­tomed to the wye/delta dual-voltage system, which is delta-wye 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. 

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

Relationship between voltage and frequency 
The relationship between voltage and frequency has been well described (see CURRENTS, August 2002), and most readers understand that a 380v 50 Hz winding is equivalent to a 460v 60 Hz winding: 
380v x 60/50 = 456v 

In other words, a 380v 50 Hz mo­tor, applied to a 460v 60 Hz power supply, will operate at the same flux and therefore produce the same torque. Since it is rotating 20% faster (60/50), the output (meaning the hp or kW rat­ing) increases by 20%. 

It is understandable that a person might look at the 380v 50 Hz / 460v 60 Hz issue, and feel safe in sending a 230/400v 50 Hz motor to operate in a 60 Hz environment. As long as the motor is connected wye, and operates at 460v 60 Hz, it produces the torque. 

Problems arise 
Now the problem begins. The sup­plier might produce a new nameplate with a confusing array of voltages and frequencies. In many cases, the im­plication is that the motor can operate at 380-410v 50 Hz, 460-480v 60 Hz, and 230v 50 or 60 Hz. 

The problem arises when that original 6-lead motor is operated at 230v 60 Hz. 

Using the same volts/Hz ratio, 230v 50 Hz is equivalent to 276v 60 Hz: 
230v x 60/50 = 276v 

That means the 230v 50 Hz delta connection can operate successfully at 276v 60 Hz. In too many cases, some­where between the manufacturer and the user, someone decided that a 230v 50 Hz winding would work at 230v 60 Hz. 

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. 

Given that the motor speed is frequency-dependent, the horsepower increases by 20% (the ratio of 60 Hz / 50 Hz). But since the motor is operat­ing well below the correct voltage, the motor only delivers 83% of rated horsepower. 
[(230/276)²] x 60/50 = .83 

Why is this problem not recognized? 
In industrial applications, more dual-voltage motors operate on 460v rather than 230v. That reinforces the perception that an IEC 230/380v 50 Hz motor can be operated successfully on 230/460v 60 Hz systems. 

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. 

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. 

Any time a 6-lead dual-voltage mo­tor is labeled for 230/460v use, we as repairers need to ask the customer what voltage they are operating at. If they are using the motor on a 230v 60 Hz sys­tem, it should be connected with 9 leads for 230/460v. For example, if the motor was labeled for 230/380v 50 Hz opera­tion, and the connection was a 2-circuit wye/delta, change the connection to a 2-and 4-wye and correct the nameplate. Better still, use EASA’s “AC Motor Verification and Redesign Program” to make the changes, and verify the densi­ties while doing so. 

Even more confusing 
If the nameplate is marked with myriad voltages, it gets even more confusing. We tend to assume that the manufacturer understands everything about electric motors. But the name­plate markings may have been dictated by the OEM purchasing the motor for international use, rather than by the mo­tor manufacturer. There may not have been engineering input into the decision as to what was put on the nameplate. 

The principles that cause an electric motor to work are straightforward. The volts/Hz ratio is one of those im­mutable facts that cannot be changed. Likewise, that torque is proportional to the square of the flux density. And that there is a linear relationship between torque and horsepower. 

If a motor is labeled for wye-delta connection and is marked for dual voltages, the ratio of the voltage must follow that square root of 3 relationship. 

Great customer service 
By contacting your customer when you recognize a conflict between the voltage rating and connection method, you do several good things: 

• Identify a potential problem 
• Prevent future failures 
• Reinforce customer perception of your expertise 
• Educate the customer about future motor purchases and repairs 

In the process, you will also find some customers who have installed transformers to step down their 460v service to 380v, not recognizing that the volts/hertz ratio is important. The more problems you solve for a cus­tomer, the more valuable you become to that customer. And that works in your favor when a competitor visits him with fresh donuts. Don’t get me wrong; I like donuts. But if a custom­er has to choose between donuts and solutions to his problems, most of the time he will choose solutions. 

Categories: Technical topics, AC motors, Stators/windings, Winding connections

Tags: Winding connections

Rate this article:

No rating

Print