“The Hidden Cost of Voltage Imbalance”

What is Voltage Imbalance?
Ideal three-phase power supplies the exact same voltage to each of the three terminals on the motor. The three-phase power that is commonly produced by a rotary phase converter is seldom perfectly balanced due to variations of the load on the system. Typically each leg will have a slightly different voltage on it, and this leads to a condition known as voltage imbalance.

What is the Cost of Operating a Motor with a Voltage Imbalance?
Voltage imbalance is bad for a three-phase motor in two different ways. First of all, the unbalanced voltage robs the motor of its efficiency, which wastes energy and reduces the amount of work the motor can provide. Secondly, voltage imbalance reduces the life of the motor because an imbalanced voltage causes an increase in the winding temperature of the motor. The winding temperature increase reduces the life of the insulation that is required to keep the motor operational.

The Effect of Voltage Imbalance on Motor Energy Efficiency (Operating at 100% of Rated Load)

Voltage Imbalance 0% 1% 2% 3% 3.5% 4%
Efficiency Reduction 0% 1% 2% 3% 3.5 4%


The Effect of Voltage Imbalance on Motor Life (Operating at 100% of Rated Load)

Voltage Imbalance 0% 1% 2% 3% 3.5% 4%
Increase in Winding Temp. 0 2 8 18 24.5 32
Percent Reduction of Motor Life 0% 13% 42.5% 71% 82% 89%

Real World Example
To help illustrate the impact that a seemingly small thing like voltage imbalance can have on the cost of your grain management system, consider the following scenario. The typical three-phase motor is around has a lifespan of around 20,000 hours when it is running at 100% of its rated load on ideally balanced three-phase power. If this motor is used 672 hours a year, which equates to 24 hours a day of use for 4 weeks total over the year, then the expected life of that motor on balanced three-phase power would be around 30 years. However, if the motor is running on a power supply with a voltage imbalance of 3.5%, a level that is not uncommon on systems using oversized rotary phase converters, then the life of the motor would be shortened to less than 6 years. Essentially, the motor would only last a fifth as long as it would have if it had been operating on balanced three-phase power.

Voltage Imbalance 0% 3.5%
Hours of Use Per Year 672
Expected Motor Life 30 Years 6 Years

It is also important to consider that when an important piece of your grain management system fails, there is an array of costs associated with it that extends beyond just the cost of purchasing a new part. There are removal and installation fees which, if the motor is significantly high off the ground like it is in a grain leg, could be quite substantial. There is also the down-time cost associated with the fact that your system is unable to operate during the time that it is being fixed. The amount of down-time required to fix the problem will vary. Depending on the severity of the problem it could just be a matter of days or instead it could be weeks. A delay of this length during the peak of harvest season could be devastating to the grain management capabilities of an agricultural operation.

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