General Overview

For years, many devices have been promoted that claim to provide energy savings, improve reliability, and increase operating life when used with 3-phase motors. In general these devices utilize solid-state switching techniques to control the voltage supplied to the motor. These devices fall into two basic categories:

• Parallel devices that claim energy savings and reliability improvements for motors that are not being supplied with continuous, balanced and perfectly sinusoidal voltage
• Series devices that claim energy savings for motors that often operate at less than rated load. Parallel Devices

As the name implies, parallel devices operate in a parallel connection with the 3-phase electric motor. Parallel devices claim to correct voltage unbalance and other power quality problems that may have a negative effect on electric motor operation. Voltage unbalance and harmonics are recognized as undesirable in a 3-phase motor power supply. Parallel devices connect inductive, capacitive and resistive elements in parallel between the power supply phases. They make electrical measurements of voltage, amperage and power factor to determine the combination of inductance, capacitance and resistance needed to make corrections in the electrical supply. They switch these elements in and out of the circuit to make corrections.

Claims are made that the devices will balance 3-phase voltage, filter harmonics, surges, and transients, and reduce electrical line losses by improving power factor.

Unbalanced voltage, when it occurs, is a critical problem to a 3-phase electric motor. For more information about problems associated with voltage unbalance refer to the Fall 2003 newsletter.

Short-term variations in overall voltage levels are seldom detrimental to the induction motor and have no significant effect on energy consumption, making it difficult to assign a value to their elimination, with one exception. A voltage sag or drop in the line voltage supplied to the motor, if of sufficient magnitude and duration, can cause the motor starter to trip, shutting off the motor. Avoiding a voltage sag shutdown of critical production machines may have significant productivity benefits if it is a common occurrence. However, relatively simple and inexpensive devices that can provide this voltage-sag ride-thru capability are available.

It is possible that a parallel device can improve the quality of the power delivered to an electric motor. The question is whether or not these power quality improvements provide sufficient value to justify the purchase and installation of the device. Long-term correction of voltage unbalance has the greatest potential benefit of the claims made for parallel devices.

Parallel Device Conclusions

The parallel devices claim to correct voltage unbalance and other power quality problems that may have a negative effect on electric motor operation. Voltage unbalance and harmonics are undesirable in a 3-phase motor power supply. It has been documented that these devices can reduce the I^2*R losses and improve the power factor of the system. Correcting these would provide a benefit to the induction motor. It is normally more cost-effective to correct these problems at their source within the facility rather than at the motor affected. It is also recognized that capacitive circuits in harmonic voltage environments create the possibility of damage to the circuits due to harmonic resonance.

Series Devices

Series devices connect in series with the power wiring to a 3-phase electric motor. They reduce the Root Mean Square (RMS) voltage to the motor by interrupting the motor supply voltage for a portion of the positive and negative half sinusoidal cycles by opening and closing solid-state switches. However, the switching produces harmonic distortions in the waveforms going to the motor.

The claim is made that efficiency is increased at reduced loads, when the motor does not need full voltage. Also, claims are made that the motor will last longer because it will operate at a lower temperature when the device is used.

In general, lowering motor voltage will result in a reduction in motor core losses allowing a lower operation temperature. However, the device is only useful on a motor running at very low loads, and then the device is only able to achieve small reductions in losses. The challenge for the series device is to get the energy savings from the reduction in core losses to exceed the losses caused by the distorted waveform supplied to the motor. Often the distorted waveform losses exceed the gains from reducing the voltage.

Another disadvantage of the series device is the fact that it is installed in series with the motor system, so if the device fails the motor stops operation. This adds one more device to the system, which reduces the overall reliability of the system.

Series Device Test Results and Conclusions

Testing and economic evaluation of four series devices at Advanced Energy has clearly shown that none of these would provide significant energy savings and would be a poor investment compared to other energy saving investments available. The best simple payback for the tested devices was 44 years for a device operating a standard efficiency motor at no-load. The devices actually use more energy at moderate and higher loads.

Purchasing a more energy efficient motor is a more cost-effective investment for saving energy. However, it has been documented that these devices can reduce the I^2*R losses and improve the power factor of the system.