Servo Voltage Stabilizer

Servo Voltage Stabilizer

A servo voltage stabilizer is a closed-loop control mechanism which serves to maintain balanced 3 or single-phase voltage output despite fluctuations at the input owing to unbalanced conditions. Most of the industrial loads are 3 phase induction motor loads and in a real factory environment, the voltage in 3 phases is rarely balanced. Say for example if the measured voltages are 420, 430, and 440V, the average is 430V and the deviation is 10V.

The Percentage of unbalance is given by

(10V X 100)/ 430V = 2.3% It is seen that 1% voltage unbalance will increase the motor losses by 5%.

Thus voltage unbalance can increase the motor losses from 2% to 90% and hence temperature also raises by an excessive amount which results in further increased losses and reduced efficiency. Hence it is proposed to take up a project to maintain a balanced output voltage in all 3 phases.

Single Phase:

It is based on the principle of the vector addition of A.C voltage to the Input to get the desired output using a transformer called Buck-Boost transformer(T), the secondary of which is connected in series with the input voltage. The primary of the same is fed from a motor mounted variable transformer(R). Depending upon the ratio of primary to secondary voltage, the induced voltage of the secondary comes either in-phase or out of phase based on the voltage fluctuation. The variable transformer is usually fed from the input supply at both ends while tapping at around 20% of the winding is taken as a fixed point for the primary of the Buck-Boost transformer. The variable point of the auto-transformer,  therefore, is capable of delivering 20% out of phase voltage which is used for bucking operation while 80% which is in-phase with the input voltage and is used for boosting operation. The wiper movement of the variable transformer is controlled by sensing the output voltage to a control circuit that decides the direction of rotation of synchronous motor fed through a pair of TRIACs to its split phase winding.

3 Phase Balanced Input Correction:

For low capacity operation say about 10KVA, it is presently seen that a double wound variac is used eliminating the Buck-Boost transformer on the variable transformer itself. This restricts the wiper movement of a variac to 250 degrees as the balance is used for the secondary winding. Though this makes the system economical, it has serious drawbacks in terms of its reliability. The industry standard never accepts such a combination. In areas of reasonably balanced input voltage, three-phase servo-controlled correctors are also used for stabilized output whereas a single three-phase variac is used mounted by one synchronous motor and single control card sensing the two-phase voltage out of three. This is much more economical and useful if input phases are reasonably balanced. It has the drawback that while severe unbalancing takes place the output is proportionally unbalanced.

3 Phase Unbalanced Input Correction:

Three series transformers (T1, T2, T3), each second of which is used, one in each phase that either adds or subtracts the voltage from the input supply voltage to deliver constant voltage in each phase thereby making the balanced output from unbalanced input. The input to the primary of the series transformer is fed from each phase from one each variable autotransformer (Variac) (R1, R2, R3) each of whose wiper is coupled to an ac split phase (2 Coils) synchronous motor(M1, M2 M3). The motor receives ac supply for each of its coils through thyristor switching for either clockwise or anti-clockwise rotation to enable desired output voltage from the variac to the primary of the series transformer, either in phase or out of phase, to perform addition or subtractions as required at the secondary of the series transformer to maintain a constant and balanced voltage at the output. Feedback from the output to the control circuit (C1, C2, C3) is compared with a fixed reference voltage by level comparators formed out of op-amps to ultimately trigger the TRIAC as per the need for actuating the motor.

 This scheme mainly consists of a control circuit, 1single phase servo induction motor coupled to a variac feeding primary of a series transformer for each phase.

  • Control circuit comprising of a window comparator wired around transistors and RMS error signal voltage amplification by IC 741 is rigged up in Multisim and is simulated for various input operating conditions ensuring firing of the TRIACs that would operate the capacitor phase-shifted induction motor is required direction that controls the rotation the variac wiper.
  • Based on the maximum and minimum values of voltage fluctuations, series transformer and the control transformers are designed using standard formula matching to the commercially available iron core and super enameled copper wire size before winding the same for use in the project.

In a balanced 3 phase power system, all the voltages and the currents have the same amplitude and are phase-shifted by 120 degrees from each other. However, it is not possible practically as unbalanced voltages can result in adverse effects on equipment and the electric distribution system.

Under unbalanced conditions, the distribution system will incur more losses and heating effects, and be less stable. The effect of voltage unbalance can also be detrimental to equipment such as induction motors, power electronic converters, and adjustable speed drives (ASDs). A relatively small percentage of voltage unbalance with three-phase motor results in a significant increase in motor losses, which entails a decrease in efficiency as well. Energy costs can be minimized in many applications by reducing the motor wattage lost because of voltage unbalance.

Percentage Voltage Unbalance is defined by NEMA as 100 times the deviation of the line voltage from the average voltage divided by the average voltage. If the measured voltages are 420,  430, and 440V, the average is 430V and the deviation is 10V.

The Percentage Unbalance is given by (10V * 100 / 430V) = 2.3%

Thus 1% voltage unbalance will increase the motor losses by 5%.

Hence Unbalance is a serious power quality problem, mainly affecting low-voltage distribution systems and it is therefore proposed in the project to maintain balanced voltage concerning magnitude in every phase, thus maintaining balanced line voltage.


A.C. Voltage stabilizers are meant for obtaining a stabilized a.c. supply from the fluctuation incoming mains. They find applications in every field of Electrical, Electronic, and many other Industries, Research institutions Testing Laboratories, Educational Institutions, etc.

What is unbalance:

Unbalance condition refers to the condition when the 3 phase voltages and currents do not have the same amplitude nor the same phase shift.

If either or both of these conditions are not met, the system is called unbalanced or asymmetrical. (In this text, it is implicitly assumed that the waveforms are sinusoidal and thus do not contain harmonics.)

Causes of unbalance:

The system operator tries to provide a balanced system voltage at the PCC between the distribution grid and the customer’s internal network.

The output voltages in the three-phase system depend on the output voltages of the generators, the impedance of the system, and load current.

However since mostly synchronous generators are used, the generated voltages are highly symmetrical and so the generators cannot be the cause of unbalance. Connections at lower voltage levels usually have high impedance leading to potentially larger voltage imbalance. The impedance of the system components is affected by the configuration of overhead lines.

Consequences of voltage unbalance:

The sensitivity of electrical equipment to unbalance differs from one appliance to another. A short overview of the most common problems is given below:

(a) Induction machines:

These are the a.c. synchronous machines with internally induced rotating magnetic fields, whose magnitude is proportional to the amplitude of direct and/or inverse components. Hence in the case of an unbalanced supply, the rotating magnetic field becomes elliptical instead of circular. thus induction machines mainly face three kinds of problems due to voltage unbalance

1. Firstly, the machine cannot produce its full torque as the inversely rotating magnetic field of the negative sequence system produces a negative braking torque that has to be subtracted from the base torque linked to the normal rotating magnetic field. The following figure shows the different torque slip characteristics of an induction machine under unbalanced supply

Induction Machine Characteristics

2. Secondly, the bearings may suffer mechanical damage because of induced torque components at double system frequency.

3. Finally, the stator and, especially, the rotor are heated excessively, possibly leading to faster thermal aging. This heat is caused by the induction of significant currents by the fast rotating (in the relative sense) inverse magnetic field, as seen by the rotor. To be able to deal with this extra heating, the motor must be de-rated, which may require a machine of a larger power rating to be installed.


The voltage imbalance can cause premature motor failure, which not only leads to unscheduled shut down of the system but also causes great economic loss.

The effects of low and high voltage on motors and the related performance changes that can be expected when we use voltages other than those noted on the nameplate are given as follows:

Effects of low voltage:

When a motor is subjected to voltages below the nameplate rating, some of the motor’s characteristics will change slightly and others will change dramatically.

The amount of power drawn from the line has to be fixed for a fixed amount of load.

The amount of power the motor draws has a rough correlation to the voltage to current (amps).

To keep the same amount of power, if the supply voltage is low, an increase in current acts as compensation. However, it is dangerous as higher current causes more heat to build up in the motor, which eventually destroys the motor.

Thus the disadvantages of applying low voltage are overheating of the motor and the motor is damaged.

The starting torque, pull-up torque, and pullout torque of major load (induction motors), based on the applied voltage squared.

Generally, a 10% reduction from the voltage rating can lead to low starting torque, pull up, and pull out torque.

Effects of high voltage:

High voltage can cause the magnets into saturation, causing the motor to draw excessive current to magnetize the iron. Thus high voltage can also lead to damage. High voltage also reduces the power factor, causing an increase in the losses.

Motors will tolerate certain modifications in the voltage above the design voltage. When the extremes above the design voltage will cause the current to go up with corresponding changes in heating and a shortening of a motor lifetime.

The voltage sensitivity affects not only motors but also other devices. The solenoids and coils found in relays and starters tolerate low voltage better than they do high voltage. Other examples are ballasts in fluorescent, mercury, and high-pressure sodium light fixtures and transformers and incandescent lamps.

Overall, it’s better for the equipment if we change the taps on incoming transformers to optimize the voltage on the plant floor to something close to the equipment ratings, which is the main concept behind the proposed concept of voltage stabilization in the project.

Rules to decide the supply voltage

  • Small motors tend to be more sensitive to over-voltage and saturation than do large motors.
  • Single-phase motors tend to be more sensitive to over-voltage than do 3-phase motors.
  • U-frame motors are less sensitive to over-voltage than are T-frames.
  • Premium efficiency Super-E motors are less sensitive to over-voltage than are standard efficiency motors.
  • 2 and 4-pole motors tend are less affected by high voltage than are 6- and 8-pole designs.
  • Overvoltage can drive up amperage and temperature even on lightly loaded motors
  • Efficiency is also affected as it gets reduced with low or high voltage
  • The power factor reduces with high voltage.
  • Inrush current goes up with higher voltage.

Get more knowledge on various electronic concepts and circuits by doing some mini electronics projects at the engineering level.

Comments are closed.