Phase Converter Alternatives

The three phase system consists of phases that are 120 degrees apart from each other and it has three wires and one neutral wire (wire grounded to earth).

That is why, it is called three phase power supply. But, why should we convert single phase system to a three phase system and, why do we need phase converter to do this? The answers is discussed here.

◈ Loss of power is low in a three phase system as compared to a single phase system.

◈ The 120 degree phase difference in a three phase power system gives a balanced load.

◈ Less heat is generated due to no frequent switching effects in three phase, whereas there is a switch connected to the secondary winding of single phase system, which results in frequent switching and reduces the motor life.

◈ Single phase systems cannot handle the heat generated due to high input horsepower (HP). HP is a unit used in the measurement of power, and it is appropriately equal to 750 watts.

The slip between the magnetic field created by the positive voltage source and the speed of the motor is low. But, whereas, if the source is negative, the slip is high. The formula for slip is:

Slip = (n_s - n_r) / (n_s)

where, n_s is the synchronous speed which is (120 + f) / p, f is frequency and p is number of magnetic poles per phase. n_r is the rotating speed of the rotor inside the motor and it is measured in rpm (rotation per minute) unit.

This device consists of a motor, which has three windings inside it. Two of the windings, act as a motor and the third winding acts as a generator. The need for a generator is to generate a balanced power in the machine circuit.

The motor generator is also used for phase conversions (AC to DC or DC to AC), 'DC' stands for direct current and 'AC' stands for alternating current. This device has a shaft near the input voltage source, to detect the variations in the voltage, so as to prevent the machine from damage due to high voltage.

A three phase motor and large number of capacitors constitute a single, small device which looks like a big capacitor, in RPC. The three terminals of the motor are connected to the output of this device. The motor used in the device has the capacity to withstand a higher HP than the output power expected.

This is a recent technology that uses microprocessor programming to control the solid state devices used in the phase converter. The input rectifier (device that converts AC to DC) of this device consists of an IGBT (Insulated Gate Bipolar Transistors), which is connected in series with an inductor.

The process of conversion to three phase power, in this device is called double IGBT conversion, as the main component used is IGBT, which is a fast switching semiconductor device operating at 10,000 Hz. In the conversion process, the AC power is converted to DC and then back to AC by IGBT. IGBT does not directly produce an AC signal.

It produces a PWM (Pulse Width Modulation Wave), which is passed through the capacitor or an inductor, because it is a fact that, capacitors block DC and allow only AC.

Digital Signal Processor (DSP) is the microprocessor used in the device, which continuously monitors the input voltage to produce a balanced three phase power. If the input voltage is considerably high, the DSP switches out, to prevent the damage due to high voltage.

In this device, the frequency or speed of the AC induction motor is controlled by the variable frequency drive (VFD). The VFD sends a modulating signal to the AC motor, thereby controlling its speed. Variable frequency drive is also known as variable speed drives, as it controls the speed of the AC motor.

This device is commonly used in fans, pumps, etc., for efficient power consumption, smooth start-up and to save electricity. Ramping is an important factor in the functioning of the motor. The VFD sets the ramp up time, reference time and ramp down time of the motor, to reduce the mechanical wear of the device using VFD.

Ramping is very important because sudden acceleration from the switch on time to the operating speed or sudden decline from the operating speed to the device off time, will cause damage to the VFD operating devices. So, to avoid this, ramping is necessary.

This device consists of capacitors and a relay switch to convert single phase to three phase. For continuous generation of three phase power, static converters are not preferred. It can be used only for small motors and low HP input.

The main drawback of this device is the inability to control the heat generated in it and it can produce only two-third torque.

Out of the aforementioned alternatives, RPCs are mostly preferred because of its efficiency, long life, low cost and continuous power supply. After all, these are the most common parameters, which people demand in the devices they use.