Three-Phase Motor

What is a Three Phase Motor?

A three-phase motor is an AC induction motor that operates on a three-phase supply as opposed to a single-phase motor that requires a single-phase supply to operate. The three-phase supply current produces an electromagnetic field in the stator winding, producing torque in the rotor winding of a three-phase induction motor with a magnetic field.

Types of Three Phase Motor

Three-phase motors can be divided into three general types: squirrel cage, slip-ring, and synchronous. Only the squirrel-cage rotor motors and the wound-rotor motors are induction motors. The rotor circuit in an induction motor does not have an external power supply. Voltage is induced in the rotor windings by the revolving electromagnetic field created around the inner surface of the stator assembly when power is applied to the stator windings.

Squirrel Cage Motors

The shape of this rotor resembles that of a squirrel cage. This is why the motor is called a squirrel cage induction motor. The construction of this rotor is very simple and robust. For this reason, almost 80% of induction motors are squirrel-cage motors. The rotor consists of a cylindrical laminated iron core with slots around the outer circumference. The slots are not parallel but are angled at an angle. It helps to prevent magnetic locking between the stator and rotor teeth. It allows for smooth running and reduces hum.

The squirrel cage rotor consists of a rotor bar instead of a rotor winding. The rotor bars are made of aluminum, brass, or copper. In this type of rotor, slip rings and brushes are not used. The construction of this type of motor is, therefore, simpler and more robust.

Slip-ring Motors

Slip ring motors are also known as wound rotor motors. The rotor consists of a laminated cylindrical core with slots on the outer circumference. The rotor windings are placed in the slots.In this type of rotor, the rotor winding is wound in such a way that the number of poles of the rotor winding is the same as the number of poles of the stator winding. The rotor windings can be connected in a star or triangular shape. The end of the rotor winding is connected to a slip ring. This type of motor is therefore called a slip-ring induction motor.

When the motor is running near the actual speed, the slip ring is short-circuited by the metal ring. With this arrangement, the brushes and external resistors are removed from the rotor circuit. This reduces rotor copper losses as well as brush friction. Compared to squirrel-cage motors, the rotor construction is somewhat more complex.

Synchronous Motors

A synchronous motor is a special type of AC motor that contains a wire-wound armature rotor assembly in the form of an electromagnet, which can be separately excited by an external DC supply through slip rings on the rotor shaft. The motor is started as a three-phase AC induction motor, and once running, DC is applied to the electromagnet on the rotor assembly. The fixed electromagnetic field of the rotor assembly locks in and rotates in unison or in step (synchronized) with the phase of the alternating current which operates it.

How Does a Three Phase Motor Work?

When the motor is excited with a three-phase supply,three-phase stator winding produces a rotating magnetic field with 120 displacements at a constant magnitude which rotates at synchronous speed. This changing magnetic field cuts the rotor conductors and induces a current in them according to the principle of Faraday’s laws of electromagnetic induction. As these rotor conductors are shorted, the current starts to flow through these conductors.

In the presence of the magnetic field of the stator, rotor conductors are placed, and therefore, according to the Lorenz force principle, a mechanical force acts on the rotor conductor. Thus, all the rotor conductors force, i.e., the sum of the mechanical forces produces torque in the rotor which tends to move it in the same direction of the rotating magnetic field.

This rotor conductor’s rotation can also be explained by Lenz’s law which tells that the induced currents in the rotor oppose the cause for its production, here this opposition is rotating magnetic field. This result the rotor starts rotating in the same direction of the stator rotating magnetic field. If the rotor speed more than the stator speed, then no current will induce in the rotor because the reason for rotor rotation is the relative speed of the rotor and stator magnetic fields. This stator and the rotor field difference are called slip. This how a 3-phase motor is called an asynchronous machine due to this relative speed difference between the stator and the rotors.

How to Wire a Three Phase Motor?

Three-phase motors are more efficient than single phase motors and are commonly found in applications requiring more than 7.5 horsepower. Although the National Electric Code does not specify specific conductor colors for three-phase current, it is common to use black, red and blue wires to identify lines L1, L2 and L3 respectively. The voltage cycle of each line lags its predecessor by 120 degrees — L2 reaches its peak voltage after L1, and L3 reaches its peak voltage after L2. Two wiring configurations, Wye and Delta, indicate the wiring methods for three-phase motors. These instructions cover a dual voltage, three-phase motor, the most common type.
Step 1
Turn off the power supplying the circuit to be wired to the motor. A three-phase motor must be wired to a three-phase supply.

Step 2
Open the motor wiring box and identify the wires within. The nine wires should be labeled 1 through 9. Some motor leads are identified by color; in this case consult the motor documentation for lead identification.

Step 3
Examine the motor nameplate for wiring information. The nameplate will specify the motor voltages and may give specific wiring information. Many motors can be wired for a high and low voltage and for either Delta or Wye (sometimes called Y or Star wiring). Wire the motor for the appropriate voltage to which you are connecting the motor.

Step 4
Make all wiring connections with wire nuts of the correct size for the conductors being used and the number of conductors being connected together. If there is a neutral wire in the conduit or cable supplying the motor, it is unused for the motor’s three-phase wiring; cap it with a wire nut. For example, use a red wire nut to connect two 12-gauge wires. Hold the bare ends of the conductors together and twist on a wire nut.

Step 5
Swap any two line connections to reverse the motor rotation. For example, move supply line T1 to T2 and supply line T2 to L1 and the motor will reverse direction. You can buy motor control switches to accomplish this change.

Three Phase Motor Wiring Diagram

Single Phase Motor Vs Three Phase Motor

Single Phase Vs Three Phase Motor

Single Phase Motor

Three Phase Motor

Power Source:

Single phase power supply

Power Source:

Three phase power supply

Structure:

Single-phase motors have a simple and robust structure. These motors usually have a cage-type rotor that produces rotations. Moreover, the stator of single-phase motors has two winding; therefore, these motors are called single-phase motors. 

Structure:

The construction of a three-phase motor is complex. These motors have a cage and wound-type rotor with three-phase winding. 3-phase motors have the following types based on structure;

  • Squirrel cage induction motor
  • Slip-ring induction motor
  • Permanent magnets motors

Size: 

The single-phase motors are big in size. 

Size: 

These motors are compact in size, and their weight is also lighter than single-phase motors. 

Power Output: 

The power output and motor amps of a single-phase motor are about 230V. 

Power Output: 

The power output of 3-phase motors is above 415V. These motors have high amps and PF ratings than single-phase motors. 

Torque generation: 

These motors are not self-starting; thus, they generate very limited initial torque. They produce starting rotation via an additional power supply. 

Torque generation: 

Three-phase motors are self-starting and generate high initial torque without any additional power source. 

Working efficiency:

The power rating of single-phase motors is low and operates on single winding; therefore, working efficiency is low. 

Working efficiency:

Since these motors operate on three windings, they are highly efficient and productive with high protection class. These motors also have low backlash and malfunction rates. 

Single-phase motor price: 

Single-phase motors are economical and reliable. Their price range is also affordable for micro businesses. 

Three-phase motor price: 

Price range is an essential factor in comparing single-phase vs three-phase motors. 3-phase motors are comparatively high priced than single-phase electric motors. These motors are designed with advanced features; therefore, they are expensive. 

Single-Phase Motor Applications:

Single-phase vs three-phase motor is differentiable in terms of applications. Single-phase motors are widely used in household appliances, lightweight machinery, toys, drill machines, and compressors. 

 

Three-Phase Motor Applications: 

Three-phase motors have a diverse range of applications in high-grade and low-volume industrial applications. Some of the common applications of 3-phase motors are as follows; 

  • Chemical Industry 
  • Automobile Industry
  • Cutting, grinding, and lathe machinery
  • Manufacturing of machining tools
  • Lifting industry (escalators and cranes)
  • Rolling and pressing industry 
  • Blowers, fans, and compressors

 

How to Run Three Phase Motor on Single Phase Power?

Three-phase motors typically have over 150% more power than their single-phase counterparts. They are self-starting as they generate a rotating magnetic field. These motors do not generate vibrations and are less noisy than single-phase motors. Unfortunately, most structures are wired on single-phase power.

Although more than one phase often supplies a building, only one phase can be used at a time. This creates problems when an application requires a three-phase motor or when only a three-phase motor is available. Fortunately, there are ways in which a three-phase motor can be “tweaked” to run on a single-phase supply.

Variable Frequency Drive

The easiest way is by using a variable frequency drive (VFD). A VFD is an electrical device that controls motors that run in adjustable speeds. It consists of a rectifier, DC link capacitor and an inverter. A VFD carries out three-phase motor to single-phase power conversion by rectifying each pair of phases to DC, then inverting the DC to the three-phase output power. This not only eliminates the rush current during motor-starting but also makes the motor run from null speed to maximum speed smoothly.

VFDs are available in different rated capacities for different motors. All you need to do is connect the power supply to the VFD’s input and connect the three-phase motor to its output.

Rotary Phase Converter

Another method of running a three-phase motor on single-phase power is using a rotary phase converter (RPC). A rotary phase converter is an electrical machine that turns power from one multiphase system to another.

These converters generate clean three-phase signals from a single-phase supply through rotary motion. RPCs are far more expensive than VFDs, so it’s rarely practical to use them for motor phase conversion.

Rewinding the Motor

The final way in which a three-phase motor can be made to run on single-phase power is by rewinding the motor. This method is also known as single phasing. It involves rewinding the electric motor using capacitors. Three-phase power enters through three sine waves that are symmetrical. These waves are out of phase with each other by 120 Electrical degrees.

To convert a three-phase motor, two of its phases are connected to the supply single-phase power. A  Phantom leg is created for the third phase using capacitors. The capacitors force an offset of 90 electrical degrees between the auxiliary and main windings. In order for the current to be balanced, the capacitors used must be of the right capacity for the load. The figure below shows the circuit diagram for conversion from three-phase to two-phase using single phasing method.

How to Check Three Phase Motor with Multimeter?

A three-phase motor converts electricity into mechanical energy via an alternating current provided by three leading power wires. The electricity is fed into the interior of the motor, where it creates a magnetic field that pushes the strator and makes it rotate, turning the motor shaft. Three-phase motors require testing with a multimeter during installation and inspections. The multimeter detects the flow of electricity and determines whether it is correct, as the wrong alternating current pattern can cause the motor strator to move the wrong way and malfunction.

Select the Phase Rotation setting on your mutlimeter.

Inspect the three-phase motor and look for terminals — where three wires connect to the motor — labeled L1, L2 and L3. Connect the meter jacks that are similarly labeled (L1, L2, L3) to the power wires.

Observe the display on your multimeter. The display will read “OK” if the power feed is flowing in the correct direction. If it reads “ER”, this means that the power feed has been reversed, which causes the motor to turn in the wrong direction.

Fix the reversed power feed by first turning off the motor and unplugging it. Switch the position of two of the wires — the order doesn’t matter — then reattach the mulitmeter leads to the wires. Turn on the motor. The mulitmeter should read “OK.” This means that the 3-phase motor is turning in the correct direction and running correctly.

How to Test a Three Phase Motor with a Megger?

The megger is the brand name of the insulation meter. The megger consist a DC generator & Ohm meter.

The quality of the insulation plays a very vital role in the hassle-free operation of electrical equipment. The insulation deteriorates with changes in the operating temperature, environment conditions, aging, etc. Therefore, it is a must to check the insulation quality periodically to ensure the healthiness of the electrical equipment.

Insulation Resistance Test of the Motor

Ensure the following safety checks before performing megger test of an induction motor

  • We must first check the meggar leads for any physical damage.
  • Check the continuity of the megger lead. We can check the continuity of leads while megger is ON. The zero resistance shows the leads are not open.
  • Wiring of the test set up must be OK.
  • Check tightness of all the connections.
  • Barricade the area where megger test is being performed.

Steps of Performing Megger Test on Motor

Step-1

Disconnect the motor from the supply source, and discharge the stator by shorting all the terminals with the ground.

Step-2

Connect the megger leads between one stator terminal and the motor body.

Step-3

Apply twice the motor operating voltage between one phase and the motor body. If the Motor rated voltage is 440 volts, apply voltage 880 volts. We should test the older motor at less voltage( about 80% of the twice operating voltage).

Step-4

Set the Megger voltage as defined in step no.3 & press the test button on megger. Note down the insulation resistance value.

Step-5

If the megger reads zero, it shows that winding is earth. Note that if the Megger value is zero, it means any of the three winding of stator may be earth. The motor stator is in star or delta configuration. Therefore, there is no need to test the Megger value (IR value) at different points of the connection terminal.

Step-6

If the insulation resistance value is high. if 440 volts motor has an IR value of more than 1 Mega-ohm and above, it means motor winding insulation is good.

Step-7

After the megger test of the motor, We must connect the winding to the earth in order to discharge the build-up voltage of the winding.

This is the way we can test a three-phase motor with a megger.

Purpose of Insulation Resistance Measurement

The insulation resistance or megger value of three-phase motor winding depends on many parameters. We know that the insulation quality deteriorates with dirt, moisture, corrosion & weather condition. Therefore, we must carry out the megger test periodically.

The periodic measurement of the megger value( IR Value) is a must for enhancing the useful life of the motor. The motor megger value can be improved if it is known before.

The three-phase motor winding can be re-varnished with suitable insulation material to increase the IR value.

In other words, the preventive maintenance of the motor and measurement of insulation resistance using megger enhance the life of the motor.