Every 3 phase motor has six (6) terminals with the supply voltage connected to three (3) of those terminals. The most common configuration of a three-phase  motor is the Delta (?) � Star (Wye) configuration with the Delta side connected to supply voltage. The terminal configuration of a 3 phase motor is shown below:

The W2U2V2 terminal set is the star side of the 3 phase motor while the U1VIW1 is the Delta side of the motor connected to the supply voltage.

The 3 phase motor is a rugged piece of equipment but as with everything man made, there comes a time when this beautiful piece of machinery fails either due to old age, misapplication, mal-operation or any other adverse cause.

The most common failure mode of a 3 phase AC motor is burnt winding or shorted winding leading to the damage of the motor. Often it is required to test the winding of the 3 phase windings with the aid of a multimeter or ohmmeter to determine whether the motor is still good or burnt or shorted.

To determine whether a 3 phase motor is still good or has gone bad, a simple ohmmeter test across the windings of the motor will reveal its true state of health. As shown below, the indicated terminal matrix (blue lines) shows the way the windings of a 3 phase motor should be tested with an Ohmmeter:

The first thing to do before testing the windings of the motor is to remove the links linking terminals W2U2V2 and the disconnect the motor from supply (L1, L2, L3). A multimeter terminals placed across this matrix of terminals will indicate the following readings for a good 3 phase motor:

(a) Terminals W1W2, U1U2, V1V2 will indicate continuity for a good motor

(b) Every other terminal combinations should indicate Open for a good motor

(c) Readings between any of the six (6) terminals and the motor frame signifying earth     

    (E) should indicate open for a good motor.

In the case of a burnt or bad 3 phase motor, this matrix of terminals should indicate  the opposite readings for a bad motor:

(a) If any of the terminal combinations W1W2, U1U2, V1V2 should indicate open then 

     the motor is bad.

(b) If any other terminal combinations should indicate continuity instead of open, then            

     the motor is bad.

(c) If the reading between any of the six (6) terminals and motor frame (E) should  

     indicate continuity, then the motor is dead.

To be able to apply an electric motor successfully in a hazardous environment, these motors must possess certain critical characteristics that make them suitable to operate in these environment without creating problems.

The table below gives the temperature codes for motors operating in hazardous environment according to NEC/CEC  & IEC letter designations are given in the table below:

The diagram below shows all the basic parts that makes up an AC induction motor. These basic parts work together to ensure the smooth performance of this highly efficient electrical device:

As shown in the diagram above, the parts 1, 2 and 8 makes up the stator of the three-phase Induction motor:

1 � Motor Frame. This is the supporting structure of the motor assembly; manufactured        

     of  iron, steel, die-cast Aluminium, resistant to corrosion and with cooling fins.

2 � Lamination Core. This part is constructed with magnetic steel plates and houses the 

     motor windings.

8 � Three - phase Windings. This comprises three equal sets of coils, one se set for each 

     phase, forming a balanced three-phase system when connected to a three-phase        

     power supply.

The rotor of a three-phase squirrel cage Induction motor consists of a set of non-insulated bars that are interconnected by short-circuit rings. What characterizes an induction motor is the fact that only the stator is connected to the power supply. The rotor is not power supplied externally and the currents that flow through it are induced electromagnetically by the stator from which comes the induction motor name.

The parts 3, 7 and 12 makes up the rotor of the three-phase motor:

7 � Shaft. It transmits the mechanical output developed by the motor.

3 � Laminated Magnetic Core. The rotor laminations have the same characteristics of the       

     stator laminations.

12 � Rotor Bars and Short Circuit Rings. These are Aluminium die castings formed as one 

       piece. They enable the rotation of the motor through induction of electromagnetic 

       current 

Below are listed other critical parts of the three-phase squirrel cage induction motor:

4 � End Shields

5 � Fan for cooling motor

6 � Fan Cover.

9 � Terminal Box. This houses the electrical terminals of the motor.

10 � Motor terminals

11 � Bearings. These support the rotation of the rotor.

To prolong the life of electrical systems and motors, regular insulation resistance testing is required. Over the years, after many cycles of operation, electric motors are exposed to environmental factors such as dirt, grease, temperature, stress, and vibration. These conditions can lead to insulation failure, resulting in loss of production or even fires.

An effective motor insulation resistance system has high resistance, usually (at an absolute minimum) greater than a few mega ohms (M?).  A poor insulation system has lower insulation resistance. The optimal insulation resistance for an electric motor is often determined by the manufacturer�s specifications, the criticality of the application where the motor is used, and the environment where it is located.

It is practically impossible to
determine rules for the actual minimum insulation resistance value of an electric motor because resistance varies according to method of construction, condition of insulation material used, rated voltage, size and type. A general rule-of-thumb is 10 Megohm or more. The insulation system of an electrical motor is said to be in good condition if:

Measured Insulation resistance is greater than or equal to 10M?

Typical Insulation Resistance Level for Electric Motors
There are no rules for determining the minimum insulation resistance value for a motor. Most data available are empirical. The ones listed below are from grundfos, a leading manufacturer of electrical motors:

Insulation Resistance Level	Insulation Level
2M? or Less	Bad
2 - 5M?	Critical
5 - 10M?	Abnormal
10 - 50M?	Good
50 - 100M?	Very Good
100M? or More	Excellent

How to Measure Insulation Resistance of a Motor
The measurement of insulation resistance is carried out by means of a mega ohmmeter � a high resistance range ohmmeter. To measure insulation resistance, a DC voltage of 500V or 1000V is applied between the windings and the ground of the motor as shown below:

During the measurement and immediately afterwards, do not touch any terminals of the motor as some of them carry dangerous voltages that may be fatal.
The minimum insulation resistance of motor measured to ground at 500V can be measured at a winding temperature of  -15�C � 20�C. The maximum insulation resistance can be measured at 500V with operating windings temperature of 80-120�C depending on the motor type and efficiency

How to Calculate Minimum Insulation Resistance of Motors
The minimum insulation resistance of any motor, Rmin, can be calculated by multiplying the rated voltage, VR, with the constant factor 0.5 M? /kV:

Regular Checks of Motor Insulation Resistance The key to prolonging the life of any electrical device is periodic checks & maintenance. The insulation resistance of stored and active motors should be checked regularly:
(a)  If the insulation resistance of a new, cleaned or repaired motor that has been stored for some time is less then 10 M?, the reason might be that the windings are humid and need to be dried.
(b) For a motor in operation, the minimum insulation resistance may drop to a critical level. If the measured value of insulation resistance is greater than the calculated value of minimum insulation resistance, the motor can continue to run. However, if it drops below this limit, the motor has to be stopped immediately to prevent harm to personnel due to the high leakage voltage

Rated Speed
The speed of operation of an AC motor when fully loaded at rated voltage is called the rated speed. It is usually given in RPM (Revolutions per minute) on an electric motor nameplate. The rated speed is the speed of the rotor.

Slip
The speed of the rotor magnetic field in an induction motor lags slightly behind the synchronous speed of the changing stator magnetic field. This difference in speed between rotor and stator fields is called slip and is measured in %.The slip  of an AC motor is a key factor and is necessary to produce torque. The greater the load (torque), the greater slip. The formula for calculating motor slip is given by:


Note that rotor speed is the same as the rated speed of the AC motor as given on the motor nameplate

The electrical power supplied to an electric motor is converted into mechanical power for powering various loads such as pumps. The power output can be rated in KW (kilowatts)  or Horsepower (HP). In the United States, the mechanical output of the electric motor is rated in HP while elsewhere, it is in KW.
Note that:

1HP = 746W = 0.746KW 1KW = 1/0.746*HP = 1.34HP

Electric Motor Horsepower (HP)
Electric motor horsepower is dependant on speed and torque. When torque
is expressed in lb-ft and speed is expressed in RPM, the following formula can be
used to calculate horsepower (HP):

From the above formula, note that any increase in torque or speed or both increases horsepower.

How to Determine Output Horsepower Overload of an Electric Motor

To determine whether an electric motor is overloaded, we require:
(a)  The electrical power input
(b)  The efficiency of the motor or
(c)  The mechanical power output or horse power rating- This can be calculated or more accurately measured.

Electrical Power Input for Single and 3-Phase AC Motors:

For a single phase electric motor, power input is given by: Power(Watts) = Voltage x Current x Power factor (P.F) For a three phase AC motor, power input is given by: Power(Watts)  = Voltage(line) x Current(line) x P.F x 1.732

Sample Scenario 1
Suppose we have a 1HP, single phase motor with power factor of 75% and efficiency 90% and is driving a fan. If the voltage and currents are respectively 123V and 10Amps, is this motor overloaded?

Let us first compute Electrical power input to motor as shown above:
Power input = Voltage x Current x P.F
                   =  123 x 10 x 0.75 = 922.5W
In horsepower we have = 922.5/746 = 1.24HP
Now we already know the electrical power input of the motor which is = 1.24HP
With an efficiency of 90%, power output is calculated thus:
Efficiency = Mechanical power output (HP)/Electrical power input(HP)
Mechanical power output = Efficiency x Electrical power input (HP)
                                       = 0.90 x 1.24 = 1.12HP

This motor is definitely overloaded!

Sample Scenario 2
A 10HP, 400V, 3-phase AC motor with power factor of 80%  and efficiency 85% takes in an average current of 16.5 Amps and is driving a pump load. Is this motor overloaded?
The electrical power input to the 3-phase AC motor is given by:
Electrical power input = Voltage x Current x P.F x 1.732
                                  =  400 x 16.5 x 0.8 x 1.732 = 9,144.96W
In horsepower is          =  9144.96/746 = 12.26HP

Motor power output = Efficiency x Electrical input
                               = 0.9 x 12.26 = 11.034HP

Again, this AC motor is overloaded!

In general, if:

The calculated or measured power output of an electric motor in horsepower(HP) is greater than the nameplate rating, then the motor is evidently overloaded!

Three phase induction motors are specified by their electrical design type. NEMA  (National Electrical Manufacturers Association) has classified three phase AC motors into design A, B, C, D. A more recent design called E has been added. These designs are suited to particular classes of applications based upon the load requirements typical of each class.

As a motor operates from no load to full load, its torque varies with speed. The relationship between speed and torque is often shown in a graph, called a speed-torque curve. This curve shows the motor�s torque, as a percentage of full-load torque, over the motor�s full speed range, shown as a percentage of its synchronous speed. NEMA classification is based on the speed-torque curves. Typical speed-torque curve for designs A, B, C, and D is shown below:



Characteristics of NEMA Three phase Motor Designs

NEMA Design A Motors
The table below shows the typical characteristics of all NEMA A Motors:

Explosion Group II Gases
Explosion group II is divided into three subgroups, IIA, IIB and IIC. Some typical gases in these groups are shown in the table below:

As shown above, the danger of gas explosion increases from group IIA to group IIC. IIC is considered the most dangerous explosion group. The higher the dangers of explosion, the stricter are the requirements to the AC motor required to be used in these explosive environment. Therefore, it is mandatory that electrical equipment required for use in hazardous environment carries a clear marking of what explosion group it belongs to

The basic steps in ascertaining the health of a three-phase AC motor are given below:
(a) General Inspections
(b) Earth Continuity and Resistance Test
(c) Power Supply Test
(d) AC Motor Winding Continuity Test
(e) AC Motor Winding Resistance Test
(f) Insulation Resistance Test
(g) Running Amps Test

General Inspections
For the three-phase motor, do the following:

(1) Check the appearance of the motor. Check for burnt, damage to body or cooling fan or shaft.
(2) Manually rotate motor shaft to examine bearing condition. Look out for smooth and free shaft rotation. If shaft rotation is free and smooth, bearing is possibly in good condition, otherwise consider replacing, repair or carry out further diagnosis.
(3) As with all testing and inspections, the motor name plate provides valuable information that will help to ascertain the true health of the motor. Examine the name plate thoroughly and compare values of running amps test (see below) with name plate value

Earth Continuity and Resistance Test
With a multimeter, measure the resistance between motor frame (body) and earth. A good motor should read less than 0.5 ohms. Any value greater 0.5 ohms indicate trouble with the motor. Further troubleshooting maybe required

Power Supply Test
For three phase motors, the expected voltage for a 230/400V system is 230V phase to neutral and 400V between each of the three phase supply lines. Check that the correct voltage is applied to the motor using a multimeter.  Ensure the terminal for power supply is in good condition. Check the connection bar for terminal (U, V, and W). For three phase motors, connection type is either Star (Y) or Delta

AC Motor Winding Continuity Test
Using a multimeter, check the continuity of motor winding from phase to phase ( U to V, V to W , W to U ).Each phase to phase must have a continuity if winding is OK. If any particular phase fails the continuity test, your motor is probably burnt.
Please see how to identify three phase windings for proper winding identification. U, V, W is a European winding designation.

AC Motor Winding Resistance Test
Check the motor winding resistance or ohms reading using a multimeter or ohmmeter for phase to phase terminal ( U to V,V to W ,W to U ).The ohms reading for each winding must be the same (or nearly the same). Remember that the three phases have identical windings or nearly so!

Insulation Resistance Test
Insulation resistance failure of an electric motor is one of the first signs that the motor is about to fail. For a three phase motor, insulation resistance is usually measured between each motor winding or phase and between each motor phase and motor frame (earth) using an insulation tester or megger. Set the voltage setting of the insulation resistance tester to 500V.  Check from phase to phase (U to V, V to W, W to U). Check from phase to motor frame (earth) (U to E, V to E , W to E ). Minimum test value of motor insulation resistance is 1 Meg Ohm (1 MO). See how to measure insulation resistance of Electric Motor

Running Amps Test
With the motor running, check the full load amps (FLA) with a suitable meter or preferably a clamp on meter and compare with the name plate FLA. Deviations from rated FLA could signify problems with the motor under test.