SPEED-TORQUE CHARACTERISTICS OF INDUCTION MOTORS

Basic Induction Motor Concepts

The Development of Induced Torque in an Induction Motor

When current flows in the stator, it will produce a magnetic field in stator as such that Bs (stator magnetic

Field) will rotate at a speed:

N sync =(120 Fe)/( P)

Where Fe is the system frequency in hertz and P is the number of poles in the machine. This rotating

Magnetic field Bs passes over the rotor bars and induces a voltage in them. The voltage induced in the

Rotor is given by:

e ind = (v x B) l

Hence there will be rotor current flow which would be lagging due to the fact that the rotor has an

Inductive element. In addition, this rotor current will produce a magnetic field at the rotor, Br. Hence the

Interaction between both magnetic field would give torque:

The torque induced would generate acceleration to the rotor, hence the rotor will spin.

However, there is a finite upper limit to the motor’s speed.

SPEED-TORQUE CHARACTERISTICS OF

INDUCTION MOTORS

The X axis shows speed and

Slip. The Y-axis shows the torque and current. The

Characteristics are drawn with rated voltage and

Frequency supplied to the stator.

During start-up, the motor typically draws up to seven

times the rated current. This high current is a result of

Stator and rotor flux, the losses in the stator and rotor

Windings and losses in the bearings due to friction.

This high starting current overcomes these

Components and produces the momentum to rotate the rotor.

At start-up, the motor delivers 1.5 times the rated

Torque of the motor. This starting torque is also

Locked rotor torque (L RT). As the speed increases,

The Current drawn by the motor reduces slightly (see

The current drops significantly, when the motor speed

Approaches ~80% of the rated speed. At base speed,

The motor draws the rated current and delivers the

Rated torque.

At base speed, if the load on the motor shaft is

Increased beyond its rated torque, the speed starts

Dropping and slip increases. When the motor is

Running at approximately 80% of the synchronous

Speed, the load can increase up to 2.5 times the rated torque.

This torque is called breakdown torque. If the load on

The motor is increased further; it will not be able to

Take any further load and the motor will stall.

In addition, when the load is increased beyond the

Rated load, the load current increases following the

Current characteristic path. Due to this higher current

Flow in the windings, inherent losses in the windings

Increase as well. This leads to a higher temperature in

The motor windings. Motor windings can withstand

Different temperatures, based on the class of

Insulation used in the windings and cooling system

Used in the motor. Some motor manufacturers provide

The data on overload capacity and load over duty cycle.

If the motor is overloaded for longer than

Recommended, then the motor may burn out.

As seen in the speed-torque characteristics, torque is

Highly nonlinear as the speed varies. In many applications,

The speed needs to be varied, which makes the

Torque vary. We will discuss a simple open loop

Method of speed control called, Variable Voltage

Variable Frequency (VF or V/f) in this application note.