Induction motors - Speed torque curve, Crawling, Methods of starting synchronous motors, Constant torque operation, Derating

Speed Torque Curve of an induction motor

The speed torque curve of an induction motor is a plot of speed on the x-axis and torque on the y-axis. When the motor is started, the initial torque is about 250% of the rated torque. This is the torque required by the motor to overcome the inertial of the standstill. As the motor picks up speed the torque drops to the pull-up torque. If the pull-up torque of a motor is less than the torque requirement of the load coupled to it, the motor will stall and over heat.

The breakdown torque is the maximum torque which can be developed by the rotor before it overheats. The breakdown torque needs to be high for loads with high inertia and which are susceptible to overloads such as conveyor belts.

The full load torque is the torque produced by a motor operating at the rated speed and load. Exceeding the full load torque causes reduction in the life of the motor.

When the motor is run on no load, the rotor speed reaches the synchronous speed. The slip becomes zero and the motor runs at zero torque

Crawling in Induction Motors

The supply given to an induction motor may have harmonics present in it. These harmonics will have their own torques in addition to the synchronous torque. Let us consider a supply with odd harmonics. The 3rd harmonic will be absent in 3 phase systems. Hence, we only have to consider the 5th and 7th harmonics. The other higher order harmonics can be neglected.

The torque produced by the 5th harmonic rotates in the opposite direction. Thus, the forward torque is given by the sum of torques produced by the primary frequency and the 7th harmonic.

The rotating field of the 5th will rotate at one fifth of the synchronous frequency (Ns/5). However, the torque produced by the 5th harmonic rotates in the reverse direction. Similarly, the 7th harmonic will rotate at one seventh of the synchronous frequency. The torque produced by the 7th harmonic is maximum at 1/7th of the supply frequency.

When some poorly designed motors are started with load, the motors may not reach the nominal speed. The motors will get stuck at 1/7th of the nominal speed.

This phenomenon is known as crawling. Crawling can be overcome by properly selecting the number of rotor bars in the rotor of the induction motor

Methods of Starting Synchronous Motors

Synchronous motors used widely in the industry. Synchronous motors provide constant speed. The synchronous motor consists of a wound rotor and a stator. The stator winding is energized from the power supply. This sets up the rotating magnetic field. The rotor gets magnetized when the field winding is energized. During operation, the rotor is in synchronism with the rotating magnetic field of the stator. Hence, the name, synchronous machine.

The synchronous machine, however, is not self-starting. The synchronous machine has to be rotated to near the synchronous speed of the stator before it can "catch" the stator field and begin rotating on its own.

There are many different methods employed for Starting Synchronous Motors.

Pony Motor

The pony motor is an induction which drives the rotor of the synchronous motor. Once the speed reaches the synchronous speed, the field winding is switched on. The pony motor is then decoupled and the synchronous motor runs on its own.

Damper windings

Damper windings or amortisseur windings are special windings which are fixed on the salient pole of the rotor of the synchronous motor. These windings work in a similar manner to the squirrel cage winding in induction motor. Thus the synchronous motor starts as an induction motor. The rotor runs at a speed slightly lower than the synchronous speed. When the speed comes close to the synchronous speed, the field winding is switched on and the rotor gets locked to the stator magnetic field and the machine runs as a synchronous motor.

Starting using Variable Frequency

Synchronous motors which are electronically controlled can be started by supplying a reduced frequency to the stator winding. This generates a slowly rotating magnetic field in the stator. The rotor of the synchronous machine is able to follow this magnetic field. Once the rotor starts to rotate, the frequency is gradually raised to the power frequency. The synchronous motor can now run at the normal frequency.

Constant Torque Operation in Motors

Constant Torque Operation refers to the operation of the motors at a fixed torque value.  The torque supplied by a motor is dependent on the load.  However, a motor which has a constant flux is considered to be running at constant torque.

For a motor to run at constant torque it has to be driven by an AC drive.  AC drives are able to vary the frequency and the voltage such that a constant V/Hz value is obtained.  The V/Hz is the ratio of voltage to the frequency in an electric machine.

When the frequency of the motor is adjusted, the stator reactance changes.  This reduces or increases the stator current.  To correct this, the voltage has to be adjusted.

The conveyor is an example of an application which requires a constant load.

Derating in Motors

Derating refers to the operation of equipment at reduced capacity or speed.  Derating in motors can be caused due to the following reasons.


When frequency increases, the speed increases and the torque decreases.  If the frequency increases by 5%, the speed increases by 5% while the torque decreases by 10%.


Voltage has a direct relation with torque.  When the voltage falls the torque also reduces.  Equipment has suddenly rotate faster or move faster due to voltage fluctuations.


As the altitude increases, the density of air decreases.  This reduces the ability of air to transfer heat and cool the motor.  Thus if the motor is to be operated above 1000 metres above sea level, it has to be derated.

Ambient Temperature

The ambient temperature is also a factor in derating.  If the ambient temperature is high, the insulation may reach its maximum temperature limit quickly.  Hence, the motor may have to be derated.