Synchronous Motor Torque-Speed Characteristic Curve

Hello friends, I hope all of you are fine. In today’s tutorial, we are going to have a look at the synchronous motor torque-speed characteristic curve. The most important feature of a synchronous motor is that it provides constant speed during its operation. The speed of the motor depends on the number of poles and the frequency of the supply connected with it. Load variation in an induction motor does not affect the motor greatly, but the load variation in a synchronous motor affects the working of the motor greatly. Load variation produces hunting (when a sudden load varied on the rotor, it started vibrating across its axis and find a new location for stability; this process is called “hunting” in a motor’s rotor), and due to this fact, the stability of the motor is also disturbed.

In today’s post, we will have a look at the relation between torque and speed of the synchronous motor and also study the effect of load variation on the motor. So let’s get started with torque-speed characteristics of synchronous motors.

In today’s post, we will have a look at the relation between torque and speed of the synchronous motor and also study the effect of load variation on the motor. So let’s get started with torque-speed characteristics of synchronous motors.

Synchronous motor torque speed curve

• A synchronous motor provides power to such loads that operate at a constant speed and system.

• Mostly this motor is connected with large power systems, so these power systems behave like large infinity bus bars for the motor, and the variation in the motor has no effect on these bus bars.

• That means how much power the motor takes from the power system, and there will be no effect on the frequency and output voltage of the system.

• The rotating speed of the motor is interlocked with the frequency of the system; due to this, the speed of the motor remains constant irrespective of the load varied on a motor.

• The given figure shows a relation between the synchronous motor torque curve and synchronous motor speed connected with the large power system

• We can see from the above-given figure that the speed of motor remains constant from no load to maximum load connected with it. The speed regulation of the motor is also zero.

SR =(n_nl-n_fl)n_fl x 100%

SR = 0%

• The equation of torque will be given as.

t_ind =kB_RB_netsinδ

t_ind= (3V_øE_Asinδ)/w_mX_s

• When the angle is δ=90°, then maximum torque exists. Normally torque, when a full load is connected, has much less value.

• In fact, the pullout torque can be three times the full-load torque of a motor.

• When the torque on the motor’s shaft is larger than the pullout torque, then the rotor will come out; the effect of the field of the stator does not remain interlocked with it.

• When the rotor gets out from the effect of the stator, it starts to slow down, and the direction of torque produced in it also reverses, and the field of the stator laps it continuously.

• The resultant enormous torque produces, first in one direction and then in the other direction; due to this, the motor starts vibrating harshly.

• The loss of harmonization (synchronization) when the pullout torque increases is called slipping poles.

The equation for a maximum of pullout torque of a motor is mentioned here.

t_max =kB_RB_net

t_max= (3V_øE_A)/w_mX_s

• From these equations, we can conclude that with the increment in field current internal generated voltage E_A increases that also causes to increment in the maximum torque.
• So if the motor operates at high field current then there will be high stability for the motor.

Effect of Load Changes on a Synchronous Motor

• To find the effect of load variation, let’s suppose that the motor’s shaft is connected with a capacitive load.

• If we increase the load connected with the shaft of the motor due to an increment in load speed, rotor rotation also decreases.

• Due to this angle, torque-induced also becomes larger (δ), so the value of induced torque also increases.

• The increment in the value of torque induced also increases the speed of the rotor again; now the motor is again rotating at synchronous speed but has a high value of angle of torque (δ).

• The given diagram shows the phasor diagram of load connected with the motor when there is no change in load.

• The internally generated voltage E_A of the motor is (køw) in this case, so these voltages depend on the field current and rotation speed of the motor.
• As the speed control by the input supply and the circuitry of field also not disturbed by any external fact so the field current is also constant with the speed that is constant due to the input supply’s constant behavior.

• So internal generated voltage should remain constant as load varies.
• But when the load varies internal voltage (E_A) abruptly decreases as shown in the given figure.

• As internal voltage decreases the jX_SI_A will increase to reach Vø from the E_A so the value of current passing through the armature winding also increases (I_A).
• You can also observe from a figure that the P.F angle (ø) also varies first it becomes less leading then more lagging.

starting torque of the synchronous motor

Synchronous motors do not have starting torque, and the rotor must be operated at synchronous speed by some other source. The torque is zero because the relative velocity between the stator and rotor MMFs is zero.

Read also:

What is the characteristic curve of a synchronous motor?

• The characteristic of a synchronous motor is called the V curve of the motor. The characteristic curve plotted between the input power factor and field current for the constant mechanical load on the motor is the shape of V and is called the inverted ‘V’ curve.

What is the relationship between torque and speed in a synchronous motor?

• The torque and speed relationship is an inverse relationship, as the rated output power of the motor is a fixed value. As output speed increases, output torque reduces proportionately. The output torque increases and the output speed reduces proportionately.

That’s all about Synchronous Motor Torque-Speed Characteristic Curve I have mentioned each and everything related to Synchronous Motor Torque-Speed Characteristic Curve. If you have any questions and query can ask in comments. See you in the next tutorial Types of Synchronous Motor.

• I also have written some related articles you can also read them are listed here.
• Introduction to Synchronous Motor
• Synchronous Motor Torque-Speed Characteristic Curve
• Synchronous Motor Starting Method
• What is Synchronous Condenser (Capacitor)
• Synchronous Motor Power Correction. 
• Types of Synchronous Motor
• Effect of Field Current Changes on a Synchronous Motor.