# Induction Motor Torque Decreasing when Load is increased

• Physicist3
In summary, the conversation is discussing the observation of a decrease in torque when load is increased in an induction motor. The person is wondering how this is possible, as they are seeing a decrease in rotor speed and an increase in rotor currents and current drawn from the supply. The conversation also touches on measuring torque with no load and the different regions of the torque-speed curve for an induction motor. The person suggests that the motor may be working in an unstable region and could be damaged. They also mention that even with no outside load, there can still be a parasite torque due to inner motor losses.
Physicist3
Hi.

Having measured an induction motor on no load and obtained a torque figure, I have noticed that when load is increased, the torque continues to decrease. How it this possible as I am observing a reduction in rotor speed and increase in rotor currents and current drawn from supply?

Physicist3 said:
Hi.

Having measured an induction motor on no load and obtained a torque figure, I have noticed that when load is increased, the torque continues to decrease. How it this possible as I am observing a reduction in rotor speed and increase in rotor currents and current drawn from supply?

If so, I can merge the threads.

I'd need to know what you mean by "load" to make sense of the question.

Are you measuring "slip" in % of synchronous speed, or RPM, or some other unit?

What is your "torque figure" ? Result of a dynamometer measurement, or a motor rating, or result of a computer simulation?

What do you mean by "on no load" ? It is counterintuitive to speak of "increasing load" and "decreasing torque" , unless perhaps you are trying to describe the torque-speed curve to left of peak .

A picture is worth a thousand words - have you plotted the results of your experiment ?
Posting same might clarify your inquiry.

old jim

How does one measure torque with no load? At no-load speed there is no torque.

Google "induction motor torque" and look at the images.

The motor rpm will increase until the acceleration torque is positive [and more than “0”].That means all the time the torque produced by motor will be more than load torque. The acceleration will be 0 when motor torque and load torque will be the same.
The motor torque vs. speed curve present 3 –significant-fragments. At first the torque will decrease as speed increases. The second portion the torque will increase, but this region it is a non-stable region since if the speed will decrease the torque will decrease more –more or less up to motor is stopping.
If the load increases [and due to this the speed will decrease] and motor torque decreases then the motor is in this region of the curve.
In no-load situation the speed is far from synchronous speed-slip very large- and any load will decelerate the rotor and reduce the torque.
In normal situation the motor will work in the 3rd region where if rotor speed decreases the motor torque will increase and will be equal to the load torque at a less velocity-more slip.
In my opinion this motor works in the second-instable-region. This could happen if the rotor is damaged: one or two bars are broken.
By the way, even in “no-load”-that means no outside load-the inner side of the motor losses will create a parasite torque-sometime significant enough.

## 1. Why does the torque of an induction motor decrease when the load is increased?

The torque of an induction motor decreases when the load is increased because of the relationship between the magnetic field and the rotor speed. As the load increases, the rotor speed decreases, causing the magnetic field to lag behind the stator field. This lag in the magnetic field results in a decrease in the torque produced by the motor.

## 2. What factors contribute to the decrease in torque when the load is increased?

There are several factors that contribute to the decrease in torque of an induction motor when the load is increased. These include the slip between the stator and rotor fields, the change in rotor speed, and the change in the air gap between the stator and rotor.

## 3. Can the decrease in torque be compensated for?

Yes, the decrease in torque can be compensated for by adjusting the voltage and frequency of the power supply to the motor. This can help to maintain a constant torque output even as the load is increased.

## 4. Why is it important to understand the relationship between torque and load in an induction motor?

Understanding the relationship between torque and load in an induction motor is important because it allows for efficient operation of the motor. By adjusting the torque output to match the load, the motor can operate at its optimal performance, saving energy and reducing wear and tear.

## 5. Are there any limitations to the decrease in torque when the load is increased?

Yes, there are limitations to the decrease in torque when the load is increased. If the load becomes too high, the motor may stall and stop functioning. Additionally, if the voltage and frequency of the power supply are not adjusted to compensate for the increase in load, the motor may overheat and potentially fail.

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