# Induction Motor Query: Current, Power, Torque & Speed

• Physicist3
In summary, when the frequency of the supply is decreased, the current and power drawn from the supply decrease. However, when the frequency is decreased below 30Hz, current drawn from the supply increases again. This is because at low frequencies, the properties of inductors become more influential on the system.
Physicist3
Hi, I am currently simulating a 3 phase induction motor system. One of the tests that I have run is to keep the motor parameters (reactance, resistance etc.) constant, and then change the frequency of the supply (50, 40, 30 Hz etc.) before measuring input parameters (current and power drawn from supply), along with output parameters such as torque and rotational speed. Throughout, I have kept the value of the voltage supply constant. Having completed the tests, I have found that when the frequency is dropped from 50 to 40, and from 40 to 30 Hz, both the current and power drawn from the supply decrease. However, when dropping the frequency below 30Hz, I have found that the current drawn from the supply increases again, yet the power (watts) continues to decrease with a constant voltage. I don't quite understand how this is possible, would it be the result of a really poor power factor at low frequency as the motor begins to stall and inductive parameters become more influential on the system?

It's properties of inductors allright.

You need to try this for real with a small transformer and Variac adjustable transformer.

Remember that flux is integral of voltage
so,
as you lower frequency with voltage held constant,
the period of each half cycle grows longer,
which means the integration has a longer interval,
which gives it a larger result, it is after all a definite integral;

which means flux will reach a level that's unsustainable in iron;
which means magnetizing current will go up to force the extra flux that the iron can't accept out through the air ;
which makes the motor hum and vibrate and get hot;

and your simulation will not emit the sounds and smoke that a real motor would.

That's why you need to take a small transformer, ammeter, and adjustable voltage source
and drive the little transformer into saturation while you watch current increase sharply at saturation point,
hear it hum and feel it vibrate and get hot, maybe even smell the hot insulation.

Look up term "Volts per Hertz" for a better explanation.

I encourage that experiment with real parts. It'll make your mind intuitively aware of what the math is telling you.

I appreciate the power of simulation, but it's a poor substitute for the visceral reality of a proper electric machinery lab.

## 1. What is an induction motor?

An induction motor is a type of electric motor that works based on the principle of electromagnetic induction. It consists of a stator (stationary part) and a rotor (rotating part) that are separated by an air gap. When an alternating current (AC) is passed through the stator, it creates a rotating magnetic field that induces a current in the rotor, causing it to rotate and generate mechanical power.

## 2. How does an induction motor produce torque?

An induction motor produces torque through the interaction between the stator's rotating magnetic field and the induced current in the rotor. The strength of the magnetic field and the speed of the rotor determine the amount of torque produced by the motor.

## 3. How does the current affect the performance of an induction motor?

The current in an induction motor is directly proportional to the amount of torque it can produce. Higher current means a stronger magnetic field, resulting in higher torque. However, excessive current can cause the motor to overheat and reduce its efficiency.

## 4. What is the relationship between power and speed in an induction motor?

The power output of an induction motor is determined by the product of its torque and speed. As the speed of the motor increases, the power output also increases. However, the motor's maximum speed is limited by the frequency of the AC power supply and the design of the motor.

## 5. How do you calculate the speed of an induction motor?

The speed of an induction motor can be calculated by dividing the frequency of the AC power supply by the number of poles in the motor. For example, a 4-pole motor connected to a 60 Hz power supply will have a synchronous speed of 1800 rpm (60 Hz / 4 poles = 15 Hz, 15 Hz x 60 seconds = 1800 rpm).

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