Electric Motor Speed Control, Formula And Calculator

Induction Motor Speed Formula?

The speed of an induction motor can be calculated using the following formula:

Synchronous Speed (Ns)=120×Frequency (f)/ Number of Poles(P)


  • Synchronous Speed (Ns) is the speed of the rotating magnetic field in revolutions per minute (RPM).
  • Frequency (f) is the frequency of the power supply in Hertz (Hz).
  • The number of Poles (P) is the number of poles in the motor.

The actual speed of the induction motor, known as the slip speed, is slightly less than the synchronous speed due to the slip caused by the load on the motor. The slip speed can be calculated using the formula:

Slip Speed=Synchronous Speed−Actual Motor Speed

Where the actual motor speed is the speed at which the motor is operating.

Keep in mind that these formulas provide an idealized representation, and actual motor performance may vary due to factors such as mechanical losses, load conditions, and other practical considerations.

This formula is the synchronous speed while the motor at full load will be slightly lower in speed than this.

What does motor speed mean?

The motor speed defines the rotation of the axial connected with the rotating part of the motor. To define it further we have to introduce another term called RPM.

Motor RPM (Revolution Per Minute) is the rate at which the rotor is revolving. It is the number of cycles in which the rotor shaft completes a full rotation each minute.

It is used for measuring the speed of turbines, motors, centrifuges, conveyors & other equipment.

We will discuss the two types of motors here. i.e. AC Induction motor and DC motor.

Induction Motor Speed

Induction motor speed is not only dependent upon the supplied voltage but also on the number of poles of the motor, and the frequency of the system.

The number of poles in an AC motor is either two or four.

Another factor that affects the speed of the Induction motor is slip. Which is the difference between the synchronous speed of the stator and the actual operating speed.

To get a motor running torque is necessary. For this purpose, a rotor rotates slightly slower than the stator magnetic field.

Is an induction motor a constant-speed motor?

Constant speed motors do not change speed and run at a uniform speed. Normally, the induction motor runs at a constant speed.

So, it is a constant-speed motor. There is a slight slip caused by the lagging of the rotor flux rate from the stator flux rate. As the value of the inductive load increases, the slip decreases.

Finally, at the rated value, the slip becomes negligible. At this time the change in speed of the motor is negligible.

So, the induction motor speed is constant. The normal slip value of the induction motor is from 3% to 5%.

Motor Speed Calculation Example and Calculator:

A Three phase induction motor works on a 50Hz power source and the number of poles of this motor is 4,

Then, Motor Speed Formula = (f *120)/p = 120*60 / 4 = 1800 RPM

So, the synchronous speed for this motor is 1800 RPM.

Let’s assume that the actual speed of the motor is 1750 RPM. To find the slip speed, we use the formula:

Slip Speed=Synchronous Speed−Actual Motor

Slip Speed=1800−1750=50 RPM

Therefore, the slip speed of the motor is 50 RPM.

Keep in mind that this is a simplified example, and the actual motor speed may be influenced by various factors such as the type of load, the efficiency of the motor, and any external resistances.

Use the below online motor speed calculator for more calculations.

How to run an induction motor at constant speed on variable load?

Running an induction motor at a constant speed on a variable load can be achieved using various control techniques and devices. Here are some commonly employed methods:

  1. Use of Variable Frequency Drives (VFDs): VFDs are electronic devices that control the speed of an induction motor by varying the frequency and voltage of the power supplied to the motor. They can regulate the motor speed according to the changes in the load by adjusting the frequency, ensuring a constant speed despite fluctuations in the load.
  2. Implementation of Closed-Loop Control Systems: Closed-loop control systems utilize feedback mechanisms to maintain a constant speed even with variable loads. These systems monitor the motor speed and adjust the input power to compensate for any changes in the load. Proportional-integral-derivative (PID) controllers are often used in these systems to ensure stable and accurate speed control.
  3. Using Mechanical Load Control Mechanisms: In some applications, mechanical load control mechanisms such as centrifugal clutches or variable transmission systems can be employed to adjust the load on the motor, thus helping to maintain a constant speed.
  4. Utilizing Flywheel Systems: Flywheels can be used to store kinetic energy, which can help smooth out fluctuations in the load. The flywheel system can supply or absorb energy as required, allowing the motor to run at a more constant speed.

When implementing any of these methods, it is important to consider the specific requirements of the application, the characteristics of the load, and the efficiency of the control system. Additionally, proper maintenance and regular monitoring of the motor’s performance are essential to ensure consistent and reliable operation over time.

How Can We Change Single-Phase Induction Motor Speed?

Changing the speed of a single-phase induction motor can be a bit more complex compared to a three-phase motor.

Single-phase induction motors are commonly used in various household appliances and small-scale industrial applications. Several methods can be used to alter the speed of a single-phase induction motor, including:

  1. Changing the Voltage: By adjusting the input voltage to the motor, you can vary the motor’s speed. Lowering the voltage can decrease the speed while increasing the voltage can lead to a higher speed. This method is relatively simple but may not be suitable for all applications.
  2. Using Capacitor Start-Run Motor: Some single-phase induction motors are equipped with a capacitor for starting purposes. Adding a variable capacitor or changing the value of the existing capacitor can help adjust the speed of the motor.
  3. Using Variable Frequency Drives (VFDs): VFDs can be used for single-phase motors to control their speed. However, this method can be more complex and expensive. It involves converting the single-phase power supply into a variable frequency AC supply, allowing for precise control of the motor speed.
  4. Changing the Number of Poles: Altering the number of poles in the motor can also affect its speed. However, this method is not commonly used in practice due to its technical complexity and cost.

When attempting to change the speed of a single-phase induction motor, it’s important to consult with a professional to ensure the chosen method is appropriate for the specific motor and application.

Additionally, adherence to safety guidelines and understanding the limitations of the motor is crucial to prevent damage and ensure efficient operation.

Which motor’s speed control is easier, AC or DC?

In general, DC motor speed control is often considered simpler compared to AC motor speed control.

This is primarily because DC motors can easily be controlled using simple methods such as varying the voltage or by using pulse-width modulation (PWM) techniques. By adjusting the voltage supplied to the DC motor, you can regulate its speed efficiently.

On the other hand, controlling the speed of an AC motor is typically more complex. AC motors are commonly controlled using more sophisticated techniques such as varying the frequency or using complex control algorithms.

The use of devices like Variable Frequency Drives (VFDs) allows for more precise control over the speed of AC motors by adjusting the frequency and voltage simultaneously.

However, the implementation of such systems can be more intricate and require a deeper understanding of power electronics and control theory.

While the basic control of DC motors might be considered simpler, it is important to note that modern technology has significantly advanced the field of AC motor control, making it more accessible and practical for a wide range of applications.

With the development of advanced control techniques and devices, the difference in complexity between AC and DC motor control has been significantly reduced in many industrial and commercial applications.

How to measure motor RPM?

By utilizing a tachometer, you can gauge the RPM (revolutions per minute) of both a motor and other rotating machinery.

This instrument offers two distinct measurement approaches. The first involves directly connecting the tachometer shaft to the motor shaft, displaying the motor’s RPM on the device screen.

Alternatively, the second method employs a laser beam. A reflective sticker is affixed to the motor shaft, and the rotating shaft is targeted with the laser. The device promptly calculates the RPM, exhibiting the results on the screen.

Can a VFD damage a motor?

While Variable Frequency Drives (VFDs) can offer significant advantages in terms of energy savings, control, and efficiency, they can potentially cause damage to motors if not used properly. Some common ways a VFD can dam

Using a device called a tachometer you can measure motor RPM and the RPM of any rotating machine. This device has two ways of measurement. The first is by direct contact between the motor shaft and the tachometer shaft and the RPM of the motor will show up on the device screen.

The second method is by laser ray. We set a small reflective piece of sticky paper on the motor shaft and then we direct the laser to the shaft while it is rotating. The device will count the RPM and show it on the screen.”

age a motor include:

  1. Overvoltage or voltage imbalance: If a VFD is not adequately designed or if the installation is faulty, it can lead to excessive voltage being applied to the motor windings, causing insulation breakdown and eventually damaging the motor.
  2. Overcurrent: Inappropriate VFD settings or sudden changes in the load can result in overcurrent situations, which can lead to overheating of the motor windings and insulation degradation.
  3. Bearing damage: VFDs can induce harmful shaft voltages that, if not properly mitigated, can cause bearing damage in the motor over time.
  4. Cable length: Excessive cable length or poor quality cables can lead to voltage spikes and reflected waves, potentially causing insulation stress and premature motor failure.

To prevent damage to the motor, it’s crucial to ensure that the VFD is appropriately sized for the motor and that the installation is done by professionals who understand the complexities involved.

Additionally, implementing appropriate measures such as installing filters and insulating the motor properly can help mitigate the risk of damage. Regular maintenance and monitoring of the motor’s performance can also help in identifying any potential issues before they cause significant damage.