How to Calculate the RPM of a Motor

How to Calculate the RPM of a Motor

When operating, monitoring, repairing or replacing a motor, it’s essential to understand its specifications. One crucial measurement is revolutions per minute, or RPM, which describes the speed of a motor. In this guide, we’ll discuss how to calculate the RPM of a motor and why it’s so important.

What Is the RPM of a Motor?

RPM is a measurement used to describe a motor’s speed. It stands for revolutions per minute and describes the rate at which the rotor is revolving, which is the number of times the rotor shaft completes a full rotation each minute. It can be used to measure the speed of motors, turbines, centrifuges, conveyors and other equipment.

Why It’s Important to Calculate RPM

Calculating motor RPM, as well as other measurements like torque, voltage and power, is essential when choosing a motor for a given application. Calculating motor speed can help you choose the right kind of motor when replacing components and help you make better repair decisions. You also need to understand RPM to control and monitor motor operation effectively.

AC Induction Motor Speeds

AC motors are designed to run at certain speeds. These speeds are the same, even across various models and manufacturers. The speed of a given motor depends on the line frequency of the power supply, rather than the voltage, as well as the number of poles it has. AC motors often have two or four poles but could also have more. The relationship between poles and the RPM of a motor has to do with the magnetic field produced in the stator poles. This field leads to the creation of magnetic fields in the rotor that relate to the frequency of the field in the stator.

You also have to account for slip, which is the difference between the stator’s synchronous speed and the actual operating speed. The rotor always rotates slightly slower than the stator’s magnetic field and is always trying to ‘catch up’ to it, which is what creates the torque needed to get the motor running.

To adjust the speed of a three-phase AC motor, you can adjust the frequency of the AC motor’s power supply using a control. Many AC controls also have a single phase input, which enables you to run three-phase motors even if you don’t have three-phase power available. Most single-phase AC motors, on the other hand, aren’t adjustable since they plug directly into a standard outlet and use the frequency that is available. 

DC Motor Speeds

DC Motor Speeds

Like AC induction motors, permanent magnet DC motors also have poles, but the poles don’t affect speed like they do with AC motors. Several other factors impact speed in DC motors, including the motor’s operating voltage, the strength of the magnets and the number of wire turns the armature has. DC motors can only perform at speeds rated for the amount of voltage available to it. 

If the battery on which a motor is running starts to die and supply less voltage, the motor’s speed will decrease. If you connect a motor to a power source, the speed will increase, although this may cause your motor to experience additional wear. You can also use controls to adjust the speed of a DC motor, which works by varying the voltage available to the motor.

How to Calculate Motor RPM

To calculate RPM for an AC induction motor, you multiply the frequency in Hertz (Hz) by 60 — for the number of seconds in a minute — by two for the negative and positive pulses in a cycle. You then divide by the number of poles the motor has:

  • (Hz x 60 x 2) / number of poles = no-load RPM

You can also calculate the slip rating by subtracting the rated full load speed from the synchronous speed, dividing that answer by the synchronous speed and multiplying that answer by 100:

  • ((synchronous speed-rated full-load speed) / (synchronous speed)) x 100 = slip rating

Then, to find the full-load RPM, you convert the slip rating to RPM and then subtract it from the no-load RPM:

  • To convert the slip rating to RPM: RPM x slip rating = RPM slip
  • To calculate the full-load RPM: RPM – RPM slip = full-load RPM

The RPM of a DC motor depends on the voltage supplied to the motor. Typically, the manufacturer of the motor will tell you the RPM you can expect at various voltages. To achieve the desired RPM, you can then adjust the voltage according to the guidelines.

Motor RPM Calculation Examples

Let’s take a look at some examples. For an AC motor, the number of poles and the frequency determine the no-load RPM. For a 60 Hz system with four poles, the calculations to determine RPM would be:

  • (Hz x 60 x 2) / number of poles = no-load RPM
  • (60 x 60 x 2) / 4
  • 7,200 / 4 = 1,800 RPM

The amount of slip varies slightly based on the motor’s design. A reasonable full-load speed for a 60 Hz, four-pole motor would be 1,725 RPM. The slip is the difference between the no-load speed and the full-load speed. In this case, that would be:

  • Full-load RPM – no-load RPM = RPM slip
  • 1800 – 1725 = 75 RPM

At 60 Hz, a motor with two poles operates at 3,600 RPM with no load and about 3,450 RPM with a load:

  • (Hz x 60 x 2) / number of poles = no-load RPM
  • (60 x 60 x 2) / 4
  • 7,200 / 2 = 3,600 RPM

At 60 Hz, a motor with six poles would run at 1,200 RPM under no load and at approximately 1,175 RPM loaded. A motor with eight poles would run at 900 RPM unloaded and at about 800 RPM under load. 12-pole motors, which are even less common than six-pole and eight-pole models, run at 600 RPM without a load, and 16-pole motors run at 450 RPM.

Motor Repair From Global Electronic Services

Motor Repair From Global Electronic Services

It’s important to understand the specifications of your equipment, so you can better operate and maintain it. Your motor’s speed is integral to its performance, and being able to calculate and control the RPM will help you get the most out of your machines.

Professional repair and maintenance services may also play an important role in helping you take full advantage of your equipment’s capabilities. At Global Electronic Services, we have extensive experience repairing and servicing a wide range of industrial equipment, including AC and DC motors, servo motors, industrial electronics, hydraulics and pneumatics and more. To learn more about AC or DC motor repair or our services, contact us today.

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