Electric motor speed: What RPM do you need?
The correct rotational speed of a electric motor starts with the speed your machine needs. Then you determine which transmission and what torque are needed to drive that machine. Therefore, simply comparing the power in kW is insufficient.
On the type plate and in our shop you will find the motor speed as RPM: Revolutions Per Minute. But what does the difference between, for example, 1500 and 3000 RPM mean for your drive system? And when should you choose a different engine speed, a frequency drive or a reducer? In this article we explain how to make that choice.
How do you determine the correct speed of an electric motor?
First determine how fast the driven shaft should rotate. With a direct coupling, it rotates at the same speed as the motor shaft. With a reducer, belt or chain transmission, those speeds can vary. Then you look at the load. The motor must provide sufficient torque to get the machine started and keep it running. Starting a loaded transport belt, for example, places different requirements than starting an unloaded drive.
Three questions are decisive for the selection:
- What motor speed or speed range does the machine need?
- What transmission is between the motor and the driven shaft?
- How many pairs are needed when starting up and during normal operation?
If you replace an existing motor and the application remains the same, the type plate and the data from the existing transmission form the basis. For a new drive system, you use the machine data or calculations from the machine manufacturer.
What is the difference between 750, 1000, 1500 and 3000 RPM?
At standard asynchronous electric motors You often encounter speed ratings of 750, 1000, 1500 and 3000 rpm. These values correspond to the speed of the magnetic rotor at 50 Hz. The nominal speed of the motor shaft is slightly lower.
| Number of poles | Synchronous rotational speed at 50 Hz | Example nominal rotational speed |
| 2-polygons | 3000 RPM | 2900 RPM |
| 4-polygons | 1500 RPM | 1450 RPM |
| 6-polygons | 1000 RPM | 960 RPM |
| 8-polygons | 750 RPM | 720 RPM |
The nominal speeds are examples. The exact value can be found on the type plate and in the product specifications.
How do the number of poles and the frequency determine the rotational speed?
At the same feed frequency, a higher number of poles means a lower synchronous speed. A 2-pole electric motor At 50 Hz, it has a synchronous rotational speed of 3000 RPM; a 4-pole version reaches 1500 RPM. If you want to change the speed of a suitable motor during use, a frequency regulator can adjust the frequency offered. The number of poles of a standard motor remains unchanged in this case.
Why is there, for example, 1450 RPM on the type plate?
In an asynchronous motor, the rotor rotates at slightly slower speeds during normal operation than the magnetic rotating field. This difference is called slip and is necessary to deliver torque. Therefore, a 1500-rpm motor can have a typeplate value of 1450 RPM have. That nominal value corresponds to the nominal tax and the specified feeding conditions. With a different tax, the actual rotational speed may change slightly.
A difference between 1500 rpm in the general indication and 1450 RPM on the type plate does not therefore indicate a defect. For a precise comparison, use the nominal product specifications.
Which gear ratio suits your application?
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How are motor speed, power and torque related?
The RPM tells you how fast the motor shaft rotates. The torque indicates how much torque the motor delivers. To assess whether a motor can drive your machine, you need both: the right speed and sufficient power. At the same power level, a motor with a lower nominal RPM produces more nominal torque. You can see this in these two motors:
| Power | Rotation speed | Couples |
| 1.5 kW | 1450 RPM | Approximately 9.9 Nm |
| 1.5 kW | 2900 RPM | Approximately 4.9 Nm |
The 1450 RPM motor provides about twice as much torque, while the other motor rotates twice as fast. A motor with the same number of kW but a different speed is therefore not automatically a suitable replacement.
This comparison applies to the declared electric motor operating values. Running an existing motor more slowly with a frequency drive Does not automatically provide more torque. It must also be checked separately whether the motor is providing sufficient power to start the machine.
How do you calculate the speed of an electric motor?
Calculating speed with frequency and pooltal
The formula is: Synchronous rotational speed (RPM) = 120 × frequency (Hz) ÷ number of poles
A 4-pole motor For example, at 50 Hz, the synchronous speed is 120 × 50 ÷ 4 = 1500 RPM.
This is the speed of the magnetic rotor. In an asynchronous motor, the rotor shaft rotates slightly slower due to slip. Therefore, use the value on the typeplate if you need the nominal motor speed, for example 1450 RPM.
Calculating the rotational speed after a reducer
A reducer reduces the speed at which the machine is driven. At a ratio of 20:1, the motor shaft makes twenty revolutions for every revolution of the output shaft.
The formula is: Output speed = motor speed ÷ reduction ratio
With a motor running at 1450 RPM and a reduction ratio of 20:1, that becomes 1450 ÷ 20 = 72.5 RPM. The output shaft then rotates 72.5 times per minute.
How can you control the speed of an electric motor?
Regulate the speed with a frequency drive
Lowering the motor speed with a reducer
Combining a frequency drive and a reducer
Frequency drive setup service
Have your frequency drive set up by the specialists at Jumatra. This way you can install the drive directly on your system.
How far can you increase or decrease the speed?
There is no fixed minimum or maximum speed that applies to every electric motor. The allowable range depends on the motor, cooling, load, and the machine being driven. WEG For example, for the W22 electric motors In the technical documentation, it is stated how much torque is allowed at different speeds. When running at a low speed for a long time, a lower load or independent ventilation may be necessary to prevent overheating. Additionally, for higher speeds, maximum mechanical speeds apply. Therefore, for your specific motor configuration, you must check both the allowed load and the maximum speed.
Can you replace an electric motor with another speed?
Yes, but only if the new speed and the available torque match the machine and the transmission. 1500-RPM motor replaced by a 3000-RPM version is not an equivalent exchange.
With the same feed frequency and transmission, the machine would run about twice as fast, provided the motor can handle the load. At the same nominal power, the faster motor provides about half of the nominal torque. At the same time, the machine may require more power at that higher speed. In our article about replacing an electric motor, you can read which specifications you need to check for that purpose.
Replacing the electric motor with a Jumatra
Do you want one? Replacing the electric motor Should you adjust the speed of your drive system? Send us a clear photo of the typeplate and specify what drives the motor. Also provide the data of any available reducer or frequency drive and tell us the speed you want to achieve. This way we can provide targeted advice on a suitable combination. Send your request via our contact page and we will help you immediately.
Frequently asked questions about the speed of an electric motor
How do you measure the electric motor speed when the type plate is illegible?
Why does the engine speed drop when the motor is loaded?
With a suitable speed meter, for example a contactless optical meter, you can measure the actual rotational speed. Use a safe measuring setup for this purpose. The result applies to the load and power during the measurement. You measure the rotational speed at the current load and power conditions. The measured value is therefore not automatically the nominal rotational speed of the motor.
Can you adjust the speed of a 230V electric motor?
That depends on the motor type. A frequency drive with a 230V single-phase power supply Often has a three-phase output for a suitable three-phase motor. This is not automatically suitable for a single-phase capacitor motor. There are special controllers for certain single-phase motors, for example versions with a permanent operating capacitor. The manufacturer must support the combination. Therefore, check the motor type and the input and output specifications of the controller; only the indication ‘230V’ is insufficient.
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