How to calculate the maximum RPM of a motor?

In summary: This is how it is able to run so close to synchronous speed without a tachometer or servo system.The torque-speed characteristic of a fan is not likely to be an exact match to the torque-speed characteristic of an induction machine, so you will have to estimate the torque required at the desired speed (assuming you don't know it) and calculate the speed-torque operating point for the fan. The fan torque-speed characteristic is likely to be unknown, so you may have to estimate it from the motor curves and the measured motor performance.In summary, there is no universal formula for calculating the maximum rpm of a fan with a blade attached, as it depends on various factors such as the type of motor, the load on the fan, and the
  • #1
Physics is awesome
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1
Hello all,

Just wondering if there is a mathematical formula that calculates the maximum rpm. If I have a fan that puts out 3 pounds of rotational force at stall speed(completely still) at the end point of a 1.5 ft blade what would be the maximum rpm of this fan with the blade attached. What would be the maximum rpm of the fan motor with no blade/load. Is there a formula for this? Also if there is an answer to this would the same formula apply to gas motors as well as a human peddling a bike?
 
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  • #2
Physics is awesome said:
Hello all,

Just wondering if there is a mathematical formula that calculates the maximum rpm. If I have a fan that puts out 3 pounds of rotational force at stall speed(completely still) at the end point of a 1.5 ft blade what would be the maximum rpm of this fan with the blade attached. What would be the maximum rpm of the fan motor with no blade/load. Is there a formula for this? Also if there is an answer to this would the same formula apply to gas motors as well as a human peddling a bike?

much of it would depend on the power, size, and a few other factors of the motor in question
 
  • #3
Physics is awesome said:
Just wondering if there is a mathematical formula that calculates the maximum rpm. If I have a fan that puts out 3 pounds of rotational force at stall speed(completely still) at the end point of a 1.5 ft blade what would be the maximum rpm of this fan with the blade attached. What would be the maximum rpm of the fan motor with no blade/load. Is there a formula for this?

Sorry it can't be done with just the information you mention.

Some motors cannot be operated without a load as they would spin too fast and damage themselves. In such cases you apply a variable load and plot a graph of rpm vs Torque (rotational force). Then extrapolate to give an estimate of the rpm with no load.

Ideally what you need is data on the torque vs rpm for both the motor and the fan from each manufacturer. Then if you plot both lines on the same graph where they cross will be the operating point of the motor and fan combination.
 
  • #4
What type of motor is this? An induction motor, or something else?
 
  • #5
If it's a DC motor, you can assume torque is inversely proportional to motor speed. The torque absorbed by the fan blades goes up approximately as the square of motor speed.
 
  • #6
Dr.D said:
What type of motor is this? An induction motor, or something else?

Good point. Some types of motor depend on the mains frequency.
 
  • #7
CWatters said:
Good point. Some types of motor depend on the mains frequency.

The two common types of AC motors are synchronous machines and induction machines. For a synchronous machine, the rotor speed is absolutely locked to the power line frequency. On a 60 Hz system (USA), a synchronous machine will run at 3600 rpm, 1800 rpm, 1200, rpm, 900 rpm, etc, depending on the number of poles.

For an induction machine, again on a 60 Hz system, the rotor speed is a little bit less than a line frequency submultiple, such as 3520-3585 rpm, 1725-1785 rpm, 1125-1185 rpm, etc, depending on the loading. For an induction machine, the slip is required in order to develop the torque required to support the load. An induction machine at no-load will run close to synchronous speed, with only enough slip to support the internal losses (bearing friction, windage, etc.)
 

1. How do I calculate the maximum RPM of a motor?

The maximum RPM (revolutions per minute) of a motor can be calculated by dividing the frequency of the power source (in Hz) by the number of poles in the motor and multiplying by 60. This formula is known as the synchronous speed of the motor. For example, if the power source frequency is 60 Hz and the motor has 4 poles, the maximum RPM would be (60/4) x 60 = 900 RPM.

2. What is the significance of calculating the maximum RPM of a motor?

Calculating the maximum RPM of a motor is important for several reasons. It helps determine the motor's speed capabilities, which is crucial in selecting the right motor for a specific application. It also ensures that the motor does not exceed its maximum speed, which can cause damage and reduce its lifespan.

3. How does the number of poles in a motor affect its maximum RPM?

The number of poles in a motor determines its synchronous speed, which is the theoretical maximum RPM of the motor. A motor with more poles will have a lower synchronous speed and a lower maximum RPM, while a motor with fewer poles will have a higher synchronous speed and a higher maximum RPM.

4. Can the maximum RPM of a motor be increased?

In most cases, the maximum RPM of a motor is fixed and cannot be increased. However, changing the power source frequency or altering the motor's design can affect its synchronous speed and therefore, its maximum RPM. It is important to consult a professional before attempting to modify a motor's speed capabilities.

5. Are there any safety considerations when calculating the maximum RPM of a motor?

Yes, it is important to ensure that the motor does not exceed its maximum RPM, as this can result in overheating and potential damage to the motor. It is also important to follow safety guidelines and use protective equipment when working with motors, as they can be dangerous if mishandled or operated at high speeds.

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