How to Save Money When Buying how to measure the rpm of a motor

Back EMF, unexpected voltage jumps, and compensating

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The data is attached to the first post. Each voltage and RPM has two values,

• with Back Electromagnetic Force
• and adjusted for BEMF.

Originally, I set the transformer to 12V and measured the RPM. When I compared it to the rated RPM, it was way off. A motor rated at 10,000 RPM @ 12V was running at 14,760 RPM! I then noticed the voltage jumped from 12V to 16.6V. Ah, the effects of BEMF.

Keeping the leads attached, I reduced the transformer until the meter read approximately 12V, and voilà! 10,380 RPM, close enough to 10K.

I re-tested all the motors with two applied voltages:

• Voltage before attaching to motor (12V or as close as I could adjust it)
• Voltage/RPM after attaching to motor; measured V jumped or dropped based on motor BEMF
• Voltage/RPM after fine-tuning voltage to around 12V:

The BEMF effect was particularly dramatic with the new can motors. They all raised the effective voltage, while the open-frame Pittman motors saw a reduction in effective voltage by about 10%, prompting me to increase the transformer output.

I measured the current at free running and stall, without adjusting for back EMF:

• 12V measured prior to applying leads to the motor,
• Do not compensate for BEMF; allow the motor to run faster than specified, measure amps
• Do not compensate for BEMF; clamp the motor shaft to zero RPM with pliers, measure amps

As expected, can motors consumed much less current than open frame motors, a fact that surprised me.

• Free running : Stall
• Open frame 0.347 : 1.1A
• Highest current draw from can motor 0.049 : 0.66A

Those little can motors got quite warm at stall, and I clamped them only briefly to avoid letting the magic smoke escape.

Understanding Motor RPM Measurement

Monitoring, repairing, or replacing a motor necessitates a good understanding of its specifications, with one of the most crucial measurements being revolutions per minute (RPM), indicating how fast a motor rotates. In this guide, we will delve into the significance of calculating motor RPM.

What is Motor RPM?

RPM is a metric denoting a motor's speed—the number of rotations the rotor shaft completes every minute. This measurement can be applied to motors, turbines, centrifuges, conveyors, and other equipment.

Importance of Calculating RPM

Understanding motor RPM, alongside other metrics like torque, voltage, and power, is fundamental when selecting a motor for a specific application. Knowing the RPM assists in making informed choices while replacing components and can guide repair decisions. It's also vital for managing and monitoring motor performance efficiently.

AC Induction Motor Speeds

AC motors are built to run at predetermined speeds, based primarily on the line frequency of the power supply and the number of poles. These motors usually have two or four poles, but may have more, with the magnetic field produced in the stator poles governing the rotor's behavior.

For further clarification on how to measure the RPM of a motor, contact us. Our knowledgeable sales team can help identify the best options for your needs.

DC Motor Speeds

Similar to AC induction motors, permanent magnet DC motors also utilize poles, but these poles do not influence speed. Instead, factors such as operating voltage, magnet strength, and the number of wire turns within the armature play a significant role. When the battery discharges and reduces voltage, motor speed declines. Connecting the motor to a stronger power source will increase speed, albeit with the risk of wear.

Calculating Motor RPM

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

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

To calculate slip, subtract the rated full load speed from the synchronous speed, divide that remainder by the synchronous speed, and multiply by 100:

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

Lastly, to determine the full-load RPM, convert the slip rating to RPM and 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

Examples of Motor RPM Calculations

For an AC motor with four poles operating at a 60Hz frequency, the RPM calculation would be as follows:

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

The slip varies slightly based on motor design; a suitable full-load speed for the above example could be around 1,725 RPM, making the difference:

• Full-load RPM - no-load RPM = RPM slip
•     = 75 RPM

For a two-pole motor at 60Hz, speeds are usually around 3,600 RPM unloaded and about 3,450 RPM loaded.

Get Help with Motor Repair

Understanding your equipment specifications goes a long way in optimizing its operation. By accurately calculating and controlling RPM, you enhance motor performance and longevity.

Professional repair and maintenance services are vital for maximizing the capabilities of your equipment. At Global Electronic Services, we specialize in repairing a broad spectrum of industrial equipment, including AC and DC motors, servo motors, and industrial electronics. To discover more about our repair services or inquire about your needs, please reach out to us.

For additional bench test engine information, contact us, and we will provide you with professional guidance.