## Speed Accuracy: AC Synchronous Motors VS Brushless Motors

om_tech_support_JT
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### Speed Accuracy: AC Synchronous Motors VS Brushless Motors

Q: I'm trying to decide between your AC synchronous motors and your brushless motor systems for a small clock application in an amusement park. I'm looking to standardize on one motor for various clock hands so I need the speed to be exactly the same regardless of load. That's why I'm looking at these motors. I see that you provide a speed regulation spec for your brushless motors, but not for AC synchronous motors. Do you have any data to confirm the AC synchronous motor's speed accuracy with fluctuations to load?

A: Due to its construction, the AC synchronous motor is designed to accelerate up to, then run at the motor's synchronous speed (1800 RPM) regardless of load fluctuation. The synchronous speed of the motor is calculated with the following formula:

120 x Hz / # of poles
# of poles = 4; so synchronous speed depends on the power supply frequency

As long as you do not exceed the motor's starting torque, the AC synchronous motor will start and accelerate up to exactly the synchronous speed. If the torque increases to above the rated torque of the motor while it's rotating, then it will not run at synchronous speed.

The aluminum knotches on the rotor inside the synchronous motor create 4 low reluctance salient poles. Magnetic flux from the stator flow easily in areas of low reluctance, so we end up with a 4 pole rotor and a 4 pole stator. In this design, the rotor is able to keep up with the synchronous speed of the rotating magnetic field as long as the torque doesn't exceed the motor's capabilities.

Here is a drawing that compares the rotor design from an induction motor and a synchronous motor, and it shows a speed-torque curve for the synchronous motor.
AC Synchronous Motor Construction, HighLow Reluctance, Salient Poles.jpg (65.78 KiB) Viewed 6130 times

aldoned
Posts: 32
Joined: Mon Sep 14, 2015 3:03 am

### Re: Speed Accuracy: AC Synchronous Motors VS Brushless Motors

Thanks for the info