Differences in overrun characteristics for AC motors

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Differences in overrun characteristics for AC motors

Postby om_tech_support_JT » Fri Nov 01, 2013 2:15 pm

Q: How do the overrun characteristics compare with the different types of AC motors that you offer, and is there a solution to improve the overrun characteristics?

A: Overrun is the number of rotations (or distance) that the motor takes until it completely stops after AC power is removed from the motor lead wires. Depending on the type of your motor (induction, reversible, clutch brake, electromagnetic brake types), the stopping characteristics may differ based on the design of the motor.

The induction motor has the most basic design and has no built-in braking mechanism. When stopping, the motor coast to a stop by load and bearing friction. In general, induction motors have the highest overrun about 30-40 rotations at the output of the motor shaft. However, you can reduce this overrun by attaching a gearhead. Assuming that you have the motor attached to a 360:1 ratio gearhead, that is about 0.1 revolutions (40 degrees) of overrun. This applies to any type of AC motor.

The reversible motor has a built in friction brake at the rear of the motor. This provides improves instant reversing characteristics by adding friction which reduces overrun. Due to this friction brake, the overrun for reversible motors is 5-6 rotations.

Overrun - AC reversible motor design, structure.jpg
Overrun - AC reversible motor design, structure.jpg (42.51 KiB) Viewed 3452 times

An electromagnetic brake motor is equipped with a power-off-activated type brake which involves an magnet coil and spring loaded brake. When no voltage is applied to the magnet coil, the spring works to press the armature onto the brake hub and hold the motor’s shaft in place, thereby actuating the brake. When voltage is applied to the magnet coil, it becomes an electromagnet and attracts the armature against the spring which releases the brake and allowing the motor shaft to spin freely. the Based on the mechanical design of this motor the overrun has been improved to 2-3 rotations of overrun.

Overrun - AC electromagnetic brake motor design, structure.jpg
Overrun - AC electromagnetic brake motor design, structure.jpg (21.04 KiB) Viewed 3452 times

Out of the motors types mentioned here, the clutch brake motor has the best overrun spec of 1 rotation. The clutch brake motor has a clutch disk, armature, and a clutch and brake. When 24VDC is not applied to either the clutch coil or brake coil, the motor shaft spins freely since the gear shaft is disengaged from the motor. When 24VDC is applied to the clutch coil, the armature of the clutch coil is drawn against the clutch disk, transmitting motor rotation to the gear shaft/output shaft of the gearhead. This design allows the motor to reach start/stop frequencies of 100 cycles per minute; compared to 50 cycles per minute for electromagnetic brake motors.

Overrun - AC clutch brake motor design, structure - brake on.jpg
Overrun - AC clutch brake motor design, structure - brake on.jpg (19.14 KiB) Viewed 3452 times

Overrun - AC clutch brake motor design, structure - clutch on.jpg
Overrun - AC clutch brake motor design, structure - clutch on.jpg (21.83 KiB) Viewed 3452 times

Below is a chart which compares induction, reversible, electromagnetic brake and clutch brake motors:

Overrun comparison between different types of AC motors.jpg
Overrun comparison between different types of AC motors.jpg (88.28 KiB) Viewed 3452 times

You can improve overrun of an AC motor (except for clutch brake motors) by simply using our brake pack to control the motor. This can reduce your total overrun to about 1 revolution. The SB50W brake pack brakes a motor by removing AC voltage from the motor, and then supplying DC voltage to the motor windings to create a stationery magnetic field which stops the motor. This function is active only for a very short time so there's no holding torque when you brake the motor. We have to limit the braking duration since it requires high current which leads to higher temperature. The SB50W can be used for frequent start/stop applications with AC motors (up to 60 cycles per minute). The SB50W will also control the direction of the motor, detect if your motor is improperly connected or overheating, and control the electromagnetic brake on an electromagnetic brake motor. It also accept a universal voltage range of single-phase 100~230VAC and works with induction motors, reversible motors, and electromagnetic brake motors from 1~90W.

For more information on our SB50W brake pack, please click the link below:
CATALOG: http://www.orientalmotor.com/products/p ... _sb50w.pdf
WEB: http://catalog.orientalmotor.com/item/a ... ries/sb50w

SUPPORT CONTACTS:
techsupport@orientalmotor.com
800-468-3982
http://www.orientalmotor.com

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