Max Start/Stop Frequency of Three-phase Induction Motors

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Joined: Tue May 10, 2016 4:48 pm

Max Start/Stop Frequency of Three-phase Induction Motors

Postby om_tech_support_JS » Tue May 10, 2016 5:26 pm

Q: I have an application which involves a conveyor that needs to run for 3-6 seconds at a time, and I need the conveyor to run 4 cycles in 1 minute. What is the max start/stop cycle frequency of your three-phase induction motors?

A: While our single-phase AC motors can start/stop up to 60x a minute when using with our electronic brake pack, unfortunately, there is not an exact max start/stop cycling frequency (cycle time) specification for our three-phase induction motors as it depends on the application and heat rise. Our induction motors are rated for continuous duty. Most of the concern is the additional heat rise due to inrush current. When an AC motor is turned on, it draws an inrush current which is much more than it's continuous rated current, and the motor will experience this for a very short duration. Current is proportional to temperature so the motor will heat up the more you start/stop the motor. We would recommend to keep the motor case temperature below 90 deg C (194 deg F). We offer either thermal protection or impedance protection for our motors which will either turn itself off or decrease current draw in order to prevent damage due to overheating. However, keeping motor case temperature lower would increase motor life. Normal operating temperature should be around 60 deg C (140 deg F). Since a difference in load could affect the overrun of the motor (OFF until complete stop) which would in turn affect the cycling frequency, the cycling frequency could vary due to different load conditions. Therefore, testing will need to be done to ensure that the selected motor's overrun characteristics and heat rise will be OK for the specific application.

When the motor is unloaded, a safe, reference value for the repetition cycle is 2 seconds minimum (or 1 second on and 1 second off) for 6~40W motors, and 4 seconds minimum (or 2 seconds on and 2 seconds off) for 60~90W motors which makes 30x a minute or 15x a minute respectively. These reference values can be exceeded as long as you can keep the motor temperature down to your desired level. This max cycling frequency (cycle time) will ultimately be dependent on the temperature of the motor. In the case of quick cycling where the motor doesn't have enough time to cool down, the heat rise will increase. In the case of a loaded motor, the actual time it takes to stop the motor will increase and inrush current also increases. For some instances, more time is necessary to cool the motor down, and thus decreasing the max start/stop frequency.

For your application, you stated that you needed to run the conveyor for 3-6 seconds and have 4 cycles per minute. This operation will have less heat rise from inrush current since you will have time to stop the motor for longer and let it cool. Our 3-phase induction motors should work fine if properly sized. Testing is the best way to find out how the motor will work in your specific application/environment. The cooler you can keep the motor case temperature, the longer the motor life will last.

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