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I'm working on an application which requires varying the speed of a conveyor that will frequently need to stop and start. More specifically, we need to first set the speed of the belt, and then have frequent cycles of 3-4 seconds on and then stop for 3-4 seconds.

I had initially thought a simple 3-phase conveyor with a VFD (for speed adjustment) and then a contactor (for start/stop) would be fine... But a few google searches seemed to indicate that this would probably damage the VFD in a short order, since the VFD needs a few seconds to start correctly.

So is there a canonical way to set up this sort of system ? The conveyor load is low (few kgs) and the speed is also low (around 1m / 10 seconds).

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  • $\begingroup$ What about not stopping/starting the motor at all, and using an electromagnetic clutch instead? $\endgroup$
    – spuck
    Dec 20, 2019 at 0:16

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A lot of VFDs are programmable to the extent that you describe. You can program several preset speeds and send a command to the VFD that will tell it to start the motor, accelerate at a preset rate to a preset speed. When you want the motor to stop, you send command is sent to the VFD, and it will decelerate the motor to a stop at another preset deceleration rate.

The difficult part will be to determine if the motor can accelerate and decelerate as quickly as you would like. However, if it can be done, a VFD can likely do it as quickly as starting and stopping with a contactor and with less stress on the motor and mechanical system.

Start, stop and preset speed selection commands can be by contact open/close or by serial communication. If you need to have more than two or three preset speeds they can be sent by analog signal or serial communications. Some VFDs may store more than two or three.

It is best to set the system up so that the fastest operation ever required is obtained at or near the speed of the motor when operating at the 50 or 60 Hz power frequency.

Is selecting a VFD, consider what brands are popular in your area of the world. Also consider what brands are offered by local suppliers that are trustworthy and helpful. Download and read the manual before buying. You need a general-purpose VFD, not a VFD that is designed for fans and pumps.

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I think the best choice is a programmable VFD with computer controller.

This is a custom choice with a variety of interfaces ;

Keypad Control (Run/Stop)

2 Wire Control (Fwd/Stop)

3 Wire Control (Fwd/Rev/Stop)

Run/Stop Serial Comms (RS485)

Frequency Ref. from Keypad

Frequency Ref. 0-10V

Frequency Ref. Potentiometer

Frequency Ref. 4-20mA

Frequency Ref. Serial RS485

Frequency Ref. Pulse Input

PID Feedback Signal 4-20mA

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  • $\begingroup$ ok that's very interesting ! so you're saying there are vfd's that can stop and start as well as adjust the speed ? would you recommend any models in particular ? $\endgroup$
    – Petrov
    Dec 20, 2019 at 1:17
  • $\begingroup$ No But I can suggest you talk to VFD sales people. $\endgroup$ Dec 20, 2019 at 1:20
  • $\begingroup$ All VFDs can start and stop the motor, it is a requirement in fact, because to do so with a contactor can harm the VFD. $\endgroup$
    – JRaef
    Dec 20, 2019 at 20:04

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