intermittent inverter running

[Bobobodopalus]

HI all,


i dont suppose anybody knows of some inverter range that might be specialized for running intermittently?


we have a project where the conveyors are wanted to run for about 5 seconds then stop for 15 seconds repeatedly.


i am telling my colleagues they need to put clutches in or this to work but apparently they really dont want to so i'm looking all over a potential solution.


if anyone has experience with doing something similar please let me know how it went, how much it shortened the inverter life span by, we could live with them lasting only a year or 2 but its obviously not feasible if the inverters die every month

 

[JesperMP]

For the VFD it is unproblematic to start and stop frequently.
Only issue is that there is the risk of a higher thermal load due to having the starting current so frequently. It gets complicated, but not imposssible, to calculate the thermal load since you have the starting current for example for 0.5 sec, then a fraction of the nominal current for 5 seconds and no current for 10 seconds.
The VFD supplier possibly has a sizing guide based on the load profile.
For the electrical motor one must take into account that there is no cooling for the 10 seconds the motor is stopped, unless of course the cooling fan has an independent motor.
For abnormal load profiles, it is recommended to protect the motor with PTCs in the windings.
Generally, if the electrical motor can take the load, so can the VFD.
If you are in doubt, size the VFD one size bigger than theoretically needed.

I think it will be much better with a VFD than with mechanical clutches.

 

[parky]

We had over 20 machines each with an infeed conveyor that indexed at the rate of about 30 times a minute, so for example it indexed for about half a second then stopped for about 1.5 seconds (we did have one that was over 50 per minute), these were 0.75kw standard motors driven by Mitsubishi inverters with the ramp times set to minimum i.e. I think 0.5. We rarely had problems either with the motors or inverters to be honest if I discount the known faults like water ingress i think in 16 years we only had two inverters fail & a couple of motors, there was no indication that this was caused by the stop/start times being so short, it normally only affects the larger standard motors.

 

[Steve Bailey]

Starting and stopping that often using an across-the-line starter is hard on the mechanical components. How often are you replacing chains, sprockets, gearboxes? How often does maintenance get called for repairs? A VFD with properly adjusted accel and decel times will reduce wear and tear on the drivetrain. The duty cycle you cite should not be a problem for a properly sized VFD.

 

[Bobobodopalus]

ok i expected the effects to be worse, will have a look at some 0.75kw VFD's, PTCs will be used.


so these conveyors will actually be belt with a pulley motor, no sprockets.
i have no idea how often the customer will perform maintenance, it will likely only be when some damage appears, although greasing, oiling etc will be expected often.

 

[Nova5]

If you are start/stopping a VFD that often you can send it a Zero speed reference instead of having the VFD open/close its contactors that often. You still have the motor thermal loading to check on.

 

[Bobobodopalus]

it would be taking it in and out of run mode, not cutting power, the ramp up and down would be in effect

 

[Tom Jenkins]

It's inrush current that usually causes damage. A VFD has a controlled current, so the thermal load on the motor windings is minimized. If you are still worried a cost-effective solution is a motor temp switch: https://www.sensata.com/products/motor-protectors/17am-motor-protector Many motor suppliers have this available as an option. The VFD should be configurable to use this as a run permissive on a discrete input to the drive.

Most conveyors are constant torque loads, so size the drive accordingly. Work with your suppliers and there should be no problems.