Frequent Start/stop Of Motor Coupled With High Inertia Load

[AB2005]

Hi Mark;

 

We are working for an energy saving project in our Corrugator plant where we have one Corrugator machine and four Flexo Graphic printing machines. We have Trim Waste Blowers at Corrugator and Flexo machines which throw the trim from machines to a trim collector cyclone away from machines. Each blower has 18KW-400V motor with star delta control. As the motor is coupled with impeller via v-belt and pulley, so the starting time of blower is almost 12sec. One of our colleague was thinking to control the Flexo machines blower in this way to start it for 5 minutes and then stop it for 5minutes i.e. 5 minutes flip flop as operationally it can be possible but technically I did not agree with him because for me, for such kind of motors, the number of start stop per hour is limited because of high inertia load. For such frequent start/stop, only soft starter or VFD can be ok.

 

Any comment on it please?

[mariomaggi]

Hi,

to save energy and to reduce acoustic noise and to improve quality of pollution, I see only one solution: VVVF drive.

Star/delta starters should ask sometime a very high current after commutation, this overcurrent should be up to 20 x In and can damage cables, contacts and trafos.

 

Best regards,

Mario

[rotomoulder]

If your blower is a forward facing unit you could try leaving the motor running but close the discharge fan side which will make the blower stall and the motor current will drop. Then when you need it again you can open it and your motor will again start to work for its living. How you open and close is up to you.

We see a big drop in motor current when our discharges block, something we dont want

Regards

Terry

[AB2005]

Hi rotomoulder

 

Blocking of discharge can be harm for motor however blocking of inlet can be possible and i am also thinking on it.

[marke]

Hello AB2005

 

I understand that your concerns about the proposal is that the number and length of starts will damage the motor

 

It is not too bad, you are describing 6 starts per hour at 12 seconds per start, and a duty cycle of 50%.

I do not think that this would be a problem for most industrial motors.

There are plenty of applications that I have seen that are much more severe than this.

[rotomoulder]

Hi rotomoulder

 

Blocking of discharge can be harm for motor however blocking of inlet can be possible and i am also thinking on it.

AB2005 I will check what happens to the motor current when the inlet is blocked and revert. However you are not correct regarding blocking of the outlet causing harm. The only thing that happens in a practical sense is that the current draw reduces.

I will arrange for some figures later today but please keep in mind that blocking the suction/inlet is easy but removing the blockage is not.

Terry

[rotomoulder]

I checked the first of our blowers that I came across with the following results:-

 

Motor Name plate 415 Volt 3 phase,3 kw,2880 rpm, 3.7 amps.50 htz

 

Running under normal load conditions ie suction and discharge of the blower open:-

2959 rpm

3.65 amps

 

With closed discharge or suction

2989 rpm

3.06 amps

 

ie a reduction where the current draw is 84% of normal.

 

Three more points.

I watched as the suction damper was removed whilst the unit was running and the effort required was far greater than the discharge damper which wanted to open under the same conditions.

 

We have never had problems with the motor when the discharge becomes blocked.

 

All of our blowers are forward facing. I cannot speak about backward facing blowers.

 

Terry