Grinding & Classification Circuits

Grinding & Classification Circuits 2017-04-04T06:57:16+00:00
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Using low tension water (4 replies)

Bill Fraser
1 year ago
Bill Fraser 1 year ago

Do you know if it’s possible to supply a mill with Squirrel Cage Induction Motor Drive using low tension motors (480V) instead MT? (4160V) when power requirement is less than 1000kw?

1 year ago

I'm not too sure this is possible, with using SCIM you will need to also have a soft starter / VSD to operate the mill successfully. A LV VSD can be used, although I am not sure if a LV motor would have enough torque capacity to drive the mill. A better solution may be MV WRIM with LRS. You will need to talk to the motor vendors in detail.

Dizzy Flores
1 year ago
Dizzy Flores 1 year ago

In general, if you run a SCIM (whether this is an LV or an MV Motor does not matter) on a VSD, you automatically have the motor's full Breakdown torque available at start – that is the benefit of having a VSD. Breakdown torques typically range from 220 – 250 % of nominal motor torque so if your LV Motor is correctly sized to offer the necessary torque at the required speed, you have plenty of headroom left to overcome the starting toque of the mill – which is typically about 150%.

Please Note that despite the fact that your motor offers up to 250% of nominal torque, you should not operate your motor within 10% of that level (i.e. overload the motor never more than say 220% as there is the risk of the SCIM to fall out of step. In high overload application you also need I2t protection that makes sure your motor does not overheat. VSDs offer that as a standard while with LRS and WRIM applications you need to add separate motor protection relays.

WRIM and LRS systems are another alternative to gain access to the motors breakdown torque at start to overcome the high starting torques. However these systems don’t offer speed control. There has been another post on this forum recently asking why there are high re-circulating loads on Ball Mills. There is always an inherent risk that the ore that will actually pass through your mill is not quite like the one that was used to do the process design. There are many mining sites out there today that don’t run at maximum capacity because they encounter high P80 rejects and as a result high circulating loads on Ball Mills. VSDs are an insurance policy that allows you to vary the speed of your Ball Mill which can result in a better grinding efficiency and lower circulating loads.

All content provided above is for informational purposes only. The author makes no representations as to the accuracy or completeness of the information provided and will not be liable for any errors or omissions in this information. The author will not be liable for any losses, injuries, or damages from the use of this information.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

Interesting analysis over the operational challenges to face whether the drive selection!

1 year ago
Gruppen 1 year ago

Below 1000 kW it is not a big problem to operate the mill with a low voltage motor. The problem is the start-up, where you have 6-7x nominal ampere which is typically not possible for the start-up time you need. Either you use a frequency converter and start up the mill slow or you go for Slip Ring motors. We do this all the time with our ball mills and no problems so far.

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