motors sharing common load 2

[Kiwijak]

Dear All Gurus

How can I drive a huge mill/crusher with two 3 phase induction motors of 90kW each. Four v-belts per motor goes onto a common 8 belt pulley that drives the crusher shaft.

How can I wire the control circuit for the motors starting gear, and what type of starting would be most suitable?

We don`t plan to change the speed of this mill, but struggle to start it with some product in it.

The motors are two identical Teco motors.

 

[edison123]

First, make sure both the motors are rotating in the right direction.

Post more information about the motors, control gears.

 

[waross]

One solution is to use a starer large enough to start both motors and use individual overload relays for each motor. Either one will stop both motors.
Check the KVA/HP code for the motors to ensure that the contactor is large enough. It may need to be larger than 2 x 90 kW.
See here for KVA/HP codes.

http://baldor.com/support/literature_load.asp?LitNumber=PR2525

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[fredt]

As a mechanical engineer I have always assumed that it was not a simple matter to split load equally between two motors coupled to the same machine. Small differences in slip in the motors could result in one being overloaded and the other taking much less load. Perhaps fluid couplings could be used to share the torque. However, I would also be very interested in the response from electrical engineers to this question.

Fred

 

[itsmoked]

fredt you are correct.

You will want two VFDs with the correct settings and set up to do this right. You need the motors to share the torque load evenly.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

I have seen many cement mills being driven by two slipring MV motors. Both the motors are identical from the same manufacture and load sharing is not a problem.

 

[waross]

I worked on the installation of a large mine mill. Each of six mills was driven by a pair of 3000 HP induction motors. No problems. Wound rotors with shared liquid rheostat starting equipment. Once a pair of motors was close to speed, a shorting contactor closed on the rotor circuit of each motor of the pair. The liquid rheostats were then switched over to the rotor circuit of the next motor to be started.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

I have done dozens and dozens of twin motor common shaft crushers, it's a very common application when the HP requirements are very large and the OEM doesn't want to make the users handle a single 600HP or 800HP motor.
300HP motors are usually something the local back-hoe operator can lift and manipulate without causing too much collateral damage (as long as it's before lunch ).

The most common method people use is a large single soft starter and separate OL relays. I have also done a lot of them where I started one motor on the soft starter and after the bypass contactor closed, I closed a 2nd contactor Across the Line, then interlock the infeed conveyor to the 2nd contactor so that you never start feeding product until you have the full HP available. This only works for Impactors however, you cannot usually do that with a Jaw, Pulverizer or Gyratory (Cone) crusher. You will want both motors on-line to accelerate them.

But (as I responded to you in another forum), it is imperative that the motors are identical with identical slip values. So since it appears that is the case, I say go ahead and do it. At 180kW, I would just use one large soft starter and 2 OL relays.

Drives are only necessary if the motors are mismatched, but then the cost becomes prohibitive unless Variable Speed provides a benefit.

Oh, and edison123's sage advice is fantastic!


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[MakeItSpin]

Belt tension is critical.

One set of belts a few thousandths deeper in the grooves will cause that motor to run faster and draw less current than the other motor.
Once V-belts are stretched and broken in it's less of a problem if the mounting is solid.

This application is common in the crusher industry. It's also common to explain to the managers why the electrician can't turn the magic knob to make the motors load share.

Several manufacturers will design and build an assembled starter to do just what you're asking for.