Replacing 30 kw motor with 60 kw motor in plate bending m/c

[CHARLESCOSTA]

A 30kw motor that was supplied along with the original 70mm
plate bending m/c has burnt out.Right now we only have a 60 kw motor as a replacement. Both are slip ring 975 rpm motors.Assuming that the factor of safety on various mechanical components like gears ,shafts etc used in the m/c is 1.5 for static strength,does any one forsee any possible damage that is likely to be caused to gear box,rolls etc by this replacement .Use of m/c is infrequent (say once in a month).

 

[MintJulep]

With very little information to go on in your post...

Motors "Burn out" when they are asked to more than they are designed for. Frequently this happens when something in the output train jams, causing the motor to stop turning. In such a condition, the motor will produce its maximum torque before it burns.

So if you replace the motor with one with twice the power, it is very likely that you will find the next weakest link.

 

[CHARLESCOSTA]

What I really want to know(not only with reference to the problem mentioned above)is the effect (on internals of gear box)of using 60kw975rpm motor for a gear box having a rating of 30 kw at 975 rpm.The gear box is used infrequently and has a factor of safety of about
1.5 for static strength.Load in both cases is same and corresponds to 30kw rating at 975 rpm.

 

[ScottyUK]

The motor will only deliver the torque required by the load. As the machines run at the same speed I see no reason why the larger motor would cause any problem UNLESS something causes the load to increase above normal running conditions. In that case the larger motor might well have enough torque to break something. Your electrical guys can probably set the motor's control and protection settings down to a level corresponding to the smaller motor so if it did reach a stall or overload condition it would trip in a similar time to the smaller one. The starting characteristics of a bigger motor are different to the original so your electrical guys need to consider how they are going to start this bigger motor - the chances are the existing controls will struggle.


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Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[JStephen]

It depends too, on how the machine is used. If capacity is determined by putting a piece in to see if it can roll it, you may have trouble.

 

[CHARLESCOSTA]

Thanks to all of you for your thought provoking replies.The 60kw slip ring motor will have double the rated torque(of 30kw motor).Since load is constant,60kw motor will supply the required loadwith less slip.So at full speed it will be exerting the same torque as 30 kw motor was.

The running speed will be slightly higher.However since pullout torque(the max the motor exerts during startup,though only for may be a second)is generally about 2.75-3 times the rated torque,60 kw motor will exert so much more torque during startup.Please comment on this difference in torque during startup.I don't really know if pull out torque explicitly determines the design of gears, shafts etc .In various books its the full load torque which is mentioned for static strength design.

 

[eromlignod]

The motor will only produce torque in response to the load. The torque rating of the motor is the *maximum* torque it can produce. Your new motor will produce the exact same amount of torque on the machine as the old one; it just won't sweat as much. You could put a motor with a million horsepower on this machine and it would run the same.

The only thing to worry about is locked-rotor torque. In that case the larger motor will max out at the higher torque and the mechanism will have to be able to handle it.

Have you considered a simple torque-limiter at the shaft coupling?

Don
Kansas City

 

[ScottyUK]

Don,

The larger motor will also deliver more accelerating torque than the smaller one so the machine will reach speed faster. Without knowing much more about the machine it is difficult to say what the consequence of this might be. You might consider a soft start to control the acceleration if you think this could be a problem. Drop a post in forum237 if you need advice on that - there a couple of guys who specialise in soft starters.


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Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[zekeman]

The load determines the forces on the elements of the gear train, not the motor power capability. The load would be both end external load and inertial. The inertial load seldom causes any burnout problems but could theoretically overload the geartrain. After the system comes to speed equilibrum, the torque load times the speed is the power delivered. If it is less than the rated power of the replacement motor after applying a safety factor, the motor is OK pending the requirement that geartrain torques (inertial plus load) at each gear are less than the design capability of the gears. If the calculated power is greater than the burned-out motor, then that was the cause of the failure in the first place. If not, motor failure may have been caused by a jam or perhaps gear or bearing failure(s)
which must be remedied before motor replacement.