tandem motors
tandem motors
(OP)
I want to direct couple two 500hp 480v induction motors to a grinding mill,one motor at each end of the shaft.I would like to use two soft starts to start the motors at the same time.Mechanically we are capable of handling the hp.I would like to know if this is possible without using fluid couplings or vfds to account for slip and speed control.The
two motors are rated at the same rpm.
two motors are rated at the same rpm.





RE: tandem motors
There are several issues here.
First off; will it work? Yes, it is done all the time. The critical thing here is that the motors must be exactly the same, as is make, model, slip %, everything. If not, one motor will drag the other.
Second; Can I use 2 soft starters? Yes, but there are additional complications. While I have made it work when someone before me has designed it with separate soft starters starting simultaneously, that is NOT the best approach. It is virtually impossible to achieve uniform current limit or ramp settings between them, so again, you end up with one motor dragging the other anyway. If this is the case, there was no point in using the second soft starter. Let me explain. If one motor is capable of accelerating the load by itself, as is usually the case with grinders by the way, you can use a soft starter on one motor, then bring the second motor on across-the-line after the load is at full speed. You will get only a brief (as in 1 or 2 cycles) inrush to magnetize the windings, but no appreciable starting current surge. I do this all the time with large rock crushers, works great and costs a lot less.
If one motor is incapable of accelerating the load by itself, then your 2 soft starter method is at risk to not work anyway. Whichever motor is allowed to have even a slight amount of voltage more than the other will attempt to bear the entire load and trip on overload. Sometimes the soft starter outputs can be close enough so that motor #2 can contribute some, but my experience is that is rare and likely a chance occurrence rather than an accuracy issue. So if you need more than 500HP to accelerate the load to full speed, use 1 soft starter big enough to start both motors. Then all you need is separate overload relays for each motor and you have a complete system.
JRaef.com
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RE: tandem motors
However, I would get a quote for starters both ways - two individual units and one for both. I expect the individual starters will be cheaper though.
The load sharing is something you should take-up with the motor manufacturer.
Honestly, I don't buy the theory that a slight inbalance between the starters would cause one motor to bear all the load. Im somewhat guessing but it's probably closer to the square of the difference. ie, 5% voltage (or current) difference between the units would be 25% sharing difference - one motor sees 63% of the load and the other sees 37% of the load. It's probably not even that much. At any rate, two good current controlled starters should easily beat 5% current difference and have good sharing.
RE: tandem motors
The two motor's design must be as close as possible to share the load. But if they are not identical the one with lower slip will take more load. The line currents will give you an idea of the load sharing. If the total load is less than 1000 HP you could have some margin to play without overloading out of allowed limits one motor (see NEMA MG1 standard).
RE: tandem motors
An interesting alternative is if a 100 HP or 200 HP motor can accelerate the load before load is thrown on it, you could belt that smaller motor to the larger motors, soft start the small motor, energize the big motors one at a time, and then deenergize the small motor. This is a rather old fashioned idea from the days when design A motors had absurdly high locked rotor current and soft starters just did not exist. In some cases the smaller pony motor had a torque converter.
I have also seen some rather new industrial machinery that does use a torque converter instead of a soft start and this method is frequently used for amusement park rides. Since torque converters can be purchased with locking clutches this is a very efficient alternative to soft starters. However, your environment might be too abrasive or corrosive for this.
My my brief experience with crushers is that I would rather be dealing with 4 smaller crushers. If something goes down you only lose 1/4 of production capacity. Also, rock crushers require enough in the way of daily maintenance that a welder needs to have a permanently connected ground cable.
Mike Cole
RE: tandem motors
RE: tandem motors