Which direction to rotate 3 phase motor in for RPC use ?

I have a Baldor 230V 3 phase 15hp motor I wish to use as a RPC. To start it, I plan to spin it up to operating RPM with a pony motor of 3/4hp. How will I know what direction to spin the RPC 15hp motor in? Does this matter? Is the RPC motor direction of rotation important? Could I screw up by rotating the RPC motor one way, then engage a switch, only to have the RPC try to slam itself into reverse?

In the future I will probably go with an American Rotary 15hp rated panel. For now I hope to use the Baldor as is with maybe a few run capacitors added in.

Thanks in advance.....Dan Miller

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The RPC will run in the direction in which you start it. It does not matter which way it runs. So spin it in the directions that is mechanically the simplest to implement.

And if you spin it the other way, all it will do is BACK UP all the machines it is feeding.

I think it depends on the hemisphere of operation. Northern is clockwise, Southern counter-clockwise.

It works the same as the right hand rule.

johnoder said:

And if you spin it the other way, all it will do is BACK UP all the machines it is feeding.

Click to expand...

and the meter !

Lock 45 said:

I have a Baldor 230V 3 phase 15hp motor I wish to use as a RPC. To start it, I plan to spin it up to operating RPM with a pony motor of 3/4hp. How will I know what direction to spin the RPC 15hp motor in? Does this matter? Is the RPC motor direction of rotation important? Could I screw up by rotating the RPC motor one way, then engage a switch, only to have the RPC try to slam itself into reverse?

In the future I will probably go with an American Rotary 15hp rated panel. For now I hope to use the Baldor as is with maybe a few run capacitors added in.

Thanks in advance.....Dan Miller

.

Click to expand...

With no capacitors installed, either direction will work fine.

OP says he will have run capacitors, therefore the pony motor and capacitors must agree. You probably can start without the pony motor and see which way it tries to run, then hook up the pony to start in that direction. If it doesn't start with the capacitors, try rope starting each direction. Hold the rope so it can jerk out of your hand without injury. Remember that the motor will be drawing high current when it is trying to start and turning slowly, so don't leave the power on for more than a few seconds to avoid burning a winding. It might be useful to do the test with it hooked on a 120 V line instead of 340. The phasing will be the same.

Someone asks legitimate questions for someone not familiar with the process and gets a lot of smartass answers and little information. No wonder PM has a reputation for being mean.

Bill

Bill 9100,

"OP says he will have run capacitors, therefore the pony motor and capacitors must agree."

I agree that the run caps will most likely create a bias as to which direction the motor will try to run. However, I have always added the caps to tune the balance as opposed to a pre-run balance guess. So my question is two fold. Does a "best balance" always favor the direction the RPC is turning? If you were to reverse the RPC would you not also end up swapping the two run caps to achieve balance again?

If you move the run capacitors over to the other line, the motor will run in the opposite direction. As to balance, my personal experience is that if you adjust the run capacitors for the load that it will normally have, balance capacitors are not needed. My advice is always the same- hook an oscilloscope to the manufactured leg and watch the amplitude and phase angle with varying load. With no or small load, the voltage will be high and the phase shift too small. As you increase the load, the voltage will drop and the phase shift will increase. If you are running on a typical 120-0-120 line with the oscilloscope common grounded, the manufactured leg should read 1.732 X the voltage from one line to ground. The phase angle should be 90 degrees at the desired load. The phase angle would read 120 degrees between legs if it was a wye connection with the reference point at the junction of the wye, but this one is from a different reference point.

If the idler motor is large enough relative to the load, you don't need any capacitors, as Jim Rozen keeps demonstrating. If you are only need to run at one load, never varying, you don't need an idler at all, just the right capacity. I ran a vertical mill for nearly ten years with a box of starting and run capacitors and a current sensitive relay to switch the starting capacity out when the motor starting load dropped off.

Bill

When belting a pony motor to an idler, it is best to not cross the belts. This defines the rotation of the idler.
To put it another way, the rotation direction does not matter. Once you have it running then connect the
load machines so the spindles run in the correct direction. If they run in reverse, swap two of the leads
going to the load motors.

9100 said:

OP says he will have run capacitors, therefore the pony motor and capacitors must agree. You probably can start without the pony motor and see which way it tries to run, then hook up the pony to start in that direction. If it doesn't start with the capacitors, try rope starting each direction. Hold the rope so it can jerk out of your hand without injury. Remember that the motor will be drawing high current when it is trying to start and turning slowly, so don't leave the power on for more than a few seconds to avoid burning a winding. It might be useful to do the test with it hooked on a 120 V line instead of 340. The phasing will be the same.

Someone asks legitimate questions for someone not familiar with the process and gets a lot of smartass answers and little information. No wonder PM has a reputation for being mean.

Bill

Click to expand...

Sir,
I am in the process of making a 15hp motor into a RPC. I could easily read the posts on this forum without joining and get the information I need, but felt compelled to join after reading your observation:
"Someone asks legitimate questions for someone not familiar with the process and gets a lot of smartass answers and little information. No wonder PM has a reputation for being mean." I am on a few forums and hardly ever look at them any more because of the level of detritus, rather than just clear well worded answers. I am learning about RPCs, and getting good information is of the utmost importance.
I have a 9" South Bend lathe and a 1956 Bridgeport knee mill. I look forward to perusing this forum to gain knowledge about all aspects of calibration, operation and maintenance of these machines.
Regards,
Doug

Bill Simmons said:

The RPC will run in the direction in which you start it. It does not matter which way it runs. So spin it in the directions that is mechanically the simplest to implement.

Click to expand...

Thank you Mr. Simmons for this information. I learned today that the idler motor has to be going fairly fast before it will spin up to speed when power is applied. I am working with a 15hp motor and had tried getting it to run on single phase by kicking the pulley to get the motor spinning and then apply power. It would slowly decrease in RPM, and being afraid of destroying the motor, I would disconnect power. Kickin the pulley to get it started was based on an article I had read in an old book from Fine Woodworking. Today, I used a drill to get it spinning fairly fast and applied power and it spun up to speed! I checked voltage between phases and it varied between 215v and 245v. Now to get on with the project by adding appropriate start capacitor and balancing the output. Having 3 phase power is a game changer for me.

15 hp is pretty large, the inrush for a self-starting could be problematic. Suggest: pony motor start. Belt it bring it up slightly above nameplate speed before energizing.