Strange Ac Drive Wiring

[plc noob]

I have encountered a ac drive setup that i have never seen before. this is a 460volt yaskawa drive connected to a 230/ 460 volt motor and the motor is wired for 230 .this a fairly old setup to my understanding . the prints on the machine show this wiring as correct and metion it being this way to reduce slip in the motor . i have never seen this before . any opinions

 

[rsdoran]

Show us what the print has please and offer as much detail on the drive and its settings as possible.

 

[plc noob]

the print is just a standard motor symbol with three wires going to a box label yaskawa ac drive and has the hp 5 hp and the voltage . there is a notation that says ac drive wired for 460 vac and dual voltage motor wired for 230 vac to control slip

 

[mordred]

Are they supplying 460 volts to a 230 volt wiring? or is the AC drive supplying 230 volts with a 460 volt input supply? could you supply make of the yaskawa drive and motor as this isn't a method of controlling slip that I'm familiar with I'd hate to think of the possible problems with your carrier frequency etc.

 

[plc noob]

it is 460 volt drive supply 230 volt motor as wired . i do not have any additional info on the drive or motor . i do know that the drive itself is about 12 to 15 years old

 

[rsdoran]

We had a discussion similar to this once, its a "cheat" per se to get more KW out of a motor. I will see if I can find the thread.

 

[mordred]

here is a link on slip principals http://www.plantservices.com/articles/2002/48.html?page=1

with the AC dive you should be able to control the slip, So Ron's suggestion that its probably due to the KW rating is probably more accurate

 

[milldrone]

plc noob,

Although not very popular this method can be used to get high speeds out of a standard motor. The VFD needs to be aproximatly twice the motor size. A custom volts to hertz ratio will need to be configured.

By using this scheeme the motor is supplied 230 volts at 60 hertz and 480 volts at 120 hertz. The torque from the motor will not drop off like it would above 60 hertz in a standard configuration.

 

[plc noob]

so wiring it by this method does not change slip at all ?

 

[mordred]

it affects torque which in turn affects slip

 

[rsdoran]

plc noob,

Although not very popular this method can be used to get high speeds out of a standard motor. The VFD needs to be aproximatly twice the motor size. A custom volts to hertz ratio will need to be configured.

By using this scheeme the motor is supplied 230 volts at 60 hertz and 480 volts at 120 hertz. The torque from the motor will not drop off like it would above 60 hertz in a standard configuration.

Thats the one, and it is not something I would do but appears to work for many.

Slip in general is necessary for torque generation and the use of a VFD (ASD) offers the possibility of minimizing slip anyway so the only gain I see is trying to "falsely" create more KW/HP then you actually have. I would rather use a larger motor and drive in small HP applications.

Just to refresh your memory "slip" is the relationship between actual speed and synchronous speed. A 4 pole motor @ 60 hertz will ideally turn 1800 rpm but if you look on the nameplates you will see many showing 1725, 1740, 1750 etc. because there is resistance by the load against the rotation of the motor shaft which creates "slip".

What happens in some situations is the load is "borderline" so the motor which should get 1740rpm may not get but 1700 or so, so people just "tweak" the system a little to get a little more performance out of the motor. Overall these days inverter rated motors can handle a little tweaking but not sure I would do it.

 

[plc noob]

will wiring it this way give you 1800 rpm on a 4 pole motor @ 60 hertz or will it be more than 1800 . in effect how much more hp is generated based on a 4 pole motor @ 60 hertz . how would you calculate the amount of increase ?

 

[rsdoran]

Take a look at these:
http://www.patchn.com/poles_%26_rpm.htm

http://www.patchn.com/motorformula.htm

Think about them and play with the numbers a little.

 

[allscott]

@ 60 HZ 230V you will see no difference, the motor is running at nameplate in this case.

The trick here is that as you go above 60 HZ the voltage increases as well. Therefore the V/HZ ratio remains the same and the torque will not drop off. I'm not sure how high I would want to go above nameplate voltage before expecting problems.

Slip isn't something that you really want to or can control. As the slip increases the motor torque increases, simple as that.

 

[milldrone]

The trick here is that as you go above 60 HZ the voltage increases as well. Therefore the V/HZ ratio remains the same and the torque will not drop off.

Allscott has it right. This is sometimes used on wood working spindles to get 5400 rpm (or higher) out of a 3600 rpm motor.

I'm not sure how high I would want to go above nameplate voltage before expecting problems.

Since the motor is a dual voltage unit, the insulation would be for the higher voltage any way.

The primary uses for such a configuration are to get the higher speed without torque loss and to get more hp in a smaller package. Both uses are detrimental to equipment.

plc noob,

The hz to speed relationship will not change. If you have a 1800 rpm base speed motor, 60 hz will still be 1800 rpm minus the slip . Does this application use speeds over 60 hz?