Rotation of idler motor

D

Don Gitzel

Aluminum

Joined

Jul 25, 2017

• Nov 9, 2020

• #1

I recently assembled a 10 hp rotary phase converter with a proven design that I use. During commissioning I found that occasionally the idler would not start. Just sat there and hummed. I noticed that the idler was rotating counter clockwise. I changed wiring to have it rotate clockwise. The idler then started consistently every time. Should the idler always be wired to rotate clockwise. If this is so, it would be great to have someone explain to me, why. Cheers.

 

rons

Diamond

Joined

Mar 5, 2009

Location

California, USA

• Nov 11, 2020

• #2

It is not so.

The occasional hum is something that happened with my last converter. The hum was followed by the magnetic contactor disconnecting because of over-current draw.
I built a new converter with about the same design but used a motor half the horsepower. No more stalls.

The stall condition has been brought up before. In theory the magnetic flux lines should always interact with the rotor bars and cause a movement. If the rotor
has a dead stall position then that is one thing. But I assumed it had to do with the way the initial conditions of the start and run capacitors.

 

JST

Diamond

Joined

Jun 16, 2001

Location

St Louis

• Nov 11, 2020

• #3

There may be some particular condition with the motor that stops it from working sometimes. Might also be a slightly weak starting circuit, the geometry may make it weaker in one direction.

Direction of idler rotation, if consistent, is not an issue.

 

D

Don Gitzel

Aluminum

Joined

Jul 25, 2017

• Nov 11, 2020

• #4

What means “the geometry”? It does seem that the idler is harder to start in counterclockwise rotation. The Code on the motor is H which I believe is for hard starting loads.

 

D

Don Gitzel

Aluminum

Joined

Jul 25, 2017

• Nov 11, 2020

• #5

What means “the geometry”? It does seem that the idler is harder to start in counterclockwise rotation. The Code on the idler motor is H which I believe is for hard starting loads. Does this mean the idler will be hard starting as well?

 

rons

Diamond

Joined

Mar 5, 2009

Location

California, USA

• Nov 11, 2020

• #6

I have a 5hp motor that hummed once in a while. I assumed that it had to do with my RPC capacitors and initial start conditions. The motor runs fine with a VFD.
Take a motor that hums like that and run it on real 3 phase. See if it hums then. My opinion that external capacitors cause it.

Geometry is the make up of the motor. Windings, rotor, stator, how it was designed.

 

JST

Diamond

Joined

Jun 16, 2001

Location

St Louis

• Nov 11, 2020

• #7

Geometry:

Yes. How the poles line up, if there is any fault, like a broken conductor in the rotor, whether the rotor is designed with skewed poles, etc, etc.

And, once in a while a rotor gets put in a motor that it actually is not intended for, maybe during maintenance, and then even though it may appear to work, there may be some odd feature of it.

 

S

Strostkovy

Titanium

Joined

Oct 29, 2017

• Nov 11, 2020

• #8

I had somewhat similar issues.

During development of my RPC I had a 10hp working idler that started with a capacitor. I also had a pony started 40hp motor, which I didn't know at the time had shorted windings. During attempts to start it, line voltage got low enough that the start capacitor of the 10hp pony would kick back in, damaging the contactor and causing unreliable starts of that section of the converter.

I'm still annoyed by that RPC. It's just sitting at the old shop ready to start but lacks a working idler. I added soft start and phase matching functionality to the point that it should start even a big synchronous motor in a vain attempt to get the short circuit to generate a third leg.

 

J

johansen

Stainless

Joined

Aug 16, 2014

Location

silverdale wa

• Nov 12, 2020

• #9

It is more likely you had a loose connection that was "fixed" when you rewired it.