# My 3 phase motor lacks power

Summary:
wiring question about 3 phase motors
I have a small 1hp 3 phase that really lacks torque, it was on a big lathe (12hp) and the entire rest of the machine was wired for 240V delta.. but this motor only has 3 wires coming into the connector box, so there are no options for changing the voltage, yet the nameplate says 220V Delta or or 440V Wye, if I'm reading it correctly?
I checked the phase to phase resistance, it's 21 +/- 1/2 Ohm, I tried running it on a VFD, still has no torque, RPM is correct for the frequency, draws 4.5A when stalled
How much torque does it have? well, you can easily grab the 2.5" pulley on it with your hand and stop it.. That's certainly not 1 hp.

Only thing I can think of is that it's meant to be 440V powered, but then how did it ever work?

Paul Colby
Gold Member
Two things I would check. The first is are all 3 windings still good? The second is does the motor have a capacitor? If so it may be configured to run on less than 3 phase power. If the cap is bad the motor would lose torque. Just a guess.

Two things I would check. The first is are all 3 windings still good? The second is does the motor have a capacitor? If so it may be configured to run on less than 3 phase power. If the cap is bad the motor would lose torque. Just a guess.
as far as I can tell the windings all have the same resistance and none are shorted to ground

I took a long thought about all the stuff that was in the control panel of the lathe, all the contactors are 120V powered, but there was a small 480-120V transformer in there unused.. I'm thinking the lathe used to be 480V in its original state, then was rewired for 240V, the 480-120V transformer deprecated and a 240-120V added

I still can't see why this motor lists 220V as an option, it's gotta just be a 440V model, no options for capacitors, or changing the winding configuration.. I am running it on 3 phase power

Paul Colby
Gold Member
The caps in the control panel. I'm guessing they are for reducing the arcing on the switch? Could one of those be bad?

More pictures would help.

Paul Colby
Gold Member
Also, since it's not the main motor, the rotation likely can be reverse. Does it work as poorly in reverse?

Also, since it's not the main motor, the rotation likely can be reverse. Does it work as poorly in reverse?
Yup.. I'm plenty familiar with 3 phase motors, just never seen a dual voltage one that didn't have 9 wires in the box
I've never seen caps on 3 phase equipment unless it's maybe for power factor correction

Paul Colby
Gold Member
Yup.. I'm plenty familiar with 3 phase motors
Cool. Then you're in good hands.

Baluncore
The motor is designed to operate in Δ = delta connected on 220V; or in Y = star connected on 440V.

When a neutral is not available, the transformer would supply a single phase control supply from two of the three phases.

The motor is designed to operate in Δ = delta connected on 220V; or in Y = star connected on 440V.

When a neutral is not available, the transformer would supply a single phase control supply from two of the three phases.
How do you connect a 4 wire wye to it when it only has 3 wires?

Baluncore
How do you connect a 4 wire wye to it when it only has 3 wires?
I depends on what "it" means.
You float the middle terminal of the Y and connect only to the ends of the arms.
The floating centre terminal is the equivalent of a neutral in a balanced 3PH circuit, so has no net current.

I depends on what "it" means.
You float the middle terminal of the Y and connect only to the ends of the arms.
The floating centre terminal is the equivalent of a neutral in a balanced 3PH circuit, so has no net current.
But how would it change the voltage rating of the motor if both the delta and wye configurations are both 220-240V between each leg? That's what's confusing me now.. It really doesn't matter if the power source is delta or wye if you're not needing the neutral, and the leg-to-leg voltage is the same

Baluncore
That's what's confusing me now..
The three individual motor windings are each rated at about 220V. When connected in Y they can be effectively used in series on the higher voltage 3PH. In delta they are effectively in parallel with each between a different pair of the lower voltage 3 phases.

Draw an equilateral triangle. The points are the supply phases. Mark the centre of the triangle as neutral. Now notice the distance (= voltage) between the centre and corners is less than the distance between two corners.

Baluncore
Read the bottom line of the motor plate pictured in post #1; ΔV 220 and YV 440.
If there are only three wires to the motor then you must open the motor terminal box and change the internal connections to switch between Y and Δ to select the different voltage.

Paul Colby and Tom.G
Read the bottom line of the motor plate pictured in post #1; ΔV 220 and YV 440.
If there are only three wires to the motor then you must open the motor terminal box and change the internal connections to switch between Y and Δ to select the different voltage.
there are only 3 wires in the terminal box!
I'm used to Baldor motors that were dual voltage, they had 9 coil wires and two ways of connecting them for either high or low voltage

Baluncore
there are only 3 wires in the terminal box!
Can you show us a picture of the wires connecting to the terminals in the box.
There should be six screw terminals, with a couple of brass connectors.

How do you connect a 4 wire wye to it when it only has 3 wires?
A three phase motor has three windings. Call them U, V and W. Each winding has two ends, say 1 and 2.

In delta, U2-V1, V2-W1, W2-U1. The phase wires, typically with 220V ph-ph, are connected at the junctions.

In Y, U2, V2 and W2 are joined*. Phase connections, typically 380V ph-ph, are connected at U1, V1, W1.

* The junction is either buried in the motor windings (permanent Y, with only three available connections), or is made by a strap in the connector box. The neutral is not connected in either delta or Y.

dlgoff
A three phase motor has three windings. Call them U, V and W. Each winding has two ends, say 1 and 2.

In delta, U2-V1, V2-W1, W2-U1. The phase wires, typically with 220V ph-ph, are connected at the junctions.

In Y, U2, V2 and W2 are joined*. Phase connections, typically 380V ph-ph, are connected at U1, V1, W1.

* The junction is either buried in the motor windings (permanent Y, with only three available connections), or is made by a strap in the connector box. The neutral is not connected in either delta or Y.
Yup, that's what I'm used to.. Not this one though, Sure it has a terminal block but only 3 wires going to the windings!
I'll try and get a pic in a bit

Looks normal here

But wait a minute...

dlgoff
The junction is either buried in the motor windings (permanent Y, with only three available connections)...
Sure it has a terminal block but only 3 wires going to the windings!
As far as I can see, those black wires are single conductors, and there are no others coming out. The motor must be permanent Y as above, and so configured for the higher voltage.

The terminal block is presumably a standard type fitted to different models. But why the plate suggests dual voltage when it’s not I can’t say. Perhaps someone has been fiddling.

The existing wires seem to come from the back of the windings, so my next move would be to open that side and locate the buried tri-connection (we call it the star point). This can be separated and the three resulting wires brought out to the terminal box.

As it happens, I’ve just this week done this to a 1932 Wadkin planer (jointer, in your parlance):

This was originally fitted with a 460V DC motor!At some point, it’s been fitted with a 3ph one.

Only three connections, so permanent star:

A likely candidate for the star point - three wires coming together from places about 120deg apart:

Resistance of each to its outer connection is about 1/2 that between outer connections:

Ends prepped:

Wires brought out, then loom re-stitched:

Paul Colby
Paul Colby
Gold Member
What about a 2:1 three phase transformer?

Like that unused one mentioned in #3?

What about a 2:1 three phase transformer?

Like that unused one mentioned in #3?
the unused transformer is 1ph... I'd need 3 individual transformers, and bigger ones too, it would just be easier/cheaper to change out the motor, which is what I plan on doing, I found a similar one with a bit less power for $50... Maybe I'll play with this one more later, or just call it a 1/8th HP motor at 240v? :P GuineaFowl, that's a nice jointer, er.. planer, watch your fingers! I'll dig into the motor eventually and see what I find there, thanks for the pics and breakdown of what you did Guineafowl the unused transformer is 1ph... I'd need 3 individual transformers, and bigger ones too, it would just be easier/cheaper to change out the motor, which is what I plan on doing, I found a similar one with a bit less power for$50... Maybe I'll play with this one more later, or just call it a 1/8th HP motor at 240v? :P

GuineaFowl, that's a nice jointer, er.. planer, watch your fingers! I'll dig into the motor eventually and see what I find there, thanks for the pics and breakdown of what you did
This 3ph/1ph stuff comes up all the time on the woodwork forum I’m on.

I often suggest swapping in a modern dual voltage 3ph motor, rather than a 1ph one. 3ph motors are smaller, lighter, smoother and cheaper. Then running it on a VFD. This way, you get soft start, overload protection, speed control, reverse, braking, etc. All useful features on a lathe, too.

Baluncore
Yup, I'm getting a dual voltage 1/2hp 1800 RPM 3ph motor, so it should have the same torque just not as fast, I'll run it on a VFD so I can make up for it, and it's belt driven too so I can play with the ratio there.. It's to rapid feed the carriage on this one.. VFD's are SOOOO nice.. I'm going to get a 10HP one for the main motor as well.. going to get a chinesium one for \$170 to start with, use my 2hp I have currently for the rapid feed, and probably get another 2hp one to sell with my current lathe just to keep it operational

This thing is 5500 lbs, 18x80"

dlgoff, berkeman and Guineafowl
dlgoff