Three Phase Motor takes too much Current

[The Metal Butcher]

Hi All,

Dad and I were playing with a new digital amp clamp multimeter the other day, and we discovered that the 5hp motor in the Sidney lathe pulls way too much juice. At idle, the combined current on L1 or L2 of the phase converter and lathe was 17 amps. The phase converter idles at about 5. I've measured the Sidney once before at 15 amps by itself with an old analog meter, so it's not the meter. It pulls around 55+ starting. I didn't have have time or material to waste to put it under a hard load, but I did yank the clutch a few times pretty fast in high gear to try to bog it slightly. And even though I never heard it bog much of any, we could spike the combined current draw into the 40s and even 50s once.

In comparison, the Lagun's 3.5/7 HP motor draws between a combined 28-35 starting, and 9-12 idling.

So what does that mean? Internal short?

I guess I should keep my eyes peeled for quality motor, especially an old Babbitt bearing one...

[Solved, see post #18 and #21]

[nickel-city-fab]

Maybe you could play with power factor correction? Basically capacitors. I feel for those who have to buy electric motors nowadays because they ain't getting any cheaper. Fortunately my local craig list is a gold mine of stuff like that.

[J Tiers]

If the RPC pulls 5A, then that suggests the 5 HP is pulling 12 or so. FLA on a 5 HP (UL) is 15.2 at 230V.

But the RPC is supplying the manufactured leg, which means it will be drawing somewhat more than its idle current. The RPC may not be perfect, and so it may not be supplying full manufactured leg current. The difference shows up on the other two legs going to the motor, power-wise.

Yes, I would expect maybe 7 or 8 A at "idle", meaning with only the motor turning, and measured going to the motor. That's with perfect balance, which is not going to be true.

You will not have good data until you measure the current on all legs going directly to the motor.

What is engaged and moving with the clutch out (as I assume you mean by "idle")?

If your "idling" actually means with spindle turning, but no cutting load, then there is quite a lot going on in the headstock, and that might not be so strange. Also there is control power, which wil add a bit.

I think I can pretty well guarantee that if you had a shorted turn, or the like, you would already know about it by the smell. I've never yet seen a serious fault of that sort that did not rapidly get worse, and I mean in a few minutes. You've already run that thing longer, if I recall correctly, and under load at that.

[The Metal Butcher]

If the RPC pulls 5A, then that suggests the 5 HP is pulling 12 or so. FLA on a 5 HP (UL) is 15.2 at 230V.

But the RPC is supplying the manufactured leg, which means it will be drawing somewhat more than its idle current. The RPC may not be perfect, and so it may not be supplying full manufactured leg current. The difference shows up on the other two legs going to the motor, power-wise.

Yes, I would expect maybe 7 or 8 A at "idle", meaning with only the motor turning, and measured going to the motor. That's with perfect balance, which is not going to be true.

You will not have good data until you measure the current on all legs going directly to the motor.

What is engaged and moving with the clutch out (as I assume you mean by "idle")?

If your "idling" actually means with spindle turning, but no cutting load, then there is quite a lot going on in the headstock, and that might not be so strange. Also there is control power, which wil add a bit.

I think I can pretty well guarantee that if you had a shorted turn, or the like, you would already know about it by the smell. I've never yet seen a serious fault of that sort that did not rapidly get worse, and I mean in a few minutes. You've already run that thing longer, if I recall correctly, and under load at that.

I measured it at 15 amps once, at the Sidney's box. That was with an analog meter which isn't 100% accurate. 15, 15, 9 for L1, L2, L3 respectively. L3 is manufactured. Idle just spinning the clutch vs headstock running doesn't change it much at all.

Yes, I've run it all day before.

[J Tiers]

I measured it at 15 amps once, at the Sidney's box. That was with an analog meter which isn't 100% accurate. 15, 15, 9 for L1, L2, L3 respectively. L3 is manufactured. Idle just spinning the clutch vs headstock running doesn't change it much at all.

Yes, I've run it all day before.

And there you have it. That 9 is likely made up from the others. The L3 is typically lower voltage , despite "balance" caps, and that unbalances the system. The phase of the manufactured leg may also be somewhat wrong, due to balance caps, and out of phase current is deceptive, it does not add up entirely as it might seem with other current. Some motors also just draw more. UL uses typical" motors to develop their ratings, and I doubt a 50 year old motor is "typical" any more.

I'd not worry too much, unless the motor starts getting way too hot, or smells burnt.

[The Metal Butcher]

And there you have it. That 9 is likely made up from the others. The L3 is typically lower voltage , despite "balance" caps, and that unbalances the system. Some motors also just draw more. UL uses typical" motors to develop their ratings, and I doubt a 50 year old motor is "typical" any more.

Ain't 50 years old.





But it shouldn't pull 50 amps under load when it's rated for 15. That's BS.

[PStechPaul]

A proper analysis would require simultaneous monitoring of P-P voltage as well as phase current under various conditions. If the manufactured leg voltage collapses when starting, it very well could draw 50 amps.

[The Metal Butcher]

A proper analysis would require simultaneous monitoring of P-P voltage as well as phase current under various conditions. If the manufactured leg voltage collapses when starting, it very well could draw 50 amps.

30 amps on the MFG leg when starting. Only a 7.5 hp converter. I will try to get some current values under load next time I run it. But I don't want to test it to death. I can replace it faster than that. I just wanted to hear the electrical gurus opinions on here for why this thing draws so damn much power.

[rdfeil]

The Metal Butcher , Sorry I am not following exactly what you are saying. I thought you said the motor would draw up to 50+ amps on startup. If that is what you said and mean, it could easily be very normal. It is not uncommon for a motor to draw 5 to 10 times the rated full load current when starting. Many things effect this including, load, voltage, voltage balance and phase relationship along with others....
Now if you are saying the motor draws 50 amps when running steady, now you have a problem!! High start currents are 100% normal for motors and different motors will have different current draws. It all depends on the engineering that went into the motor design....

[The Metal Butcher]

The Metal Butcher , Sorry I am not following exactly what you are saying. I thought you said the motor would draw up to 50+ amps on startup. If that is what you said and mean, it could easily be very normal. It is not uncommon for a motor to draw 5 to 10 times the rated full load current when starting. Many things effect this including, load, voltage, voltage balance and phase relationship along with others....

Right. It does start at over 50 amps. Not a huge issue, though it is considerably higher than the aforementioned 7HP.

Now if you are saying the motor draws 50 amps when running steady, now you have a problem!! High start currents are 100% normal for motors and different motors will have different current draws. It all depends on the engineering that went into the motor design....

Yes. I can make it draw 40-50 amps just by putting it under a load (engaging the clutch).

[J Tiers]

30 amps on the MFG leg when starting. Only a 7.5 hp converter. I will try to get some current values under load next time I run it. But I don't want to test it to death. I can replace it faster than that. I just wanted to hear the electrical gurus opinions on here for why this thing draws so damn much power.

............................................But it shouldn't pull 50 amps under load when it's rated for 15. That's BS.

Right. It does start at over 50 amps. Not a huge issue, though it is considerably higher than the aforementioned 7HP.

Yes. I can make it draw 40-50 amps just by putting it under a load (engaging the clutch).

Hold on there.....!

Are you saying it pulls 50A under load "continuously"?

Per your earlier posts, you said you could "spike it" into 40 or 50 A by suddenly applying a load. I would regard that as perfectly normal...... so long as it comes back down once equilibrium is again established. That's only 3x FLA, and perfectly normal for a sudden load change. The motor may draw double that when starting, but unless the meter has a "peak hold", you will not see the peak, you will see a lower number.

As for drawing "power", we have no idea how much power it is drawing. All we have is current readings. We do NOT know what the voltage was when that current is drawn.

Normally, an unloaded motor will draw 40 to 50% of the FLA at idle, although some may draw less, as little as 25%. But the phase angle relative to voltage is so large that the net power is low. The "power factor" (Cosine phase angle) is as low as 0.1.

What is your local voltage? Low voltage will generally cause a larger current, and that can even occur at idle.

There are a lot of variables here. You have a motor that is running from an RPC, where the current on one phase is known to be low, and the voltage is almost sure to be low and/or at a somewhat incorrect phase angle.

That motor would likely draw its expected current when supplied from a good 3 phase source. An RPC is an "OK" source. They work well, but are not the same as a well balanced powerco 3 phase. The motor is not operating under the normal conditions, and so will not draw the normal currents. The unbalanced voltages which are inevitable will likely lead to excess circulating currents in the motor, and those have to come from somewhere.

If you really suspect a problem, then pull the motor and let a motor shop check it. If there is anything funky about it, they should find it.

[rdfeil]

I am with Jerry on this. Spiking the motor with a sudden load IE: dumping the clutch, WILL cause a high spike, which will return to a lower level quickly. Another thing that add unknowns to this mix is the phase converter. Phase converters want constant loads for constant performance. By spiking the load you are causing current, voltage and phase changes which ALL interact. As long as the motor current is 15 amps or less when you are in a constant condition (Not starting or bumping the clutch etc...), you are good!! Motors can take a lot of abuse as long as they do not get excessively hot or the overload conditions do not instantly melt wires or arc through the insulation. Do not worry about the differences between the two motors.

Also, remember the rated current on the nameplate is "Full Load" current only. Starting current and accelerating current can and will be MUCH higher.

[Ringo]

Hi All,

Dad and I were playing with a new digital amp clamp multimeter the other day, and we discovered that the 5hp motor in the Sidney lathe pulls way too much juice. At idle, the combined current on L1 or L2 of the phase converter and lathe was 17 amps. The phase converter idles at about 5. I've measured the Sidney once before at 15 amps by itself with an old analog meter, so it's not the meter. It pulls around 55+ starting. I didn't have have time or material to waste to put it under a hard load, but I did yank the clutch a few times pretty fast in high gear to try to bog it slightly. And even though I never heard it bog much of any, we could spike the combined current draw into the 40s and even 50s once.

In comparison, the Lagun's 3.5/7 HP motor draws between a combined 28-35 starting, and 9-12 idling.

So what does that mean? Internal short?

I guess I should keep my eyes peeled for quality motor, especially an old Babbitt bearing one...

Since I been adjusting caps on my own RPC, and researching the data from others, L1 will read lower than T1.
Assuming L1, L2 is your single phase side, and T1, T2, T3 is your 3phase side, then the total amps on 3ph side reads higher than 1ph side. Don't ask me why, it just shows up that way on a amp clamp.
There is certain amount of VooDoo, Black Majic, and false readings in a RPC. One research item said to balance your caps to equalize your 3ph amp legs idling the (5hp) load motor. I did that and later turned on a small grinder, the amps on L1 went down with the grinder coming on. That right there tells me there is voodoo & false readings in a RPC.
I wouldn't get too wrapped around axle on this.

[The Metal Butcher]

Interesting. I did not know that the current could be spiked over nameplate. Well I got some free brake rotors, I will turn them to dust and see if I can't stall it out. Results... possibly later today. Stay tuned.

[Rich Carlstedt]

Some motors are Pigs !
Especially if they are 2 speed motors.
Like all things---tradeoffs are made during manufacturing or in the design department
That motor was made in Korea...just because a GE stamp is on there, does not mean it's A OK
Rich