Sparks between shaft and casing on DC motor

[Wildseuil]

Hi everybody,

First of all, sorry if there are some mistakes in my english, I'm french...
We've recently bought a DC motor (400V/400kW/3200rpm) as spare part. The original motor was smaller (250kW/3000rpm). This motor is driven with an ABB variable speed drive, which is designed for the bigger motor.
As usual, we proceeded an autoset on the VSD, everythig is OK, but there is a strange phenomenon : at low speed (<500 rpm) there are electrical sparks between the shaft and the casing. The voltage between the shaft and the casing in hard to measure but it is approx. 150VAC. Then we removed the earthling conductor, and placed rubber strips under the motor, to have an insulation between the ground an the motor. We measured the voltage : approx. 150VAC / 150Hz between the casing and the ground.

We think this is due to the input voltage, and we have several ideas to solve the problem :
- placing a filter (RL or RC maybe?)
- Adding a copper brush on the shaft to evacuate this current through the casing

Has anyone ever seen sth similar?
Ar our ideas good?

Thanks a lot for your help!

L. MICHEL

 

[squeeky]

Dear Wildseuil,

I'm surprized your not swamped with answers. This is AC (Alternating Current) and not DC (Direct Current). Depending on the age of the VSD you may get sparking as the VSD uses high frequencies in a PWM (Pulse Width Modulation) to create what it calls sinusoidal waves. Not really true.

The motor purchased should have been specified for a VSD.The non drive end bearing should be insulated from the endshield by type or by an insulating ring.This will stop circuitating currents but now cause the shaft to get a raised potential to earth. When it is high enough it jumps the gap. Your lubrication on your Drive End bearing must be good as it is acting as an insulator. Watch for increasing vibration as the bearing will be doing micro flashing and eventually fail, sometimes very quickly.

This can also come from a magnetized shaft or fixed position where someone has used a magnetic clock guague to do some alignment. This acts as a DC magnetic source. It now spins passed a metallic circuit at 3200 rpm and starts to generate electricity. YOu can only pick this up with a Gause meter that measures small magnetic parts. It should have a wand. Try to move it in all planes as you need to cross the magnetic circuit to get a reading and if you are inphase you may not pick it up.

There are specially made grounding systems. In your region Improseal make a ring that goes on the shaft end and earths the shaft. I believe that SKF may now have made one. In SOuth Africa there is Whyko. You cannot use a normal electric brush. People have tried hard and soft compounds but they are designed to run at a percific current density. Without the current they glaze over and the resistance increases and eventually becomes an insulator. There is another company that seels a gold filiment brush that works. They even sell a 1 ohm resistor with a high watt rating to put in series so you can put an occiliscope across it to see the voltage (Current). We prefere to use these. You must polish the shaft at the contact point or your gold brush will turn to dust very quickly. Very expensive mistake.

I have tried to upload a page showing the ring type. If it does not come up, please let me know and I will try again.

 

[dArsonval]

L. MICHEL;

Your French to English is very readable.

A "spare part" DC motor (interpreted by the reader as 'Used Equipment') is immediately suspected of issues and potential problems.
Not knowing what the history of the motor may be, or is... alludes that everyone is shooting answers in the dark.

Typically... before placing a used motor in service, the entire device requires a methodical inspection of its electrical integrity,
and any discrepancies found during this inspection need to be corrected.

As for "measured voltage"? It's likely all about the "Current" in this instance.

Any amount of electrical current passing between the motor housing (casing) and shaft of the device
has already ruined (destroyed) the bearings of the motor and has further rendered it not usable for industrial purposes.

Others will chime in on drive data, power supply, etc.

Products (arrangements) such as grounding brushes have claimed their status among solutions to these "drive" problems.

Electrically insulating the motor bearing journals has been a popular choice in solving shaft current issues and most likely the most "maintenance free" decision.

Enjoying a nice sunny day [for once] here in the U.S. Midwest.
And always enjoying this forum.

John

 

[Skogsgurra]

Dear Wildseuil

There were suggestions that you should have mistaken an AC motor for a DC motor.

I am not so sure that you were mistaken. Il est bien un moteur Courant Continu, n'est-ce pas?
(Excuse my French, but this way we may clear any possible misunderstanding).

It is very common to have a high voltage between shaft and frame in DC motors that are driven from a thyristor controller AND that has Insocoat bearings on DE AND NDE.

The reason is that the winding is on the rotor, so that the coupling capacitance between winding and rotor is high. That is especially tru for the BMI range of motors, mostly because they usually have a long and slender rotor.

I have measured more than your 150 V, but usually the frequency is 300 Hz and not 150 Hz. If the rectifier is half-controlled (three diodes and three thyristors), you may get 150 Hz.

I have had this on large drives on gantry cranes (in l'Havre and also in Elisabeth, USA). It may need some special investigation before one can say exactly what needs to be done. But a pair of heavy duty brushes from frame to shat usually cures the problem. Phone me if you want to discuss the problem, oui, je parle francais, but I think your English is quite sufficient.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[squeeky]

Dear Guys,

My error. Where I am we use VSD for an AC variable speed drive. I know some countries state VFD. From the answers the Englsih passes the engineering understanding but the terms may differ. Sorry.

 

[Wildseuil]

Hi everybody,

Thanks for your answers.
I just wanted to add some details :
Yes, we have a DC motor, probabely not designed for a use with VSD (!) as it's a refurbished one...
At the moment, we only use the motor with no-load.
The sparking problem appears only at low speed. Over 500rpm, the voltage begins to decrease and the phenomenon is no more visible. An other thing we noticed is that the frequency of this current remains very constant (150.0 Hz +/- 0.2).
In the meanwhile, we have tested another power source : just a couple of variable threephase source + diode bridge and capacitor, to have a quite clean DC source (ie without using PWM), then there are no problems anymore; the volage between shaft and frame is less than 1V

That confirms it's due to the input, coming from the VSD.

Anyway VSD is useful (especially for speed regulation, and reading parameters), so we'll try to find a brush or the InproSeal.

Don't hesitate if you have other ideas, but anyway thanks for the tips!
I'll come back as soon as I have other information.

L.MICHEL

 

[Skogsgurra]

As I said, it is a quite common problem and does not have much with the age of the motor to do.
Tha fact that the voltage
1. is 150 Hz and
2. is higher at lower speeds
shows clearly that the VSD (for clarity; Variable Speed Drive, can generally be either a VFD or a thyristor controller) consists of three diodes (uncontolled) and three thyristors (controlled), a so-called semi-controlled rectifier.

Such rectifiers produce a base voltage from the uncontrolled rectifiers and this is bucked by the thyristors so that the voltage is kept low at low speeds. That means that the firing angle is close to 180 degrees and that the voltage changes more violent when running at low speed than when running at higher speeds. So, your observation that shaft voltage is high at low speeds is correct.

Since there are three controlled thyristors, you will have three firing events during each 50 Hz cycle. That means three times 50 Hz - so your observation that the frequency is 150 Hz is also correct.

Not much to worry about, except that tha shaft voltage may cause bearing problems. And that it may be unpleasant/dangerous to living creatures. Including human beings.

Questions:
Is the armature winding insulation OK? How many megohms?
What are the exact bearing types at DE and NDE? Are they insulated?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[podobing]

Wildseuil,
A grounding brush would (Should) eliminate the voltage, but place the brush on the drive end of the motor. The bearing on the NDE (Non Drive End) should probably be insulated to prevent shaft currents from becoming a problem. Check with the manufacturer, but I would never think that it would be a good idea to have both bearings insulated - in the event of a grounded armature.
Bon Chance
Dave