We have several 2400 volt induction motors in our plant. There is a theory in the plant that there is a shock hazard if you touch a motor lead wile the motors is coasting down to a stop. Some of the motors supply big blowers and can take 15 minutes to stop.When ask about I said that there was nothing to it but i would ask. I also was told by the millwrights that it was unsafe for them to work on equipment wile we were megging it.

Both fears have some basis in fact.

Meggers can deliver harmful shocks. Even an annoying little zap can result in secondary accidents, such as falls.

A spinning motor without power is called... a generator! Sure, some motor designs are more efficient generators than others, but the effect is very real.
In a more unusual application, passing power through a motor has been used to correct power problems. Again, this is a highly engineered application, and isn't a DIY fix for power quality problems!

Reno (et al), what about an AC motor that uses an AC or DC field winding? If the field winding doesn't have any voltage then there are no lines of flux and the rotor is just a spinning mass of steel and copper? (My American Electrician's Handbook is packed in the storage unit...)
I do agree on the megger. Get in the wrong place at the wrong time and the electricity could also kill you, right? In any case it wouldn't feel nice...

AC induction motors will act as generators until their magnetic field has decayed. The decay time is measured in cycles and maybe seconds but never in minutes.

A megger will never cause a stopped AC induction motor to start.

"A megger will never cause a stopped AC induction motor to start."

True, but I think that the concern was for electrical shock.

I have been shocked by a megger. It is similar to automotive ignition shock, high voltage low amperage. Hurts! don't try this at home.

Let's put our 'thinking-heads' on chaps!

During operation of an induction motor, the 3-ph stator windings create regularly rotating fields in a rotor placed within them, creating currents in the rotor's cage conductor bars. Variation in magnetic linkages, [ because of 'slip' ], induces current in these bars in a direction which tends to counter the stators' fields.

Now turn the motor off. All three supply phases are cut, the stators' fields collapse because no current is flowing in the windings. When it collapses, the rotor fields must collapse too, as they are induced in the rotor bars - hence the name "induction" motor.

If we now, during rundown, [ theoretically, of course, in the interests of Science! ], re-arrange the motor connections by shorting a stator terminal to another stator terminal via a 'theoretical' electrician, what happens? Does he receive an electric shock?

Depends on the speed of the rotor, and whether any residual field is retained in the rotor, I think. Immediately after switch off, residual fields may exist in the rotor which could exite the stator fields to generate a voltage. The fields' rapid initial collapse means that new fields would have to form in the stator windings upon our theoretical electrician's shorting of the terminals. Without the regular poco 50-hertz supply creating the rotating field, what you could get is a rapid series of voltage spikes as each bar passes by each field winding, reducing in intensity as the speed falls away.

Not, I deign, a regular 50hz three-phase sinewave voltage, anyway.

Enough volts to shock? Don't know is the answer! -but probably yes if the residual motor speed is high enough and residual rotor magnetic fields actually exist.


Alan




[This message has been edited by Alan Belson (edited 08-18-2006).]

Remember that magnetic steels have hysteresis. When the magnetizing field is removed, the steel still acts as a weak permanent magnet. (We've all seen this too often when we use our pliers and screwdrivers around a strong magnet, after which the tool is forever picking up metal filings.) On a 2400 volt motor, I have little doubt that this residual field is quite sufficient to generate lethal voltages.

[This message has been edited by SolarPowered (edited 08-18-2006).]

As to the meggar....remember that the perfect insulation is also a perfect capacitor. When you are doing the dielectric absorbtion tests on a motor and developing a polarization index, you are essentially timing the capacitive charging rate of what has become essentially a large capacitor. So depending on the mass of the motor, you can be building a substantial charge in the winding that is totally out of proportion to the initial output of the meggar unit itself.
Very dangerous.

"unsafe for them to work on equipment"

Sounds like you have good by millwrights!

The power factor correction and harmonic reducing capacitors on switched power supplies and UPS systems (especially better quality ones) have a disturbing tendancy to excite induction motors, allowing them to act as generators until all their kinetic energy is spent. In the case of large chillers, you have to consider the KE of all that water, too, and any KE of the machinery plant motors are connected to...

This generally has a very bad affect on the motors and, well, pretty much everything else connected to the bus. Hopefully the engineer took things like this into account when designing the plant, but it's almost impossible to accurately model and predict things like this.

A couple of years ago, I wanted to know how long it would take for the voltage to drop off on our 60 and 75 HP tunnel fans. I think it took 2 to 3 seconds after I stopped the fans, definately less than 5 seconds. Of course they keep spinning much longer than that.
Joe

Coasting AC induction motors offer 2 very real risks as mentioned by others.
1) Residual magnetism. Depending on the design, this can be very substantial. So much so in fact, that island mode wind generators can take advantage of it to enable self-excitation systems (Google that and you will see). They need caps to charge it up high enough, but without the RM the caps would have nothing to work with.

2) PFC or Starting capacitors. If the caps are still on-line when the motor is disconnected and coasting, they will maintain the excitation current and allow the motor to continue as a generator.

So a good rule for a millwright (or anyone else for that matter) who is NOT familiar with the magnetic hysterisis properties of that particular motor's core laminations, or does NOT know if PFC caps or Starting caps are connected, it to ASS-U-ME that there is "potential for potential" on that coasting motor.