high current on an async electric motor

[orense]

Hi,

I know this is a plc forum, so this is a bit off-topic. However I know there are lot of smart electrical engineers here, so I will give it a shot :

We have a motor starter that feeds a 690 VAC async motor. The motor runs a mud pump.

The motor was initially made for 480 VAC, but it got rewinded when the vendor found out they had delivered for wrong voltage.

Now one of the breakers in the starter cabinet burned, due to something that looks like high temperature, which should mean high current. It was running with water through the pump.

We have been troubleshooting while running the pump on water only, after changing the breaker.
The motor is a 300 horsepower motor, and while running on water it is running on very low load. However, the current is very high. We have been measuring on all three phases, and the current is almost three times as high as expected.
The rpm has barely dropped, so that also indicates that the load is low, since slip is somewhere around 5 - 6 % on full load.

Do any of you know what can be the reason for the high current ?

We have also checked temperature with heat gun, and motor does not seem to get hot, only that the current is high.

Motor has been meggertested with approved values.

This is a star / delta starter, and the connections have also been checked.
During start up the star / delta sequence works well, it is only the steady state current while running that ends up too high. It lands at around 150 amp, but should be at around 50 - 60 amp.

Thanks for any contributions

 

[jawolthuis]

What are the Line-to-line voltages at the output of the starter while the motor is running?

Also, Has the motor ever run properly since installation? The Star/Delta windings may be wired out of phase.

 

[orense]

Thanks, not sure about the voltage. I will check with the site.

The motor has been running, but we have never measured any other current that seems more reasonable.

Can you explain what you mean by star/delta wiring out of phase ?

Thank you!

 

[jawolthuis]

I don't know exactly what you have, but I was envisioning a 12 lead motor with the half-windings hooked up to the wrong phases.
envision the attached diagram with 1 & 4 swapped around with 9 & 12...

 

[Gil47]

A 300 HP motor equates to 220 KW
on 690 volts that looks to me like 230 to 250 amps full load
The pump is designed to pump mud but you are testing it with water
so it is running at light load there fore very little slip
A motor running light loaded with little slip is going to have a bad power factor
if the motor is running at speeds say 1000 rpm or less it will have an even worse power
factor

My view is that a motor of that KW with no load connected and running at 3000 to 3600 rpm
would probably run with higher than 50 amp but at about 0.8 or lower power factor

I have seen a 12 pole motor at about that 220 kw
running unloaded with a power factor of 0.4 made the current look like it was loaded

 

[pal]

AS Gil47 says the 3 phase current is about 250A full load - with a star/delta connection the current in each of the 6 legs will be 155 to 165A at full load . As well as checking the voltage at the starter , check the voltage at the motor . The bigger the volt drop on the cable run , the greater the current is to compensate .

 

[orense]

A 300 HP motor equates to 220 KW
on 690 volts that looks to me like 230 to 250 amps full load
The pump is designed to pump mud but you are testing it with water
so it is running at light load there fore very little slip
A motor running light loaded with little slip is going to have a bad power factor
if the motor is running at speeds say 1000 rpm or less it will have an even worse power
factor

My view is that a motor of that KW with no load connected and running at 3000 to 3600 rpm
would probably run with higher than 50 amp but at about 0.8 or lower power factor

I have seen a 12 pole motor at about that 220 kw
running unloaded with a power factor of 0.4 made the current look like it was loaded

Thanks for answers ;

The motor is running on a 60 hz power plant.

The motor is running at 1800 rpm without load.
With water the rpm has dropped down to 1795.

I have been told that at full load the motor should consume approx. 225 A

I am not sure about the power factor.

 

[Liam Moran]

Can it be assumed that you have a tacho fitted to me able to measure a difference of 5 RPM?. I would expect for the supply frequency you mention that the no load speed be less than 1800 RPM. 1800 RPM would be the speed if the motor were synchronous.

 

[jraef]

Can it be assumed that you have a tacho fitted to me able to measure a difference of 5 RPM?. I would expect for the supply frequency you mention that the no load speed be less than 1800 RPM. 1800 RPM would be the speed if the motor were synchronous.

Yes, absolutely, in fact 1795RPM would be very low slip, confirming what Gil47 said. Unloaded, that 150A of current you are seeing is mostly the result of the PF being at around .2 or maybe even less. Your "expectation" of it being 50 or 60A is what is false here, there is almost no way on accurately prediction unloaded amps on an AC induction motor, and the number is meaningless anyway, except in a historical context for trending, i.e. tracking bearing wear or winding damage.

Mud pumps are designed around a flow of high viscosity fluids. Using a low viscosity fluid means the impellers will barely couple with the load even though the flow may appear high. So your motor likey doesn't even think it is working yet.

Things like breaker terminals burning are actually more likely attributable to the star-delta starter, a well known source of issues like that. Even if you have it connected correctly, the unloaded motor can, during an open transition from star to delta, have enough residual magnetism to re-connect before its magnetic fields collapse. So because there is a phase shift between star and delta, if that happens it's like connecting two generators out of phase with each other. There is a massive voltage spike at transition, followed by a massive current and torque spike. The voltage spike makes the contactor arc last longer, burning the contacts and sometimes, especially on something like 690V, it may cause a flashover at terminal blocks, such as breaker terminals. Because that transition spike is always followed by the current /torque spike, I'd have some concern for your mechanical equipment as well.

 

[orense]

Can it be assumed that you have a tacho fitted to me able to measure a difference of 5 RPM?. I would expect for the supply frequency you mention that the no load speed be less than 1800 RPM. 1800 RPM would be the speed if the motor were synchronous.

Hi Liam Moran,

Thank you for your answer.

The 1800 rpm was never measured as far as I know.
The 1795 was measured with a tachometer as you assumed.
This was part of the troubleshooting that was conducted.

 

[orense]

Yes, absolutely, in fact 1795RPM would be very low slip, confirming what Gil47 said. Unloaded, that 150A of current you are seeing is mostly the result of the PF being at around .2 or maybe even less. Your "expectation" of it being 50 or 60A is what is false here, there is almost no way on accurately prediction unloaded amps on an AC induction motor, and the number is meaningless anyway, except in a historical context for trending, i.e. tracking bearing wear or winding damage.

Mud pumps are designed around a flow of high viscosity fluids. Using a low viscosity fluid means the impellers will barely couple with the load even though the flow may appear high. So your motor likey doesn't even think it is working yet.

Things like breaker terminals burning are actually more likely attributable to the star-delta starter, a well known source of issues like that. Even if you have it connected correctly, the unloaded motor can, during an open transition from star to delta, have enough residual magnetism to re-connect before its magnetic fields collapse. So because there is a phase shift between star and delta, if that happens it's like connecting two generators out of phase with each other. There is a massive voltage spike at transition, followed by a massive current and torque spike. The voltage spike makes the contactor arc last longer, burning the contacts and sometimes, especially on something like 690V, it may cause a flashover at terminal blocks, such as breaker terminals. Because that transition spike is always followed by the current /torque spike, I'd have some concern for your mechanical equipment as well.

Hi Jraef, thank you for reply !

I do not have the in depth knowledge about electrical motors, but have received this information from my office. Seems like they are making false assumptions then.

I did see trending of the motor during startup, and there was a significant peak in current when the star/delta switchover happened. Actually way higher than the start current in star.

When the motor starts, the measurement on each phase goes up to approx. 750 amps. Then when it is coming down, the delta kicks in. When that happens the current peaks up to almost 1250 amps before it comes down to approx 150 and goes steady state on that value while running. It is for a very short moment at such high value.

If the star/delta does this, what is the remedy? Other type of starter ?

Thanks !

 

[Gil47]

It does not surprise me that the current at star to delta transition momentarily peaks over 1000 amps in you case

( Wye to Delta )
Star delta is commonly used to start loads of high inertia that take time to run up
Although now days Soft starters would be favoured in order to remove that transition spike

In older starters the transition of the clapper type contactors with large contact clearances could have
been close to 1 second giving time for arcs on contacts to quench
But with modern contactors with small clearances the transition times can be a quarter of that time
with the arc on one contact not quenched before the next contactor closes

A situation as I describe normally blows two or more fuses and often destroys the main contacts
in either the star or delta contactor

When using star to delta with modern contactor a suitable timer should be considered to allow
a longer transition delay of the delta contactor closing after the star contactor opens

 

[shooter]

On star the motor should reach its normal rpm, then switch over.
starting amps is 2 times nominal
So i think you have a serious problem, the spike when switching over is because the phases are not in sequence so please check connections and direction of windings (Can not be done by you needs windingcompany.
especcialy when it is rewinded. Most probably one of the windings is not connected properly.

 

[jraef]

Despite the fact that people THINK the starting current is lower, star-delta is ALWAYS going to have a transition spike that is at LEAST as high as a DOL level, but can under the right circumstances be much much worse, as you are seeing in your case. The best one can hope for is that it is too fast for the meters to read it, which is why some people believe it works. It's an illusion however.

Again, I'm going to agree with Gil47, a solid state soft starter is a much better way to accomplish the reduction in starting current, there is no transition spike at all, plus you get other benefits in most cases. The ONLY advantage that star-delta has ever had is that it is slightly less expensive than solid state, but even that is becoming less and less of a factor as the volume of solid state starters increases and the number of people understanding the dangers of star-delta makes the sales volume of star-delta decrease.

I for one stopped specifying or even allowing star-delta (wye-delta here in North America) on my projects over 20 years ago, even though at that time the difference in cost was greater. Now it should be a "no-brainer", yet the ability to save a few percent in the cost still wins out with those who are short-sighted. That often turns out to be OEM equipment vendors who gravitate to the cheapest way to satisfy a vague specification, even though it is usually the end user who pays for the long term operating expenses when their equipment life is foreshortened. So instead of simply specifying "reduced voltage starting" and end up with the cheapest way out from the OEM, specs should be more specific and say "starting shall be accomplished using reduced voltage solid state technology employing a minimum of 6* SCRs." Adding the specific names of products that have good LOCAL support in your area is also a good idea, because it can be very frustrating to end up with some unknown brand they bought over the Internet from China and when you need help with something, they are impossible to get hold of.

*The 6 SCR portion is to forego another new disturbing trend in cheapness within the soft starter community, using fewer SCRs and purposely unbalancing the starting current. Don't do that either unless perhaps it is a very easy starting centrifugal (low viscosity) water pump...

 

[DickDV]

I'm with you 100% on that latest post, jraef.