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Motor inrush current question

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tobias

Fri Apr 19 2013, 11:17PM

Registered Member #1956 Joined: Wed Feb 04 2009, 01:22PM
Location: Jersey City
Posts: 172

How is a 7.5 HP big?
Direct on line start is commonly used up to 15 HP.

Steve: VFDs will rectify the AC input. You can still play with them on single phase, just make sure not to push them to the nominal power.

Dr. Slack

Sat Apr 20 2013, 06:42AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Steve Conner wrote ...

Interesting, are you driving a 7.5hp load? If you're just running the motor unloaded, the steady state current should be less than 8A.

Not necessarily. Some induction motors draw high current at no load. At nominal rotor slip frequency, the cage inductance is high enough to keep circulating currents down. As the slip frequency drops, the currents can rise and the cage iron saturate.

Steve Conner

Sat Apr 20 2013, 03:13PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

tobias wrote ...

Steve: VFDs will rectify the AC input. You can still play with them on single phase, just make sure not to push them to the nominal power.

I checked this out. It's not so much the lack of 3 phase power as the voltage. Both VFDs are rated for 380 to 500V AC input. With 240V input the DC bus voltage is too low and it just sits in undervoltage lockout.

Of course I could use a voltage doubler to make 600V DC from the 240V line.

HV Enthusiast

Sun Apr 21 2013, 02:35PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Unfortunately, a soft-start controller, etc... cannot be used due to physical constraints, start-up delays (the motor must spin up extremely quickly for system redundancy), as well as noise. (VFDs are switching supplies and can induce a lot of high frequency switching noise)

The primary concern for this large inrush current is that it presently fails MIL-STD-1399 which only allows a certain % of inrush current vs. overall power of the motor.

We'll likely have to apply for a waiver on this. The engineers at the motor manufacturer state this is normal for this particular motor wired in a delta fashion.

Steve Conner

Sun Apr 21 2013, 06:29PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Sounds like a tough application!

radiotech

Mon Apr 29 2013, 06:00AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Your motors work out to 8 HP or 6.1 kW. How are you measuring the per phase
inrush current, and what is the voltage at the motor terminals corresponding to
the current reading ? It may be a question of how stiff your motor feeders are
and how many kvars are on the system. A clamp-type kilowatt meter may
show what the problem is. This will show power factor, loaded and unloaded.

Also, unwise to re-energize a rotating motor. the motor may try to dump
its inertia back to the power line because essentially a fast spinning motor is
an induction generator that will draw magnetizing vars from the power line.
An 8 HP motor, especially if 3000 RPM, doesn't have too large a shaft , and if it has
a big pulley, it may snap off.

As to the Wye-Delta starters, and the resistance starters, they are rare now. We saw those
when diesel plants had to start motors. A 100 HP genset might get knocked off
when a 20 HP motor was thrown on line without current limiting.

I've been away from VFD's for 10 years. But then, they were very advanced and
fully capable of tuning the load parameters to start any load.

Steve Conner

Mon Apr 29 2013, 07:33AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I remember once working on a wind tunnel controlled by a 1950s vintage Ward-Leonard set. The starter had a huge lever that had to be pulled towards you to start the motor in wye, and then shoved forcefully in the other direction to change it over to delta. The technician warned me that if the overcurrent trip were to trigger, it could break a wrist. Not sure I quite believed that.

radiotech

Mon Apr 29 2013, 11:13PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

We has a Korndorfer starter that need a certain touch. The motor was 6600 volt
and drove a 500 Volt 1500 Amp generator.

Long after the machinery was taken out, the starter remained on the
primary line, just locked out. One night, on my shift, a rat crawled in,
got across the top stabs, and caused a blast that took the circuit down,
and blew the door open.

Old electrical equipment is full of surprises.

johnf

Thu May 02 2013, 08:03AM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

Okay
I have many 3 phase motors here @ home with the largest in service a 15kW (shaft output) unit on my compressor.
If you want to find the instantaneous start current you only have to measure the phase to phase resistance.

now depending on motor coupling and various other factors this is why most motors that have a large inertial mass have star delta starters where you start in star so that the back EMF bucks the supply --all of this dependant on the rotor slip.
so my compressor in star to start draws only 130 ish amps for a few mS then the amature starts moving and this settles back to around 45 amps per phase as the rotor continues to accelerate. about 2.5 seconds later the contactors change over to delta and the current again spikes to 100 amps per phase for a few mS until the amateur has attained the slip angle due to no load at 5 seconds the loading valve closes and allows the compressor valves to operate and start compressing air.
any failure of non return air valve, loading valve leakge is an instant test of the three 80A motor rated HRC fuses on the switchboard.

so my point in all of this is it depends on fault current available from the supply, your wiring capacity to handle, etc whether you star delta start or direct online your motor.
here the regulations do allow direct online up to 15kW but you are limited to one start per hour. So in my case fro a compressor I have to have the star delta starter.
and no I'm not going for a soft electronic starter I'm in a high lightning area and these things die even when they are not in use

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