Hello Guest it is April 03, 2020, 03:28:14 AM

Author Topic: lack of power when using invertor ?  (Read 4288 times)

0 Members and 1 Guest are viewing this topic.

lack of power when using invertor ?
« on: January 16, 2015, 12:20:37 PM »
Hello chaps,
Opinions are welcome!!
I have a benchtop lathe ( chinese thing ) which i have changed to Mach3.
the original motor was a 750 watt 220 volt single phase motor.
I have changed this to a 1.1 kw 3 phase motor run by an invertor.

It seems to me that the new motor does not have the power that the original
motor had. Perhaps it is just my imagination but for example turning 50mm dia
delrin rod at 1500rpm i can only comfortably take a 0.5mm cut depth  at a feedrate of
0.35 per rev .
Does this sound about right  ??
when I used the machine with the 750 watt motor i seldom used more than
900 rpm because the gearbox was to noisy. but still at 900 rpm i could take a
reasonable cut 

regards

fred
 
think of the trees-- use both sides of the computer paper
Re: lack of power when using invertor ?
« Reply #1 on: January 16, 2015, 04:03:46 PM »
Could be quite a few things causing this. Again, as in your past posting with this same drive, you are not specific about the actual problem. I assume the motor is slowing down with deeper cuts. Sounds like low motor torque.

1) The VFD could not be setup properly for the motor specs (kw, voltage rating and rpm).

2) Low cost VFD's operate in what's called V/F (volts-frequency) mode, this means the output voltage to the motor reduced  as the frequency is reduced to lower motor rpm. When this happens motor developed torque is reduced exponentially... slow down the frequency enough and the motor can stall under load. This concept is a not understood by non electrical people that don't work with VFD's, then wonder why their motor is stalling or runs like crap.

The 750W, 220V motor you had operated at line voltage (220V) and developed 100% torque. I assumed the lathe speed is changed via gears or pulleys. I would use the lowest gear ratio to keep the motor rpm's in the high end to get max torque... then use the speed control pot to "trim" the rpm's to the desired lathe chuck speed. 

Industrial VFD's such as Siemens and Toshiba have a "vector mode" of operation to eliminate the torque problems with V/F mode. But you are talking about a $1500 drive instead of a $200 drive. Cheap Chinese drives are even worse for motor torque control and are garbage.     

Offline BR549

*
  •  6,925 6,925
    • View Profile
Re: lack of power when using invertor ?
« Reply #2 on: January 16, 2015, 04:15:48 PM »
Now another question is (;-) Is the 3phase motor a 4 pole (3600rpm) or a 2 pole (1800rpm) . The 2 pole will have more torque at a lower rpm than the 4pole. 

Best to use a 2 pole then overdrive the Freq to get the higher rpm if needed.

Just a thought, (;-) TP
Re: lack of power when using invertor ?
« Reply #3 on: January 16, 2015, 08:53:20 PM »
Yes, I forget to mention 2 and 4 pole motors. A 1800 rpm motor can be over driven, however, the torque curve decays rapidly as the motor exceed it's rpm design limit. So it's there is no free lunch so to speak. The other issue - and more dangerous - is the 1800 rpm rotor is larger and heavier as compared to a 3600 rpm motor. Over speeding an 1800 rpm motor can run the risk of the rotor windings de-laminating and flying apart.

I had a customer that tried to over speed a 75hp, 1800 rpm NEMA motor on a large industrial blower system... he reconfigured the drive and max rpm limit as a cheaper way to go instead of changing a $4200 motor. He looked like a hero to his boss. Worked for a couple of days. Then on day #3, at 2900 rpm's the rotor self destructed and locked dead... the 150 pound blower fan's inertia caused the motor to rip off the base destroying the blower housing. $25,000 worth of damage... priceless!  
Re: lack of power when using invertor ?
« Reply #4 on: January 17, 2015, 07:05:57 AM »
Ok Guys - please bear with me while i try to give more info.

Thanks GEH for your expert observations. firstly - your first post - " motor slowing down with deeeper cuts"
yes that is exactly it. If I take a deeper cut or a bigger feed the motor will not turn the chuck.
In my opinion your "low motor torque " describes the issue.

Regarding the quality of the inverter- I wouldnt describe it as a "low cost chinese"
It is an omron type 3g3MX2-A  made in jAPAN. I note your comments about the motor setup
and i will address these to the Omron serviceman on Monday . I really dont know if it is v/f mode or not.
- but understanding your speed/torque I will check the motor settings again and try and work through
gearing for more torque

The motor plate says 3phase induction motor 1.1 kw 1400 rpm 50 hz  (am i presuming correctly that this is
2 pole motor ?)

But the unanswered question still is . At 1.1 kw am i expecting to much for a cut ( in plastic ) of bigger than
.5 MM ?? just looking for opinions from others.

You chaps on the forum . Cutting 50 mm plastic at 1500 rpm what depth of cut do you achieve and what
KW is you motor.
In my engineering shop of course i have much bigger machines - similar size and power to Colchester Truimph
and to take 7mm to 10mm roughing cuts are not unusual  . At 1500 rpm cutting PTFE or UHMWPE the
streamer of swarf will land about 2 to 3 metres away which is what we want .



think of the trees-- use both sides of the computer paper
Re: lack of power when using invertor ?
« Reply #5 on: January 17, 2015, 10:12:26 AM »
I think i found something

I obtained a rev counter and when the spindle is turning at 1200 rpm
the motor is only turning at 500 rpm.
 
If I crank up the speed using S1600 i get a message "set to max - to fast for the pulley?? ( or something similar)
Is there somewhere to tell it what the pulley ratio should be.
regards 
While i search the forum for pulley ratio
fred
think of the trees-- use both sides of the computer paper

Offline mc

*
  •  380 380
    • View Profile
Re: lack of power when using invertor ?
« Reply #6 on: January 17, 2015, 10:25:21 AM »
TP, you've got your 2 and 4 poles mixed up. The more poles, the slower the motor turns.

How is your VFD controlled by Mach?

Offline BR549

*
  •  6,925 6,925
    • View Profile
Re: lack of power when using invertor ?
« Reply #7 on: January 17, 2015, 11:09:14 AM »
MC you are so correct (;-)

(;-) TP
Re: lack of power when using invertor ?
« Reply #8 on: January 17, 2015, 02:50:14 PM »
Check the Omron drive specs and it can be setup for vector mode or V/F mode with either CT (constant torque) or VT (variable torque) setting. For lathe operation I would use CT.

Some things to consider...

Most industrial motors are TEFC (totally enclosed fan cooled) with a cooling fan attached to the shaft in the endbell. Operating a motor at slow speed can result in overheating due to lack of cooling air. I don't know what type of 1400 rpm/3 phase motor you are using but running at less that 50% speed it might require external cooling blower. It doesn't take long for a motor to overheat especially under a load.

Second issue is the motor design... Motors used with VFD's are "inverter duty rated" which means the stator and rotor winding are designed with special insulation for frequency modulation and voltage spiking that occurs. Regular motors can be used but the is more probability the winding insulation will break down and short. Putting a 3 phase line reactor (choke) on the VFD output, between the drive and motor, can help eliminate spiking.  


  
« Last Edit: January 17, 2015, 03:03:47 PM by geh7552 »
Re: lack of power when using invertor ?
« Reply #9 on: January 17, 2015, 08:27:11 PM »
Fred,
If your motor is turning 500 rpm and the spindle is going 1200, then you have a large pulley on the motor and a smaller one on the spindle. So the motor is running at about 500/1400x50 = 17.85 Hz which greatly reduces motor torque, then you are gearing UP by 500/1400 = 35.7% of the motor torque to the spindle!  So there is nothing left. I don't know what the maximum spindle speed you want is, but this is a bad combination.