Single Phase Motor Chatter

[velocette]

Hi this is fishing expedition put out this as a bait in October 2011

Some of the noise comes from the motor if it is "Single Phase" they do not have perfectly smooth rotation Something to do with
50 Cycles per second = 3000 per minute. Motor speed 1425 rpm don't ask me how this is I do not know.

Can anyone shed any light on this

Yes I live in part of the world that uses 50 Cycles per Second

Eric

 

[JorgensenSteam]

Single phase motors vibrate somewhat because they do not produce constant torque around the entire 360 degrees of rotation like a 3-phase motor does.

 

[Ken I]

Fishing expedition - O.K. I'm hooked - now can you land the fish...

First consider a three phase system - consider a magnet rotating in a delta (triangle) set of windings - will generate three phases of alternating current - connect these to a similar winding motor and it generates a rotating field - hey presto a motor.

This motor will spin at the same speed as the "generator" less a bit for slippage so for 50Hz you get 3000 rpm (more like 2850 under load because of slip).

This is for a "single pole" motor (a bit of a misnomer as its actually two poles per phase - but this is the convention).
Up the number of poles to 2 then you have the rotor within a hexagon of windings and the motor will only rotate at half the synchronous speed down to 1500 rpm (in your case 1425 {rated} after slippage.

Single phase can only "push - pull" on the rotor - hence the need for a "starter" set of windings which drops out when it is up to speed but is still like a single piston engine and cannot generate constant torque throuout the 360° of rotation.

Same rules the number of poles determines the speed relative to synchronous -
1 pole 3000 rpm @ 50 Hz or 3600 @ 60 Hz
2 pole 1500 rpm @ 50 Hz or 1800 @ 60 Hz
3 pole 1000 rpm @ 50 Hz or 1200 @ 60 Hz
4 pole (fairly rare) 750 rpm @ 50 Hz or 900 rpm @ 60 Hz.

The slip is because the "squirrel cage" rotor (a series of aluminium rods cast through the rotor) act as windings - the rotating field induces a voltage (and hence a current) to flow in them, inducing a reactive magnetic field which allows the rotor to be towed around by the rotating magnetic field.

The rotor cannot match the speed of the rotating field because at that point the relative motion between the rotor and the field would be zero therefore no induced reaction - so the slip is mandated by the physics.

Typically slip is about 5% of synchronus speed at full load - at 50Hz this 5% slippage is inducing current in the squirrel cage at 2.5 Hz at max. torque - so when you switch on a stationary motor the squirrel cage is standing still (for a moment) and the induced current is therefore at 50 Hz - since the current induced is going to be proportional to frequency (approximately) then starting torque is going to be 1/20th of rated torque.

This is why squirrel cage motors have such lousy starting characteristics.
For the same reasons if you overload a squirrel cage motor the slippage increaces (beyond optimum) and the reaction torque drops rapidly hence a stall.
(A D.C. motor by comparison will just continue to deliver until it saturates or hot copper runs out of the windings.)

Now you can see why variable frequency drives improve motor performance, not only does it give you the opportunity to vary the speed, it also allows you to ramp the frequency which improves starting torque and ramp up torque to near full rated torque across all frequencies.

Single phase squirrel cage motors are not a good choice for driving a lathe. Try adding a bit of flywheel mass to the motor - this should help to dampen out the cyclic nature of the beast.

Clear as mud ?

Regards,
Ken

 

[velocette]

Hi Ken Thank you for explanation on why we have "Single Phase Chatter". Even I can understand your clear and informed answer.
I had figured out it was "slippage" you have made it clear how it works.

My lathe had a single phase motor with a chattering and rattling six speed gearbox in close attendance with the ability to transfer this chatter to finishing cuts and some speed and feed rates.

The alternative was to fit a DC motor and KB Electronic speed control and set it up according to the instruction manual.
This is a successful conversion "Self Praise Interlude" resulting in a very quiet and smooth running lathe with finger tip control of speed and feed.
Yes there is a DC motor on the leadscrew as well.

A small aside to this conversion is that with an ampererage meter on the AC supply there is less current used than when running on the Dc motor Than the AC motor.
May be able to recover the cost of the DC conversion in power saving. Forty years seems about right

Flywheel would have been a cheaper and simpler option. 20/20 vision in hind sight yes I have that to.
Unfortunately the conversion excercise has made me into a DC Motor Addict. The little buggers are everywhere in my workshop now

Eric

 

[Ken I]

Power is power is power.

A DC motor is probably going to be a little less efficient - an ampere reading is misleading on an A.C. device due to "power factor"

A pure inductor can have volts, amps but no watts due to the power factor being zero.

So any measurement of A.C. power without a wattmeter or knowledge of the device's power factor can be misleading.

Its normally on the nameplate as PF or cos(psi) - nomally at rated power - it gets worse as the load drops off - you will often see an AC motor amps not hugely different from no load to full load - what is changing is its power factor.

Watts = Amps x Volts x Power Factor

More mud.

D.C. is the way to go on small aplications although in my world of robotics we can vector a 3000 rpm AC motor to a millionth of a revolution the technology keeps getting sillier.

AC servos have permanent magnet rotors so there is no slippage and the three pahse vectored voltages can be brought to a given vector value at 0 Hz - this holds the motor at the vector - by moving the voltages you can vector the motor to any position thhrough any speed - fun stuff.

Ken

 

[Don1966]

Great explaination Ken. I would like to add that VFD drives have very good torque and has now exceeded the DC motor performance. Since the price of solid state devices have drop you can pick them up cheap. The problem with DC motors is the maintence because of the brushes and communitor ware. AC drive have come a lone way from there first introduction years ago. Now with the vector drives with PWM, algorithms programmed into them generate a motor model of the perticular motor connected to it. These motor are also more efficient. They can also produce more torque and low speed then do DC motors. By controlling the frequency to voltage ratio they maintain the motor flux and adjust it as required by the motor slip factor to generate torque. This make them one of the best drives on the market. AC drive do have a high frequency hum to them, but this is small.
I hate to say this but power is power it takes 746 watts to create one HP the same is true for he DC motor,in other words HP on AC is the same as HP with DC. We multiply current x voltage for DC to get HP and we multiply voltage times current x efficency to get AC except for 3 phase, it is the same as single phase except
we will add, multiply by the square root of 3 which is 1.73. One motors HP is no motor then the other.

Regards Don

 

[Don1966]

Looks like we posted on top of each other Ken pretty much the same reply.

Don