Motor help needed!

[bazp]

Hi Guys!

I have a Seneca Falls Star lathe that is currently without a motor. The motor I received with the lathe is far to big and cumbersome to use, plus it is so badly wired that I don't want to go anywhere near it!

So, what sort of motor should I replace it with? I'm assuming that a 1 HP, single phase of some description. My knowledge of motors is limited to what I picked up at uni (when I wasn't asleep, that lecturer was particularly boring!).

Any help gratefully recieved!

Ta
Baz

[Deleted]

I have used single phase motors from both Screwfix and Machine Mart for a number of years. I have 2 lathes with one of each at the mo; the Screwfix is a 2HP one and is a bit noisy but has lasted for 6 years of daily industrial use. The DOL starter that came with it was crap: the stop button wasn't long enough inside the case to reach the relay! Soon fixed though.
I have a Machine Mart motor on an ML7 (with clutch) which is fairly quiet but the overload relay cuts in after about 3 hours constant use, I had to turn it up above the standard setting to get the motor to run for more than 15 minutes initially. Again the DOL starter is pretty flimsy but it works OK.
I used to have a pretty big Herbert mill fitted with a 2HP Machine Mart motor which was amazing, so quiet you could almost only hear the belts.
For the price you can't go to far wrong with either.

[chris vine]

Hi Baz,

You might think about a 3 phase motor with a variable speed inverter drive. This will be quiet (except perhaps for a high freqency whine at some speeds).

There is Gavin Oseman who is always advertising on the private ads part of the site. He seems to specialise in the inverters at low prices. I am sure he would advise on a suitable motor.....

Chris.

[bazp]

What is the advantage of using a three phase motor with inverter over a single phase one? How many poles would be best, 2 or 4?

[baggo]

Hi Baz,

I replaced the single phase motor on my ML7 with a ½HP three phase motor and inverter. Basically three phase motors are smaller and more efficient for the same power output. The inverter allows you to control the speed, direction etc. of the motor easily. It's possible to vary the spindle speed from barely moving to several times normal speed by merely turning a knob. A lot of the time it saves having to keep moving the drive belt on the stepped pulleys although for very low spindle speeds it pays to drop the belt down to the lowest speed setting. The motor loses a lot of it's power at very low speeds so may stall otherwise if you try taking heavy cuts.

John

[chris vine]

Hi,

I bought one of the Newton Tessla drives, motor and inverter. Yes it was a bit more expensive than the diy option but it is very good.

On their set up the motor torque is maintained (almost unchanged) down to the lowest speed. Plus it gives you a bit higher top speed.

I find on a myford S7 that most of the time I just leave the belts set for the highest speed.

Chris.

[houstonceng]

Bazp

The key advantage of a 3-phase mptor vs a 1-phase is that the 3P has a smooth rotational speed, whereas the 1P motor actually rotates in a series of pulses at the mains frequency. This can lead to paterning on the workpiece.

Driving the 3P from a Variable Frequency Drive (VFD) also adds the advantage of speed control - not available on the 1P for reasons which I won't explain here.

Contrary to popular belief, 3P motors are not always physically smaller than 1P motors of the same rating.

It is also not recommended that a 3P motor be speed controlled outside a range of 1/2 to 2 times it's normally rated speed. ie the VFD is adjusted to give 25Hz min to 100Hz maximum. At the lower speed, the fan may not give enough cooling to the motor and, if you want to run lower, an auxiliary cooling fan is recommended. One must also note that at certain VFD frequencies, resonant vibrations may become noticable, hence the recomendation to be careful about maximum speed in addition to being careful not to operate the driven mechanism beyond its safe max. eg if the Lathe has a nirmal max speed of 2000RPM when driven by a 1450RPM motor at 50Hz, feeding it with 100Hz would give a potential max speed of 4000RPM which might exceed the bearing capabilities.

Most VFDs have "torque boost" and other compensations in-built in order to attempt to maintain persformance across the speed range. It may be possible to damage the motor if too high a boost is selected - especially at lower frequencies where the impedance of the motor is lower. Good quality VFDs usually have limitations in-built to avoid adverse effects on the motor, however, it must be said that not all 3P motors are suitable for VFD use and somer manufacturers of motors make specials for VFD systems.

Trust this helps.

[houstonceng]

Sorry. Forgot to add that if the recommended 1P motor is 4 pole (the most usual) giving theoretically 1500RPM - practically circa 1450RPM - then a 3P 4 pole motor should be used.

[Laurie_B]

Something to consider before purchasing an inverter unit on its own,is whether the motor and the inverter unit are compatible.Units such as the Newton Tesla system have a dedicated motor supplied so the problem doesn't arise.
But with older,ex-industrial machines fitted with a 3 ph motor/s it is importantant to check the motor nameplate to see if the motor windings can be connected in 'star' or 'delta'. This will be shown typically on the nameplate as "220/250V-380/440V" It is also referred to as 'dual voltage'.
Some inverters offered for sale only produce a 240V 3ph output which is ok if the motor terminals can be reconnected to a 'delta' configuration.These are not suitable for motors which are only configured for 415V 3ph.

[chris vine]

Hi Andy,

Yes you are quite right. If I run the motor too slowly for a long time it get much too hot. It is fine for a short while, eg screwcutting without having to select a lower belt speed.

I suspect that the method of keeping the torque as high as they do at almost zero speed involves quite high currents, together with the ineffectiveness of the fan at low speed.

Some dual voltage motors are for 440 or 720V (if I remember correctly). I don't know which industries use this system but I have a motor in the workshop labelled like this. Probably not common on machine tools....?

Chris.