Varibale Speed for Drill Press (Induction motor?)

[Steve WanParticipant @stevewan33894]

Hi all machinist fans,

 

Just curious whether has anyone able to lower the press drill speed to less than 100 rpm for reaming? Not by mechanical means but by changing the induction motor voltage and also read on website the frequency too.

 

Could I use the ceiling fan control and mount parallel with the supply lines and make the variation? Will the motor slow down and gets heated up even faster?

Also the motor comes with a huge capacitor, will it affacts the frequency?

 

AC motor is warm at full speed, not sure with lower speed will make any damages as AC motor has no torgue.

 

My induction motor is only 1/3 HP.

Thanks inadvance any useful suggestions.

 

Steve Wan

[Steve WanParticipant @stevewan33894]

[Keith LongParticipant @keithlong89920]

Hi Steve

If your drill is fitted with a single phase induction motor then it’s a one speed device. The only thing that will change the speed is varying the frequency of supply and the motor will have been designed for the nominal supply frequency of the country that it was originally intended for. You’ll also have starting problems at different frequencies, as you won’t get the correct phase relationship between the currents in the starting and running windings to give you the necessary rotating magnetic field.

3ph motors are a totally different beast proposition and will work with variable supply frequencies, but the speed torque characteristic may not be what you need without the proper control circuitry. Another thing to bear in mind with slowing a motor down is that you also slow down the cooling fan fitted to it which can cause the motor to overheat. In that case you might have to fit a separate cooling fan and motor.

If you really want to slow a single phase induction motor driven machine down (or speed it up) the only way is by altering the mechanical drive system.

Keith

[colin hawesParticipant @colinhawes85982]

Hi Steve, I consider it unnecessary to use a hss reamer at such a low speed provided you use lubricating oil  on it, the lowest belt speed should be ok. To run an AC motor at a different speed requires a frequency change and this would damage it due to the change in reactance.    

[Steve GarnettParticipant @stevegarnett62550]

Whilst all of the foregoing is true, it’s not going to explain to Steve how the ceiling fan motor gets slowed down!

 

Essentially this works by the controller limiting the amount of power in each cycle of mains that the fan motor gets. Since the fan presents a fixed load to the motor, then what happens is that because there’s less power, the motor slips out of synchronisation with the supply frequency and runs slowly. If the fan were removed, the motor would speed up again, but with less overall power available.

 

If you tried this with your drill, then I suppose that a similar scenario would result. When you were reaming your hole, the drill would slow down under load, but since the load would be variable, you wouldn’t be able to determine the position of the controller easily, if at all. My guess would be that in fact, you’d be most likely to get yourself in a situation where you were completely out of control of what was happening, and that wouldn’t be good at all. So the real message here is – don’t even attempt to do it!

[Ian S CParticipant @iansc]

On a n old fan that I repaireda few years ago, the speed control was obtained by switching in a choke (looks like a transformer, but only one winding, and two wires), with this switched in there was a drop in speed of 20/30%.  The motor was 40watts 230V, and its shaft power would be reduced by at least that percentage.  That system is a no goer for bigger motors.  Other fan motors have two windings.   Ian S C

[John HaineParticipant @johnhaine32865]

Almost certainly the fan uses a “torque motor” which is made having a higher resistance rotor than a standard induction motor.  These will slow down according to the stator current (less = lower) but correspondingly are rather inefficient and get rather warm.  All induction motors run asynchronously*, the rotor going slower than the stator fields, but torque motors are very asynchronous.  Standard induction motors just stall if you try to operate them in this mode, and then get very hot. 

 

Torque motors are used where you need very simple speed control but efficiency isn’t important; or when you just want to exert a constant torque at variable speed.  I used to have an audio tape deck with such motors driving the reels – normally the takeup reel exerted just enough torque to roll up the tape; whilst the feed reel was actually driven backwards by the unrolling tape and the torque kept it tensioned.  Fast forward or rewind involved increasing the appropriate motor current to drive the reels fast.

 

*unless they are “synchronous induction motors” in which case they start like induction motors but when close to synchronous speed the rotor locks into synch with the stator field operating as a reluctance motor.  Ordinary induction motors with “squirrel cage” rotors can’t run synchronously.

[Ian S CParticipant @iansc]

The motor was an ordenary little shaded pole motor you’ll find in most all fans. Ian S C

[Steve GarnettParticipant @stevegarnett62550]

I think that I have used a slightly different definition of ‘synchronous’ to you, Ian.

 

If the supply frequency is fixed, and the rotor max speed (no load) is achieved, then even though the rotor speed is slower than the stator fields, the relationship between them is fixed and to all intents and purposes, they are (by my definition) synchronised – they won’t slip out of that relationship. If you increase the load, then that initial relationship is broken, and that’s what I meant by ‘slips out of synchronisation with the supply frequency’.

 

I find this to be a more useful definition of ‘synchronous’ for people who want to get a basic grasp of what’s going on with these motors rather than the more pedantic ‘normal’ definition…  (no I’m not accusing you of being a pedant!)

Edited By Steve Garnett on 19/01/2011 10:14:23

[Ian S CParticipant @iansc]

The choke is in series with the motor windings, and it appears that it drops the voltage in the field, there by reducing the motors power, the drag of the fan sets the revs, it is posible to use a choke with tappings to give more than one speed reduction. It seems the modern method is to use a capacitor, some were about 4-8uf for a similar effect. This is way OT, the nearest i’v heard of speed control on a single phase bench drill was in ME a good many years ago, the chap had found that if he wired his motor so that he could switch in the starter windings, he could drop the motor speed, I think he had a 2 pole motor, so changing it to4 pole halved the speed, it worked quite well for some time, until one day the start windings gave up. This gives an idea, if one was into rewinding motors (its tedious, rather than difficult), and there was enough room, the start winding could be removed, and rewound to match the run windings. Just a thought, I’v rewound a few motors but never thought of making a 2 speed one. Ian S C

[Steve GarnettParticipant @stevegarnett62550]

Can’t help thinking that it would be easier to fit a small three-phase motor and controller than go through all that. With the best will in the world, this would be an experimental motor at the end of it. Fine if you want to do it, but if you are trying to get a result from your machinery and do short runs at low speed, then the former approach is the one to go for. My pillar drill has a backgear system, and accordingly goes down to 50rpm anyway, so for me it wouldn’t ever be an issue.

[Trevor Williams 1Participant @trevorwilliams1]

Following all the previous comments about changing the speed of a single phase motor by putting a choke (resistance) etc in series with the supply – which is a none starter due to the large power loss this would intoduce, apart from being very dodgy without the required Electrical knowledge. Why don’t you tackle the job the easy way and make a bigger pulley(s) to reduce the speed, much easier and a lot safer too. The other Electrical solutions would be very costly to implement.

[Ian S CParticipant @iansc]

I agree Trevor, the choke or similar speed reduction method is a non starter, it can only be used for motors 40 watts or less, and it relies on a constant load to set the speed.

A jack shaft would be one way to go, it will involve an extra belt, but the pullies are smaller. You don’t need to go as far as a mates vintage (1920s) American made pillar drill press, it has a back gear, and goes down to 5rpm, it also has power feed. Ian S C

[Steve GarnettParticipant @stevegarnett62550]

Haven’t heard back from Steve, but this has to be a pretty small drill, one way or another. Just how much you could easily do in the way of a physical adaptation of it to include either extra or larger pulleys is not clear, but I don’t think it would be that easy – which may well be why he discounted the idea in the first post. I still think that a small three-phase motor and inverter (even though it would cost a little more) would provide a physically easier, and more timely solution, I’m afraid.

[Steve WanParticipant @stevewan33894]

Hi folks,

I have been reading all your valuable info. Many thanks!

Actually, I got a triac switch from a spoilt water-heater and wanted to fix to my drill press to lower the speed more but since I got so much negative feedbacks which is good so I have decided to apply to my soldering iron insteadfor a variable heat control. Nowchecking the application and its fixture.

My press drill was using 2 pulleys, the lowest rpm around 500. Now I made a smaller pulley with 2 belts which inturn makes the speed much slower when all 3 pulleys are linked. The frequency is not adjusted, running at full speed as it was.

Wondering I could have the chance to view photos of the slow down mechanics of Steve Garnett’s drill press? Sounds interesting as I don’t think they’re in production now.

I appreciate all your efforts here

Steve-Singapore

[Author]

Posts