Will this control work for me?

[fnbrowning]

I am looking for a controller that would reduce the rpm's of an induction motor.

The motor is ½ H.P. 120v, 8 amp, 1700 rpm, TEFC induction motor used on a drill press.

I would like to reduce the motor speed to approximately 1000 to 1140 rpm.

 

Now I have an idea that may work, and I’d like someone to review it.

 

MOST IMPORTANT! This controller would not have to *start* the motor, only control it.

The controller would never be used to reduce the rpms to less than 50 percent.

Because of the flywheel effect of the pulleys and rotating quill, the motor takes several seconds to spin down when switched off.

A controller could be placed alongside the normal A/C line, incorporated with a Single-pole double-throw switch.

When speed control is required, the Single-pole double-throw switch would be tripped on the idling motor, thus interrupting the current flow for no more than a second, and throwing the load over to the controller. Thus, the motor would *never be allowed to spin down,* and thus would not be restarted, just spun back up - to the speed setting of the controller!

 

Given these narrow set of conditions in a controlled environment where no uninstructed personnel would be using the machine, would this type of controller described below work?

 

“Solid State Variable Speed AC Motor Control.

Designed for Speed Control of: Shaded Pole, Permanent Split Capacitor and AC/DC Electric Motors.

Full Wave Control Circuitry with RFI Filter.

{Not to be used with Capacitor Start or Capacitor Run Motors}

Typical Applications: Fans Air Conditioners Fireplace Blowers Attic Fans Humidifiers Ventilators”

 

The circuit in their literature appears to use one or more Tiac, Diac, Capacitors, Resistors, and an RFI coil.

[marke]

Hello fnbrowning

 

Your description of the motor speed control system sounds like a voltage controller only. This principle can be applied to "fan" motors that have been designed for variable voltage control. These are a special type of induction motor that has a high resistance rotor. If you apply a variable voltage controller to a shandard induction motor, you will overheat the rotor and cause it to fail.

The other problem of the variable voltage controller is that you are not directly controlling the speed, you are reducing the torque output from the motor. The speed will be determined by the load lorque curve. A drill has a variable load torque requirement and so the speed will vary with the load. (It will appear as an undersized motor so is not ideal, especially as you would generally require a lower speed with a larger drill bit needing a higher torque.)

The best way to vary the speed, is to replace the motor with a small three phase motor and inverter.

 

Best regards

[fnbrowning]

Originally posted by marke

Hello fnbrowning

The best way to vary the speed, is to replace the motor with a small three phase motor and inverter.

Best regards

I know you have the very best intentions when you post your final recommendation, but it is cost prohibitive for the small hobbiest!

 

It would be cheaper for me to 'toss away' my current motor and just purchase one with a lower rotational speed. Cheaper, but inelegant and wasteful of resources. {mainly mine!}

 

I am wondering why there are no discussions of microprocessor control of AC induction motors. Motorola boasts of microcontroller control of 1-phase AC induction motors at their web site. For instance, see:

http://e-www.motorola.com/webapp/sps/site/...deId=03nQXGrrlP

 

What about microprocessor control? Is there something that could be bread-boarded together using hobbiest or experimenter electronic parts?

 

Remember: I only need to control the upper third of the rpm range.

[marke]

Hello fnbrowning

 

First, there are a number of different types of motors employed in this equipemnt, the most common being the induction motor and the universal motor. (The universal motor has brushes, the induction motor does not)

The induction motor is a pseudosynchronous machine and is gpoing to run at the frequency of the supply. The only real way to control the speed of an induction motor, is to alster the frequency of the supply applied to it. The cheapest way to achieve this, is to use a small low cost three phase induction motor and a low cost inverter. (cheaper to buy than to try to build yourself!!)

A universal motor can be controlled by varying the voltage applied, but this will be at a loss of torque. Generally, for a drilling machine, you want more torque at low speed, and this can only really be achieved mechanically.

 

A discussion on a micro based speed control system is a great idea. I will happily contribute to such a discussion if you wish to start one, but the first thing to establish, is what sort of motor you are trying to control.

 

Best regards,

[fnbrowning]

Originally posted by marke

A discussion on a micro based speed control system is a great idea. I will happily contribute to such a discussion if you wish to start one, but the first thing to establish, is what sort of motor you are trying to control.

 

Best regards,

Great!

 

The motor is ½ H.P. 120v, 8 amp, 1700 rpm, TEFC induction motor. Removal of two protective covers revealed:

 

1 start capacitor, 200mfd, 125 volts.

 

4 wires connected to 12v supply.

Wires marked 1&3 connected to hot or black leg.

Wires marked 2&4 connected to neutral or white leg.

 

I would like to reduce the motor speed to approximately 1000 to 1140 rpm.

 

As unusual as it may seem, for my application, I do not need more torque as the speed drops. I will use the lower speed for one thing only, and it's not for drilling into steel or iron, or a high torque application.

[GGOSS]

Hello fnbrowning,

 

Based on the description you have given, I suspect your motor is a standard cap start or cap start/cap run induction motor. If so, you should be aware single phase speed controllers are not suitable for connection to these types of motors.

 

Most drill presses I have been involved with make use of pullies and v-belts to couple the motor and drill. If yours is of this type you might consider changing the pulley ratios to obtain your desired speed.

 

Understanding that you wish to keep costs to an absolute minimum, the above 'mechanical' solution may provide the best overall result.

 

Regards,

GGOSS

[marke]

Hello fnbrowning

 

Yes it would appear that your motor is a standard capacitor start induction motor and will not be suitable for variable voltage control.

i assume that

4 wires connected to 12v supply.

should read 120 volt?

You could alter the speed by mechanical means as GGOSS, the easiest may be to use pully ratios, or by changing the frequency and voltage applied to it. If the motor is a cpa start cap run, then altering the frequency is less of an option as the reactance of the capacitor is frequency dependant.

One way of achieving this is to use a large power amplifier coupled to the motor via a transformer to step the voltage up. Another way is to use a PWM inverter, but this would need to be filtered if the capacitor remained in circuit. The bottom line as far as I can see, is that it would be cheaper to buy a three phase motor and speed controller than to try to build this sort of technology yourself.

 

Best regards,

[fnbrowning]

GGOSS, marke;

 

Thank you for all the good information. I appreciate your efforts.

 

I guess I have to give up.

 

Reasons?

 

As to: >>"consider changing the pulley ratios to obtain your desired speed."<<

 

Of course, my drill press DOES use pulleys. Actually, ribbed pulleys & belts. Just doesn't have a speed ratio I want.

If you've ever tried to find pully cones outside of the machinery they mount in, you know that these parts are not normally available to general retail.

Oh, ONE pulley or two as a SPECIFIC replacement part, but no pully sets dimensioned out for the selection of the retail buyer.

 

As to >>"use a small low cost three phase induction motor and a low cost inverter"<<

I'm assuming the low $$ means as a business cost, not the hobbiest out-of-pocket.

Last price I saw for a 1/2 horse 3-phase combo motor/controller was $600+ U.S. PLUS shipping.

Don't know about your economy, but in mine, that is not low cost! ;b;

 

I guess all that's left is to try to find an olde, used, 1/2 horse induction motor with a NEMA type 56 frame that spins around 1000 rpm and bolt that up.

 

*sigh* I don't think I like electric motors!

[marke]

Hello fnbrowning

 

Sorry we could not be more helpful.

I understand your financial requiremnts, but to actually build something to alter the speed wouod cost even more than the expensive motor and controller. It is not a case of perception of cost, more a case of reality.

Good luck in your quest.

Best regards,

[Guest]

it may be worth you looking into a washing machine motor as some of these are made with two sets of windings to enable spin and wash cycles to be performed by one motor.

I have just got one that will run at 800rpm on one set of windings and 1250 on the other the case is slightly larger but this works fine for me

 

just an idea

Adam