I built a Leslie (rotating drum using factory unit out of old Thomas Organ) using the factory motor for my guitar rig (SRV -Stevie Ray Vaughn tone) . Is there a simple circuit to control the speed that will be reliable? I have used a Hardwick Hindle Inc ceramic type # KS-8852 L20 600 ohm .288 amp series 5A rheostat controlling a 117 volt 60 cycle 35 wt AC motor and it works. I have ran it for a minute at approx half speed and neither motor nor rheostat get hot but I will use this at least 45 minutes at a time. Thanx

 

You must have a universal motor, not an AC motor. It is not possible to control any AC motor that I know about using a series resistance. Since the rheostat is working, why try something else?

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What about using a lamp dimmer?

It would work but you might need to add a snubber network in parallel (100nF capacitor in series with a 100hm: resistor) with the motor to protect the dimmer.

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Right, a lamp dimmer is designed for the resistive loads of incandescent light bulbs.

There are similar controls available from lamp stores for controlling the speed of ceilng fans. They might be more suitable since they are designed to control the fan motors.

Bob

 

Ceiling fan controllers usually have a small inductor coil / auto transformer, in the controlunit which has different taps and can be switched in series with the fan to obtain different speeds.

Other type of controllers use a couple of capacitors which are switched in series with the fan motor.

You can try and see if it works.

edited for typo's.

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Agreed but a lamp dimmer will be alright for driving small motors (<50W) providing a snubber network is added. Most lamp dimmers are designed to control loads up to 250W. Therefore they'll be alright for small inductive loads providing the dimmer is de-rated accordingly and adequate suppression is added to limit the back EMF.

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The rheostat is much larger than a normal light dimmer. the light dimmer switches I have seen state " not for controlling motor speed". I have ran the rheostat and it gets hot but no wire turns red like in a blow dryer. The rheostat is ran in series like on a sewing machine. Should I try the cap/ resistor but what wattage resistor and electrolytic or non polorized cap? Thanx for all your help.

 

You should obviously use a non-polarised capacitor as it's AC and ensure it's rated for at least 350VDC.

Calculating the value of capacitor to use in't easy as you need to account for the motor's inductance.

How much did your lamp dimmer cost?

Can you easilly buy another one if you break it?

Lamp dimmers are cheap where I live so I would try it and if it breaks then I've not lost much.

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I haven't tried a lamp dimmer but the dimmer won't handle the amps as the motor has 5A marked on it 5 amps probably being drawn on lower speeds. Does that help?

 

5A for a 35W motor?

Are you sure you're not talking about the reostat?

At 117V that's 585VA!

I'm not going to calculate the power factor but it's terrible, in fact so bad I wouldn't believe it!

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I know this is an old thread, but for anyone who is interested, I have designed AC motor control circuits using a FET. There are many advantages to using a FET instead of a Triac. One main reason is No Hum (well very little). Triac controlled AC motors are noisy at low speeds, since triacs conduct when voltage reach a certain voltage level (phase delay controlled).
FET controlled is quiet because FET is switched on at each zero crossing and is switched off after a timed period (e.g. on for 5ms of both +VE and -VE half cycles.

If anyone is after some help in designing a quiet AC fully variable speed controller compatible with inductive motors (ideal for fans, but will also work with resistive loads) then post any questions, comments e.t.c.

 

I am interested, sounds like a great project. I was searching around for a triac circuit or confirmation that a light dimmer would work for a 1.5amp 115vac motor. Would the FET circuit be good for this?

 

Yes it will work on 115V, and since main control component is a FET, then can drive not only 1.5Amps inductive loads, but much higher. You will also need a mains power capacitor (motor absorption capacitor) which should be rated at approximately double the mains RMS voltage. Capacitance depends on :
1. type of motor since PSC motors require a lower capacitor value, then a shaded pole.
2. The current rating of motor.
E.g I have run a 0.75A shaded motor with 15MFD Cap
and a 0.2Amp PSC motor with 4MFD

The FET has to have a maximum working limit at appro 3 x RMS. This is so mains spikes don't damage FET, when fet is off and don't reach avalanche breakdown voltage.

Main advantage of FET control over triac control, is motor is much quieter, especially at lower speeds. This is because the FET is switched on at mains zero crossing, and switched off after a specific timed period.
E.g after 3mS for a slow speed, and 9.4mS for full speed. The FET on time for minimum speed is determined by power rating of motor. You must not go over 9.4mS for full speed, otherwise you will miss the mains zero crossing point.
My next post (if you are still interested), I will post a circuit and discuss programming a microcontroller to control FET.

 

Medelec35:
Sure, we are interested!

 

I built a lamp dimmer that was able to control a vacuum cleaner to blow air into a forge hearth.