Reduce voltage for a single phase induction motor - Page 1
Forum section: electronics / beginners

author: robsims
author_rank: Frequent Contributor
timestamp: September 16, 2022, 11:24:58 pm
content: Hi,I have a 120 volt 60Hz single phase induction motor. At 60 volts it reaches it's rated RPM and the motor doesn't get too hot. At 60 volts it also has enough torque for the purpose i want to use it for. Mains voltage is 120 volt and i want to reduce the voltage so i can let the motor run at 60 volts. How can i reduce the voltage without using a transformer or variac? I want to place the voltage reducing components in a small box which i will attach to the motor housing

author: james_s
author_rank: Super Contributor
timestamp: September 17, 2022, 01:27:03 am
content: Why do you want to reduce the voltage? When you reduce the voltage on an induction motor the current it draws increases which causes the motor to be less efficient.

author: Whales
author_rank: Super Contributor
timestamp: September 17, 2022, 06:25:38 am
content: If it's a low power induction motor: a "motor run" capacitor in series is a common solution. Not sure how many watts this is feasible up to, but probably much less than 100W.

author: armandine2
author_rank: Frequent Contributor
timestamp: September 17, 2022, 08:26:15 am
content: I wonder if you have a Capacitor-Start and Capacitor-Run set up or just Capacitor-Start with centrifugal switch?I believe (IIRC) the torque is proportional to the voltage squared - so you are losing a lot of torque at 60Volts.I don't have practical experience here - but you could drop the 60 Volts with a capacitor of course - though they do say all voltage dropping capacitors fail.

author: armandine2
author_rank: Frequent Contributor
timestamp: September 17, 2022, 09:19:15 am
content: I wonder too if it is possible to create a dual voltage set up with existing connections?

author: robsims
author_rank: Frequent Contributor
timestamp: September 17, 2022, 10:05:10 am
content: Ok leave efficiency and torque out of the equation for now. And the motor doesn't get too hot on 60 volts as i already said. What is the best way to reduce mains voltage? Please, if you have knowledge or experience with this please let me know

author: Siwastaja
author_rank: Super Contributor
timestamp: September 17, 2022, 11:27:35 am
content: If you have confirmed this particular motor operates satisfactory at 60V AC, then the options are:* Transformer (can be autotransformer for reduced size)* TRIAC dimmer* Series capacitorAll have their pros and cons. Triac dimmers are very popular for AC induction fan motor control but cause a buzzing sound I hate. Transformer is IMHO best, by far, but it's also the largest/heaviest, even if it's autotransformer.Suitable capacitor may be quite big, too.If it needs to be small, try triac controller.

author: MrAl
author_rank: Super Contributor
timestamp: September 17, 2022, 11:35:48 am
content: Quote from: robsims on September 16, 2022, 11:24:58 pmHi,This is one of those things where you have to include a little more information in order for anybody to try to figure out a good solution or solutions.In particular, the rated current when operated at 60v would be nice, or even at 120v would be nice.For example, if your motor draws 200 amps at 60 volts nobody here would recommend using a series capacitor but if it only draws 0.1 amps i bet there would be.AC rated capacitors are sometimes a bit more expensive, but if you only have to buy 1 it's not too bad.

author: robsims
author_rank: Frequent Contributor
timestamp: September 17, 2022, 11:47:37 am
content: Many thanks Siwastaja, I'll figure out what the best option is.If anyone has other solutions please share

author: BeBuLamar
author_rank: Super Contributor
timestamp: September 17, 2022, 11:48:33 am
content: Why do you want to reduce the voltage? I don't think you even save electricity doing so.

author: armandine2
author_rank: Frequent Contributor
timestamp: September 17, 2022, 12:01:55 pm
content: Dropping ac voltage in a single phase system is a standard question in Engineering Science courses.Typical question in the UK is how do you use a product from the USA in the UK.The question normally assumes a pure resistance load and expects you to size a suitable series capacitor - to drop 240V to 120V.The analogy should work for you, I guess. You are assuming it is really made for 60VAC but you have 120VAC.May be, because of the motor inductance, a parallel capacitor and inductance arrangement in series would be better than a simple series capacitor. No experience here. Involves more ac calculations.For a simple illustrative series capacitor example:First, you use the motor rated power to calculate the existing resistive load and use the fact that the circuit with series reactance must operate at its correct rating, so the current will remain the same as before. That is in your case finding what current it runs at with 60VAC, which you are now so happy with.Use Pythagoras to calculate the voltage drop - using a phasor diagram. The Value of the reactance is then found. Xc= Vc/I and from Xc= 1/2 pi f C.

author: robsims
author_rank: Frequent Contributor
timestamp: September 17, 2022, 12:17:59 pm
content: With a triac you control the motor speed if i'm not wrong, but i need to reduce the voltage

author: robsims
author_rank: Frequent Contributor
timestamp: September 17, 2022, 12:20:29 pm
content: Please don't ask why i want to reduce the voltage. Also o don't want to hear how it will affect motor efficiency etc. i just want to know what the effective way is to reduce the voltage

author: Circlotron
author_rank: Super Contributor
timestamp: September 17, 2022, 12:26:09 pm
content: Quote from: james_s on September 17, 2022, 01:27:03 amIf the motor is fully loaded then what you say is quite true.If the motor is only partially loaded an appropriate reduction in supply voltage will reduce the supply current, save energy, and improve the power factor.What happens is that at part load the motor is running above full load rpm and closer to (but never at) synchronous rpm. The frequency of the induced current in the rotor is very low so the rotor's reactance is way different to the rotor end ring resistance so the rotor has a low power factor and consequently so does the stator. Thus the stator copper losses are higher than need be at low load. If at part load we reduce the supply voltage the motor slows ever so slightly toward full load rpm, the rotor induced frequency approaches the ideal value and the current pulled by the motor decreases. Of course, if you decrease the voltage too far the motor slows down too much and the supply current goes off the chart and the motor will very quickly burn out.

author: Zero999
author_rank: Super Contributor
timestamp: September 17, 2022, 12:32:25 pm
content: Just one question: why? If it's for safety's sake, use an isolation transformer, otherwise don't bother: run it at full mains voltage.A capacitor in series won't work very well. The voltage across the motor will vary with the load. The inductance of the motor should also be taken into account. If the wrong value is selected, it can form a resonant circuit and increase the motor voltage, which will damage it.A transformer is the only practical way.An induction motor won't run very well with a TRIAC.

author: robsims
author_rank: Frequent Contributor
timestamp: September 17, 2022, 01:05:57 pm
content: many thanks zero999

author: Siwastaja
author_rank: Super Contributor
timestamp: September 17, 2022, 01:09:52 pm
content: Quote from: robsims on September 17, 2022, 12:17:59 pmIt reduces average voltage the motor sees. Motors have significant amount of inductance so it really doesn't matter if you actually reduce the voltage amplitude, or "chop" it on/off 100 times a second like a TRIAC dimmer does.Effect of the triac dimmer is the same as with any other way that reduces voltage - transformer, series resistor or series capacitor: motor speed is reduced, motor runs at higher slip, and at lower efficiency, but when it is a fan motor designed to run at higher slip, it is OK, because with a fan, the mechanical load reduces quadratically with reduced RPM.If you just need to "reduce voltage" "no questions asked", I'd say a transformer is your only option, it's the only one which only controls voltage regardless of other parameters, no compromises or assumptions (as long as it's properly rated, of course).

author: CaptDon
author_rank: Super Contributor
timestamp: September 17, 2022, 01:14:45 pm
content: I am guessing you want to run a fan at lower speed to reduce noise? A very lightly loaded small induction motor will tend to run near rated speed until you reduce the input to about 80% of rated input voltage then the speed will rapidly drop. If the motor is lightly loaded (shaft mounted fan blade) you will get reliable starts at about 40% normal voltage and above. This trick only works with small (1 amp full load) squirrel cage motors that have no start winding and/or start capacitor. The motors most often used the way you want to use it are of the 'Impedance Protected' variety which is a motor that can survive forever with a locked rotor. The capacitor in series will work with the motor I suspect you are using. And to clear up a point, the triac controller reduces speed by reducing the amount of time per A.C. cycle that voltage is available to the motor. Depending on the design of the controller the motor may or may not see peak line voltage. In some designs the peak voltage seen by the load will be reduced, in other cases the peak voltage may show up at the load but the overall duty cycle is reduced. Now, all you need to do is figure out what value of capacitor will give you the motor speed you are looking for. We ran reduced speed fans in the consoles at the radio station where I worked. We just took a little 50VA control transformer with dual 120VAC primaries and wired them in series and then connected the fan to the mid-point, Presto..60VAC!!

author: CaptDon
author_rank: Super Contributor
timestamp: September 17, 2022, 01:20:57 pm
content: Induction motors WILL run reliably from a triac control circuit if the control circuit IS DESIGNED for that purpose!! There must be at least 100 million triac 'fan controllers' out there driving variable speed ceiling fans!!! Again, the controller must be a 'motor speed' controller of proper design. Some use a full wave bridge in conjunction with an SCR to avoid unsymmetrical waveforms which can saturate an iron core transformer or motor and destroy it.

author: BeBuLamar
author_rank: Super Contributor
timestamp: September 17, 2022, 04:11:42 pm
content: Quote from: Circlotron on September 17, 2022, 12:26:09 pmOK transformer is the best way but you don't want to for whatever reason just like why you want to run the motor at lower voltage. Use an inverter designed for 120VAC to output 60VAC. The inverter would convert the AC to DC then back to AC using a PWM circuit.

author: robsims
author_rank: Frequent Contributor
timestamp: September 17, 2022, 08:21:34 pm
content: Ok so it looks like best option is a transformer. I'll look for one and rewind the secondary coil so i get 60 Volts out of it. Or I'll buy a 10 amp Variac

author: Zero999
author_rank: Super Contributor
timestamp: September 17, 2022, 08:33:38 pm
content: Quote from: CaptDon on September 17, 2022, 01:20:57 pmA small induction motor driving a fan is a special case. Try using a TRIAC based phase controller on something else such as a lathe or air-conditioner and watch it get very hot, very fast. The original poster didn't say what the motor is driving and he doesn't want to reduce the speed, just the voltage for some reason.

author: CaptDon
author_rank: Super Contributor
timestamp: September 17, 2022, 08:50:06 pm
content: A 10 amp variac, well, now we have some more data finally. So the O.P. is messing around with probably a 1/2 horsepower motor. So far I.I.R.C. he never stated the horsepower of the motor and was standoffish to answer any question about what the unit was or why he wanted to reduce the voltage. It all still makes little sense of why you would run a bigger (say over 1/10 hp) motor at half of its design voltage? This is why I assumed it to be a smaller fan motor at reduced speed as is often common with audio gear and fans in production recording studios. Wish the O.P. would have been up front about what he/she was doing. We could have given much better answers instead of speculation. Maybe he is using the motor to spin propaganda and didn't want us to know, although in that case he would want the motor to run well above its rated speed.

author: Zero999
author_rank: Super Contributor
timestamp: September 17, 2022, 09:20:31 pm
content: Quote from: robsims on September 17, 2022, 08:21:34 pmYou've not said why you want to reduce the voltage.You've already said you don't want to reduce the speed and torque. I hope you're aware of the fact the motor will just draw more current, at a lower voltage, if the speed and torque remain the same, so it won't save any power. A Variac doesn't have any isolation between the secondary and primary, so it doesn't gain anything safety-wise. The circuit connected to the output needs to be treated with the same precautions as the mains.

author: james_s
author_rank: Super Contributor
timestamp: September 18, 2022, 12:34:46 am
content: Quote from: robsims on September 17, 2022, 12:20:29 pmWhy don't you want to tell us why?

author: robsims
author_rank: Frequent Contributor
timestamp: September 18, 2022, 02:21:23 am
content: I'm going to co put a polishing wheel on it. At 60 volts it runs good, doesn't get hot and delivers the power needed to polish. If i run it at 110 volts it runs noisy and is quickly getting hot. The motor is chinese made

author: Brumby
author_rank: Unknown
timestamp: September 18, 2022, 08:10:37 am
content: Sounds reasonable.

author: Circlotron
author_rank: Super Contributor
timestamp: September 18, 2022, 12:32:37 pm
content: Quote from: robsims on September 18, 2022, 02:21:23 amI had a bench drill at work like that. At full voltage it growled and vibrated and carried on. Dialled the voltage way down with a variac and it shooshed right up but was still plenty powerful enough for the small holes I was drilling. Horrible split phase motors.

author: Zero999
author_rank: Super Contributor
timestamp: September 18, 2022, 01:04:00 pm
content: Quote from: robsims on September 18, 2022, 02:21:23 amHow about a series inductor? Try a fluorescent lamp ballast.

author: BeBuLamar
author_rank: Super Contributor
timestamp: September 18, 2022, 01:47:37 pm
content: Quote from: robsims on September 18, 2022, 02:21:23 amIf it has a capacitor it may have a running capacitor that is too large. Try something with smaller value.

author: robsims
author_rank: Frequent Contributor
timestamp: September 18, 2022, 02:25:49 pm
content: Yes circlotron knows exactly what i'm talking about. Sorry to say but these Chinese made motors are terribly engineered. They are trash. A series inductor is an option, but i'm going to purchase copper winding wire and make the inductor myself. I'm going to try a smaller capacitor first and see what it does. But just like zero999 said if the wrong value is selected, it can form a resonant circuit and increase the motor voltage, which will damage it.

author: Zero999
author_rank: Super Contributor
timestamp: September 18, 2022, 06:26:01 pm
content: A smaller run capacitor shouldn't' cause it to blow up, unless it's left running stalled. My concern about it forming a resonant circuit was when the capacitor is connected in series with the motor.If you're going to make your own inductor, the only sensible way is to use a laminated iron core. Rewinding a small mains transformer to make it into an inductor might be an option.

author: robsims
author_rank: Frequent Contributor
timestamp: September 18, 2022, 07:28:53 pm
content: I tried a capacitor in series with the motor. it worked but the motor was making a light humming sound. So the only thing left is making an inductor. I'm going to rewind a small transformer

author: BeBuLamar
author_rank: Super Contributor
timestamp: September 18, 2022, 08:58:45 pm
content: Does the motor has a capacitor? If it has replace it with something of smaller value. The capacitor is needed only for starting but they leave it in the circuit because it's expensive to include a centrifugal switch to disconnect it from the circuit when the motor is running.