## Potentiometer for motor speed control- any electrical engineers around?

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Mrxyz
Posts: 582
Joined: Wed Feb 29, 2012 6:12 am

### Potentiometer for motor speed control- any electrical engineers around?

Hi
Perhaps someone with electrical background may be able to help !

So I have a motor which is connected to a propeller whose speed needs to be controlled. So I need an on off and variable speed control. I was 'told' to use a potentiometer but what I bought from eBay burnt out.
I cannot use a PWM - pulse wave modulator.
A rheostat/potentiometer are the only options.
The battery is 8.4V, 3000 mAh NiMH.

Motor is a Traxxas Titan 12t 550
Max power 40.24 W @ 7772 rpm
Max torque 72.16 Nmm
Motor setting - timing - fixed, Brushes - internal, Springs - internal
Gearing 26/76

Happy to provide any other information needed.
I am not sure what to buy or where to start looking.

Any suggestions?
Last edited by Mrxyz on Sat Feb 10, 2018 8:58 pm, edited 1 time in total.

oldcomputerguy
Posts: 3126
Joined: Sun Nov 22, 2015 6:50 am
Location: In the middle of five acres of woods

### Re: Potentiometer for motor speed control- any electrical engineers around?

Can you provide specs of the motor? Kinda hard to make a recommendation without them.
It’s taken me a lot of years, but I’ve come around to this: If you’re dumb, surround yourself with smart people. And if you’re smart, surround yourself with smart people who disagree with you.

Mrxyz
Posts: 582
Joined: Wed Feb 29, 2012 6:12 am

### Re: Potentiometer for motor speed control- any electrical engineers around?

Mrxyz wrote:
Sat Feb 10, 2018 8:11 pm
Hi
Perhaps someone with electrical background may be able to help !

So I have a motor which is connected to a propeller whose speed needs to be controlled. So I need an on off and variable speed control. I was 'told' to use a potentiometer but what I bought from eBay burnt out.
I cannot use a PWM - pulse wave modulator.
A rheostat/potentiometer are the only options.
The battery is 8.4V, 3000 mAh NiMH.

Motor is a Traxxas Titan 12t 550
Max power 40.24 W @ 7772 rpm
Max torque 72.16 Nmm
Motor setting - timing - fixed, Brushes - internal, Springs - internal
Gearing 26/76

Happy to provide any other information needed.
I am not sure what to buy or where to start looking.

Any suggestions?

Mrxyz
Posts: 582
Joined: Wed Feb 29, 2012 6:12 am

### Re: Potentiometer for motor speed control- any electrical engineers around?

oldcomputerguy wrote:
Sat Feb 10, 2018 8:20 pm
Can you provide specs of the motor? Kinda hard to make a recommendation without them.
I have updated my post.
Thanks

just frank
Posts: 1455
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Location: Philly Metro

### Re: Potentiometer for motor speed control- any electrical engineers around?

I'm not an EE but i have taught electronics to college students....

IMO you will NOT find a potentiometer to put in line with your motor that will do the job. Inevitably, the 'pot' would need to dissipate almost as much energy/heat as the motor unless it was at 100% speed.

One way to do this would be to set up an emitter follower circuit with a power transistor rated for the needed current.

https://www.electronics-notes.com/artic ... llower.php

You would then use the pot to regulate the speed, but the current through the motor would be larger than that through the pot by factor \beta.

In this case the power transistor, rather than the pot will dissipate the extra energy....the circuit will be in efficient at medium speeds. You will need to heatsink the transistor and maybe provide a small fan.

A more turnkey (and modern) approach would be to buy an integrated variable voltage DC-DC-converter...

like this: https://www.amazon.com/dp/B00CNXLDCQ/re ... B00C0KL1OM

probably not ok for your app, but something similar should be available if you hunt around.

Posts: 161
Joined: Fri Oct 18, 2013 6:07 pm

### Re: Potentiometer for motor speed control- any electrical engineers around?

Curious about why you can't use pwm?

No matter what, the heat dissipation is going to be important.

scotthew
Posts: 58
Joined: Mon Mar 09, 2015 7:34 am

### Re: Potentiometer for motor speed control- any electrical engineers around?

You probably want to specify why you can't use PWM. Most modern off the shelf motor controllers, like the one prior poster has suggested will use a variant of PWM (e.g. switching). Some designers will not use PWM out of high frequency RF/noise considerations, in which case a non-switching linear amplifier will be required. These are more expensive, and likely not as efficient,which is probably fine for what you are doing. Or you can make your own controller out of a transistor and a couple of other components.

Using pure resistance to set voltage can work, but prior poster is correct that you may be disipating far more power through the resistor than the motor.

Jack FFR1846
Posts: 7403
Joined: Tue Dec 31, 2013 7:05 am

### Re: Potentiometer for motor speed control- any electrical engineers around?

You don't want to just waste the battery capacity in a linear solution (potentiometer directly hooked to motor or through a transisitor or FET). You want something that won't be dissipative. One way that you said won't work is PWM. For this, the voltage from the battery pack is turned on/off at high speed (100's of kHz) and the duty cycle (time on/total time of a period) determines how much % of input voltage reaches the motor. But as mentioned, this could cause noise, although I sort of doubt it will be an issue. This is how many motors and virtually all LEDs are controlled.

Another way is with a switching power supply. Since you want to reduce the speed, you're talking a buck converter. You'll need to design something that can take a reference input. That's where the pot comes in. It gets connected such that it is the target that the power supply will regulate to. With a buck converter, the output voltage is always lower than the input voltage.

Yet another way is to use a circuit that will both buck and boost. The advantage here is that as the battery gets low, the converter continues to provide power to the motor by boosting the battery from the motor. This can be done with a 4 switch, single inductor buck-boost or with a SEPIC converter or by configuring a buck converter as a flyback.

For some useless trivia, A123 started life building battery packs for exactly this, but including a boost converter in their RC car battery packs circa 10 years ago or so. They've sold off that business.

Note that any of these power converters essentially do work on PWM to convert the voltage from input to output, so if that's a huge problem, you're back to a linear which will not cause noise but will dissipate power reducing your battery life.
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Mrxyz
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Joined: Wed Feb 29, 2012 6:12 am

### Re: Potentiometer for motor speed control- any electrical engineers around?

Sun Feb 11, 2018 8:22 am
Curious about why you can't use pwm?

No matter what, the heat dissipation is going to be important.
Hi all,

The motor and circuit have rules which prohibit a pwm.

Mrxyz
Posts: 582
Joined: Wed Feb 29, 2012 6:12 am

### Re: Potentiometer for motor speed control- any electrical engineers around?

Jack FFR1846 wrote:
Sun Feb 11, 2018 8:54 am
You don't want to just waste the battery capacity in a linear solution (potentiometer directly hooked to motor or through a transisitor or FET). You want something that won't be dissipative. One way that you said won't work is PWM. For this, the voltage from the battery pack is turned on/off at high speed (100's of kHz) and the duty cycle (time on/total time of a period) determines how much % of input voltage reaches the motor. But as mentioned, this could cause noise, although I sort of doubt it will be an issue. This is how many motors and virtually all LEDs are controlled.

Another way is with a switching power supply. Since you want to reduce the speed, you're talking a buck converter. You'll need to design something that can take a reference input. That's where the pot comes in. It gets connected such that it is the target that the power supply will regulate to. With a buck converter, the output voltage is always lower than the input voltage.

Yet another way is to use a circuit that will both buck and boost. The advantage here is that as the battery gets low, the converter continues to provide power to the motor by boosting the battery from the motor. This can be done with a 4 switch, single inductor buck-boost or with a SEPIC converter or by configuring a buck converter as a flyback.

For some useless trivia, A123 started life building battery packs for exactly this, but including a boost converter in their RC car battery packs circa 10 years ago or so. They've sold off that business.

Note that any of these power converters essentially do work on PWM to convert the voltage from input to output, so if that's a huge problem, you're back to a linear which will not cause noise but will dissipate power reducing your battery life.
I cannot use a converter.
I have to use a potentiometer in line. But I am not sure where can I find one which will control the above motor and battery?
FWIW, the motor will run for less than 2 minutes at a time. And in order to save weight, I cannot add a heat sink.

Mrxyz
Posts: 582
Joined: Wed Feb 29, 2012 6:12 am

### Re: Potentiometer for motor speed control- any electrical engineers around?

just frank wrote:
Sun Feb 11, 2018 8:03 am
I'm not an EE but i have taught electronics to college students....

IMO you will NOT find a potentiometer to put in line with your motor that will do the job. Inevitably, the 'pot' would need to dissipate almost as much energy/heat as the motor unless it was at 100% speed.

One way to do this would be to set up an emitter follower circuit with a power transistor rated for the needed current.

https://www.electronics-notes.com/artic ... llower.php

You would then use the pot to regulate the speed, but the current through the motor would be larger than that through the pot by factor \beta.

In this case the power transistor, rather than the pot will dissipate the extra energy....the circuit will be in efficient at medium speeds. You will need to heatsink the transistor and maybe provide a small fan.

A more turnkey (and modern) approach would be to buy an integrated variable voltage DC-DC-converter...

like this: https://www.amazon.com/dp/B00CNXLDCQ/re ... B00C0KL1OM

probably not ok for your app, but something similar should be available if you hunt around.
I have to find a potentiometer to control the speed.
Where can I find one?

Found this site;
https://www.banggood.com/search/potenti ... ml?nobrd=1

Thanks
Last edited by Mrxyz on Sun Feb 11, 2018 5:15 pm, edited 1 time in total.

Epsilon Delta
Posts: 7430
Joined: Thu Apr 28, 2011 7:00 pm

### Re: Potentiometer for motor speed control- any electrical engineers around?

Digikey or any other electronics supplier.

A 0-10Ohm 50W rheostat should do the job and are available. (I'm guessing on the Ohms and Wattage, I think I'm being conservative here but not sure). Any such rheostat will probably be larger than you want and may be disturbingly expensive. You say you can't budget space or weight for a heat sink. but the heat has to go somewhere.

A PWM solution is almost certainly smaller and lighter, and no matter what the motor documents say the motor won't care if you do it right. Never-the-less the right pot will do the job.

FlyingMoose
Posts: 377
Joined: Wed Mar 04, 2009 10:48 pm

### Re: Potentiometer for motor speed control- any electrical engineers around?

If you can’t use a heat sink, maybe you can build a compartment to insert a small ice cube, which you replace before each 2-minute run.

livesoft
Posts: 61944
Joined: Thu Mar 01, 2007 8:00 pm

### Re: Potentiometer for motor speed control- any electrical engineers around?

FlyingMoose wrote:
Sun Feb 11, 2018 5:22 pm
If you can’t use a heat sink, maybe you can build a compartment to insert a small ice cube, which you replace before each 2-minute run.
A small cube of dry ice will be way cooler.
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dowse
Posts: 331
Joined: Tue Sep 10, 2013 3:10 pm

I (and others) would be curious to know the reason for the requirements. Nonetheless, this part, while not cheap at $76, should do the job: https://www.digikey.com/product-detail/ ... -ND/824085 Note - this is a 6 ohm part. I would have chosen a 2 ohm, but the lead time is 16 weeks. Maybe if you search around you can find one in stock at another supplier. The result is that you will not be using the full range of the rheostat, so the control will not be as fine as it could be. My take on this is that for a 40W motor, a few ohms should be enough to drop the voltage (and motor speed) to zero. You want to utilize a good portion of the range of the rheostat for control. Note - the temperature of the package could get as high as 500 deg. F in free air with the motor set for 0 speed, and the rheostat dissipating all of the power. Be careful! Mrxyz Posts: 582 Joined: Wed Feb 29, 2012 6:12 am ### Re: Potentiometer for motor speed control- any electrical engineers around? dowse wrote: Sun Feb 11, 2018 6:13 pm I (and others) would be curious to know the reason for the requirements. Nonetheless, this part, while not cheap at$76, should do the job:

https://www.digikey.com/product-detail/ ... -ND/824085

Note - this is a 6 ohm part. I would have chosen a 2 ohm, but the lead time is 16 weeks. Maybe if you search around you can find one in stock at another supplier. The result is that you will not be using the full range of the rheostat, so the control will not be as fine as it could be.
My take on this is that for a 40W motor, a few ohms should be enough to drop the voltage (and motor speed) to zero. You want to utilize a good portion of the range of the rheostat for control. Note - the temperature of the package could get as high as 500 deg. F in free air with the motor set for 0 speed, and the rheostat dissipating all of the power. Be careful!
The project is for a competition which has a lot of restrictive rules! I wish I could use the PWM!

AND - THANKS for your detailed and quick replies! Appreciate it...........I have learnt so much from these forums - lots of non-financial questions answered! You all are awesome!

Epsilon Delta
Posts: 7430
Joined: Thu Apr 28, 2011 7:00 pm

### Re: Potentiometer for motor speed control- any electrical engineers around?

dowse wrote:
Sun Feb 11, 2018 6:13 pm
I (and others) would be curious to know the reason for the requirements. Nonetheless, this part, while not cheap at \$76, should do the job:

https://www.digikey.com/product-detail/ ... -ND/824085

Note - this is a 6 ohm part. I would have chosen a 2 ohm, but the lead time is 16 weeks.
...
My take on this is that for a 40W motor, a few ohms should be enough to drop the voltage (and motor speed) to zero. You want to utilize a good portion of the range of the rheostat for control. Note - the temperature of the package could get as high as 500 deg. F in free air with the motor set for 0 speed, and the rheostat dissipating all of the power. Be careful!
It's been a long time since I used a brushed DC motor so I may have this all wrong. 40W @ 8.4V is 4.8A and 1.75Ω. I'd expect 0-2Ω in series to throttle from say 20%-100% power. That may or may not be enough.

On further consideration you can probably get by with a 10W resistor.

If the pot does not include a switch you will also need one in series. 10Ω in series would still drain the battery, even if the motor does not turn, although I'd expect a decent quality motor to spin with 10Ω in series.

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### Re: Potentiometer for motor speed control- any electrical engineers around?

> The motor and circuit have rules which prohibit a pwm.

Sounds like the "rules" prohibit modern technology. Maybe two plates in a bucket of salt water solution?

Slacker
Posts: 569
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### Re: Potentiometer for motor speed control- any electrical engineers around?

Any way you can post the relevant section of the rules governing motor control?

jharkin
Posts: 1681
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Location: Boston suburbs

### Re: Potentiometer for motor speed control- any electrical engineers around?

That motor is a brushed DC electric motor used to power radio controlled cars. They are not as common anymore as most hobby radio control equipment has moved to brushless DC that require special motor controllers that perform electronic commutation (easily identifiable as the motors have 3 input wires, not 2).

The standard way to control one is with a brushed motor Electronic Speed Control ("ESC"). This is the appropriate controller designed for that motor:

https://www.amazon.com/Traxxas-Electron ... B0046LBJCM

Note that the speed controller is designed to take its input signal from a hobby R/C receiver output channel. If you are trying to control it some other way you are going to have to design your own circuit that can supply it with the input signal its looking for - which is a 3-6 volt 50Hz PWM waveform with a 1520uS neutral (typically 900uS for full off, 2100uS for full on).

A Hobby radio control transmitter and receiver just to run this motor can be very inexpensive. And would be a plug and play solution with the control above and your motor.

https://www.towerhobbies.com/cgi-bin/wt ... LXGPTF&P=0

dowse
Posts: 331
Joined: Tue Sep 10, 2013 3:10 pm

### Re: Potentiometer for motor speed control- any electrical engineers around?

jharkin wrote:
Mon Feb 12, 2018 9:59 am
That motor is a brushed DC electric motor used to power radio controlled cars. They are not as common anymore as most hobby radio control equipment has moved to brushless DC that require special motor controllers that perform electronic commutation (easily identifiable as the motors have 3 input wires, not 2).

The standard way to control one is with a brushed motor Electronic Speed Control ("ESC"). This is the appropriate controller designed for that motor:

https://www.amazon.com/Traxxas-Electron ... B0046LBJCM

Note that the speed controller is designed to take its input signal from a hobby R/C receiver output channel. If you are trying to control it some other way you are going to have to design your own circuit that can supply it with the input signal its looking for - which is a 3-6 volt 50Hz PWM waveform with a 1520uS neutral (typically 900uS for full off, 2100uS for full on).

A Hobby radio control transmitter and receiver just to run this motor can be very inexpensive. And would be a plug and play solution with the control above and your motor.

https://www.towerhobbies.com/cgi-bin/wt ... LXGPTF&P=0
The OP has already said that the competition rules don't allow for use of efficient controllers. I can only think that the competition must be to see who can toast marshmallows the fastest while simultaneously reducing the speed of the motor.

I found a more reasonable part that might work:

https://www.amazon.com/Ceramic-Potentio ... B00K82TJMA

Not sure if it matters, but it is a variable taper as opposed to a linear taper. It might be wise to set up a small fan nearby to cool the pot, assuming the rules allow it.

nordsteve
Posts: 636
Joined: Sun Oct 05, 2008 9:23 am

### Re: Potentiometer for motor speed control- any electrical engineers around?

Is the prohibition specifically against PWM or against all forms of control other than a pot?

Repeating others, it would be useful to read the specific rule to understand the constraints you are under.

Jack FFR1846
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Joined: Tue Dec 31, 2013 7:05 am

### Re: Potentiometer for motor speed control- any electrical engineers around?

Do the rules limit motor speed? I ask that because a circuit to do that would be much different than just a potentiometer. For example, when starting or accelerating, you want full current from the battery to provide maximum torque. Assuming that max speed is the limit, then coming up with a speed limiting circuit that will stay out of the way until you hit that max speed will allow the car to perform far better than throwing essentially a resistor in series with the battery.

Or do the rules say that you have to put a potentiometer in series with the motor to limit the max speed?

In general, motor speed is related to voltage.....max torque is related to current. So reducing the battery voltage can accomplish this, if that's allowed by the rules and the numbers work out. This can also be done by the number of turns in the motor. But if either of these is prohibited, then ok. If they're free....can you rearrange the cells in the battery pack? Instead of series, remove series cells and build parallel strings? Just a thought.
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jharkin
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Location: Boston suburbs

### Re: Potentiometer for motor speed control- any electrical engineers around?

dowse wrote:
Mon Feb 12, 2018 10:51 am
The OP has already said that the competition rules don't allow for use of efficient controllers. I can only think that the competition must be to see who can toast marshmallows the fastest while simultaneously reducing the speed of the motor.
Fair point - but maybe knowing what that motor was designed to be operated with may give him some insight.

Its interesting that they are being supplied car motors to drive a prop. Probably just where cheap in large lots. I assume we are all making the same guess there that this is some kind of organized completion like battlebots, or maybe a university design contest. It would help to know more of the actual rules and application.

barnaclebob
Posts: 2792
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### Re: Potentiometer for motor speed control- any electrical engineers around?

My brother's old RC car used to have a mechanical speed controller that was driven by a servo. I think it had 3 speed settings, would that work for your purpose? You could just manually move it instead of using a servo.

something like this:

http://www.activepowersports.com/traxxa ... FUQAvD_BwE

Mrxyz
Posts: 582
Joined: Wed Feb 29, 2012 6:12 am

### Re: Potentiometer for motor speed control- any electrical engineers around?

jharkin wrote:
Mon Feb 12, 2018 1:27 pm
dowse wrote:
Mon Feb 12, 2018 10:51 am
The OP has already said that the competition rules don't allow for use of efficient controllers. I can only think that the competition must be to see who can toast marshmallows the fastest while simultaneously reducing the speed of the motor.
Fair point - but maybe knowing what that motor was designed to be operated with may give him some insight.

Its interesting that they are being supplied car motors to drive a prop. Probably just where cheap in large lots. I assume we are all making the same guess there that this is some kind of organized completion like battlebots, or maybe a university design contest. It would help to know more of the actual rules and application.
So sorry to not clarify this issue from the beginning.
The competition is for my son's Science Olympiad and their rules are online, but I can find and post them. For full disclosure, my son has done almost all the work including motor selection, designs, etc. I think he went through around 20 plus design options before selecting one which worked best. And he has spent more hours and money working on these projects than I want him to !!. DW is quite shocked at how much this costs. But it is definitely worth it.
I was trying to help him figure out ways to control the speed of the motor as there is a penalty if the hovercraft is too fast or too slow.
Again, many thanks for all your help.

nordsteve
Posts: 636
Joined: Sun Oct 05, 2008 9:23 am

### Re: Potentiometer for motor speed control- any electrical engineers around?

I'm guessing he's making the hovercraft. Here's the relevant rule:
Electrical components shall be limited to batteries, wires, motors, switches, resistors, potentiometers, capacitors, mechanical relays, fans, and blowers. Brushless motors and integrated circuits are not permitted unless they are an integral part of or embedded into commercially available fans used for cooling electronics or computers.

dowse
Posts: 331
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### Re: Potentiometer for motor speed control- any electrical engineers around?

nordsteve wrote:
Mon Feb 12, 2018 2:49 pm
I'm guessing he's making the hovercraft. Here's the relevant rule:
Electrical components shall be limited to batteries, wires, motors, switches, resistors, potentiometers, capacitors, mechanical relays, fans, and blowers. Brushless motors and integrated circuits are not permitted unless they are an integral part of or embedded into commercially available fans used for cooling electronics or computers.
Ah, now we are getting somewhere. OP, what a fun project for your son! Most likely, the adjustment range will not need to be 0-100%, but rather a smaller range around some nominal setting. This tells me that not a lot of power needs to be dissipated in a pot. Alternatively, I'm guessing that the 8.4V battery pack is a typical arrangement of 7 cells connected in series. This gives you the possibility of selecting the voltage supplied to the motor in 1.2V increments by opening up the pack and wiring to individual cells. One of those might correspond to a sweet spot for the hovercraft speed. The voltage could be made selectable with a rotary switch. No unwanted heat to dissipate. Also,in combination with this, maybe some fine speed control can be accomplished with adjustable air baffles. Just some thoughts to share. Your son must make the final choices. Good luck and have fun!

Mrxyz
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### Re: Potentiometer for motor speed control- any electrical engineers around?

dowse wrote:
Mon Feb 12, 2018 3:50 pm
nordsteve wrote:
Mon Feb 12, 2018 2:49 pm
I'm guessing he's making the hovercraft. Here's the relevant rule:
Electrical components shall be limited to batteries, wires, motors, switches, resistors, potentiometers, capacitors, mechanical relays, fans, and blowers. Brushless motors and integrated circuits are not permitted unless they are an integral part of or embedded into commercially available fans used for cooling electronics or computers.
Ah, now we are getting somewhere. OP, what a fun project for your son! Most likely, the adjustment range will not need to be 0-100%, but rather a smaller range around some nominal setting. This tells me that not a lot of power needs to be dissipated in a pot. Alternatively, I'm guessing that the 8.4V battery pack is a typical arrangement of 7 cells connected in series. This gives you the possibility of selecting the voltage supplied to the motor in 1.2V increments by opening up the pack and wiring to individual cells. One of those might correspond to a sweet spot for the hovercraft speed. The voltage could be made selectable with a rotary switch. No unwanted heat to dissipate. Also,in combination with this, maybe some fine speed control can be accomplished with adjustable air baffles. Just some thoughts to share. Your son must make the final choices. Good luck and have fun!

Yes - it is a very interesting project with my son!!
There were around 6 designs for the body, around 10 for the skirt of hovercraft, 4 batteries, 3 motors, and 4 for the position of the motors! He was able to design all of it and took many weeks but now he has got it right!
The rules this year want exact time for reaching the end and penalty for going too fast or slow!
So he needed some way to slow it down.
He tried to block the air flow but it did not work consistently.........He also needs to try some baffles which need to be adjustable...

But,
He plans to try the rheostat first, which I think will melt.....
and then try to 'open' the battery and wire the individual batteries with switches to select them as needed.

THANKS!

PS: and yes I will keep you all posted on whether these options work or not!

Theseus
Posts: 423
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### Re: Potentiometer for motor speed control- any electrical engineers around?

Not an EE here but I read all responses with a lot of interest - despite that I understood very little if anything.

Just wanted to say I continue to be amazed at the depth and the breadth of our BH community and people’s willingness to help answer questions.

BH is literally the first place I turn to when I need to ask some thing.

Thank you all for making this forum so enjoyable!!

dowse
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### Re: Potentiometer for motor speed control- any electrical engineers around?

Sounds like he is well on his way! It's great that he is doing so much himself. By all means, let him run his own experiments (safely), but you might want to be an observer when he tries the rheostat. Be ready to pull the plug on it if it starts to smell like it's burning.

dowse
Posts: 331
Joined: Tue Sep 10, 2013 3:10 pm

### Re: Potentiometer for motor speed control- any electrical engineers around?

Just thought of another approach. For learning, here are a couple of questions to pose to your son:

I understand the rules allow contact with the sides of the track. Is there a way that can be used deliberately to control the speed?

Do you know how the track is designed? If so, what features could be added to the vehicle that would make contact with the sides and slow it down? Would they slow the vehicle in a controlled, predictable manner?

Epsilon Delta
Posts: 7430
Joined: Thu Apr 28, 2011 7:00 pm

### Re: Potentiometer for motor speed control- any electrical engineers around?

It appears that the rheostat is to be adjusted to a single value and left there. If that is the case once he knows the resistance that is needed the rheostat can be replaced by a fixed resistor or resistors. That might allow greater design flexibility, since fixed resistors are usually smaller and better at heat dissipation. It could also opens up interesting possibilities such as building his own resistor in the form of glowing nichrome wires wrapped around (non-flammable) parts of the hovercraft.

Mrxyz
Posts: 582
Joined: Wed Feb 29, 2012 6:12 am

### Re: Potentiometer for motor speed control- any electrical engineers around?

dowse wrote:
Tue Feb 13, 2018 9:05 am
Just thought of another approach. For learning, here are a couple of questions to pose to your son:

I understand the rules allow contact with the sides of the track. Is there a way that can be used deliberately to control the speed?

Do you know how the track is designed? If so, what features could be added to the vehicle that would make contact with the sides and slow it down? Would they slow the vehicle in a controlled, predictable manner?
Yes, you can try to do so, and in fact he did so in one event. But this is not a very reliable method.
Thanks

Mrxyz
Posts: 582
Joined: Wed Feb 29, 2012 6:12 am

### Re: Potentiometer for motor speed control- any electrical engineers around?

Epsilon Delta wrote:
Tue Feb 13, 2018 2:52 pm
It appears that the rheostat is to be adjusted to a single value and left there. If that is the case once he knows the resistance that is needed the rheostat can be replaced by a fixed resistor or resistors. That might allow greater design flexibility, since fixed resistors are usually smaller and better at heat dissipation. It could also opens up interesting possibilities such as building his own resistor in the form of glowing nichrome wires wrapped around (non-flammable) parts of the hovercraft.
This is new to me. Are there any links to how to use such wires?
The speed of hovercraft will need to be set during the actual event itself and not known prior to that!
So he need to set up the hovercraft to have adjustable speed. Thus a potentiometer - but it can get too hot and in fact, 2 burnt out already. So they were not the right type or that this is not a good method. Plan to buy the 10 ohm, 50W potentiometer from Amazon and try it out.
If that does not work, then buy smaller battery packs and wire them with switches to have some control........
Thanks

dowse
Posts: 331
Joined: Tue Sep 10, 2013 3:10 pm

### Re: Potentiometer for motor speed control- any electrical engineers around?

Glad to hear he has already thought about different ways of controlling the speed and why some should be ruled out. That's the point - think through the problem and the pros and cons of different possible solutions.

Consider the selectable voltage method in combination with the pot adjustment. With the pot set a minimum, find the lowest voltage selection that results in a little too much speed, then adjust the pot to obtain the desired speed. That should minimize the power that the pot has to dissipate and therefore limit the heat. Also, the 50W part is designed to withstand high temperatures.

Epsilon Delta
Posts: 7430
Joined: Thu Apr 28, 2011 7:00 pm

### Re: Potentiometer for motor speed control- any electrical engineers around?

Mrxyz wrote:
Wed Feb 14, 2018 7:24 am
Epsilon Delta wrote:
Tue Feb 13, 2018 2:52 pm
It appears that the rheostat is to be adjusted to a single value and left there. If that is the case once he knows the resistance that is needed the rheostat can be replaced by a fixed resistor or resistors. That might allow greater design flexibility, since fixed resistors are usually smaller and better at heat dissipation. It could also opens up interesting possibilities such as building his own resistor in the form of glowing nichrome wires wrapped around (non-flammable) parts of the hovercraft.
This is new to me. Are there any links to how to use such wires?
The speed of hovercraft will need to be set during the actual event itself and not known prior to that!
So he need to set up the hovercraft to have adjustable speed. Thus a potentiometer - but it can get too hot and in fact, 2 burnt out already. So they were not the right type or that this is not a good method. Plan to buy the 10 ohm, 50W potentiometer from Amazon and try it out.
If that does not work, then buy smaller battery packs and wire them with switches to have some control........
Thanks
You could use a fuse to protect the pot.

It probably makes sense to use a fixed resistor in series with the pot. This will protect the pot in two ways. First this limits the current when the pot is turned to zero ohms. Without a fixed resistor you will get high current, which is probably when the pot is being destroyed. Second it means that some of the energy is dissipated in the fixed resistor rather than the pot.

I'd probably start with 2 x 2Ω 25W resistors in series (for a total of 4Ω). If that reduced the speed too much a single 2Ω 25W resistor and if that was still too slow I'd put the 2 resistor in parallel (equivalent to 1Ω). Similarly when tweaking the pot start with it set to the maximum and slowly lower the resistance until you get the needed speed.

IIRC a good reference would be an old (e.g. 1980) ARRL handbook. (American Radio Relay League -- the ham radio organization). Many public libraries should have one. New ones probably have the information but will have more on digital electronics and less on building your own resistors.

The glowing nichrome wires was a bit tongue in check. There's really no reason to build your own resistors. Many power resistors and potentiometers are actually coils of nichrome wire wrapped around a ceramic core. About 1m of 20AWG nichrome wire is about 5Ω. The electricity only cares about the gauge and length not the exact shape, so you could wrap the wire about any handy object, such as tail fins. The wire will get hot. Putting it on the outside will help with cooling, but it also risks burning your fingers. If you get the gauge and length right it will get red hot (nichrome wire is used in toasters, hairdryers etc) which may get extra points for looking cool, but may also draw a lifetime ban for exposing red hot pieces of metal where they can be touched. Adults have gotten much grumpier since I was a lad.

acanthurus
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dowse
Posts: 331
Joined: Tue Sep 10, 2013 3:10 pm

### Re: Potentiometer for motor speed control- any electrical engineers around?

acanthurus wrote:
Thu Feb 15, 2018 12:33 am
I think user just frank's suggestion of an emitter follower with a small pot is worthy of some more discussion. That was my initial idea when I read PWM was verboten. Whether a single transistor or a darlington would be necessary I don't know. I think it would be an OK solution depending on how much Vbe loss is acceptable down from the battery voltage.

Also, there are other forms of on/off modulation besides pulse width. PFM might be a creative way around badly written rules.
Someone posted the relevant excerpt from the rules above, which do no allow any transistors, diodes, ICs, etc. Only batteries, wires, switches, relays, resistors, capacitors and potentiometers are allowed. In other words, no active components allowed at all. Otherwise, of course much more efficient designs could be utilized. It seems the contest organizers wanted to keep the electrical system simple and avoid giving an advantage to those with even a small amount of electronics expertise.

acanthurus
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Mrxyz
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Joined: Wed Feb 29, 2012 6:12 am

### Re: Potentiometer for motor speed control- any electrical engineers around?

Update:
Could not find a potentiometer.
Could not find battery pack of NimH which do not exceed 9V. Smallest I found was 4 V (4 cell). And yes, I am not allowed to open the pack and rewire them. So, I could use two 4 V batteries but half speed is too slow.
So used baffles (black and orange colored vertical sliders) which my son 3D printed to size.These can be adjusted up or down to control speed. They are on the back of the pusher propeller.
Rudders were adjusted to allow the hovercraft to move straight, and not rotate around.
Thus, with the above 2 methods, speed was controlled.
Now will update you once the competition is over.