# Small Electric Motor with variable speed/torque setup

Discussion in 'The Projects Forum' started by xuno_gil, Mar 17, 2016.

1. ### xuno_gil Thread Starter New Member

Mar 17, 2016
3
0
Hello all, this is my first post here so try to be nice, in turn I will try to be as descriptive as possible.
I am trying to build a miniature type of hoist/wench for a project. By miniature I mean that the motor will have a max torque of about 3Nm and power of about 33W or 1/25hp.

So this is what I need the hoist to do: I need the hoist to be able to lift the determined weight, sounds simple enough right? Well I wish it were that simple. Well what if I said that I needed the hoist to lift a changing weight at a constant velocity? Not a greater weight cause that will be maxing out the motor but from the max weight raising it at the same rate even if the weight decreases.

I hope this makes sense, what would be the best way to do this? I was thinking of a sewing machine pedal which varies the speed of a sewing machine motor, but that changes the speed not the torque, are there variable torque motors? I know power is a product of speed but torque is independent so I don't see how a variation in speed will automatically be a variation in torque. I don't need to have the motor sense the change in weight and thus carry the torque but I do want it be be human controlled via a pedal mentioned above or something like that.

There are other things I need to have the system do as well but I wan't to get a clear understanding of this (motor) part first. I know it will require some wring up and all but I first want to know if its mechanically possible simply by wiring up a pedal type thing to a motor correctly.

Anyways, any help will be greatly appreciated, thanks before hand!

2. ### ronv AAC Fanatic!

Nov 12, 2008
3,401
1,457
To do this you need some feedback - something to tell you how fast the motor is turning. Once you have that you can make a circuit to speed it up if it is slower than what you set the speed to be or slow it down if it is to fast.

3. ### TheButtonThief Active Member

Feb 26, 2011
219
38
Have you considered using a frequency inverter like the one's SEW make:
http://www.seweurodrive.com/dr-motor/umrichter.html

Their B track motors (230V 1ph: 0.33 to 3.0 hp) have in-built encoders:
http://www.seweurodrive.com/dr-motor/einbaugeber.html

How the drives work are pretty simple, to maintain a constant motor speed with a varying load, motor speed is detected, if speed drops due to increased load then power to the motor is increased. Power to the motor is controlled by means of PWM and because speed sensing and power modulation is updated 1000's of times every second, there is no discernible change in speed.

4. ### #12 Expert

Nov 30, 2010
16,703
7,339
Let's start with this one. torque = 5252 x HP/R.P.M. therefore torque is not independent of power and speed.
These two requirements are contradictory. An AC induction motor synchronizes with the power line frequency and provides pretty much the same speed for all loads within its operating power, much more stable than a human can operate a pedal.

If you need more than one speed, then you go with the variable frequency drive.

Last edited: Mar 18, 2016
5. ### xuno_gil Thread Starter New Member

Mar 17, 2016
3
0
What I mean when I say "I do want it to be human controlled" is that lets says that the weight does not change but instead remains constant and instead I want the motor to vary in how much assist it provides in lifting the weight. I would like the pedal, or whatever, to be the mechanism in which I, as the user. can control the amount of assist provided. So instead of the weight changing it would be a case where the net force will change, and ideally the rpm of the motor will remain constant despite the change in net force. So for example, if I press the pedal all the way down the motor will do 100% of the work, I press the pedal 50% of the way the motor will do 50% of the work etc. .

I really like the idea of the SEW product line, but I am afraid it might be over kill for what I am looking to do and the system needs to be portable. If I needed to develop some control system that could read engine speed and thus adjust current accordingly to maintain a constant speed, I could definitely look into that. I guess not knowing much about motor option out there I am not sure what technological capabilities are out there for small motors. An AC induction motor might just be the best option then, given all that I am requiring, will an simple AC induction motor work?

Thanks all!

Jul 18, 2013
10,851
2,522
You cannot vary a single phase induction motor, unless very small shaded coil type and they are only good for small loads, fans etc.
What you say sounds contradictory, for a constant weight you want the net force to change and the motor rpm remain constant?
What do you mean by net force?
If the weight changes then motor torque (current) will increase accordingly and the voltage will have to be increased to maintain a certain motor rpm.
If you want to detect a certain load then you will require feedback in the form of a load cell etc.
Max.

7. ### xuno_gil Thread Starter New Member

Mar 17, 2016
3
0
What I mean by net force is the net force on the weight that I am trying to lift with the motor. A weight suspended by a string not accelerating has a net force of 0. Mass x Gravity is pulling it down and the tensile strength of the wire is pulling it up by the same quantity of force (M x G) if the weight start to move (accelerate) the net force is no longer 0 and is + or - dependent on direction it moves. I want a motor setup that can adjust the force it applies to the weight (vary the amount of torque) dependent on a input device like a foot pedal that a person can control.

8. ### #12 Expert

Nov 30, 2010
16,703
7,339
A variable frequency drive will do that. A lot of motors synchronize to the frequency of their power supply and add more force when necessary to maintain their speed. This is an inherent property of some motors. If you want to do this with a motor that doesn't compensate for the changing load, you have to make a speed detector and feedback amplifier to change the applied voltage. Pick one.

9. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,973
1,135
You might find a windscreen-wiper 12V DC motor (rated ~80W from what I read) easier to control than an AC motor, and it comes with reduction gearing which would help with your 3Nm torque requirement.

Last edited: Mar 19, 2016
10. ### BR-549 Well-Known Member

Sep 22, 2013
2,181
419
"A weight suspended by a string not accelerating has a net force of 0. Mass x Gravity is pulling it down and the tensile strength of the wire is pulling it up by the same quantity of force (M x G) if the weight start to move (accelerate) the net force is no longer 0 and is + or - dependent on direction it moves."

What causes the weight to move? And what direction is the movement? Is the direction up or down? Or is the direction sideways?

This will go a lot quicker and easier if you show us what your trying to do.

What is the weight? Why is it changing? What is the time duration of the operation?

Is the weight moving in more than one direction?

Photographs can answer a lot of questions.

This will determine the proper feedback. And feedback will be needed. If we knew exactly what your doing, we might be able to exploit an inherent feedback.

Although this is rare.

Last edited: Mar 19, 2016

Jul 18, 2013
10,851
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That was something that puzzled me in the OP description, Unless it is meant that the motor and the pedal assume total control, i.e. if no foot pedal then the total work is done with the motor.
Otherwise it is shared, that is the only explanation I can see, unless missing something?
Max.

12. ### BR-549 Well-Known Member

Sep 22, 2013
2,181
419
My understanding of torque is different from his, other terms might be too.

I'm not sure I understand the setup yet.

13. ### ebeowulf17 Active Member

Aug 12, 2014
685
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The only way to change the force applied by your system and also have the rpm remain constant is for something else to change how much force it's applying.

So, assuming someone is lifting an object and your device is helping them lift it, if the device starts helping less, the rpm of the motor (speed of the rising object) will slow down unless the person lifting the object starts lifting harder.

You can make a system to provide a controlled amount of lifting assistance, or a machine to provide a controlled lifting speed, but you can't control for both at the same time because they're inextricably linked.