Electric motor - variable torque vs speed

[solvejskovlund]

TL;DR Summary

What kind of electric motor / controller provide a wide range of torque at the cost of speed without destroying the motor?

I'm bothered by a winch being very slow at light loads and was wondering if a different motor / motor controller / gear could increase the low load speed without sacrificing the high torque at low speed? And also without destroying the motor?
Is there a chance that the right combination of motor, controller and (worm) gear, the output could be controlled in the range 800Nm@5rpm to 10Nm@400rpm? (If my calculations are correct, both of this scenarios should require the same power.)

What kind of motor / controller is more suitable for such application?
Available power is DC 24V, max 20A.

The usage of worm gear is to prevent the wire from being unrolled when no power is applied, even under load.
I'm hoping for a controller that automatically increases torque when higher load is detected, but does not go above the max power allowed, which I expect to result in reduced speed. I'd also appreciate the ability to set max torque and max speed at the controller for scenarios where those should be limited.

 

[Baluncore]

Do not overspeed the DC motor used on a winch. In my experience, the commutators tend to overheat and disintegrate.

One good solution is to install a variable gear ratio drive.

You might consider a hand crank for use only when the winch is disengaged from the motor.

 

[Averagesupernova]

Permanent magnet fields or shunt wound motors have good speed regulation. The solution is to size the motor such that the fastest you ever want to wind the winch at a given torque is able to be handled by the motor. Reducing speed would involve a pwm type drive to reduce the voltage to the motor.
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This might be a place to start.
https://www.dartcontrols.com/

 

[solvejskovlund]

One good solution is to install a variable gear ratio drive.

Do you happen to have a link to such variable gear ratio drive?

 

[solvejskovlund]

Maybe an AC motor could do the job? I've tried to read up on 3-phase AC motors. First impression is that speed can be controlled by frequency, so if there is a current sensor and a motor controller that adjusts speed (frequency) aiming for constant current, wouldn't that be a solution?

 

[Baluncore]

We do not know what system you have, or how handy you are as a mechanic.

Maybe there is a planetary gear that would give you the required ratios.

 

[solvejskovlund]

I know how to use an angle grinder and a drill.

I'm sure there are planetary gears with usable ratio. But how would that help? Is there a way to adjust the gear ratio of planetary gears? Maybe if two motors are connected to the planet gear, the difference between their speed can cause variable ratio, but wouldn't that put the momentum from one engine onto the other rather than the output shaft? Maybe this is where the ratio comes into play?

The challenge here is that max torque is wanted at such low speed that it's a danger of burning the coils. And if that is compensated by a high ratio gear, there is a risk of too high rpm at low load. On the other hand, they say about electric cars that the max torque is at 0 speed. I think cars use 3-phase motors, hence I try to learn how they work.

 

[Baluncore]

I'm sure there are planetary gears with usable ratio. But how would that help? Is there a way to adjust the gear ratio of planetary gears?

The ratio depends on which shaft is the input, which is the output, and which is braked. Band brakes are used in automatic transmissions to change the ratio.
https://en.wikipedia.org/wiki/Epicyclic_gearing#Fixed_carrier_train_ratio

You may also consider a wide range hydraulic speed adjuster, where the input shaft pump has a variable flow rate, ±maximum, while the motor has a fixed volume/rev. Those systems are used in some zero-turn mowers. The limit is then set by the maximum hydraulic pressure applied to the output drive motor.

How often will the system be used?
Does it justify the investment?

 

[Averagesupernova]

We haven't been given anything for load specs, specific speeds of the wire rope, what an existing winch looks like, etc. until we see that, everything suggested is simply an avenue towards a possible solution that the OP will have to figure out.

 

[solvejskovlund]

Specs for speed and load is given in initial post: 800Nm@5rpm to 10Nm@400rpm
That is on the output shaft after gear. I think the gear has to be of worm type because of hanging load. Other gears in addition to the worm gear is a possibility.

The only number that is a requirement is the 800Nm. The 5rpm is about the speed I'd guess a 500w motor to be able to hold at that torque with ~85% efficiency. With such high load I have time to wait for it to do it's work. It's not often I'll need it to run at max load. And that is the argument against the electric winches I find in the stores - they run at the slow speed of max load even with zero load - they run at constant speed rather than constant power. When I give it a quarter load (which probably will be the most used condition), I'd like it to run at ~4x speed. I don't want it to run at that slowest speed all the time just because I need it to be able to do that heavy lift once in a while.
The 400rpm is to indicate that the speed at low load should be much faster than with high load. If it's 200 or 600 rpm doesn't really matter, and the 10Nm is what I'd guess a 500w motor could be able to pull at 400rpm.

Winch will be used many times a day, every day for some weeks when I'm doing some project with heavy lifting. Then it may be several months without any use at all.

Hydraulic sounds too complicated.

I was reading up on automatic transmissions using planet gear and brakes. That sounds complicated too.


This is the electronics forum. To my surprise no one has commented on whether or not the requested range of speed and torque is withing range of any kind of electric motor. Is that because to wanted specs are so far from reality that it's not worth commenting, or is it that no one knows?

 

[Baluncore]

The only number that is a requirement is the 800Nm. The 5rpm is about the speed I'd guess a 500w motor to be able to hold at that torque with ~85% efficiency.

It is not that simple.
You are trying to get from where you are now to somewhere different. But we do not know where you are now, because we have not seen the layout of your existing system. That is why we think in generalities, and why we suggest replacing the entire system.

Hydraulic sounds too complicated.

Yet it is probably the simplest solution, with the highest speed ratio. It is bidirectional, with neutral, and has the greatest power to weight ratio.

We do not know what you are trying to do, because we are not mind readers, while you are trapped into using what you know, which has not worked to date.

Why do you use a wire on a winch drum, when you could use a simple loop of chain?

 

[solvejskovlund]

Here is a sketch of the shed. There are several locations for lifting points. Winch will be used for lifting as well as horizontal pull (which actually is a slope)

 

[Tom.G]

The speed-torque curve you are after could be satisfied with a compound-wound motor.
There four basic motor designs in common use:

​

1) Induction - Only AC supply,​

basic speed determined by AC frequency and winding geometry​

​

2) Shunt Wound - AC or DC, constant speed under load​

Field and Rotor are each fed with line voltage​

​

3) Series Wound - AC or DC, Speed ≅ 1/(√Torque)​

Field and Rotor are connected in series & fed from the line voltage​

​

4) Compound Wound - AC or DC, a combination of 2) & 3),​

i.e. two field windings, one connected in series with the rotor and the other connected to line voltage​

Obviously you don't want #1, Induction motor, or #2 Shunt Wound.

That leaves #3 & #4.
Number 3 is out because at zero torque the speed approaches infinite!

So the Compound Wound it is. The slope of the speed/torque curve is determined by the ratio of the flux contributed by each of the two field windings.

Time for you to use Google search for the power rating and speed/torque curve you desire.
Places to start learning the above details can be found:
https://www.google.com/search?hl=en&q=speed+curve+series+motor

Hope this helps!

Cheers,
Tom

 

[solvejskovlund]

Thank you. Now I know what to read up on.