Forcing electric motor in opposite direction rotating

Thread Starter

leod

Joined Dec 21, 2020
13
Hello Forum

I am looking to assemble a set of el. motor, converter, control panel and sensors (maybe some other parts too). Because I am not from this field I need help and info if below is possible:
- on motor axis I attach disc with rope
- on touchscreen display I choose power 0.1 kW and motor start to rotate in one direction with power 0.1 kW
- I pick-up rope and start to hold with force which is strong enough to stop rotation, motor stops but I feel motor is still pulling
- I start to pull rope with force 0.12 kW, so motor starts to rotate in opposite direction
- I lose the rope and motor continue to rotate in first direction
- on touchscreen display I choose power 0.2 kW and I repeat all again

It is possible to assemble motor set like described above?
Will motor burn or it can be stopped this way and forced to rotate in opposite direction?
What type of motor is best choice for this? maybe servo motor or other?
Which other parts are needed - converter? sensors? other?

Thanks

Leo
 

MrChips

Joined Oct 2, 2009
22,468
Welcome to AAC!

In theory it might seem correct.
In practice the motor winding will overheat and burn up if it cannot withstand the stall current. Also you will have a difficult task designing how the rope is to be attached to the motor shaft.
 

Thread Starter

leod

Joined Dec 21, 2020
13
Thanks for reply.

It is possible to avoid burn by using another type of motor or maybe add other parts to resolve this problem?
 

LesJones

Joined Jan 8, 2017
2,836
It is a strange situation to supply a motor with a specific power. In most situations a motor is supplied from a voltage source. The voltage source may have a current limit. (This current limit is not the same thing as fuses or thermal overload.) Can you explain why you are using a constant power source. What type of motor is an el. motor ?
Also you say " pull rope with force 0.12 kW, " Force and power are not the same type of unit. If you were talking about an PM or shunt wound DC motor force on the rope would be related to the motor current.

Les.
 

Thread Starter

leod

Joined Dec 21, 2020
13
Thanks for reply.

El motor - electric motor.
Power source is constant because it is plugged-in regular network 220V, like in flat or house. Probably there must be regulator between, which increase or decrease power output of motor.
I am looking for motor which can work with different power, like rotating power from 0.1 kW to 2 kW, depends what we choose on control panel display.
Pull rope with force which is equivalent to 0.12 kW, like 100N.

Which motor should be used to achieve requirements? Which motor doesnt burn in case outside factor block it meanwhile it rotate? Which motor allow outside factor to make it rotate in opposite direction meanwhile it rotate in original direction?
 

MisterBill2

Joined Jan 23, 2018
7,687
You need to understand that in most motors the limiting of current is caused by the counter-EMF generated by the rotation. That is why motors draw much more current as they start.
The one exception to this is what is known as a stepper motor, but even a stepper motor does draw more current when stalled.
What would be the purpose of this "exercise" of a motor? If there is actually some useful purpose then an air-driven motor will be the better choice, because it will not overheat when it is stalled or driven in reverse.

The nature of this post seems similar to another recent post, and so I seriously question the validity .
 

Thread Starter

leod

Joined Dec 21, 2020
13
Thanks.

Can stepper motor be driven in reverse when already rotating (so it goes against rotating direction)? or it burns?
 

DNA Robotics

Joined Jun 13, 2014
596
March water pumps use a magnetic drive. You could rotate the output in the opposite direction and the motor wouldn’t care. It would just keep spinning. It may use more amps if you want to measure it..

(38) MARCH water pump TEST AC-4C-MD 230VAC - YouTube

But, if the pull rope is attached to the shaft, and the shaft is spinning, how do you catch the rope to pull it?
If you want the rope to disengage, think about the rope pull start mechanism on a lawn mower engine.
At about 8:10 in this video, it shows the part jumping out to catch the flywheel.
(38) HOW TO REPAIR Older 3.5-5HP Tecumseh Recoil / Pull Start - YouTube
 
Last edited:

Thread Starter

leod

Joined Dec 21, 2020
13
Thanks for reply.

I saw magnetic motors are used only for waterpumps, so I am not sure about this ... Motor must offer possibility to pull axis in opposite shaft of rotating without burning and to program different work power levels and torque levels.

I would like to attach rope on shaft disc, and then shaft disc to axis. On control panel I choose certain speed and torque and motor starts to pull. If I pull rope in opposite direction of rotating with force which is stronger than motor rotating power, then motor still "try" to rotate in its direction, only because I pull stronger it doesnt. Once I losen rope motor continue to rotate in its direction, because there is no more force to prevent it.
 

MisterBill2

Joined Jan 23, 2018
7,687
A stepper motor can be stalled or pulled in reverse with no issues.

BUT NOW, before there is any more help offered, what is the purpose of this proposed application?

It seems to be a lot like substituting an electric motor for a counter-weight in an exercise machine.

It also seems to be quite similar to a recent thread about a string rewinder.
 

LesJones

Joined Jan 8, 2017
2,836
For a PM (Or a shunt wound motor with the field current kept constant.) motor the speed has a reasonably linear relationship to the voltage. The torque has a reasonably linear relationship to the current. You would have to design the power supply to tolerate reverse voltage on it's output while maintaining a constant current. You still seem to be confused about the relationship between power and force. (You need to refresh your memory by re reading your school physics books.)

Les.
 

MisterBill2

Joined Jan 23, 2018
7,687
@L.J. Also a PM motor would have a similar relationship and a bit less heat being operated in the stalled mode.
What the TS needs is a servo motor designed for high loads and low speeds. Most motors do not do well when stalled for long periods of time.

And what the rest of us need is an explanation of what the application of such a setup would be.
 

MaxHeadRoom

Joined Jul 18, 2013
21,962
, but even a stepper motor does draw more current when stalled.
Wrong! .
A stepper motor is ideally ran at exactly the plate stated current throughout the whole RPM range, any decent designed stepper drive is capable of this.
Max.
*REF: Superior Electric Design engineers Guide to Stepping motors.
 

MaxHeadRoom

Joined Jul 18, 2013
21,962
@leod A DC brushed motor can be ran down to a stall condition as long as the manuf maximum rated continuous stall current is observed.
This would have to be monitored by some means.
Max.
 

MaxHeadRoom

Joined Jul 18, 2013
21,962
The old way to run a stepper with constant current was the series resistor which dropped less as the motor inductive reactance increased.
Now the modern way is to use a PWM form of drive that uses a P.S. with much higher voltage than the motor rated value, but the mean current to the motor is kept at a constant over the whole RPM range using the PWM method.
Max.
 

Thread Starter

leod

Joined Dec 21, 2020
13
Yes, it is for exercise machine.

I am new on this forum, this is my first thread, in case there is another similar one with good info I would appreciate link.

Honestly I really dont know much about motors and all necessary parts, because of this I appreciate every info about which motors and parts, how to assemble and how to find person or company who can do this set for me.
 

MisterBill2

Joined Jan 23, 2018
7,687
A more effective and easier to control concept would be to use an induction motor as DC energized brake while pulling the rope out, and then powering it as a motor to retract. An induction motor makes a quite effective brake with much less power than running it, and thus much less heating. but you would need a senspr to know when it stopped moving.
 

MisterBill2

Joined Jan 23, 2018
7,687
You can verify it with almost any induction motor. Sin the shaft and see how easy that is. Then apply 12 volts DC, or even six, and it takes a lot more effort. And for some serious excitement, put a long cord on a 1/4 HP induction motor and plug it in, and after it is running, unplug it and quickly shrt circuit the line plug with a screwdriver. Instantly the inertia of the rotor is transferred to the whole motor body and it rolls across the floor.
 
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