I'm working on a project that involves the use of a 24v DC motor to occasionally rotate a shaft. Most the time the motor won't need to operate and the shaft will just spin freely.
The problem that I'm having is that despite the motor being non-locking; it provides a noticable amount of mechanical resistance against the shaft being rotated freely. If I disconnect the motor, the shaft roatets easily again.
I know that the motor driver I'm using isn't providing the motor a voltage to create this resistance and so the only thing I can imagine is causing it is when the shaft is rotated, the motor which at this point is freewheeling, is generating a voltage back to the cytron board working almost like a dynamo. This voltage is being redirected back to the motor and whilst it's not enough to spin the motor, it's enough to feel a resistance when spinning the shaft thats linked to the motor.
Does this sound like whats going on? and if so, how would I solve this problem? I though maybe use a diode inline with the motor? (I know the use of a diode would limit the motor to only going in one direction but thats fine as the motor doesn't need to be bi-directional). Any help is much appreciated. Thanks.
Additional info in case it's relevant:
Microcontroller: ESP32
Motor driver: Cytron MD20A (6-30v dc, 20a)
Motor: [Motor
](http://350W DC Electric Motor 24V 3000RPM Gear ratio 9.7:1 13.3 N.m Razor Dirt Quad 849505536337 | eBay)
You've given a completely broken link for the motor.
If the motor is brushed, then yes it has loads of friction and will have completely unacceptable
drag.
Brushless motors are loads better, but also have some magnetic drag.
If you overspeed the motor while connected to a powered-down motor driver you'll probably
fry the driver completely too...
Only a three-phase induction motor or variable reluctance motor can avoid motor drag, and they
are complex to drive and harder to source. And they avoid the issue of a powered-down driver
being vulnerable when the motor is spun fast.
If your shaft only turns one direction then a Sprague clutch is probably what you want, however!
bradj98:
so the only thing I can imagine is causing it is when the shaft is rotated, the motor which at this point is freewheeling, is generating a voltage back to the cytron board working almost like a dynamo. This voltage is being redirected back to the motor and whilst it's not enough to spin the motor,
It is true that the motor will act as a generator but the voltage does not get redirected back. It just takes energy to generate a current. The faster the motor is turned the more it will generate.
If the motor coils are open circuit then there won't be any current and the resistance to motion will be minimised. Of course the full mechanical friction will still exist. If the motor coils are short-circuited then the current (and the resistance to motion) will be maximised.
Some motor controllers have the option of placing the motor in either of those states. The short-circuit state is normally used to provide a braking effect to bring the motor to a stop more quickly. The open circuit state allows the motor to freewheel.
Robin2:
It is true that the motor will act as a generator but the voltage does not get redirected back. It just takes energy to generate a current. The faster the motor is turned the more it will generate.
If the motor coils are open circuit then there won't be any current and the resistance to motion will be minimised. Of course the full mechanical friction will still exist. If the motor coils are short-circuited then the current (and the resistance to motion) will be maximised.
Some motor controllers have the option of placing the motor in either of those states. The short-circuit state is normally used to provide a braking effect to bring the motor to a stop more quickly. The open circuit state allows the motor to freewheel.
...R
Just before I read this I tested what would happen if i just joined the 2 motor wires together completely ignoring the driver and what you've said is bang on: the short circuit state is whats causing the extreme resistance to motion.
I tested continuity between the output pins of the driver and they are in a short circuit state whenever the motor is non-operating.
Having narrowed down the issue to the driver keeping the motor in this closed ciruit state, what would you reccomend as being the best way to solve my problem? I know you mentioned some drivers offer the option for either state but after looking at the data sheet I don't think that's the case with mine.
Would it be bad if I used a transistor to switch the main power to the driver on and off depending on whether or not I need the motor to operate? Logically it would solve the problem as the output pins are only shorted where there's power to the module but then there is that voice telling me it would be very stupid to keep switching the driver on and off every 5 seconds.
The best option for your motor has already been stated.
For freewheeling, use a mechanical or electromechanical clutch to disconnect the motor shaft from the driving shaft. Otherwise you will destroy the motor driver.
Sorry about the faulty link for the motor. It was because pulled it up using my purchase history on ebay and the item is no longer listed. Please see below the description of the motor.
Posted by MarkT
If you overspeed the motor while connected to a powered-down motor driver you'll probably
fry the driver completely too...
The rotational speed of the shaft won't be any faster than 20rpm (it moves pretty slowly). I know it's still probably not good practice use this set up but I couldnt source a clutch operated motor that met my specification and was within my price range. As you can see from the screenshots the motor I'm using is actually intended for E-bikes.
Posted by MarkT
Only a three-phase induction motor or variable reluctance motor can avoid motor drag
Motor drag isn't an issue, as you can see from my reply to Robin2, it's the closed circuit state that's creating the huge resistance to motion.
Posted by jremington
The best option for your motor has already been stated.
For freewheeling, use a mechanical or electromechanical clutch to disconnect the motor shaft from the driving shaft. Otherwise you will destroy the motor driver.
As mentioned above, a motor of this spec is likely to cost alot more than the one I picked up for less than £50. Given, I could end up spending what I've saved replacing motor drivers but I'm happy to take that chance. And If the driver does get fried then I'm sure there's still an alternative solution to spedning hundred on a clutch motor.
BUT ONLY open or close the relay when the driver is off and the motor is stationary - i.e. when there is no current in the wires to the motor.
Should I add a delay inbetween switching the relay and powering the motor?
So something like:
// Operating the motor
digitalWrite(Relay, HIGH);
delay(200);
digitalWrite(Motordir, HIGH);
ledcWrite(motorPin, 255);
//Stopping the motor
ledcWrite(motorPin, 0);
delay(200);
digitalWrite(Relay, LOW);
Or is a delay not necessary? (My thought process is that given the slow speed of the relay there could still be current in the circuit before switching the relay).
Posted by jremington
Sounds like a really excellent, well thought out plan!
I've said that using a motor with a clutch isn't viable solution for me. If you have no other solution to offer then please refrain from posting unhelpful, sarcastic comments as I'm sure your time and expertise could be better put to actually helping somebody.
