Brushed DC motor - what happens if you reverse the direction? - Page 1
Forum section: electronics / projects

author: ivonenand
author_rank: Contributor
timestamp: February 08, 2018, 06:49:37 am
content: Hi Guys,I'm making a small electric scooter, mostly from aliexpress parts, but I will be making my own motor controller. My scooter will have a 300W brushed DC motor running on 24V. I'm thinking of implementing an electric brake to it so my question is this. What happens if the motor is spinning in one direction and you apply the voltage in the opposite direction? Will the motor brake faster than if I short circuit it's windings? I'm drawing up a driver and I need to know which one of these three motor drivers to go for:1. Simple low side FET with a freewheeling diode on the top side2. Low side fet + high side FET for braking3. Full bridge driverThanks for any advice,Ivo

author: amyk
author_rank: Super Contributor
timestamp: February 08, 2018, 12:23:59 pm
content: Quote from: ivonenand on February 08, 2018, 06:49:37 amYes it definitely will. Be careful of overheating, however.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 08, 2018, 03:51:55 pm
content: Assuming a permanent magnet motor.Yes, it will brake faster, but most likely too fast - you lose the traction, saturate the motor iron, current is excessive; no point doing that. Unless you want to burn some rubber.Most likely, you are going to limit the braking level so that even just shorting it out completely is too much - so you "short it" with a PWM signal instead. This causes the motor inductance to work like a boost converter, providing regenerative braking by charging the battery. This is what you want.At very slow speeds (think about 5 km/h), boosting up doesn't work as well anymore, after a certain point you just completely short the motor with 100% duty cycle, so that the rest of the energy to stop is converted in heat in the motor windings. At even lower speeds than that (practically, think about 1 km/h, or just keeping still), you would need to carefully inject some reverse polarity, with small duty cycle so that you still provide braking torque, but don't let the motor actually run reverse - you need an actual position feedback for this to work. This is probably something not worth implementing - use a mechanical brake to stop completely / stay still.TLDR: use a half bridge to drive the motor. You get positive and negative torque - braking is regenerative, which is nice! Use a current sense feedback to prevent excessive motor currents outside the motors designed operating area, and to avoid blowing FETs.

author: Terry01
author_rank: Frequent Contributor
timestamp: February 08, 2018, 04:03:12 pm
content: Why don't you use a complete motor and control unit from a power chair or even an electric mobility scooter. Then everything is all set up for you and if you get one of the ones you can programme from a laptop it is very easy to set up to suit you perfectly.There are 100s of them on Ebay or the likes. Quite reasonable prices too.

author: C
author_rank: Super Contributor
timestamp: February 08, 2018, 05:05:19 pm
content: A brushed DC motor is also a brushed DC generatorThink of riding a circle on an angle.Across the angle takes X current. As you start climbing the angle, current increases until max current of up the angle.As you start back down from top current starts decreasing until you get a max generated down the angle.Speed is magnitude of current range.You want to always control current & monitor current.Power is how much work you are doing. To much power in motor or generator mode harms motor.So your 24V is a limit and your controller is a +/- current source with a power monitor (V*I) to protect motorThis is a full H bridge.You will need an additional Half-Bridge connected to a resistor to keep your supply from increasing out of limits. Regen breaking will make 24V supply increase. On increase above 24V, You could charge battery up battery charge limit, but will need the half-birdge resistor to hold/keep from over current into battery.Current control is like gas pedal & brake pedal.

author: ivonenand
author_rank: Contributor
timestamp: February 08, 2018, 07:16:27 pm
content: Hi Guys,Thanks for the great input, really appreciate it. Yes, I could get a ready made controller from aliexpress for a few $, but then I don't get to play around much with the controller. I will have a speed sensor on the wheel (reed relay, like the one used on bicycle computers) and if I have my own controller I can implement an RPM control loop. Not sure how well it will work, but I'll give it a shot; with only 300W on the motor I'm sure I can't get killed, no matter what the software does :-)I will go with a half bridge then. One more question, at what PWM frequency are these brushed motors usually run? Is the frequency in the 15-20kHz range, or something slower?Thanks again!Ivo

author: Benta
author_rank: Super Contributor
timestamp: February 08, 2018, 07:25:01 pm
content: QuoteUsually at around 3...4 kHz. At full speed it doesn't matter as you'll have 100% duty cycle, but at partial speed torque vanishes completely at higher frequencies, mainly due to iron losses.

author: technix
author_rank: Super Contributor
timestamp: February 08, 2018, 07:31:02 pm
content: You should add a solenoid or pneumatic piston actuated friction brake on top of the electric brake, also high power resistors. High power resistors are used as resistive brake when regenerative braking runs the risk of damaging the battery over its maximum charging voltage, and provide half of the emergency brake. The friction brake serves as half of the emergency brake, the parking brake and low speed brake, so you can forgo the delicate control at below 5km/h and just start applying the friction brake at lower speeds or complete stop. Do keep an emergency brake button and a tilt switch handy, and when the emergency brake button is pressed or the tilt switch detected you falling over it applies simultaneously maximum resistive brake and maximum friction brake to stop you from crashing too hard (which can put your life on the line) at the cost of burning some rubber.A cherry on top feature that requires a method to sense the speed and some software: cruise control. In cruise control mode the software keeps a feedback loop so the speed is kept constant. Extra cherry on top is adding some distance sensors so in all modes it can help you keep a safe distance with the object in front of you, and in cruise control mode keep distance with all objects around you.

author: C
author_rank: Super Contributor
timestamp: February 08, 2018, 08:07:21 pm
content: Might want to keep is simple.With a car or cycle you control power which is current for motor, Operator controls power. No speed control needed as it is an operator function.With power controlled safely a speed input becomes a cruse control.With out power control being first it is very easy to have speed control request 1000% power which will kill motor.A half bridge will prevent using some power to stop when regen is not enough. Keep in mind that regen is using motor as generator and if you ask for more regen then motor can provide then you are just motor in reverse. Regen current + motor current just needs to stay less then limits of motor current.

author: Benta
author_rank: Super Contributor
timestamp: February 08, 2018, 08:44:26 pm
content: Quote from: C on February 08, 2018, 08:07:21 pmYou lost me.Brushed DC motors are voltage controlled devices when it comes to speed. Power (or rather current) draw is only defined by the mechanical load ( = needed torque).

author: C
author_rank: Super Contributor
timestamp: February 08, 2018, 09:58:26 pm
content: Quote from: Benta on February 08, 2018, 08:44:26 pmIf you want to protect a motor you control it's current. A motor starter does this, dropping voltage to limit limit current to safe levels to get load moving before increasing voltage. Huge motors connected to AC can have many steps on start.On start a DC motor goes into over current and will fry if load takes too long to spin up the speed of a voltage setting. As motor spins faster back emf builds to limit current drawn.Controlling current keeps motor in safe limits.First sensor should be current.Second sensor should be motor temp.after these to you could do speed sensor with the first two setting the limits.This is not new stuff, just simple DC servo theory from way back.

author: Benta
author_rank: Super Contributor
timestamp: February 08, 2018, 10:06:55 pm
content: OK, now we're on the same page.

author: Terry01
author_rank: Frequent Contributor
timestamp: February 09, 2018, 12:07:23 am
content: Another idea that may be worth 10 minutes of your time is looking at brushless motors? I am still really new to electronics so can't give you all the names of components etcetera that make the motor up. I am a power chair user though so can definitely give you real world experience and feedbackI have a 6mph chair that runs on brushed motors and an 8 1/2mph chair that runs brushless. The brushless chair from what little I do know has the simpler motors mechanically. Maybe more expensive though, i'm not sure? The brushless motors are almost silent when running, have more torque right through the speed range and for me as a user are the better option.Like I say though I don't know enough electronics wise to say one way or the other.One site you may find a lot of info on and a source of great help is "wheelchairdriver.com".Good luck in your venture!