PWM affects only speed or torque as well ?

Hello everybody

Before continue to adruino question, i need an aswer to the following question.
I seached in the internet and I found 2 different answers related to motor speed control. There are many guys who report that when they reduce the speed of their motors with a typical PWM driver, torque is reduced as well. Some of them say that the PWM reduces the average voltage (due to duty cycle) and thus, the current is also reduced and accordingly the torque is reduced too.

However there are many other guys who say that a PWM driver contoller affects only the speed of a motor ( voltage control) but the current is staying constant. Thus the PWM is NOT affecting the torque (and the current) but only the speed (and the voltage).

I'm confused ? what is really happening with the typical PWM controllers we use in projects where we need to control the speed of the load in a brushed DC motor but we need the torque to stay constant ?

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There are many guys who report that when they reduce the speed of their motors with a typical PWM driver, torque is reduced as well. Some of them say that the PWM reduces the average voltage (due to duty cycle) and thus, the current is also reduced and accordingly the torque is reduced too.

That is correct. PWM switches the power on & off at a (relatively) high speed. i.e. If the power is off most of the time, you're not feeding as much energy into the motor and your not going to get as much energy out.

In some situations, at low speeds the motor may not have enough torque to start unless you use a higher PWM value to start the motor and then back-off to get the desired speed.

gspir:
Some of them say that the PWM reduces the average voltage (due to duty cycle) and thus, the current is also reduced and accordingly the torque is reduced too.

However there are many other guys who say that a PWM driver contoller affects only the speed of a motor ( voltage control) but the current is staying constant. Thus the PWM is NOT affecting the torque (and the current) but only the speed (and the voltage).

I'm confused ? what is really happening with the typical PWM controllers we use in projects where we need to control the speed of the load in a brushed DC motor but we need the torque to stay constant ?

  • PWM reduces the average voltage. Motors have lots of inductance and lots of inertia, so effectively they respond to just the average voltage.

  • Speed is largely determined by the voltage, so as PWM reduces the average voltage it can also reduce the speed.

  • Torque is determined by current. So you might expect that if PWM reduces the voltage, that it would also reduce the current and thus reduce the torque. This analysis however is misleading, because as the speed is reduced you need less voltage to maintain the same current (and hence torque).

While it's certainly true that for any given speed you can get more current and torque if you have more voltage, the reality is that if you might burn out the motor, or trip a breaker/fuse, in doing so. Say for example you applied full voltage to a large DC motor while holding it stalled. You might get an impressive amount of current and torque, but you can't sustain that without something overloading or burning out.

The bottom line is that at reduced speed you CAN get full load current and full load torque at reduced voltage levels (for example using PWM).

  • Torque is determined by current. So you might expect that if PWM reduces the voltage, that it would also reduce the current and thus reduce the torque. This analysis however is misleading, because as the speed is reduced you need less voltage to maintain the same current (and hence torque).

Thank you guys, i unserstand the basic principle but it still is not clear if finally a PWM speed controller affects the speed or the speed and the torque as well.

To be more practical. If I took for example this PWM controller to drive for example a 200rpm (rated) 12V dc motor with rated torque 5kg.cm, the output torque will be the same in all speeds for a constant load ?

I need to know this becouse I'm making a p[roject where constant torque is essential.

Hi,
For Permag DC motor,

Tom.... :slight_smile:

gspir:
Thank you guys, i unserstand the basic principle but it still is not clear if finally a PWM speed controller affects the speed or the speed and the torque as well.

To be more practical. If I took for example this PWM controller to drive for example a 200rpm (rated) 12V dc motor with rated torque 5kg.cm, the output torque will be the same in all speeds for a constant load ?

I need to know this becouse I'm making a p[roject where constant torque is essential.

Perhaps this will help:"power = torque x speed". Power is the average volts X average current. If you change the average voltage by PWM, that also changes the average current because the motor resistance is constant. Therefore if you need "constant torque" You need to increase the voltage of the PWM when the pulse time is reduced.

By changing both pulse rate and pulse voltage, you will maintain constant torque and will change motor speed.

My thoughts.

Paul

The key point to note is that "PWM" by itself is not enough information.

You need to know the decay mode used by the motor driver or H-bridge in question.

slow decay and fast decay behave differently, both are non-linear and mix torque and speed
responses. The duty cycle of voltage at the motor terminals does not, in general, match the
PWM duty cycle when using decay modes, and is affected by several factors including the PWM
frequency.

There is a more linear way to do things, by synchronous rectification, then the motor terminal voltage
is always defined, and its duty cycle exactly matches the PWM drive. This give a linear speed
characteristic and works much like a varying analog control voltage. Its used principally in
servo-motor setups where you need active braking and linear behaviour round the control loop
(though often servomotors have an inner current-control loop using a current sensor). It has the
advantage of much stiffer speed control, but the expense of rather less efficiency due to iron
losses in the motor. It also requires the PWM frequency to be high enough or it doesn't work
well.

You can also think of "synchonous rectification" mode as like class D audio amp - for instance
it is naturally used for driving voice-coil linear motors, as the analogy with an audio speaker is
very close.

erhaps this will help:"power = torque x speed". Power is the average volts X average current. If you change the average voltage by PWM, that also changes the average current because the motor resistance is constant. Therefore if you need "constant torque" You need to increase the voltage of the PWM when the pulse time is reduced.

By changing both pulse rate and pulse voltage, you will maintain constant torque and will change motor speed.

I tried to figure what you said, in a simplified diagram. Please check and tell me if I'm right. So, when you have a constant load of 5kg and you need your 5kg.cm 12V motor (just an example) to can handle this load in all different speeds (from 1% to 100%), then your PWM driver should be able not only to change the rate of the pulses but also change the voltage in a way that the average voltage should stay 12V. Is it what you are saying ?

Lets suppose you have a 12V DC, rated 100rpm (geared), rated 5kg.cm motor.

In diagram 1, if you put 12V, the motor will run at 100rpm, with 5kg.cm
In diagram 2, if your PWM-driver average voltage is 6V, the motor will run at 50rpm, with 2.5kg.cm (simplified)
In diagram 3, if you PWM driver average voltage is still 12V, the motor will run at 50rpm (due to pulse cycle) but with constant torque of 5kg.cm (simplified)

Ok you may have some losses etc, but the main concept is this ?

Paul_KD7HB:
Perhaps this will help:"power = torque x speed". Power is the average volts X average current. If you change the average voltage by PWM, that also changes the average current because the motor resistance is constant.

Completely wrong I'm afraid - the winding resistance is constant, but most of the voltage across the
motor is countering back-EMF, nothing to do with the winding resistance at all, but the motor speed.

torque proportional to I,
speed proportional to V - IR

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I think I will agree with MarkT, since the winding resistance is constant and yes there is always a back-EMF but this theoritical analysis cannot asnwer if a market PWM speed driver affect the torque or not . My first question in this topic is still unanswered i think. In practice, when you buy a PWM speed driver, (lets talk with examples), like this one : https://goo.gl/yrHrF2

When I put the speed down to 20%, the output torque of the motor would remain 5kg.cm or not ? The answers in internet varies as I said. Some people say it will affect torque, some others say that PWM drivers affect ONLY speed, not torque. Do My diagrams above (even simlified enough) describe in simplicity how a PWM works ? (especially diagram 2 and 3).

thanks

No, the diagram you need is of what really happens in most motor drivers that use slow-decay, fast-decay
or mixed decay modes - there can be 4 phases per PWM cycle, on-time, slow-decay, fast decay, and idle,
in the most general case - the average voltage and current are complex functions of pretty much everything.

Something of the complexity is evident in some of the datasheets for motor drivers, for instance: https://www.maximintegrated.com/en/design/reference-design-center/system-board/6219.html/tb_tab1

MarkT:
Completely wrong I'm afraid - the winding resistance is constant, but most of the voltage across the
motor is countering back-EMF, nothing to do with the winding resistance at all, but the motor speed.

Yes. This is precisely why in my earlier reply I stated: "as the speed is reduced you need less voltage to maintain the same current (and hence torque)." Back EMF is very important.

Regarding the importance (or otherwise) of winding resistance. The truth is that it tends to be more important (and back EMF less so) in very small motors only.

MarkT:
The key point to note is that "PWM" by itself is not enough information.

You need to know the decay mode used by the motor driver or H-bridge in question.

slow decay and fast decay behave differently, both are non-linear and mix torque and speed
responses. The duty cycle of voltage at the motor terminals does not, in general, match the
PWM duty cycle when using decay modes, and is affected by several factors including the PWM
frequency.

There is a more linear way to do things, by synchronous rectification, then the motor terminal voltage
is always defined, and its duty cycle exactly matches the PWM drive.

Hi Mark. I agree with what you're saying about needing to know exactly how the PWM is used, however I think it is over complicating the OP's simple question. In all of the various modes of operation it really just comes down to one thing, whether it's continuous or discontinuous current operation.

Any time that you have continuous current operation, the average voltage is simple function of the duty cycle. Of course in 'fast decay' modes (I assume that means returning the current back to the supply and hence reversing the voltage instead of freewheeling it) you are more likely to get discontinuous current. But no matter what mode you're using, it all comes down to whether or not you have continuous motor current.

gspir:
but this theoritical analysis cannot asnwer if a market PWM speed driver affect the torque or not . My first question in this topic is still unanswered i think.

Sometimes the answer to a question is "it depends". Say I ask you a very simple question: "Does the power dissipation in a resistor increase or decrease as the resistance value increases?"

If you answer an unequivocal "it increases" you are wrong, and similarly if you answer "it decreases" you are also wrong. Without seeing the whole circuit, the only correct answer is "It depends. Show me the circuit".

In some sense both of the things that you had been told (prior to asking here) about the effect of PWM on torque are true. I tried to summarize this in my first reply #2 above, but let me repeat it.

1. For any given operating conditions, if you reduce the voltage (for example by using PWM) you will reduce the current and hence the torque.
2. HOWEVER, as the speed is reduced you need less voltage to produce the same current (and hence torque).

Perhaps it will help to give a concrete example.

Say I had a DC motor operating with a steady 48 volts DC, and it was running at 2000 RPM with a 10 amp draw and had back EMF of 40 volts.

At this operating point the output power is approx 400 Watts, the torque approx 1.9 Nm and we can deduce that the overall series resistance (windings brushes etc) is 0.8 ohms. Further, lets assume that the load torque is constant (with speed) just to keep things simple.

Now say that I suddenly change from a steady 48 DC to PWM with 60% duty cycle (though still 48 volts peak) and assume a simple switch and freewheeling diode circuit.

The immediate effect (not exactly instantaneous because of inductance, but relatively quickly) is that the current and torque will decrease. And as the torque is now decreased, the motor will begin to slow down.

As the motor slows down however, it's internally generated voltage (the back EMF) decreases accordingly, and this causes the current to increase again. Eventually it slows to the point where the current rises back to 10 amps and the original torque value is restored.

As 60% duty cycle gives us about 28 volts, and the series drop at 10A is still 8 volts (as per original operation point), we can deduce that the motor will settle at whatever new speed gives only 20 volts back EMF. Normally this would be half the speed for 40 volts (the original operation point), so the motor will slow to 1000 RPM in this example, and still maintain full load torque.

Hi!

I have finished a design that uses PWM from a UNO that greatly increases' torque at low rpm as well as providing speed information, without an encoder or pid loop. Basically a current sense resistor is between motor- and ground with motor+ coming from a high side pnp Darlington that is driven from an UNO after level shifting.
The UNO samples the current after hardware filtering and uses the current demand to add to the PWM command. With no load a motor draws low current so little or no compensation is added. Add a load and the current goes up increasing the PWM duty cycle in the right proportion to buck the load and keep the shaft speed almost constant!
This technique is called IR compensation and the theory is to add voltage to the motor to compensate for the IR drop in the motor. To build, make a pwm drive with a sense resistor, filter the pwm with a filter and read the analog voltage. Scale this value and add to the PWM
command and you can have vastly improved low speed torque with very little effort. I use a pot to scale the compensation until the motor becomes twitchy at low speed (too much positive feedback in that case) then back it off a bit.
To read the motor speed or RPM the motor+ is divided by resistors and read by the UNO. A voltage reading requires pausing the PWM for a few milliseconds then reading the divided voltage, then turn PWM back on.
Now I have motors that seem almost like torque motors along with speed sense. Gears or pulleys can either have a more direct ratio or eliminated altogether.
I will be documenting the design for upload. By-the-by I developed this design for use on home sewing machines so heavy fabrics can be sewn at low speed without stalling :slight_smile:

Ciao

Hi,
Can you share your code and schematic please?

As this will probably help the OP.

Please read the first post in any forum entitled how to use this forum.
http://forum.arduino.cc/index.php/topic,148850.0.html

Thanks.. Tom.. :slight_smile:

How about this ? Good stable torque

https://fr.aliexpress.com/item/1-Pcs-N20-DC-12V-100-Rpm-Gear-Motor-High-Torque-Miniature-Motor-Gear-Box-Shaft/32695520982.html

ted:
How about this ? Good stable torque

https://fr.aliexpress.com/item/1-Pcs-N20-DC-12V-100-Rpm-Gear-Motor-High-Torque-Miniature-Motor-Gear-Box-Shaft/32695520982.html

I've got the 600RPM output ones on order from ebay, shouldn't be long before they arrive.


Tom.. :slight_smile:

Hi Tom
So you are doing the same project ?

ted:
Hi Tom
So you are doing the same project ?

No, not really, just looking for some stuff for my brother for his model railway. and me to play around with.

Very relaxing just playing around with stuff.

Tom.. :slight_smile: