A dimmer of suitable wattage would work fine to lower the speed. The problem you then have is you lose the torque as well. PWM controller the way to go for speed control without losing slow speed grunt. Cooling can also be an issue when running slower than rated.investigating the idea of a speed control for an angle grinder and potentially a jig saw for a stone cutting / polishing
Is it as easy as using a lighting dimmer as shown or is there some trickery I don’t understand?
Ok so no dimmer, will you be good enough to point me in the direction of PWM controllerA dimmer of suitable wattage would work fine to lower the speed. The problem you then have is you lose the torque as well. PWM controller the way to go for speed control without losing slow speed grunt. Cooling can also be an issue when running slower than rated.
Or if you have a variac sitting around even simpler...
Not on a universal motor far to much torque lostOr if you have a variac sitting around even simpler...
Explain?Not on a universal motor far to much torque lost
Explain?
Torque is provided by current in an electrical motor. If the motor requires more torque then it will try and pull more current, assuming the variac can supply this (because they are limited to a maximum based off their windings) then you shouldn't lose torque.
Or if you have a variac sitting around even simpler...
Something along these lines would probably do the trick https://www.amazon.com/SMAKN®-0-220V-Modulator-Electric-Controller/dp/B00UKK50UKOk so no dimmer, will you be good enough to point me in the direction of PWM controller
Because you reduce the back emf, only way to maintain torque would be speed sensing and uping the voltage to increase overall power
It will slow the motor but will sacrifice the torque, though i agree a bigger variace might counter this its also not ideal due to practicality and would cause far higher current across the windings
@mtt.tr - this isn't a dig at you personally but those statements are just wrong (at least my understanding of what you are trying to say is, sincere apologies if I have interpreted them wrongly), there's enough bad information on the internet lets try not to propagate more on this forum since it's one of the best out there.
Warning - there be some maths and equations ahead...
Using @Parm's example of controlling say an angle grinder then if we use an example of a 2kW motor running at 240V then the motor's internal construction will have appropriate insulation to handle at least 240V and the wires should all be thick enough to handle the current required for 2kW (8.3A). Realistically it (should) have been designed to handle much more than this since that will be the rated power when up to speed rather than start-up currents which we all know to be higher. So lets assume for the sake of argument (ignoring start-up conditions) that the motor's internal construction will definitely survive 240V and 10A which is the most that it can handle at full power off mains.
Time for some equations: here's a relatively reputable source (MIT) to provide some equations*. Power-tools (the subject of this thread) tend to be universal motors, as pointed out, so can be described by the equations given for DC motors in these course notes.
https://ocw.mit.edu/courses/mechani...icles-13-49-fall-2004/lecture-notes/lec13.pdf
The three equations of importance are under section 13.2:
Where:
- e = Kpw
- V = e + RI
- T = KpI
Equation three shows that the only thing that influences the torque is the current flowing through the windings. More current = more torque, simples - we've already determined in our example that we can't exceed the design limits of the motor so doesn't matter if we run at 10V or 240V if we can supply 10A then we will get the same torque out of the motor.
- K = motor constant (constant)
- p = air gap magnetic flux per pole (effectively constant for a given motor)
- w = motor's angular velocity (output of the motor)
- e = back emf (variable)
- V = applied voltage (variable under our control)
- R = armature resistance (effectively constant for a given motor)
- I = armature current (variable under our control, indirectly)
- T = developed torque (an output of the motor)
Now if we look at what happens when we limit the voltage (through use of a variac for example, or by some form of PWM which is limiting the average voltage).
We find that the back-emf is a by-product of the motor's specific characteristics and angular velocity. Substituting equation 1 into 2 then you get: V = Kpw + RI. Now K, p, R and I are effectively constant for a given motor operating at maximum torque. So we now find that V is proportional to w - i.e. by limiting the applied voltage we are only affecting the speed that we can achieve.
If you further substitute 3 into this then you get:
V = Kpw + RT/Kp
V - Kpw = RT/Kp
If you assume a load of things are constant for a given scenario (V, K, p and R) then you see that torque is inversely proportional to the motor's angular velocity. I.e. the faster that the motor spins the less torque it has - which is why you often here the phrase bandied about that electric motors develop all of their torque from stationary. Which is why electric vehicles accelerate so quickly from a standstill. If you consider this point and equation 3 then from stationary the motor develops maximum torque and therefore requires maximum current in order to get moving.
Trying to cover off a point about PWMs (and why they don't necessarily operate below a certain threshold) - motors have an inherent inertia to them - both physical and due to the magnetics involved. You have to overcome that to start them moving.
If we consider the DC case for simplicity for the moment then at low duty cycles you are applying the maximum voltage for a brief period of time. The motor itself is basically a massive inductor which opposes changes in current so slows down how quickly the current can be pulled in from the supply. If the duty cycle is low enough then basically the motor never has enough chance to get enough current in it to start the thing moving. This energy must go somewhere so since the motor hasn't started turning the windings are in effect resistors and simply start heating up - which is sometimes when you can hear them buzzing if things don't start up properly. As soon as the motor is spinning the back-emf rises and the current pulled by the motor drops to satisfy equation 2.
With AC then things are basically the same except the voltage you are chopping for the PWM is now varying so in order to get consistent speeds you need to either chop significantly faster than the AC frequency or synchronise yourself to the AC frequency. You can chop things nice and quickly because the motor is a big inductor and therefore can't respond quickly so smooths things out nicely because of it's inherent inertia. The smaller the motor the smaller the inductor so the quicker you need to chop things.
Wow - that took a long time to write, hopefully some people can follow it and it's of use, if there are any questions feel free to ask and I'll do my best to bamboozle you. Or if someone spots a mistake let me know
Also saves me having to dig out my lecture notes from the Real Professor Green