PWM vs VFD for pump control

This question is not entirely related to BrewPi but close… I notice that larger brewery pumps often come bundled with VFD’s (Variable Frequency Drives) for controlling the speed. I can’t find any good information on what the fundamental difference between this and using a PWM output from eg a BrewPi via SSR to control speed. I’m guessing constant torque is the main pro for vfds, but does that really matter for a brewery pump? All you want to do is slow down from max output… are there any other pro’s? Like more precise control of the low end, like around 10% of max? suggests that the rapidly alternating voltage from PWM control can be harmful for electro motors.

For a more expert response you’ll need @Elco.

Thanks, hadn’t seen that one. But I guess it just adds to my confusion :). The PWM they are referring to is the PWM-part of the VFD, not pulsing an AC SSR. I was also under the impression that instead of just pulsing DC to simulate a variable frequency AC, they (VFDs) also have a circuit that create a “smooth” sinus, that would counteract the effect they are talking about. I guess what I’m referring to is the inverter, and a good quality inverter should output a stepped sine close to a pure sine wave.

A motor turns when the windings in the stator are powered in succession, pulling the rotor to a new position. A DC motor changes which windings are powered electronically or with brushes.
An AC motor just uses the 50Hz cycle of the power grid usually.

With PWM, you give the motor power in short bursts. This can cause problems with alternating the windings or can reset the internal motor controller. The TD5 pumps we sell have an internal controller that just takes the PWM signal as an input signal and manages power to the windings. So you could say it has an ‘internal VFD’.

A VFD (variable frequency drive) does what you expect from the name: it changes the frequency at which the alternating windings are powered.
A VFD in its common usage takes 50Hz single or 3-phase power and outputs it at a frequency other than 50Hz, changing the speed of the pump.

VFD’s take their input signal to set the speed as 0-10V or a PWM signal.
230V 50Hz pumps are not designed to be powered with a PWM interrupted power line, which is why a VFD is put in between.