# Poor Man's Variable Frequency Drive (VFD) = CRAZY!!

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#### harvie

##### Member level 5
Lately i've been in need of limiting the RPMs of single phase synchronous motor. I've tried using light dimmer, but it didn't worked out really well. So i've began reading about synchronous motors and VFDs. I understood that you have to change frequency of AC voltage, not it's duty cycle. But commercial VFDs are expensive devices built from expensive components (most of them cost one or several times more than my motors).

I am experimenting with synchronous fans of different sizes, small synchronous motor from coffee machine, small motor from microwave oven and 50l hobby air compressor. I'd like to see if i can regulate RPMs of some of these motors. Some of them for fun, some of them for profit and some of them (fans and compressor) to make them run quietly.

:bsdetector: So this is what i've came up with. (And yes, i know it's some pretty serious crazy----)

[from http:// i . imgur . com/yK32GXP.jpg]

I think that while this is pretty f------- up, it might work with some motors, while it might fail terribly with others. What do you think? Please be gentle, remember it's just for fun.

This is not about how to build proper VFD, but how to build wannabe VFD for \$10 in few hours.

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#### c_mitra

##### Advanced Member level 5
Yes, if you skip alternate cycles you end up with a frequency half of the original.

The major problem will be how to get a rather continuous set of frequencies- say 20,30, 40,45 etc Hz starting with a mains frequency of 50Hz?

After you remove alternate cycles, you should use a massive filter so that the output looks roughly like a sine wave. Else the motor will only get some periodic kicks.

#### betwixt

##### Super Moderator
Staff member
What you are doing is limiting the power to the motors and relying on their load (or friction) to slow them down. Not only is that difficult to control, it makes the speed highly dependant on the load. Phase control of a synchronous motor is also likely to create large current spikes and EMC!

What you really need is full continuous cycles but at a different frequency.

Brian.

#### KlausST

##### Super Moderator
Staff member
Hi,

You talk about "single phase synchronous motors": they usually are very low power, for analog clocks for example.

Most motors driven by a single phase are brushed motors, or three phase asynchronous motors with a motor capacitor (feeding the third phase).

But most are brushless DC motors, or three phase asynchronous motors (also brushless)

Mind the difference between synchronous and asynchronous.
*******

A three phase asynchronous motor needs a control device wich controls voltage and frequency.

A single phase brushed AC motor only needs voltage control (no frequency control).

Klaus

#### c_mitra

##### Advanced Member level 5
But most are brushless DC motors, or three phase asynchronous motors (also brushless)
Brushless DC motors are actually a misnomer, because the external DC voltage is converted into 3 -phase AC voltage before it enters the motor.

#### KlausST

##### Super Moderator
Staff member
Hi,

I agree.

Indeed a so called "brushless DC motor" (BLDC) in most cases it just a 3 phase synchronous motor.

Some of them have built in driver circuit. Then the input really is DC. But to me it seems this in the minority.

Klaus

#### c_mitra

##### Advanced Member level 5
You need to adjust the supply voltage with the frequency to maintain a respectable torque. When you cannot change frequency in a linear step, the whole purpose is rather defeated.

#### harvie

##### Member level 5
The idea is that you don't change it at all. You start with f/2 and continue like that for the whole time.

#### Warpspeed

##### Advanced Member level 5
Are you sure these motors are actual synchronous motors and not plain ordinary induction motors ?
Usually synchronous motors are only fitted to things that must run at a very constant fixed speed, such as clocks and timers.
Fans and compressors never use synchronous motors.

The problem with slowing single phase induction motors is that they never run smoothly at greatly reduced speeds, they can become jerky or noisy, its why commercial VFDs always use three phase motors even in very small sizes.

The suggestions above to use a dc motor, preferably a brushless dc motor are very sound. Much easier to control, and the motor will run smoothly and trouble free, with much higher torque down to very slow speed.

If it has to go REALLY slowly, a stepper motor driven with low frequency ac may be worth thinking about.

#### SunnySkyguy

##### Advanced Member level 5
this would end up saturating core with large currents due to any DC content if not symmetrical.
INterleaving cycles also cuts starting torque by duty cycle.

VFD's use low ESR active switches to drive the motors with PWM half bridges to make pseudo sine waves to reduce Eddy current losses.Then 3 phase gives smooth torque at wide speed ranges.

#### FvM

##### Super Moderator
Staff member
Of course the motors mentioned in post #1 a are asynchronous motors, either shadow-pole or capacitor-supplied (in case of the compressor motor).

The reference to cycloconverters doesn't exactly hit the point because you can't generate arbitrary phases from a single phase supply.

The waveforms sketched in post #1 are not real because the phase shift between voltage and current with inductive loads isn't considered. A triac switches off at current zero crossing, not voltage zero crossing. Trigger patterns must be well designed to achieve the intended operation. It's easy to get DC output and burn a motor, as suspected by SunnySkyguy.

Nevertheless I believe that the suggested drive method can work to some extent. Torque will be low because the rotary field generation means (shadow pole or motor capacitor) is designed for 50 Hz. Torque is even limited when operating one of these single phase motors with a real VFD. Drives requiring a relative high starting torque like a piston compressor will possibly fail.

It would be interesting to see results of actual tests.

#### SunnySkyguy

##### Advanced Member level 5
For constant torque, VFD's ramp up the voltage with frequency, to achieve a more constant V/F ratio for limiting current and torque.
Motors often use 5x to 8x starting current, so using full power at 1/4 speed would burn out the motor using your FW rectified approach

and .... lof lots of hum from the pulsed torque.

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