Hi

Got a single phase 370W induction motor that is coupled to a pump, I am planing to control the pressure directly trough the motor (not the valve method), so i looked around for a single phase VFD which is hard to get because not many manufacturers makes them with single phase output and they are a bit to expensive, so now i am thinking of using PWM to do the job, i got a few of these that can handle 10-15 Amps, what i have heard is that its a bad idea to control an induction motor using PWM, to my experience the PWM does not reduce the motor speed instead it reduce the Torque which under load will cause the motor to run slower (Correct me if i am wrong), What is the side effect of using the PWM method ? any better ideas beside the VFD method ?

Thanks in advance guys

 

http://ww1.microchip.com/downloads/en/AppNotes/00967A.pdf

I've found this app note to be very useful for the application that you mention.

How were you planning on using PWM? Pulsing a single triac on the motor input? What kind of motor is this? When I hear single phase motor, I think Permanent Split Capacitor, but that's just because of my experience.

 

The problem with PWM is that it is equivalent to reducing the motor voltage to slow the motor. This leads to a large slip value in an induction motor which generates a large induced rotor current and possible overheating of the rotor. That's why VFD is the preferred method of controlling the speed of an induction motor.

 

The problem found with controlling a 1ph split phase motor with a VFD is they tend to drop out of run at lower rpm, and/or a sudden load.
This is why 3 ph motors are prevalent in VFD use.
A small 3ph motor and VFD of that size should not be to expensive?
Max.

 

The problem with PWM is that it is equivalent to reducing the motor voltage to slow the motor. This leads to a large slip value in an induction motor which generates a large induced rotor current and possible overheating of the rotor. That's why VFD is the preferred method of controlling the speed of an induction motor.

Wouldn't the water in the pump take care of the heat in the rotor ?

 

Another answer is a treadmill motor or other DC motor, controllers abound, or easy to make a PWM controller.
Max.

 

http://ww1.microchip.com/downloads/en/AppNotes/00967A.pdf

I've found this app note to be very useful for the application that you mention.

How were you planning on using PWM? Pulsing a single triac on the motor input? What kind of motor is this? When I hear single phase motor, I think Permanent Split Capacitor, but that's just because of my experience.

You guessed right

 

The problem found with controlling a 1ph split phase motor with a VFD is they tend to drop out of run at lower rpm, and/or a sudden load.
This is why 3 ph motors are prevalent in VFD use.
A small 3ph motor and VFD of that size should not be to expensive?
Max.

Max
The main point of this is that i want to know why its a bad idea of using PWM on induction motors, curtschow mentioned slip which makes much sense.
I've seen other AC motors used in fans that are controlled using PWM, i am not sure tough if they where induction motors or universal motors,

 

Maybe those were ECM motors as used in HVAC systems etc, these are P.M. rotor motors.
In an induction motor, the rotor current depends on induced voltage provided by the stator, in a VFD a form of PWM is used to create a pseudo sine wave that is modulated by the switching frequency, the pulse width decides the energy or torque and the low frequency sine wave frequency the rpm of course.
Max.

 

Most of today's industrial VFD's (inverters) are PWM devices. Induction motors are ran with VFD's (PWM)- no problem. They control speed, primarily with frequency, voltage is adjusted to improve efficiency as b1u3sf4n09 noted.

 

If you read and understand the app note I posted, it is possible to achieve what you want without a VFD:

As you decrease the frequency of the motor, you decrease the speed along with it. The only caveat is that you reduce efficiency of the motor if all other variables remain static. To counter this, you generally will vary the voltage in a linear fashion. For a 120V, 60Hz motor, you will decrease the voltage 2 V/Hz.

Edit: I apologize for posting a vague formula. n is motor speed, f is motor frequency, p is motor poles (static value)

 

Here's a link to Automation Direct (they usually have really good deals on drives and PLC's: http://www.automationdirect.com

Here's a catalog (it was too large to upload): http://www.automationdirect.com/static/catalog/13-vfd-variable-frequency-drive-ac.pdf

As I noted previously, standard induction motors are ran, regularly, with inverters. However, if you plan to run the motor much slower than its rated speed, you should use a motor rated for inverters.

Almost forgot: Stay away from AC Vector drives, they must have a matched motor designed for vector control.

 

Wouldn't the water in the pump take care of the heat in the rotor ?

Depends upon how good a heat conduction path there is between the pump and the motor rotor. If it's only through the steel motor shaft, then it probably won't help much.

What type of PMW do you have that can handle 10-15A? Aren't those for DC where you need AC?

But you can just try a standard AC lamp dimmer circuit to see how it works, with the caveat that the motor rotor could overheat.

 

qitara, what is your motor's voltage? Is the pump a positive displacement or centrifugal?

crutschow has a good point, if your motor is series wound, a rheostat is all you need, especially when we're talking about a half horsepower motor.

 

Almost forgot: Stay away from AC Vector drives, they must have a matched motor designed for vector control.

If by vector you mean sensorless vector I would have to disagree, I have been installing VFD's of all makes for some years now, and never used vector rated motors, I try to use 4 pole motors and run up to around 120Hz max.
I work mainly in the CNC field and use the sensorless vector for better control down to very low rpm for spindle control etc.
I do encourage the use of 3 phase inductor or choke on the VFD output also to make it easier on the VFD and the motor.
Most decent makes also have auto tuning which gets you in the ball park.
Max.

 

If by vector you mean sensorless vector I would have to disagree, I have been installing VFD's of all makes for some years now, and never used vector rated motors, I try to use 4 pole motors and run up to around 120Hz max.
I work mainly in the CNC field and use the sensorless vector for better control down to very low rpm for spindle control etc.
I do encourage the use of 3 phase inductor or choke on the VFD output also to make it easier on the VFD and the motor.
Most decent makes also have auto tuning which gets you in the ball park.
Max.

Vector drives shouldn't be confused with v/hz scalar or torque control VFD's. Vector drives are commonly called AC servo drives, they are controlled by field orientation and do, indeed, require a vector motor that is specifically matched to the drive.

I'll edit to, hopefully, make a bit more sense of my words for you: The motor is an induction motor, but it must have tach feedback and the motor and drive should be closely matched in HP and performance ratings. Whereas, (I'm sure you know) one can get away with all kinds of matches using the typical inverter drive (VFD) with no regrets.

 

In my world AC servo's are two types, BLDC and AC sinusoidal, both motors are practically identical physically but the commutation is different.
VFD's usually come in V/hz or simply variable frequency and Sensorless Vector and then Sensored feedback by P.G. or encoder.
Who's are the AC servo drives you mention?
Max.

 

Rockwell, Reliance, Allen-Bradley, to name the most common. As for me, I was an authorized electronics service technician for over twenty drive manufacturers for a number of years. In house, I performed component-level (board) repairs. In the field I was a troubleshooter and technical advisor. That's where my experience lies.

 

I still have all my old Reliance and ElectroCraft (now Rockwell) motor catalogues and also worked with A.B. but I must say I have never come across any Sensorless vector motors by them, All the ones I have used have either used encoders or resolvers back to the drive.
But back to the VFD's they either use simple variable frequency or field orientation sensing which is usually described as Sensorless Vector which is quite common now.
Apart from using A.B. servo's and VFD range I am not aware of any other kind that A.B. carries?
Max.

 

No offense, but I didn't say sensorless. Those were your words. I stated that feedback (tach) was required. Allen-Bradley PowerFlex 700.

Again, no offense Max, but your terminology doesn't sound like 3 phase industrial. What is your background? (If you don't mind.)