DC Motor adjustable speed drive

[W@ttson]

Hello,

The benefit of utilizing an encoder with an AC flux vector drive is so that you can have closed loop control and get refined control down to 0RPM and produce FLT at that low speed. The DC motor inherently has this ability due to its construction of independent field and armature windings (what the field orientation control attempts to mimic in AC motors).

Question: What is the benefit of utilizing an encoder with a DC motor and DC adjustable speed drive as compared to the drives open loop control?

 

[retirede]

Open loop basically has no way to keep a constant speed (if that's a requirement) under varying load. In the old days, DC drives used a simple tachometer for feedback. Encoders were much more expensive and would have been overkill.

 

[Besoeker]

Open loop basically has no way to keep a constant speed (if that's a requirement) under varying load. In the old days, DC drives used a simple tachometer for feedback. Encoders were much more expensive and would have been overkill.

I agree. In the early days we used a BD2510 0.1 V/rpm mainly for drives in the paper making industry. Steady state was reckoned to give 0.1% speed holding.
Older systems had differential face plate regulators. Transient response wasn't the most dynamic but steady state had zero speed error.

 

[W@ttson]

Open loop basically has no way to keep a constant speed (if that's a requirement) under varying load. In the old days, DC drives used a simple tachometer for feedback. Encoders were much more expensive and would have been overkill.

What about the "armature voltage feedback"? This is a setting in the drive and uses the EMF produced from the motor to regulate speed. Wouldn't this be an open loop set up that allows for speed regulation and constant speed control? I imagine its just the speed at which it can respond is the difference??

 

[retirede]

What about the "armature voltage feedback"? This is a setting in the drive and uses the EMF produced from the motor to regulate speed. Wouldn't this be an open loop set up that allows for speed regulation and constant speed control? I imagine its just the speed at which it can respond is the difference??

Accuracy is also a factor. IIRC, 5% is about the best you can do using armature voltage feedback. If that's acceptable for the application, you're good to go.

 

[Besoeker]

What about the "armature voltage feedback"? This is a setting in the drive and uses the EMF produced from the motor to regulate speed. Wouldn't this be an open loop set up that allows for speed regulation and constant speed control? I imagine its just the speed at which it can respond is the difference??

For a plain shunt wound motor that would regulate armature voltage. Armature voltage is approximately proportional to speed but the speed would vary somewhat with load.
And if you needed field weakening it just gets too complicated.

 

[W@ttson]

Thanks to all the replies!

 

[Besoeker]

Thanks to all the replies!

An interesting topic, especially for old-fartimers like me............
So thank you for bringing it up.
I'm an old guy in my seventies.
I grew up with variable speed DC drives. Ward-Leonard, thyratron and a few other technologies. But mostly SCRs as you guys call 'em........

 

[Phil Corso]

... I'm an old guy in my seventies... /QUOTE]

So, that explains the crankiness! It also explains my mine... I'm 83!

BTW... anyone know what's wrong about being 83?

Phil

 

[Besoeker]

So, that explains the crankiness!

Me? Cranky??
Get a life.

 

[Besoeker]

But, back on topic..........
Differential face plate speed regulators.

The speed reference was the Master Motor. A synchronous machine. That sent the speed requirement into one input of a differential gearbox. The other input was from actual motor speed.The difference adjusted the arm on the diff box and varied the resistance of the supply to the motor field thus adjusting the required motor field.

 

[Open Neutral]

An interesting topic, especially for old-fartimers like me............
So thank you for bringing it up.
I'm an old guy in my seventies.
I grew up with variable speed DC drives. Ward-Leonard, thyratron and a few other technologies. But mostly SCRs as you guys call 'em........

What? No ignatrons?

 

[GoldDigger]

What? No ignatrons?

That's ignitron.

Sent from my XT1585 using Tapatalk

 

[Besoeker]

That's ignitron.

Not even I predate them....................

 

[zbang]

I grew up with variable speed DC drives. Ward-Leonard, thyratron and a few other technologies. But mostly SCRs as you guys call 'em........

And here I was reading about the humble metadyne yesterday (the London Underground used them for a while on some of the trains). And while otherwise looking around, I tripped over a nice write-up of the rotary converter by one "Tony S." .

 

[RumRunner]

Hello,

The benefit of utilizing an encoder with an AC flux vector drive is so that you can have closed loop control and get refined control down to 0RPM and produce FLT at that low speed. The DC motor inherently has this ability due to its construction of independent field and armature windings (what the field orientation control attempts to mimic in AC motors).

Question: What is the benefit of utilizing an encoder with a DC motor and DC adjustable speed drive as compared to the drives open loop control?

If the speed (output) of the motor is monitored remotely (sensors located away from the controller) and this information is fed back to the controller, that would be classified as closed loop.

If you are only monitoring the EMF (assuming this is done locally) without using a servo of some sort and not located at the driven machine, then that would be open loop.

Both systems have advantages and disadvantages and their difference have been cited by other posters.

Open loop=easier to install, cheap maintenance, less accurate

Closed loop= more complicated and expensive, high accuracy.

Using an encoder for DC Motor or other machines. . . whether it is used for torque adjustment, speed or temperature control is irrelevant.

Encoder is basically a digital-to-analog converter (DAC ) or analog-to-digital (ADC) converter which could be done through algorithm within the controller program itself.

 

[Ingenieur]

An advantage of a dc motor is design speed
100 hp at 50 rpm, common for heavy lift elevators/hoists
t = 5252/50 x 100 = 10500 lb ft
assume a 4' drum speed is 630 fpm
no gears or elaborate speed control required
lower wear since mech speed id 50 rpm

ac motor
rpm = freq x 120/poles = 7200/poles (for 60 Hz)
to get to 50 rpm you need 144 poles, not reasible
even 600 rpm is 12 poles
expensive construction
t = 5252/600 x 100 = 875 lb ft
you need 10500/875 = 12:1 gearing
a vfd could be used but at 5 Hz not efficient
higher losses with a gearbox

the dc motor is much larger though

advantages to both

 

[Besoeker]

An advantage of a dc motor is design speed
100 hp at 50 rpm, common for heavy lift elevators/hoists
t = 5252/50 x 100 = 10500 lb ft
assume a 4' drum speed is 630 fpm
no gears or elaborate speed control required
lower wear since mech speed id 50 rpm

ac motor
rpm = freq x 120/poles = 7200/poles (for 60 Hz)
to get to 50 rpm you need 144 poles, not reasible
even 600 rpm is 12 poles
expensive construction
t = 5252/600 x 100 = 875 lb ft
you need 10500/875 = 12:1 gearing
a vfd could be used but at 5 Hz not efficient
higher losses with a gearbox

the dc motor is much larger though

And vastly more expensive.

 

[Ingenieur]

And vastly more expensive.

depends on the application
elevators and hoists, not so much

 

[Besoeker]

depends on the application
elevators and hoists, not so much

The lowest speed DC motor I dealt with was 60-180 rpm. It was in a steel mill in Bilbao and driving the first section of the rolling mill. It was huge and weighed over 100 tonnes. And that's an intermittent duty too.