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Single phase amp reading.

Single phase amp reading.

Single phase amp reading.

(OP)
I have a small single phase motor with no name plate. I want to use a variac transformer to control speed, so I want to read amps while it is running but I don't want to cut open the cord. Question.. can I get an accurate reading from around the entire cord? This includes live, neutral and ground. The reading I get around the cord is almost five amps. Should I size a larger variac or will a five amp suffice?

Ward Holloway, Jr, PE

RE: Single phase amp reading.

You can use a clamp on amperemeter around each individual wire. The current fed by the hot wire should measure the same figure as that around the return, trough the neutral wire.
The size of the Variac should match at least the current demanded by the motor driving the load. Check the terminal wire size of the motor and  figure out if that current measured is not overloading the motor.  I am assuming that the motor will  work around 110-120 Volts.

RE: Single phase amp reading.

The speed of an induction motor, in general, can't be controlled by reducing the line voltage.

RE: Single phase amp reading.

You are talking about a DC motor, right?

RE: Single phase amp reading.

(OP)
This motor is 115VAC c-frame motor. I was told a variac could be used to control speed. Is this correct!!

Ward Holloway, Jr, PE

RE: Single phase amp reading.

Hello WardHolloway

As a general rule, with an induction motor, you can not vary the speed by varying the voltage. There are however special induction motors that you can vary the speed by varying the voltage. These are a high slip induction motor with a high resistance rotor and are generally designed for fan control and are an integral part of the fan assembly.

If the motor is a universal motor, (it will have brushes) then yes, you can control it's speed by controlling the voltage.

You can not measure the current by putting a clamp maround the whole cord. You need to put the clamp around the active feed only. (one core)

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Single phase amp reading.

(OP)
Is there any possible way to control the speed of a single phase, 115VAC c-frame motor? It runs a small conveyor.

Ward Holloway, Jr, PE

RE: Single phase amp reading.

Sorry.  Not really.  If you want some changes with a 1Ph you do it with some mechanical transmission.

Three phase motors are very easy to control the speed on but NOT single phase.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Single phase amp reading.

A 3 ph c-frame motor of the same size, with the addition of a VFD that can take 120 single phase and generate 240 3 ph, is likely the two-prong solution to your problem.  Check out KB Electronics, one of several mfrs of such VFD's.

BK

RE: Single phase amp reading.

If you are reading 5 amps by putting a clamp meter around the entire cable, including hot neutral and ground, you have a serious issue that should be resolved. The reading should be zero around the entire cable.
You may have a bad neutral connection at the motor and some or all of the motor current is returning via the metalic machinery, not the neutral conductor or the ground conductor. Such a situation may rsult in lethal voltages appearing on the frame of the motor or conveyor.
Regardless of the cause, it should be investigated and repaired.
respectfully

RE: Single phase amp reading.

There are a few types of 1 phase motors that can be controlled by varying the voltage; the Universal motor as Marke said, and the Shaded Pole motor. Universal motors are like the ones used in hand drills; they are actually DC motors with brushes and built-in rectifiers to accept AC input. Shaded Pole motors are the type you would find in ceiling fans that are often controlled by dimmer switches which simply vary the voltage. This is why people often think that you can control any 1 phase motor with a dimmer; and a variac is essentially a giant dimmer.

Unfortunately Shaded Pole motors are almost completely useless when it comes to putting them on conveyors. Universal motors technically could work, but you would be hard pressed to find one suitable for that kind of use.

As others have said, replace it with a 230V 3 phase motor with the same frame, then get a 115V 1 phase input - 230V 3 phase output VFD. In the long run you will save yourself a lot of headaches.

RE: Single phase amp reading.

I'm not sure why three phase is needed at all. The link I provided above has VFDs for 115V single phase motors of small sizes.

RE: Single phase amp reading.

Those look like 3-phase inverters to me. Some models can accept single phase input but they don't control a single phase motor.

As already stated, use a VFD and 3-phase motor. This is the only solution that makes sense. Most manufacturers have VFD's that will work for your application.

RE: Single phase amp reading.

stevenal,

Note that "... Optidrive 1 is for use with PSC (Permanent Split Capacitor) or Shaded-Pole Single-Phase induction
motors" not all single phase motors.

Also: "To ensure reliable starting, the Optidrive 1 initially ramps the motor voltage up to rated frequency and voltage, before reducing the frequency and voltage to the desired operating point..."

The Optidrive may do the job, but if it was my project, I would feel more confident with a 3-phase motor.

RE: Single phase amp reading.

Hi stevenal.

I thought of those too then remembered "most" single phase motors are imbued with start windings and start switches that know nothing about starting or not starting - just speed.  So of course if you slow a motor down just enough the start will kick in. I don't want to contemplate what happens after that. flush

I have seen those used well on refer condenser fans and wine chillers.  They are permanent split cap and are just fans which alleviates a bunch of accuracy needs.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Single phase amp reading.

Do not use a single phase AC motor on a conveyer. The starting almost always is tough and a voltage controlled single phase AC motor is at its weakest just there.

You can, if you have the time and money for it, test single phase voltage control. Just to convince yourself that it doesn't work. Then buy a decent three-phase motor and a VFD.

Single phase AC with voltage control is for fans. They may even be very good with fans because of a fan's speed/torque characteristic - which is very different from that of a conveyer.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Single phase amp reading.

(OP)
Update: Vendor claims the motor is a shaded pole single phase. The motor is rated at .7 amps. This is a conveyor... a very small one. I will attempt the variac solution as many post indicate this will work with a shaded pole motor.
Thanks for the everyones help.

Ward Holloway, Jr, PE

RE: Single phase amp reading.

Honestly, I wish you luck...

An AC motor want to run at a speed that = freq x 120 /#poles.

This means you need to vary the frequency to control the speed.

If you just reduce the voltage the motor will still want to run at rated speed. But it won't be capable of producing rated torque. So, as you apply load the motor will start to slip and slow down but the slip is very uncontrolled so you'll have no real control and the speed will likely vary A LOT as the load changes or as the line voltage fluctuates.

A fan has a load torque that increases with the speed^3. So, if you build the motor right you can make the speed controlled via the voltage. As you increase the voltage you get more torque which allows the motor to turn the fan faster. Works well. Unfortunately, your conveyer will have the same torque requirement at all speeds - it's not likely to work well at all. Reduce the motor torque and you'll be all over the place speed wise.

RE: Single phase amp reading.

"I will attempt the variac solution as many post indicate this will work with a shaded pole motor."

I don't see these posts. Did they get deleted?

Please change "Your" to "You're" in my previous post.

RE: Single phase amp reading.

Does this help stevenal?


Quote (jraef):

There are a few types of 1 phase motors that can be controlled by varying the voltage; the Universal motor as Marke said, and the Shaded Pole motor.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Single phase amp reading.

That doesn't say that a conveyor can be speed controlled using voltage control.

A fan, yes. A conveyor - no. No way.

Why don't people listen...?

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Single phase amp reading.

Now Skogs... Did you not try a few stupid things after your parents told you that it was stupid/dangerous?bombcry

It could actually be a conveyor that moves feathers one at a time and so some setting will actually work.  The OP isn't clear on the configuration.

     

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Single phase amp reading.

I agree. Although I said a shaded pole motor can be controlled with voltage only, I have never seen one used on other than a fan. The classic example is the paddle fan on your ceiling.

That must be one tiny little conveyor... moving pimentos into an olive stuffer perhaps?

RE: Single phase amp reading.

Pimentos or feathers... The conveyor will still be either running or standing still. Maybe, there is some voltage that will make it run at some intermediate speed. But come sun (or come rain) and that speed will change.

Yes smoked, I tried making black powder. Several times, actually. And suddenly - it worked. That's how I lost my hair. First time.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Single phase amp reading.

itsmoked,

No it doesn't help, since in his same post jraef said "Unfortunately Shaded Pole motors are almost completely useless when it comes to putting them on conveyors."

Still can find only one post that recommends he try this, and that one only suggests he try it to show it won't work. Hardly qualifies as "many."

RE: Single phase amp reading.

(OP)
It is a shaded pole motor about ,7amps. It is on a conveyor.
I decided to try the Optidrive or something simular.
Thanks for the help

Ward Holloway, Jr, PE

RE: Single phase amp reading.

(OP)
Update...
I called the Optidrive vendor and as soon as I told him what I was doing he almost refused to sell it to me. I also researched replacing the motor-gearbox and found out it's a custom designed unit for this conveyor. It appears my only option is to replace the motor-gearbox and design a custom mount for it. I may not have the time for this.
If anyone has any ideas for speed control of the original motor please post it.
Thanks

Ward Holloway, Jr, PE

RE: Single phase amp reading.

Can the motor and gearbox be separated?  If so, the motor is almost certainly a standard frame size. If not, you may be able to buy a three phase "Frameless" replacement.

RE: Single phase amp reading.

Being really small, as conveyors go, maybe you can just get a large stepper motor or servo motor and some inexpensive off-the-self controller with a pot adjustable.

Strap the direction and feed in any speed pulse you want.

http://www.geckodrive.com/

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Single phase amp reading.

Use a PM DC motor? This is well within the power requirements for this type of motor and it is easy to vary speed of them.
 

----------------------------------
  Sometimes I only open my mouth to swap feet...

RE: Single phase amp reading.

With a shaded pole motor, reducing the voltage will educe the torque capacity of the motor. If you reduce the torque below the torque required by the load, the motor will begin to slow and will continue to slow until the torque of the load falls below the torque produced by the motor.
With loads such as pumps and fans, the torque increases as the speed increases and so the reduced torque will cross over the load torque at some speed. This is what determines the operating speed.

With the conveyor, the speed torque curve will be very different, however if the torque does increase with speed, then there may be some potential for variable voltage control.
With torque control on an application where the load torque may change with load, the speed will change with load.
Speed stability could then be improved by the addition of some form of speed feedback and PID control, but at the end of the day, it is probably going to be cheaper to fit a three phase motor and three phase speed controller.

Best regards,

Mark Empson
http://www.lmphotonics.com

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