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.
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.
RE: Single phase amp reading.
RE: Single phase amp reading.
Ward Holloway, Jr, PE
RE: Single phase amp reading.
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.
Ward Holloway, Jr, PE
RE: Single phase amp reading.
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.
RE: Single phase amp reading.
BK
RE: Single phase amp reading.
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.
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.
RE: Single phase amp reading.
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.
http://web1.automationdirect.com/adc/Home/Home
Look at AC Drives and AC Motors.
RE: Single phase amp reading.
Your right, the drives I linked above have three phase output. I offer this link instead: http://www.anaconsystems.com/data/opti_1.pdf
Both the input and output are single phase.
RE: Single phase amp reading.
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.
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.
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.
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
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Single phase amp reading.
Thanks for the everyones help.
Ward Holloway, Jr, PE
RE: Single phase amp reading.
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 don't see these posts. Did they get deleted?
Please change "Your" to "You're" in my previous post.
RE: Single phase amp reading.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Single phase amp reading.
A fan, yes. A conveyor - no. No way.
Why don't people listen...?
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Single phase amp reading.
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.
That must be one tiny little conveyor... moving pimentos into an olive stuffer perhaps?
RE: Single phase amp reading.
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
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Single phase amp reading.
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.
I decided to try the Optidrive or something simular.
Thanks for the help
Ward Holloway, Jr, PE
RE: Single phase amp reading.
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.
RE: Single phase amp reading.
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.
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Sometimes I only open my mouth to swap feet...
RE: Single phase amp reading.
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