Single phase amp reading. 1

[WardHollowayPE]

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

 

[aolalde]

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.

 

[sreid]

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

 

[BobM3]

You are talking about a DC motor, right?

 

[WardHollowayPE]

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

 

[Marke]

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

 

[WardHollowayPE]

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

 

[itsmoked]

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

 

[stevenal]

http://www.hitachi.us/Apps/hitachic.../hitachi/forbus/powerequipmentsystems/&nId=iD

 

[bklauba]

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

 

[waross]

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

 

[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. 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.

 

[stevenal]

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.

 

[LionelHutz]

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.

 

[sreid]

An easy place to purchase VFD's and motors is Automation Direct.

http://web1.automationdirect.com/adc/Home/Home

Look at AC Drives and AC Motors.

 

[stevenal]

LionelHutz,

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.

 

[CJCPE]

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.

 

[itsmoked]

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

 

[Skogsgurra]

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

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[WardHollowayPE]

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