AC motor question?

Thread Starter

boydage

Joined Oct 7, 2016
82
I am a EST in NZ and have just got my hands on a motor that is unusual in the fact it is a 1 phase motor with a cap but no centrifugal switch to take it offline.

I believe its a 2.2kw or so I was told. Main winding is 2.2 Ohms and Start Winding is 5.5 Ohms the Cap that came with it was grabbed of another motor.

It runs, and starts fine. I have only bench run it for less than a few minutes as there is no centrifugal switch (the cap came off a switched start winding) and I am reluctant to burn anything out.

I have absolutely no idea what size capacitor to buy for this motor and have no idea how to work it out. Especially in the fact it must be a permanent run type. The motor definitely needs it, it wont rotate without the Cap unless I help it along. Anyone have an idea on what I could use on this motor that wont burn out?
 

MaxHeadRoom

Joined Jul 18, 2013
23,634
For a 2.2 Kw motor, It could have had a an external current relay to operate the start winding.
PCM or Permanent Cap Motors usually are the smaller (1/2hp) variety and the winding are the same.
Test to see if it runs without the start cap once up to speed, do this by disconnecting the cap, once the motor is up to speed and see it it retains RPM and torque.
What is the size of the cap you are using, electrolytic start caps are only intended to be in circuit for a few seconds, the smaller value start/run caps are oil filled paper rather than electrolytic such as start only caps and are often left in circuit during run.
 

Thread Starter

boydage

Joined Oct 7, 2016
82
Hi. Thanks. The one it starts with that I have on it now is a 250v 250MFD but like I say I have only run it with the cap connected for a short time just to see it start and run. I was concerned it might burn the cap out.

I have run it without the cap connected, infact I got it spinning with a finger twist. But I am unsure if it has full torque or not, I will put the load on it and then run it, and disconnect the cap. Will measure the current too.

So I worked out the internal wiring. There is a white wire that comes out of the motor windings, unterminated on the block that I thought was there for reversing. Perhaps its for something else.

All of the motors in this workshop are out of woodworking machines >1.5kw. Most of them are fine, just missing a start Cap. All of the other motors have a centrifugal switch. Except for this one. I have never ever seen one with any form of external relay, infact I have never seen an external start relay before ever lol. So I am unsure what to do here.

Hey is it possible that a motor might have an external start relay put inside the starter box? Thoughts? I would like to get it going its the only one that will fit this machine. Willing to build a disconnect or put one in.
 

Attachments

MaxHeadRoom

Joined Jul 18, 2013
23,634
That size of cap sounds like a start only cap, it is likely to blow if left in circuit.
If that in fact is the correct cap, then you need an external current/start relay.
 

Thread Starter

boydage

Joined Oct 7, 2016
82
Oh its about 5% chance its the original cap. Ok. I will look at either a mechanical centrifugal switch (I have a few motors) or a relay of sorts. I guess the start winding only needs to be online for a second max huh?

Good to talk this one out mate thanks
 

MaxHeadRoom

Joined Jul 18, 2013
23,634
In some cases the start cap is kept in circuit, but value is usually 15μf or less, and oil filled paper, at least none-electrolytic.
 

Thread Starter

boydage

Joined Oct 7, 2016
82
Hey Max,

So tonight I ran it with the fan attached. It draws 15A - seems quite high but I also dont know my clamp is very accurate. I do know the rotor and stators are not original matched but hey, it runs, and has lots of torque. I do know its a 2.2kw I wonder if it will last TBH.

Question is, if you look at the drawing I made of the windings do you see how to reverse the direction? Like, the five wires disappear into the windings. The white is common with the neutral. B2 is common with the phase. and I get the resistances shown in my drawing, including 7.0 ohms between phase and B1. But without picking the input windings out I cant see the easy way to reverse it. Surely the white wire is how its done, I just cant see it to save myself. I know I only need to reverse one winding.

What am I missing here?
 

MaxHeadRoom

Joined Jul 18, 2013
23,634
If you want to reverse permanently, then if the bell housings each end are identical, physically dismantle,, reverse the stator end for end, and reassemble.
 

Thread Starter

boydage

Joined Oct 7, 2016
82
Hi thanks for that. This is a weird type of motor with one end flat so reversing the stator would be tough. I think I will split out the windings and reverse one of them where they join on the common. Good though it runs
 
Top