Hello:

I have a question with respect to a motor belonging to an old coffee grinder I am currently working on.

It's a single phase 220v/1450 RPM 1/2 or 3/4 HP (?) motor with a squirrel cage rotor, main and auxiliary windings and a start capacitor, originally using a centrifuge switch for the the auxiliary winding.

I have measured the current with the motor running idle with no load and the value obtained was 2.50 amperes.

I disconnected the centrifuge switch and started the motor by manually connecting the auxiliary winding/capacitor for a just a second once powered on to be able to measure the current just before the centrifuge disconnects (would this be the peak starting current?) and the value measured was 10.0 amperes.

My specific question to the forum is about the starting mechanism, which I'd like to replace (it's broken and no replacement parts are available) but I'm at odds as to what to use.

I have come across two options:

1. an amperometric relay
2. a voltimetric (sp?) relay

I'd be very obliged to receive any input with regard to this matter.
Thanks in advance.


CIV

You cannot do it with a current relay (amperometric?), the load current may vary to much. What you may be able to do is use what is called a "potential relay" which is probably what you were trying to describe as voltimetric. These are commonly used in 1 phase refrigeration or submersible pump motors where the environment makes mechanical switches unreliable. Do a search on that term, but here is an example.

http://www.grainger.com/Grainger/wwg/itemD...emId=1611630458

Hello JRAEF:

Thanks a lot for the quick reply.
Much appreciated.

> You cannot do it with a current relay (amperometric?) ...
Yes, I think this would be what I know as an amperometric relay.
It's a sealed coil that has to be placed in a vertical position for it to work properly.
Starting current brings up the relay contacts which connect the auxiliary winding/capacitor and drops by gravity to disconnect as the motor spins up and current drops.

> ... the load current may vary to much.
Would this happen if not starting it under any load?
The problem is that these relays require that their specs be matched to the motor's start and run currents as there are minimum 'pick up' and maximum 'drop out' currents for each relay model.

> What you may be able to do is use what is called a "potential relay" ... ... trying to describe as
> voltimetric.
Yes.
I think this would be what I know as a voltimetric relay.

> These are commonly used in 1 phase refrigeration or submersible pump motors ...
I have come across PTC type relays (solid state, no moving parts) for AA units but they have a requisite with respect to having to cool down (minimum 60s according to spec) before the motor can be stopped and started again.

> Do a search on that term, but here is an example.
Thank you very much for the input.

Best regards,

CIV

Hello:

Another question if I may.

Would it be possible to start the motor with just a switch?

Something like a DPST push button switch to power the unit (it's 220v) coupled to a SPST momentary switch to power the start winding for just a pulse, working both at the same time and on the same axis.

Or maybe a DPST relay actuated from a DPST push button which would latch the 220v power to the unit with one pole and connect the start winding for just a pulse at the same time. It would have to withstand the start current though.

Is this too far fetched or just impractical because of cost?


Thanks in advance for any comments.

Regards,

CIV

Probably possible, but I am no expert on 1 phase motor design. There might be a consequence of leaving the start capacitor in the circuit too long.

You could use a timer to disconnect the start winding after a short period of time.
Measure the time taken for the motor to reach full speed and set the timer to that time.

Best regards,

Hello:

Thanks for the replies.
Much appreciated.

I've thought of using a timer but I found two things to deal with:

1. I'd have to add a relay anyway to cope with the 10A starting current
2. I recall the spin up time (with the old centrifuge switch) seemed to be about 1.5 second or less. I don't know of a timer that would work for 1 second and/or fractions.

I do remember a professional kitchen appliance (ham/meat/cheese slicer, like in a deli) that used a knob that worked ina way similar to what I described in my previous post: a permanent switch for mains and a spring loaded momentary switch for the start winding, both on the same axis with a (mechanical) block to disble repeating the momentary manouver unless you turned it all the way off first.

But I have not found one of these.

Thanks for the input.
Best regards,

CIV

Hello Sawbona,
I am a new member in this forum.
My suggestion is that you should not use any switch for manually starting the motor. Using of Timer is a good idea. There are many timers available in market with minimum time base (0.1sec - 2sec). You should select a timer in which your desired preset time no greater than 60-70% of total time of timer (time base). This will provide you excellent break of winding supply. I have an experience that if you select the target time (preset time) of timer less than 5-10% of time base, the chance of error increases +/-15%. You should also use a 15A relay for making/breaking of winding supply because the relay contact of timer can't meet your requirement
Sorry for poor English.

Hello:

Thanks for your reply and the timer data, a very interesting tidbit to know about.

> ... not use any switch for manually starting the motor.
A few Berkel old models (deli slicing machine, German or Dutch made) had such a switch and apparently it worked well enough, but you are right and I agree, it is a risk and you could lose a motor's windings.

> Using of Timer is a good idea.
I'll look into it.

> Sorry for poor English.
It's good enough for me to understand you perfectly well.
Thanks for the effort.

Best regards,

CIV

Hello:

I have been able to solve the problem described in my original post by means of a current relay similar to the ones used in AC compressors (it's actually a replacement part). It 'picks up' at about 7.2A and 'drops out' at about 6.0A.

I have been testing it for a while and it seems to be working quite well.

I have placed a 120º C thermoelectric switch (similar to a Klixon 7AM031) for protection of the main winding from overheating but I still have to fuse the motor itself for overload protection of the auxiliary winding in case the relay ever fails and gets stuck in the 'up' position or the rotor locks up for any reason.

My guess is that it would have to be a 250v 'slow blow' type fuse but I am at odds as to whether the rating should be 10A or higher (the start up current measured in at 10A).

I'd be very obliged to hear from the forum in this respect.


Thanks in advance,


CIV