Single phase motor

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AEgoddard

Joined May 9, 2022
11
Hi all, I'm in need of some advice please. I've got a single phase motor that just sits and hums when switched on but runs when given a helping hand. I replaced the start capacitor which had gone open circuit but it still does the same thing. So next thing to check is the starter winding but am a little confused as to what terminals. If anyone could assist I would greatly appreciate it as this is at work. I shall upload a couple of pictures of the terminal box and wiring diagram. Thanks is advance.

Arthur
 

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Ramussons

Joined May 3, 2013
1,278
From the wiring diagram, Blk - Red and Brown-White are the Main coils. If we consider the phasing, Blk and White are the corresponding ends of the 2 coils.
For low voltage, they are in parallel Blk-Red || Wht-Brn, For high voltage Blk-Red <-> Wht-Brn.
Now, where is that Capacitor connected?
 

MaxHeadRoom

Joined Jul 18, 2013
25,469
What happens if you give the shaft a spin when simultaneously applying power?
If it runs OK then it confirms no start cap in circuit.
 

shortbus

Joined Sep 30, 2009
9,444
What happens if you give the shaft a spin when simultaneously applying power?
If it runs OK then it confirms no start cap in circuit.
Or a bad centrifugal switch. One that is in an open configuration when motor is not rotating. I just had to put a new motor on one of my belt sanders suffering from the same condition.
 

shortbus

Joined Sep 30, 2009
9,444
So what provides the initial phase shift? ;)
I'm not debating that. If you don't have a NC centrifugal switch the will be no phase shift. The TS stated he replaced the cap with a new one and still no start. The same thing I did, when I pulled the endbell off one of the switch contacts was totally missing. by turning the shaft fast it would start but not using phase shift from the cap.
 

MisterBill2

Joined Jan 23, 2018
12,440
So what provides the initial phase shift? ;)
The phase shift for a non-capacitor induction motor is caused by the higher impedance of the start winding. Or it might be lower impedance. At any rate it provides enough phase shift to start the motor spinning. Capacitor motors have more starting torque and so they can use a smaller (cheaper) motor for the same application most of the time. And the capacitor scheme is less expensive.
 

crutschow

Joined Mar 14, 2008
30,127
If the centrifugal start switch has failed, you could kludge a fix by adding a momentary toggle switch in it's place.

To get fancier, you could use a 3-position ON-OFF-Momentary_ON DPDT switch.
You press the Momentary-ON position to connect both the motor power and the capacitor contact.
After it starts the normal ON then connects only the motor.
 
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MaxHeadRoom

Joined Jul 18, 2013
25,469
Hi all, I'm in need of some advice please. I've got a single phase motor that just sits and hums when switched on but runs when given a helping hand.
An obvious cap problem is if you are able to spin it in either direction at start and it runs in equally as well either way.
 
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