[TheForge] Electric Motor Q?

Sun Jul 21 22:21:00 2002

A capacitor start induction motor has the start winding only in circuit only
at start-up. The phase of the start winding with respect to the run winding
kicks the motor one direction or the other. The motor as shown in the
diagram uses a starting relay. This is most common to sealed motors such as
refrigerator compressors and it works by sensing the current in the run
winding. the start relay is normally open circuit. When power is first
applied, the motor is not spinning and the current is higher than the normal
run current. This pulls in the start relay and closes the contacts applying
power to the start winding. Other motors use a centrifugal switch mounted on
the armature. That switch is closed below about 500 rpm and open above that.

I routinely test a motor that won't start by wrapping a cord around a
pulley, power the motor and give the cord a yank. Not for the faint of heart
as it can be dangerous. If the motor comes up to speed after a goodly yank,
the starting circuit is defective. Usually it is the starting relay or
centrifugal switch, occasionally the capacitor. With the starting circuit
disconnected, an induction motor can be 'kick started' in either direction.

The diagram for reversing was a new one for me. To make it run in reverse,
and inductor (secondary of a transformer) was switched into the start
winding in place of the starting cap. (yes the voltage on the unconnected
primary would go  to about 600v)  A capacitor will cause the current to lead
the voltage while and inductor will cause the current to lag the voltage.
The change in phase angle in the start winding will cause the motor to go
forward or reverse. The way I have seem this most commonly done though is to
break out the start winding completely. Motors that are made to be reversed
use only a starting capacitor in the start winding but the polarity of the
start winding is reversed. These motors have jumper straps which select the
way the start winding is connected and therefore the direction it will start
in.

For breaking any induction motor, disconnect the line and apply a low
voltage DC to the main winding. Turns it into a hysteresis brake.

Three phase motors are something else again. No starting circuit or
capacitors. The motor can be reversed on the fly by swapping two of the
three input phases. This will cause a very high current surge if the motor
is reversed while running and for most three phase motors full voltage 'plug
reversing' is not recommended.

Oppie
Electrical Engineer but still can't forge weld
----------------
----- Original Message -----
From: "Adam Whiteson" <[email protected]>
To: <[email protected]>
Sent: Sunday, July 21, 2002 8:38 PM
Subject: [TheForge] Electric Motor Q?

| I came across this interesting article (there are a number of interesting
| items at this site if you haven't already discovered it)
|
| "http://www.homemetalshopclub.org/news/sep01/sep01.html#Featured Article"
|
| (note: the URL includes the final word "Article")
|
| Anyway it's about reversing one phase induction motors and dynamic
| braking.  If you read the article, then my question concerns fig 2 about
| which he says that you can't reverse the motor while spinning because the
| start relay stays open.  Why not just  wire it so that is so that the
| reverse switch applies power directly to the terminal of the start
inductor
| ?   You would still have to reroute the start relay since, when the motor
| slows it will kick in and if its not wired properly it will apply power
| directly to the starting cap, bypassing the inductor. But this doesn't
seem
| hard to do.
|
| Another issue is that of running 110v through the secondary of the
| transformer to get the needed start inductance, won't this generate a 600v
| potential on the terminals of the primary?
|
| Adam
|
| _______________________________________________
| http://mailman.qth.net/mailman/listinfo/theforge
| theforge mail list group photo site is
| http://www.photoaccess.com
| Login:  [email protected]
| password:  anvil
| ___________
|
|