Single phase motor automatic reversing circuit

Thread Starter

Mathieu

Joined Nov 1, 2008
1
may anyone help me with a circuit to reverse the reverse the direction after every 5 seconds. I'm doing a research on project that will use a motor and i want it to work like a washing machine motor.
 

jpanhalt

Joined Jan 18, 2008
7,699
If you want to reverse an electric motor while it is running, use a DC or 3-phase motor. Most reversible single-phase motors will simply keep running in the same direction, if you try reversing them while they are running. In the process, they may make a funny noise and you may burn out something, but they will not instantly reverse directions.

John
 

scubasteve_911

Joined Dec 27, 2007
1,202
If you want to design this properly, you need to create a dynamic model of your system. You can then simulate your system's response in Matlab.

For a PMDC motor,

KVL for Electrical Equation

Va(t) = Ra*Ia(t) + L(dIa(t)/dt) + w(t)*Kv

Va = voltage armature
Ia = armature current
L = armature inductance
Ra = armature resistance
Kv = voltage constant for motor, V/speed
w(t) = speed in radians/second

Mechanical Equation

dw(t)/dt = (Kt*Ia(t))/J -(D*w(t))/J - TL(t)/J

J - inertia load
Kt - torque constant N/A
TL - torque load
D - dynamic coefficient of resistance

You can take the laplace transform, then apply various functions of Va, ie step, then inverse to determine time domain response.

Steve
 

awright

Joined Jul 5, 2006
90
Speaking in broad generalities, a common appliance single phase motor has no preferred direction of rotation during running. It runs in the direction intended by the manufacturer of the machine because it is started in the preferred direction by the start winding (with or without a start capacitor, depending upon the motor type). In the absence of a functioning start winding and switch, the motor would simply sit there without rotating after application of power, humming loudly due to the very high "locked rotor" current until it burned out after a few seconds.

In fact, you can start a motor with a burned out start winding by getting it rotating in the desired direction by pulling on a rope wrapped around the shaft just prior to applying power. Interesting, but definitely not safe, not practical with most loads, and not recommended. (I smashed the back of my hand into my drill press table while doing this trick a long time ago and never repeated it.)

The start winding is switched out of the circuit by the centrifugal switch (or sometimes a current sensing relay) a fraction of a second after power is applied and the motor has come up to the speed at which the transformer action of the stator field acting on the shorted turns embedded in the rotor (in the form of the cast aluminum conductors, often with cooling fins on the ends) creates magnetic flux in the rotor to provide torque. The position of the magnetic flux in the rotor must be shifted away from direct alignment with the stator field for torque to be generated, and this shift in position is dependent upon the rotation of the rotor.

Because the start winding is out of the circuit above a fraction of running speed, there is nothing you can do to force the motor to reverse except slow the motor down enough for the centrifugal switch or current relay to reconnect the start winding (this is the click you hear as a motor coasts down after power is disconnected). Then you would have to reverse the polarity of the start winding to get the motor running in the opposite direction.

This trick would draw more than normal starting current for longer than normal starting time and, as others have observed, overheat the motor, possibly to the point of failure if you did it repeatedly.

Many other motor types CAN be reversed while rotating but it must be done with knowledge of the motor operational limitations and thermal characteristics. Motors used in many modern high efficiency washing machines are not conventional single phase motors but are, rather, multi-pole "switched reluctance" motors that must be used with a matched active driver/control circuit. They are capable of bi-directional operation over wide speed and torque ranges and might be more suited to your project. However, getting data on the control methodology might be daunting.

Back to the drawing board...

awright
 

scubasteve_911

Joined Dec 27, 2007
1,202
Forgot to mention, from your simulations, you will be able to calculate average power into the motor. If this is equal or less than the operating power, you should be okay.

steve
 

leftyretro

Joined Nov 25, 2008
394
I don't know why people ask questions and don't follow up...
It could be because they are overwhelmed by all the answers and were assuming that their simple question would have a simple answer. Reality can be a bit nasty sometimes. ;)
 
Last edited by a moderator:

Pich

Joined Mar 11, 2008
119
A small sinlge phase AC induction motor can be reversed by bringing it to a stop and simply reversing the start winding. Fractional and small HP < 10HP 3 phase AC motors can be reversed on the fly by simply swapping any 2 leads using special reversing contactors (interlocked).
 

nagaloo

Joined Jan 27, 2009
29
There are instant reversing single phase motors which either use a special starting switch or relay/electronic device. But they would never be rated to reverse every 5 seconds. Typically they are on gate/door openers. I have seen specially made washer motors that use a 3 ph and universal/brush motor all in one case. But I think there is 1ph to 3ph vector/PWM drive for the wash cycle.
Stew
 

Bernard

Joined Aug 7, 2008
5,228
I remember such a motor, reversed about every 2 sec, guess about 1/8 HP; was used to sweep a radio direction finder ant. One end of shaft had a magnetic brake,reversing was by DPDT relay, with some controls for timing [1944 USN ].
 

Bernard

Joined Aug 7, 2008
5,228
' Forgot to add ref back to Pich, page 10. brake works equally well on AC motors.
 
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