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Changing a motor's direction each time power is supplied

StewartVivonne

Mar 15, 2014
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I hope I can get some help here, I'm a heating engineer with not much electronics savvy, and I need to modify the motors on one of our products.

It's a motor that turns, controlled by a timer. so it turns for 10 seconds every hour. because it always turns in the same direction it sometimes gets itself in knots (it is used for vacuuming up wood pellets in a silo)

So I'd like a relay, a DPDT relay for example that will change the ploarity of the motor each time it is supplied.

It is a 220V motor with 2 coils, one working coil and one starting coil (wired to the condensor)

Any help will be greatly appreciated

Thanks

Stew
 

Fish4Fun

So long, and Thanks for all the Fish!
Aug 27, 2013
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Stew, Welcome to the forum! I understand what you want to do, but there are a few questions that need to be addressed before anything more general that "wire it like a two switch light circuit" can be given as an answer....

Code:
1) At what level do you want to implement the control:  

a) In the logic that controls the motor.....

b) On the  power side of the motor control....

Code:
2) You state that the motor is 220v, and that it
has a starting coil and a run coil, but you 
haven't given quite enough information....
What type of motor are we dealing with:

a) Single Phase 220Vac ...
     if so, do you know for a fact that it is reversible?  
     A large number of single phase AC motors are 
          simply not designed to be reversed. 
b) Three Phase 220/240Vac...
     if so describe the existing control circuit...
c) 220v DC motor....
     while unlikely it is possible, 
     so, again, describe the existing control circuit
d) AC/DC Servo....
     again, while unlikely, describe the existing control circuit

Code:
How big a motor are we talking about?  
     1/2hp, 5hp, 50hp?    
     While the size of the motor may not 
     impact the logic of the design, 
     it could play a large role in how 
     you might actually realize your goal.

Best thing to do is report back with all of the information on the motor's name plate, and give as complete a description as possible of the existing drive control...ie, is it an analog timer/relay...is the controller part of a larger control system...etc, etc.

If the motor is the vacuum motor itself, it is likely a very high RPM motor designed specifically to turn in one direction and may not be reversible...or perhaps it is part of a belt/gear driven system and is a more standard AC motor?????

In any case, a bit more information is requisite....

Fish
 
Last edited:

StewartVivonne

Mar 15, 2014
4
Joined
Mar 15, 2014
Messages
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Thanks for your interest

The motor is 220volt single phase. It can be reversed as I've already tested reversing the cables that supply the motor and it turns in the opposite direction.

It is a 30watt motor

The inversion needs to be done at the motor/condensor side, There's no complicated logic or timing just need it to turn one way when supplied and then the other way the next time.


The motor is on a 'mole' that sits in a silo of wood pellets and turns to pussh the pellets into a vacuum that sucks the pellets off to a hopper next to a boiler.

This mole sometimes turns. It shoudn't, but it does. And because it always turns the same way, after a while it windes the vacuum tube into nots and then ther'es a loss of vacuum pressure and breakdown.

I could put a "tail' on the mole that would activate a mechanical switch if the mole turns. But it would be at risk of mechanical failure, it's a clumsy solution.

I'm currently away from the office at an expo, when I get back I can draw and scan a wiring diagram for the motor and it's connections to the condenser
 

StewartVivonne

Mar 15, 2014
4
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Mar 15, 2014
Messages
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been reading about 'flip flop' circuits and bedin to understand a bit better what you mean by logic
 

Kiwi

Jan 28, 2013
471
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Jan 28, 2013
Messages
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Motor reverser 1.jpg

Hope above circuit could be of use to you. Those 20 Series Finder step relays are great. They mechanically change state, and latch, every time you apply an input to A1 and will stay in that state until they receive another input, even if the power is turned off.

The Limit Switch could be;
1 - bypassed, so that the Finder relay changes state every time power is applied,
2 - a micro switch that works on a cam if the "mole" gets too far off centre.

Good luck.
 

duke37

Jan 9, 2011
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5,364
The motor will have two windings, one with a series capacitor and four wires. One winding will need to be reversed as shown in the diagram but the other winding will need to stay the same.
 

duke37

Jan 9, 2011
5,364
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Jan 9, 2011
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That looks better from a motor point of view. Remember that the start winding will have a series capacitor.

I do not know anything about the impulse relay. but you must ensure that the motor is not 'started' when it is aleady running in the other direction.
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
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I don't know whether this will be useful to you or not, but I designed a circuit to do what you want - to prove to myself that I could.

My priority was low cost. I was aiming for simplicity as well. I'm not sure whether I achieved that, but I can't see any way to simplify it further without affecting reliability.

attachment.php


The circuit is powered directly from the mains and should be considered live. It must be rigidly mounted in a non-conductive enclosure with only the wires accessible.

These wires are marked CN1~8. CN1 and CN2 are the Phase and Neutral inputs to the circuit, and CN3~8 are the six connections to the output relay K2, which control the external equipment.

It provides a DPDT (double-pole double-throw) aka DPCO (double-pole changeover) relay output which changes over to the opposite state each time the circuit is powered up.

The circuit must be powered up for at least 2 seconds on every run, to give the capacitors time to charge up. It must be switched off for at least 60 seconds after each run, to give them time to discharge. If this requirement is not met, there is a possibility that the circuit will not change over properly.


The circuit takes power directly from the AC mains supply using coupling capacitor C1 and diode D1 to generate a positive half-wave rectified but unsmoothed voltage peaking at about 27V DC on the anodes of D2 and D3.

R1 is a fusible resistor that protects D1 and the other circuitry if C1 fails short. C1 must be an X2 rated component, designed for continuous connection across an AC mains supply. R2 discharges C1 when power is disconnected, for safety.

The current circuit state is retained by K1, a DPDT latching relay with two 24V DC coils. This relay retains its state, even without power, and only changes its state when the coil for the opposite state is energised.

Positive voltage peaks at D2 and D3 anodes charge C2 and either C5 or C6, depending on the state of K1. Initially assume K1 is in the position shown in the schematic, the "reset" state, and the current from D3 will charge C4. C5 will not be charged. The voltage on C4 is passed to the "set" coil, and into R6, Q3 and R7.

When mains power is initially applied, C4 charges up in about 0.5 seconds (the charging current is limited by the reactance of C1). C2 also charges up. Once the voltage across C2 reaches about 7V, Q1 conducts and holds C3 discharged. C3 does charge (via R6) briefly at power-up but only to about 0.1V which is not enough to trigger Q2 into conduction.

While power is present, the circuit remains in a stable state, with Q1 conducting and holding C3 discharged, and keeping Q2 and Q3 OFF. C4 charges up to about 27V.

When power is removed, the half-wave-rectified voltage across D1 disappears quickly. C2 discharges through R3, Q1, and R4, and when its voltage has dropped below about 7V, Q1 turns OFF. This allows C3 to be charged up by current from C4 flowing through R6.

After power has been removed for about a second, C3's voltage reaches about 0.7V, and Q2 starts to conduct. Q2 and Q3 are connected as an SCR; once Q2 conducts, Q3 is biased into conduction and supplies more current to Q2's base. Both transistors saturate and Q3's emitter pulls down to about 1V. The 27V across C4 is applied across the 'set' coil of K1 and causes K1 to switch over.

This changeover action in K1 does not affect the supply to the 'set' coil, because that voltage comes from C4 which was charged when power was present previously. C4 discharges into the 'set' coil of K1, flipping it to the 'set' state.

Once the voltage on C4 is too low to sustain conduction in Q2 and Q3, they turn OFF. K1 is now in the opposite state.

When power is next applied, K1 will cause the current from D3 to charge C5 instead of C4. Then when power is removed, the same action will occur, but this time C5 will be charged, and the 'reset' coil of K1 will be activated.

The second set of contacts on K1 control K2, which has a 230V AC coil. K2 drives the external circuit. The specified relay is rated for 8A and 230VAC on its contacts.

When power is applied, if K1 is in the 'set' state and K2 is energised, there will be a short delay before K2's contacts change over. The pull-in time for K2 is not stated on the data sheet, but it should be less than 0.1 seconds. So K2's contacts will actually be in the wrong state for a short time when mains power is applied.
 

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