Controlling a hoist with a garage door logic board

[chief201]

I have been reading old posts concerning garage door circuits and think I may have found my answer, but want to verify it with the members. I intend to control a hoist motor with a Liftmaster medium duty commercial garage door operator logic board. I studied the wiring diagrams and made a temporary connection to a 120v power supply. After identifying that the close button needed a photo eye setup to properly work, I solved that problem with a pulse generator. While performing a bench test I discovered that both lead wires to the capacitor for the open and close circuits showed 124v when either button was pressed. I couldn't wrap my head around this until I saw the old posts that seemed to indicate that this may be normal, that both leads will be hot going to the capacitor. Is this correct, and if so, does one lead initiate before the other to determine the direction. Obviously, I'm not very versed in electronics, knowing just enough to get me intro trouble. Once I've determined the board will work for this, I'll deal with the compatibility of the hoist motor, which has two unlabeled capacitors. At this point I'm assuming it works like the operator motor. Thanks for your help.

 

[Diver300]

It sounds like the circuit for an induction motor.

An induction motor won't start if run from a single ac power supply. It needs at least two live supplies, out of phase with each other. "Out of phase" means that the peaks of the alternating voltage occur at different times in the cycle.

The out of phase supplies are often achieved by having one live supply fed with a capacitor in series, known as a "capacitor-run" motor.

To reverse the motor, the other supply is fed with a capacitor in series. It's common to have capacitor permanently connected between the two live supplies, A and B. When power is fed to A, the capacitor takes current to B, changing the phase as it does so, and the motor runs one way. When the feed to A is removed, and a feed to B is connected, the capacitor takes current to A, and the motor runs the other way.

It sounds like you have this circuit. The capacitor won't change the voltage much, and if you don't have the motor, so that all you have is a voltmeter, there will not be any change of voltage, so you will read the same voltage on the output terminals for both directions.

I think that you can just connect the the motor the two output terminals (and the neutral connection as well) and you will have your reversing motor.

If the motor that you are trying to run only has two terminals, it's not the simple induction motor and that won't work, and you'll have to post some photos or more details if you want an answer.

 

[chief201]

Thanks. It sounds like it will work. I can't find a wiring diagram for this particular hoist configuration, but it's a Pittsburgh 2,000lb with two capacitors and an emergency stop button in the pendant remote. I haven't had a chance to trace the wires, but all indications are that all these motors use three wires. If my reasoning is correct, I should be able to connect the board's two output leads to the two capacitors and connect the board neutral to the motor neutral. The board, with the addition of limit switches, will then control the up, down, and stop features with a single momentary push of a button, I hope.

 

[rjenkinsgb]

The garage door motor probably takes one amp or so.

The 2000lb hoist need 14A running, according to the manual, and the start current will be rather higher than that.
The relays and wiring in the garage door unit will not stand that much current.

You would need to add extra relays to stand the motor current, either in place of the existing ones or use those to operate additional ones.

This is the basic principle of a small reversing AC motor as in a typical basic garage door unit; the "switch" would be two separate relay contacts for forward and reverse.
When running either way, one winding is directly connected to power and the other is fed through the capacitor, which causes a phase shift and defines the direction:

And this is a diagram for a smaller Pittsburgh hoist, probably quite similar to yours:


Although it is drawn very differently, the actual wiring is the same. The capacitor is permanently across the top and bottom motor wires, the centre wire is neutral and the live/hot is connected to either the top or bottom motor wire via the control switch.

Though yours may have contactors or power relays in the motor junction box, rather than the full power going through the pendant switches. If it does, you could use the garage door controller relay contacts to emulate the pendant button contacts.

And the manual for the 200lb one, for reference and electrical power specifications:

https://manuals.harborfreight.com/manuals/62000-62999/62770.pdf

Having said all that, I'd highly recommend using a more suitable remote control - only having alternate directions & no guaranteed "Stop" on a hoist that may carry enough weight to crush someone is downright dangerous & may breach safety laws.

If anyone could ever get near enough what it lifting be at any possible risk, it would need an emergency stop system with duplicated circuits to be legal in the UK / Europe; it would be in the higher risk category there is.

 

[chief201]

If I am reading this control board correctly, it appears that it already has a 30A relay for the open (lift) circuit and a 10A relay for close. Possibly the 10A relay assumes a no-load start and run condition. I would think the hoist would be similar, but I'll check the no-load amps prior to connecting. The capacity of the hoist is not an indication of the proposed loads. My platform and carriage assembly weighs about 50lbs. and because of its low profile design, 250lbs. positioned in the center of the 3'x3' platform is about as much load that I'm comfortable with. It's original intent was to lift construction material to a second floor deck of my house for a renovation project and became so handy that I decided to upgrade and make it permanent. And yes, I'm sure my wife and I will occasionally ride it, as well. For safety's sake, there is an emergency power shut off switch that will not not run through the control board, and I've been mulling over ways to incorporate an automatic braking system that would engage should the carriage start to descend to quickly.

 

[rjenkinsgb]

Relays and contactors have different capabilities based on the type of load being switched.
Thermal rating - how much current they can stand with the contacts close.
AC1 rating - for switching resistive loads such as heating.
AC3 rating - for switching inductive loads and specifically motors.

Looking at the big relay, it's rated 20 / 30 A resistive load; ie. what it can stand with the contacts closed.

It also has a rating for when switching motor loads (AC3 class) and that is 2HP; 1500 watts, at 250V, so about 6 - 7A rating for a motor.
The smaller relays do not have an AC3 figure given.

However, (if I've found the correct Chamberlain manual) the garage door motor is only 400W and the PCB tracks & wiring connecting the relay power circuits are likely to be sized based on the expected current load for that motor; the tracks are not particularly large.


An induction motor typically takes around 5 - 6 times its rated current for a short time as it starts and accelerates to working speed.
It will also give a significant inductive "kick", a high voltage spike that causes contact arcing, if the contacts open while current is flowing. (The board has R-C snubbers across the two sets of motor windings to damp the voltage spikes.)

That means motor control relays/contactors generally seem over-sized in well designed equipment, so they can cope with the current surges and spikes without problems.


I'd use the garage door board to switch a couple of properly rated contactors, a "reversing pair" setup fitted with electrical and mechanical interlocks, plus a thermal overload to protect the motor if something jams.

eg. Something like a pair of these:

LC1D18F7 | Schneider Electric LC1D Series Contactor, 110 V ac Coil, 3-Pole, 18 A, 7.5 kW, 3NO, 690 V ac | RS

uk.rs-online.com

fitted with an interlock kit:

LAD9V2 | Schneider Electric TeSys Mechanical Interlock for use with LC1 Series | RS

uk.rs-online.com

And the coil for each fed via the auxiliary NC contact on the other.

Add a thermal-magnetic overload set to suit the motor power.

I'd also strongly recommend you read through this:

https://www.motorcontrolwarehouse.co.uk/media/pdf/E-Stop%20safety%20measures%20-%20MCW.pdf

I originally thought you were referring to something like an engineering / workshop hoist.

Now you have said people may be travelling on the hoist system, you are likely in to some very serious legal consequences and liabilities if you do not take "all possible precautions" for the safety and reliability of both the control system and the mechanical systems involved.
It is definitely a SIL Category 4 system, as regards safety requirements.

In some parts of the world it may be a criminal offence simple to allow anyone to ride on such a system, if it does not have formal safety approval and certifications.