Why Does My Single Phase Motor Hoist Fail to Lift Loads Consistently?

[ashokeez]

I have used a single phase motor (220 V 50 hz) in a hoist application. It is used to lift a given load up & down, but being a single phase motor, it sometimes never lifts the load & starts to rotate in single direction only i.e never lifts the load. When you press UP button, it rotates downwards, same for DOWN button.

The thing is, the motor takes the easiest path and never lifts the load, while UP button is pressed, but instead rotates in opposite direction. Its a brake motor, whenever brake is released, the load/weight pulls down & motor tries to rotate in the same path. This is not happening always, since sometimes the motor is capable of lifting the load upwards.

Please guide me to come out of this problem.

thanx in advance.

 

[xez]

It seems as if you're using the wrong motor and motion
control system for the job if it's being so unreliable.

In general for hoists, it's very preferable to have a
counterweight of the same or similar weight of the object
being hoisted attached to the same rope as the hoisted
object on the other side of the pulley the load is attached
to.

This way with the counterbalance there's almost no
static net force on the motor driving the hoist, so there's
little or no load generated torque forcing the
motor's rotation when the motor is at rest. Thus the
only job of the motor is to overcome the friction and
inertia of the system to accelerate the motion of the load
and decelerate it when it's desired to stop the motion.

Of course by the use of gears / pulleys or lever / cam
action one can use a heavier or lighter counterweight
than the actual load so long as the overall static force
on the motor shaft is very much lessened by the balancing
forces of the load and the counterweight.

Also if the motor's starting torque is not sufficient to
cause a starting rotation in the desired direction of motion,
it would be beneficial to use techniques like gearing
or higher ratio pulley systems such that the
torque force on the motor's shaft due to the load is
substantially lessened so that the motor's starting torque
shall be sufficient to overcome whatever force / friction /
inertia the load represents to the motor in any desired
direction of motion and starting position.

Of course using a motor with higher starting and sustaining
torque could help matters.

Using motion control devices like flywheels and
variable clutches / transmissions could also help the
motor more reliably drive the load, but the complication
of adding these is usually prohibitive compared to just
doing one of the more straightforward solutions like
using a more suitable motor and pulley / gearing /
counterweighting system.

Good luck.

 

[Danger]

Good response, Xez.
I would add only that if the mechanism itelf can be reworked in such a (drastic?) fashion, the use of a non-reversing system such as a lead-screw would be appropriate for both ease of lifting and saftey (no external braking system needed). Alternatively, this same motor which is giving him trouble could be used to power a hydraulic pump.

 

[ashokeez]

It seems as if you're using the wrong motor and motion
control system for the job if it's being so unreliable.

In general for hoists, it's very preferable to have a
counterweight of the same or similar weight of the object
being hoisted attached to the same rope as the hoisted
object on the other side of the pulley the load is attached
to.

This way with the counterbalance there's almost no
static net force on the motor driving the hoist, so there's
little or no load generated torque forcing the
motor's rotation when the motor is at rest. Thus the
only job of the motor is to overcome the friction and
inertia of the system to accelerate the motion of the load
and decelerate it when it's desired to stop the motion.

Of course by the use of gears / pulleys or lever / cam
action one can use a heavier or lighter counterweight
than the actual load so long as the overall static force
on the motor shaft is very much lessened by the balancing
forces of the load and the counterweight.

Also if the motor's starting torque is not sufficient to
cause a starting rotation in the desired direction of motion,
it would be beneficial to use techniques like gearing
or higher ratio pulley systems such that the
torque force on the motor's shaft due to the load is
substantially lessened so that the motor's starting torque
shall be sufficient to overcome whatever force / friction /
inertia the load represents to the motor in any desired
direction of motion and starting position.

Of course using a motor with higher starting and sustaining
torque could help matters.

Using motion control devices like flywheels and
variable clutches / transmissions could also help the
motor more reliably drive the load, but the complication
of adding these is usually prohibitive compared to just
doing one of the more straightforward solutions like
using a more suitable motor and pulley / gearing /
counterweighting system.

Good luck.

Thanx XEZ for the reply.
Generally hoists/cranes don't have counterweight measures. Also in my case there is a space constraint where i can't employ counterweights. Again the motor is rated 2 times higher than the required capacity. Its a one ton hoist, but suspended load is only 500 kgs (0.5 ton). This was done not to overload the motor and for long life. As you told rightly, i used a high rated motor, so that no problems encountered. Roughly the motor uses only 50% of its capacity. And the hoist is standard one bought in market and used for various applications acros various fields. Therefore I cannot modify/redesign any parts and the hoist is proven in other similar areas. The point to be noted is most time motor functions in desired direction & very well lifts the suspended load. Sometimes it does not & rotates in opposite direction.

Now the issue is, when it's able to run in desired direction, is there any possibility to enforce the motor externally by some method to run in desired condition? Or whenever the motor lifts, can we simulate the same condition, all the time?? so that the motor will not assume its own direction & rotates in forced direction, either ccw or cw?

Any suggestion?? please!

 

[ashokeez]

I agree with you that lead screw mechanism will do good. Actually it was in my earlier design of lead screw driving mecahnism. But any lead screw for that purpose, the system has be guided or channelised. otherwise the lead screw will fracture, when the system is in suspension or floating in the air like a hoist. There should be no disturbances to the lead screw & it should be fixed firmly such that no dynamic radial load acts on it. Good suggetion, Danger!

 

[Danger]

Do you mean that this is a 'gallows' type lift, where the actuator forms one side of an inverted triangle? If so, then I agree with your assessment of a lead-screw's vulnerability. My solution to that would be to rigidly mount the screw mechanism in a vertical channel and use it to drive a mechanical linkage to the lever arm. If the 'follower' is tightly fitted to the channel, there should be almost no side force on the screw.
A couple of other things come to mind. The first would be to add a ratchet mechanism to prevent downward motion when you want it to go up. The second would be to simulate the use of a counterweight by putting a variable-tension booster spring in parallel with the actuator.
(I haven't worked out the details for either one of those yet.)

 

[ashokeez]

The ratchet mechanism will do prevent downward motion but how to release it when you want the load to come down?? I remember a circuit which determines the direction of rotation of single phase motor & forces the motor to rotate in desired direction. I don't have the circuit & partlist details now.
That circuit is slightly advanced than hoist pendent circuit. Hoist pendent will have an "UP" & " DOWN" switch, which will correspond to that direction of rotation, either CW or CCW, which will be triggered by a capacitor.

 

[xez]

Well what specific technology kind of motor is it?
Different motor types (e.g. single phase AC, three
phase AC, DC, capacitor start, shaded pole, et. al.)
have different failure modes and concerns associated
with them.

For capacitor start motors, for instance, it's quite common
to have capacitors that are either improperly installed or
which are somewhat degraded and need replacing.
Since the capacitor specifically controls issues of starting
and initial rotational orientation it could be an item
to consider in the face of such problems as yours.

You mention that your motor is rated for twice the
load weight that you're using, which is a good thing.
I will just mention, though, that the torque available
when starting is different than the torque available
when running at speed, so it's possible to have a motor
which has ample torque when running but not so when
starting. Since you say this motor and hoist mechanism
is commonly used together for that purpose, and is rated
for loads twice as high as your application, I'll assume
that all aspects of the motor capabilities are suited to
your usage (starting/running torque, duty, temperature,
lifetime, et. al.).

It is a common situation with motors to have one or
more windings which may simply be defective due to
broken connections or melted wire or short circuits due
to winding insulation defects.
It's certainly common for an old or abused motor to
develop such troubles, but it's also possible for relatively
new motors to unfortunately have such problems.
Depending on your particular motor's technology
it might be very reasonably expected that symptoms
of a partially or fully failed phase winding would be
exactly the problems that you are experiencing --
inconsistent starting, inconsistent running direction,
lack of available torque/power, et. al.

Other things to verify would be:
* Are you running the motor at its intended
power frequency?

* Are you attaching the motor to the AC power lines and
to its direction control etc. with proper connections
so that there are no missing phases, reversed wires,
broken wires, connections made to terminals that are
incorrect for the power voltage/phase that you're using,
et. al.? Certainly a 240VAC motor winding attached to
a 120VAC circuit, or motors with some reversed phase
connections, et. al. might exhibit problems like you see.

* Is the brake working properly, and when it is supposed
to be disengaged, is the motor permitted to turn freely
in either direction without friction?

* Is the coupling / belt / pulley on the motor shaft
working smoothly and not causing undue friction or
misalignment when operating in any direction with any
attached load?

* Do you smell burning / melting type scents near the
motor when it has been operating for a few operations
over a few minutes?

* Is your input voltage and current / power capacity
within specification? Sometimes having other equipments
drawing much power or connecting through too many
lossy connections / cables can rob the motor of needed
power, and some places just have unreasonably low
power capacities due to utility undercapacity.

* Is the motor properly lubricated?

* Is the hoist connected with the proper arrangements
of cables / pulleys / attachment points? Certainly
accidentally switching pulley tracks or cable attachment
points or some other such mechanical error could result
in improper mechanical gearing of the motor to the load
and thus exceeding the motor's capacity.


I suspect that you'll find that there is either a mechanical
or electrical fault that's been causing your problem;
inspect everything closely, double and triple check even
if your first glance and intuition is that everything is
correct.

And, of course, be safe; having a hoist that's liable
to unexpectedly drop its load or motor that may
run amok isn't a very safe situation, so keep well clear
of potential dangers due to unexpected failures / motions
until you're certain that the safety / operational
problems have all been identified and resolved!

Thanx XEZ for the reply.
Generally hoists/cranes don't have counterweight measures. Also in my case there is a space constraint where i can't employ counterweights. Again the motor is rated 2 times higher than the required capacity. Its a one ton hoist, but suspended load is only 500 kgs (0.5 ton). This was done not to overload the motor and for long life. As you told rightly, i used a high rated motor, so that no problems encountered. Roughly the motor uses only 50% of its capacity. And the hoist is standard one bought in market and used for various applications acros various fields. Therefore I cannot modify/redesign any parts and the hoist is proven in other similar areas. The point to be noted is most time motor functions in desired direction & very well lifts the suspended load. Sometimes it does not & rotates in opposite direction.

Now the issue is, when it's able to run in desired direction, is there any possibility to enforce the motor externally by some method to run in desired condition? Or whenever the motor lifts, can we simulate the same condition, all the time?? so that the motor will not assume its own direction & rotates in forced direction, either ccw or cw?

Any suggestion?? please!