replacing 3 phase motor

[kiko]

people ...
quesion .....
there was a car lift , that is old ...and it was 3 phase driving a hydraulic pump to move up..gravity down..

this motor was a 1.5Kw motor 3 phase..

the lift was moved to a domestic garage , so no 3 phase supply in house ..
the motor was replaced with a 1.5Kw single phase motor .

is this the same thing ?

it seems to struggle ....
lifts the plate ok empty .
with a small car it struggles and will only lift if the hydraulics are reduced and it will lift it slowly
with a larger car ,4 x 4 , no chance ....i currents like 30 and 40 amps flicking about ...

is this a hydraulic or a electrical power problem ......??
any useful ideas to help me solve the problem ....?

thank you for you time and attention ...


Kiko

 

[plugsandsparks]

You will need a motor with the same torque characteristics and speed to make it work the same, KW is only one element

 

[kiko]

yes , speed i was thinking of .. but torque not sure how to match that up ....
kinda confused , thinking 1.5kw was 1.5kw ....but i see not ...
ill get some info off the original 3 phase info plate and the new single phase one to match it up ...
and post again here ...
or anything else you might need .........

 

[Lucien Nunes]

If the power and speed are the same, the running torque will be the same (power = speed x torque). But the starting torque of a single phase motor might be nowhere near as good as a 3-phase, depending on what type of starting it uses. Lifting a dead weight puts the full load on the motor at starting and calls for excellent starting torque, so you can't assume it's ok to replace the 3-phase motor with any single-phase motor of the same power.

If it's taking 40A, something is wrong, but it could be all sorts - the supply voltage drop is too great, the motor is not running up to speed because it's unsuitable for the starting duty, or the motor is faulty. Was the motor new or known to be good when installed? Has it been operated without correct overload protection? (I wonder how it managed to draw 40A...)

 

[kiko]

the motor was new ..no problem there .. about the speed ...i will have to check it out

the motor is only rated for about 8 amps anyway...I was thinking that it might be a speed issue here ..possibly ..
this machine belongs to a friend of mine .. so i will have to go and check this weekend...

thing is that the hydraulic flow can be regulated through a valve ... controlled by a lever
it would make the table rise faster or slower ,,,by pressing more .. or less
...so when it was lifting a small car , i could pull the lever back to restrict the flow ... based on an ammeter i was watching and so lift the car ..
full on it would not have done it , so there is a hydraulic input as well .. possibly based on flow and internal diameter of the hose supplying the fluid .. . the bigger the better .. i think ...

that was my first confusion , if it was an electrical or hydraulic problem ... or a bit of both ...


i'll get more info to aid the thought process ............

already , i got a few ideas .. so thanks to alll

 

[alanmcm]

Check the amount of poles the motor has. If the old motor was a 1.5kw 2 pole motor and you've installed a single phase 1.5kw 4 pole motor then you've got a major speed difference. Check the old plate and get a single phase motor to match.

 

[Lucien Nunes]

But if it was 4-pole instead of 2-pole it would run at half the speed and have double the torque, so it would lift slowly but the weight of car would make less difference than before and it wouldn't go into overcurrent. Possibly the other way round though.

 

[kiko]

More info
the original motor was a

3 phase
six pole motor linked in Delta
1.5 Kw
2840 r.p.m.


the new motor
single phase
1.5kw
2810 r.p.m.

they're both kinda similar, but very different ...the small difference in rpm would not be the problem
the fact the the motor is wired in delta might be ...

i just don't know ,, but i see that there is no way that it will lift a car with this single phase motor
it could be a power thing if the single phase motor is rated at 8.5 amps and the 3 phase at 6.5..(per phase i guess) that would be a difference .. and yet both 1.5 Kw...

more thoughts??

 

[Marvo]

It's possible that other electrical components on the lift are 380v and aren't operating on 230v such as safety dead-lock solenoid or electric hydraulic bypass valve/ check valve etc. My advice is to call in someone who knows these systems intimately. There's no room for mistakes with this type of equipment, if something fails someone could get killed.

 

[kiko]

no ,, although i see your point ..and I thank you for your input , there are no other things in circuit . ...


at the moment the limit switches etc, are not in place ...the safety aspect of it will be sorted later ,,
we just want it to go up with a car on it first .

 

[marconi]

More info
the original motor was a

3 phase
six pole motor linked in Delta
1.5 Kw
2840 r.p.m.


the new motor
single phase
1.5kw
2810 r.p.m.

they're both kinda similar, but very different ...the small difference in rpm would not be the problem
the fact the the motor is wired in delta might be ...

i just don't know ,, but i see that there is no way that it will lift a car with this single phase motor
it could be a power thing if the single phase motor is rated at 8.5 amps and the 3 phase at 6.5..(per phase i guess) that would be a difference .. and yet both 1.5 Kw...

more thoughts??

The synchronous speed of an unloaded induction motor is given by the formula w = 120 x f /p where w is rotational speed in rpm, f is supply frequency and p is number of poles. Thus a 6 pole motor running off a 50 Hz supply has a speed of 120 x 50/6 = 1000rpm.

It is curious therefore why you have quoted a speed of 2840rpm for the original 3 phase motor.

You have fitted a 2 pole single phase induction motor which has a synchronous speed of 120 x 50/2 = 3000rpm.

When loaded the rotor of an induction motors 'slips' with respect to the synchronous magnetic field and thus turns at a slightly smaller angular speed.

I suspect you have swapped the motor incorrectly for a 2 pole instead of the required 6 pole machine. A 2 pole motor will develop only a third of the mechanical torque as a 6 pole for the same electrical power input of 1.5kW since power = torque x angular velocity. This would explain why the motor is struggling.

In addition, you are running the hydraulic pump at three times its correct rate so I doubt it is performing as it is designed to do.

 

[Lucien Nunes]

I doubt very much that the original motor was 6-pole and would go by the RPM rating that it was 2-pole. 6-pole motors are bigger, heavier, more expensive and offer no advantage when driving a hydraulic pump. Perhaps the OP meant 6-lead?

I cannot recommend operating the lift with any safety devices and interlocks defeated, so I am unwilling to advise further.

 

[Darkwood]

no ,, although i see your point ..and I thank you for your input , there are no other things in circuit . ...


at the moment the limit switches etc, are not in place ...the safety aspect of it will be sorted later ,,
we just want it to go up with a car on it first .

Firstly, you are making alterations to a car ramp, in doing so you are required to meet all current regulations regarding this equipment under the EU LV directive, been an old ramp it probably falls far short of current requirements.
You have also shown your hand that you struggle to understand the basics of motors, this is not a crime in itself, we all learn somewhere but it worries me your attempting to do work without some of the basic knowledge expected here.
Regarding the quoted post - I feel your blasa attitude to the control and safety limits very worrying too and a few members here have also expressed their concern, you are messing about with equipment which IMHO should only be worked on by a trained ramp engineer knowledged in the relevent BS60204 standards for machinery control and the relevent regulations that cover both the hydraulics and and mechanical side of this ramp.

On that note, and for your safety I am closing the thread as it is becoming clear the best advice has already be given and cannot be expanded on and that is to get someone in competent to work on ramps.