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Running a three-phase motor from single phase mains.

Diver300

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A few years ago I wrote this:- https://www.electro-tech-online.com...atic-miller-system.100563/page-7#post-1309053

I now think that using loads of capacitors and a voltage sensitive relay to run a three-phase motor like that is not the best way of doing it.

The lift had got some water in it, leading to condensation in the electrics, and it was tripping the ELCB, so I decided I had to move the control gear. I would need a large box to house the capacitors, so I looked up how much an inverter would cost.

I found that an inverter was only around £80 from Amazon. https://www.amazon.co.uk/gp/product/B07M7NMCVH/

That was a lot less than I paid for the 20 capacitors that I had needed to run the vehicle lift before. When I bought the capacitors, I don't think that I had understood how much inverters had come down in price. The last one that I had bought was around 30 years before, when I paid around £1000 for a < 1 kW inverter.

The inverter is now on the wall and working. I had changed the plastic housing of the control box before I had screwed it to the wall and clipped the cables in place. The contactor in the control box is only used to switch the low voltage control signal to the inverter. The inverter is powered as soon as the control box is switched on, and runs when it gets the control signal.


And here is a car on the lift:-


It has the additional advantage of having a soft start.

Previously, as mentioned here https://www.electro-tech-online.com...atic-miller-system.100563/page-7#post-1333975, the arrangement took lots of current. With the inverter it takes far less. I guess that the power factor is better. I have also slowed it down to 40 Hz instead of 50 Hz, in which brings the current down further, and I'll be changing the isolator to a fused spur in due course.

The house lights no longer dim as the lift starts up. All in all, it's a huge improvement on what I had before. I should have fitted an inverter from the start. There will be few applications where using lots of capacitors is in any way better than using a VFD.

The setup is not perfect. The cheap inverter has a very small but noisy fan that runs whenever it's turned on, whether the motor is running or not, but most of the time that I'm using the lift, it'll be turned off. The inverter also takes a few seconds to turn on.

Before the motor would run, there was a lot of setting up to do, with a very badly translated instruction book, and a terrible user interface. I had to set the normal motor frequency, in this case 50 Hz, in one of the parameters, and that was set to 400 Hz to start with. At that setting, it wouldn't put more than about 25 V on the motor and it didn't run. Initially the ramp rate was set really slow in both accelerating and decelerating, so those had to be changed as well.
 

MaxHeadRoom78

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Looks like one of the Chinese Huanyang, The 400Hz setting is generally used with the 2pole 6.000 to 24,000rpm spindle motors.
 

Diver300

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Looks like one of the Chinese Huanyang, The 400Hz setting is generally used with the 2pole 6.000 to 24,000rpm spindle motors.
It is a Huanyang inverter. I wondered what application would use 400 Hz.
With their popularity, I would have thought that they could afford a proper translation of the instruction book into English.
 
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JimB

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MaxHeadRoom78

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Here is mine, driving the high speed spindle on my milling machine.
JimB
BTW, did you confirm the earth connection on the 4 pin motor socket is connected to the motor frame?
Invariably the connection is missing!!
 

MaxHeadRoom78

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With their popularity, I would have thought that they could afford a proper translation of the instruction book into English.
From what I have gathered, the factory just produces them and uses their sales outlet to market them, but have no specific technical support whatsoever.
English or otherwise.
 

JimB

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BTW, did you confirm the earth connection on the 4 pin motor socket is connected to the motor frame?
A good point Max, I must have checked for continuity on the four pins in order to identify the motor windings.

But...
Invariably the connection is missing!!
You are correct, I have just been out and measured it.

This of course leads me on the the ethical dilemma, do I re-wire the thing to add an earth connection to the fourth pin?

In normal use, the body of the motor is earthed through being metalically attached to the body of the mill, which is earthed, so I have no immediate worries about getting a "belt" off the thing.

Having said that, there are other ways in which the wiring for the high speed spindle could be improved.
But not this afternoon.

JimB
 

MaxHeadRoom78

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Generally for the HS spindles, the custom VFD cable is recommended, not so much for the regular style of 3ph motor.
What i have done in the past, is to remove the socket and wire in a nearest suitable connection to the motor frame.
As long as you have a earth conductor in the cable.
The problem relying on the mechanical mounting for earth can be when the spindle is traveling on a moving/gantry axis, or other for example, the earth grounding is not a positive connection.
.
 

rjenkinsgb

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when the spindle is traveling on a moving/gantry axis, or other for example, the earth grounding is not a positive connection.
A very good point.

That in turn can lead to reduced bearing life, if there is any leakage - resistive or capacitive - which causes current to pass through bearings.

A bypass connection is essential for both safety and machine life, if earthing is via moving parts.

Reference info:

 

unclejed613

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I wondered what application would use 400 Hz.
aircraft use 400hz three phase power... it allows for physically smaller power transformers in avionics equipment.
 
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MaxHeadRoom78

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aircraft use 400hz three phase power... it allows for physically smaller power transformers in avionics equipment.
That is how the 24krpm spindle motors are made, but they do not have sufficient inductive reactance below 6krpm, if so used, end up with a burn out!
 

unclejed613

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That is how the 24krpm spindle motors are made, but they do not have sufficient inductive reactance below 6krpm, if so used, end up with a burn out!
motors are used for "selsyn" (self synchronizing) controls where two motors are wired in parallel, and one motor used as a control input, the other as an actuator. when power is applied, there is no starting torque, so neither motor moves. when force is applied to the input shaft of the control motor, the actuator motor follows it. if there is resistance to the motion of the actuator, it is felt in the control shaft. this allows for connecting aircraft controls electrically to the actuators in the aircraft control surfaces without the mechanical levers and wheels and steel cables used in small aircraft. it also provides manual feedback to the pilot. since the motors are designed to be used in a "stalled" state, they are designed with adequate reactance to operate in that state. the motors have a stator winding, to which power is applied, and the rotors also have a set of wye connected windings, which are connected in parallel between the controller and actuator.
 
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unclejed613

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A bypass connection is essential for both safety and machine life, if earthing is via moving parts.
that explains the leaf spring with a graphite contact i see at the ends of some motor shafts. the leaf spring being attached to the motor case, and the graphite contact centered on the end of the shaft....
 

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