Single Phase Induction Motor Wiring help needed

[blacklight186]

Dear All,

Our company has a single phase induction motor Baldor make model VL5023A. It's a 1HP, 1725RPM, 1Ph, 60 Hz, 115/230 VAC single phase motor. More info can be found here https://www.baldorvip.com/servlet/productInfoPacket/VL5023A.pdf The last page of the pdf has the wiring diagram. LOW & HIGH refers to wiring for 115 & 230 VAC. STD & OPP refers to the directions of rotation (forward & reverse).

Due to the nature of our power supply in Dubai, we are forced to supply it 120 VAC, 50 Hz. Our end client in Saudi has 120 VAC 60 Hz. Anyway,for testing purposes in Dubai, we found that wiring it to LOW STD config (to accept 115VAC) caused it to draw a large amount of current (15.5 amps) along with heat & noise on no load. Wiring it to HIGH STD config (to accept 230 VAC) but still giving it only 120VAC caused it to draw a very minimal current of 2.3 Amps on no load.

I contact the Baldor motor company and their Sales rep. said that the overheating we faced was due to mismatch of the Hertz - at high torque, the starting winding was not getting disconnected from the circuit as it should, since the centrifugal switch was not opening (due to the hertz being less). This was causing the starting winding to remain in the circuit even when the high torque was reached.

But they could not explain why wiring it for 230 VAC but supplying it only 120 VAC (50 Hz) solved the problems. Please could someone help us with this?

Thanks,

blacklight186

 

[Darkwood]

Reduction in the hertz will reduce the speed of the motor hence the centrifugal switch dosn't open circuit and starting current always flows, luckily your wiring 120v to 230v connections as doing 230v to 120v would burn out your windings but that aside what you have essentially done is put 2 x 120v windings in series which would normally allow for 230v connection but because you are only running at 120v you have effectively given the run windings only half the voltage they require, this may by sheer luck of design give you extra speed but at the cost of greatly reduced torque and the motor may stall and burn out when loaded if run like this under load.

 

[pushrod]

No expert by any means and don't know if this is any help but here are my thoughts for what they are worth. Synchronous speed (so actual rotor speed as well) will be directly proportional to frequency. From Ns = f/P. Because it is a centrifugal switch that will only operate when a certain speed is reached, so maybe the critical speed is not being reached to throw the switch.

With regard to the current draw when it is set up for low voltage one of the windings is wired out so there will be a lower impedance and consequently the same current will be drawn with the lower voltage .

Wiring it for 230V but supplying it with only 120V to my mind would drastically reduce the torque under load, although the initial speed might not be affected.

Edit - apologies for repeating some of what you said Darkwood - i opened the page and then went and had my tea - unaware that you had replied!

 

[Darkwood]

No worries pushrod done it myself plenty times but this time i beat yer lol, but with regards to your post the wiring plan he links you to shows a duel voltage motor and all hes done is put 120v through 2 windings which each winding is wound for 120v in series they can cope with 230v thus hes under voltaged by 50% on the set-up and as we both stated it will work until load is added and the torque may not cope unless the motor is way oversized for the job, and the reason the centrifugal switch works at 120v is probably because its set at about 85% speed which is just hovering on the difference between 50 and 60hz

 

[pushrod]

Hi darkwood, I was just pleased that when I read your post, after mine, that we were both basically singing from the same song sheet . I only replied because i thought no one else was going to. Motors are interesting, but definitely not a strong point for me .

 

[blacklight186]

Hi darkwood & pushrod, thanks a lot for your replies.. I posted yesterday but I don't know where the post went :S

When the motor is wired for 230 VAC but I'm only supplying it 120 VAC, shouldnt the current be higher than what it would have been if I had supplied it 230VAC? Per the datasheet, the no load current at 230VAC should be 4.6 Amps. When I supplied it 120 VAC, it was drawing only 2.3 Amps.

Also, there are three windings in the diagram - 2 for 'phase A' and 1 for 'phase B'. Which two of these windings are in series when wired at 230 VAC? Also, what is causing the centrifugal switch to open when I'm supplying it 120 in this case... I thought the Torque produced was less?

thanks again guys...

 

[Darkwood]

A duel voltage 1ph motor has 2 run windings which when connected to 230v are wired in series so reducing the voltage to half will reduce the current to half also. R=V/I therefore R=120/2.3 therefore R=52ohms, R=230/4.6 therefore R=50ohms,
but normally 120v is connected in parrallel across 2 windings and we now know 2 windings in series is approx 50ohms so 1 winding will be 25ohms.

The lowest winding on the diagram is the start winding the 2 windings on the same plane are the run windings, normally you would only have one run winding but this is a duel voltage motor so has two to allow two supply voltages as a choice.

 

[blacklight186]

thanks darkwood. So, is the centrifugal switch opening when I supply 120 VAC 50 Hz to the 230 VAC 60 Hz setup and if so how?

 

[Darkwood]

Because you are only 10hertz shy of the 60hz setup the centrifugal switch will be set at 85% speed as a ball park figure so you are just on the borderline or its speed threshhold when you supply 50hz and not 60hz,the voltage difference will only effect the field strength and subsequently the torque, it may be just down to the lower field strength which allows the motor to hit the threshhold of the centrifugal switch. As a possible future solution you could use a 3ph induction motor via a single phase inverter which will give you speed control in excess of the supply frequency and no need for integral starting winding either and it can then be programmed for 60hz on dispatch if its software does not have auto frequency detection. This will give excellent motor control and overload protection too with many extra options available, although it may come down to cost but may be worth looking into.
What ever method you decide your connections you are using shouldnt be employed if possible as you are supposed to be testing the motor as it wouldn't have correct overload protection and hovering around the threshhold may bring it back into play if load is added to motor as torque isnt full, this might lead to the eventual damage to motor.