60Hz US Motor in UK

[Onesandnoughts]

I have a compressor fitted with a 230v 60Hz single phase cap-start, cap-run motor. It is meant to run at 3450 rpm. Although it will run here in UK on 50Hz, after a couple of minutes the caps begin to smoke! I think that the reduction in operating frequency means that the motor does not reach sufficient speed for the centrifugal switch to cut out the start side. Apart from replacing the motor, is there anyway to solve this ?

 

[electricuwe]

I guess you have already catched the problem. You could tray to readjust the cetrifugal switch or to diconnect the capacitor by another means.

 

[Marke]

If the motor was designed for 230V 60Hz and you run it on 50Hz, the curent will be much higher than it should be and you will have a burnt out motor. If you are lucky, the motor may have been wound for 50Hz, but this is unlikely. I would suggest that rather than spend a lot of time and money playin to sort out the cetr switch, you are better of just to replace the motor.
Mark Empson

http://www.lmphotonics.com

 

[SteveKW]

In my shop we would take the springs and use a pair of wire cutters to cut into the spring and separate a few coils (making sure not to cut or nick the wire) for any motor that was headed overseas. This would weaken the spring so the start windings come out a bit earlier. Another trick for your application would be to decrease the diameter of the pulley on the motor so the load is decreased. It may take a few more minutes to come up to pressure but the compressor would still work.


Make sure to tell us if this works!

 

[Onesandnoughts]

Thanks for the replies so far. I forgot to mention that this is a direct drive unit, so the motor is mounted directly on the compressor pump, so pulley changes are not an option. Replacing the motor is also awkward due to having to find same mount etc and expensive from what I have seen, so please tell me more about 'modifying' switch springs!

 

[Marke]

Before you spend too much time, check that the motor is capable of operating continuously on 50Hz. There is a very real probability that it will burn out due to excess flux. As you reduce the frequency applied to a motor, you also need to reduce the voltage proportionaly. In this caes, if it is wound for 230V 60Hz, you would need to operate it at 190V 50Hz. The KW rating is also reduced by an equal amount.
50Hz motors are physically larger than their 60Hz counterparts. They need more iron. Mark Empson

http://www.lmphotonics.com

 

[Onesandnoughts]

Marke - Thanks for that... i had seen something about voltage reduction before now you mention it. A local motor 'specialist' suggested that I strap a 40 microfarad cap across the mains input and that "would solve my problem" - needless to say I have not experimented with this idea! Are you suggesting that a rewind may be the solution?

 

[Marke]

Tes it may be necessary. I would recommend that you uncouple the motor from the compresor and try to run it under open shaft conditions. If necessary, you could put a temporary switch in series with the start winding and measure the open shaft current draw. If this is too high, then you have to either rewind, reduce voltage or replace motor. Typically, I would expect the open shaft current to be no more than 60% or rated current for a small motor, but as low as 25% on a large one. If the no load current is significantly higher than 50%, I would start looking sideways.
Good luck. Mark Empson

http://www.lmphotonics.com

 

[SteveKW]

Putting a RUN capacitor in parallel with the start capacitor and centrifugal switch would be a better option for making the motor capacitor run/capacitor start. That would help to also bring the power factor down (lower the running current). As for weakening the springs, take a pair of dykes (wire cutters) and cut into the coil of the spring so one or two coils separate. This lengthens the spring so the switch will open up at a lower RPM.

 

[jbartos]

Suggestion: It appears that the capacitor values could also be varied to achieve the proper function at 50Hz. Some calculations and experimentation may be needed. If you can contact the motor manufacturer tech support, it just may be ready there. To counterbalance decrease of frequency by the capacitance, the capacitor value shall be increased.
Xc=1/(2 x pi x f x C)

 

[Mikifuz]

How much power are we talking about? If it's about 1HP you could try to make an inductive valtage attenuator... I mean an inductance in series with the motor to reduce it's voltage. It's very easy and usefull, all roof-fans uses it!
Take the core of a transformer 1/3 the size of the motor and make enough turns with the same wire size as the motor has. The goal is to keep the factor voltage/frequency (V/F) constant.
You still need to deal with that centrifugal switch but the motor will not burn out.
Good luck!

 

[SteveKW2]

Typically, lowering the voltage like you describe can be done only on blowers/fans and centrifugal pumps. All those loads usually decrease with a decrease in speed. They are the exception to the rule. Not true with a compressor. Lowering the voltage or limiting its current would stall a compressor motor and she would still burn up ... just a bit slower though!

 

[Marke]

Onesandnouhts, I am concerned that there are confilicting views being posted her and it is important that you do things correctly in order to prevent failure.
The problems that I see are:
1. The reduced frequency with the same voltge will increase the flux in the iron and probably cause the motor to fail due to increased iron loss. see

http://www.lmphotonics.com/faq.htm#60_50

2. The centrifugal switch not operating due to lower speed.
3. Starting capacitor.

1. The flux in the iron should be kept at the design flux which will require that either you reduce the supply voltage by the same ratio as the frequency reduction, or you rewind the stator with more turns.
2. The switch can be modofied by altering the springs to operate at a lower speed.
3. The start capacitor may need to be increased in value to give the same phase shift. I suspect that you will get away with it though.

Adding capacitors across the windings will reduce the current from the suplly and will apear to be improving the situation, but this is just power factor correcting the motor and does not affect the flux in the iron so it will have no bearing on the shortened motor life.

If the motor has been wound to operate on both 50 and 60Hz, then you have no problem.

I have personally experienced a motor on a coolant pump that lasted about 6 weeks between failures. This motor was amnufactured in the USA and was stamped 230V 50Hz on the name plate. After the third failure, we came to the conclusion that rather than wind for 50Hz, then manufacturer had just change the name plate. We then altered the stator turns to accommodate the change in frequency and the motor operated faultlessly for 15 years until the machine was retired.
Mark Empson

http://www.lmphotonics.com

 

[Onesandnoughts]

OK gents - thanks for all this good gen! The motor is rated at 6hp and pulls a max of 20 amps at start. It is obvious that there is no easy way ahead here and maybe I ought to investigate the cost of a rewind and continue my search for a reasonably priced replacement motor (fruitless so far :-( though).

 

[jbartos]

Suggestion: Since the UK voltage has not been posted, it is assumed to be the same as for 230V (however, it could also be 220V). The Marke's link states that the voltage at 50Hz is supposed to be reduced from 230V to 191V (or possibly from 220V to 191V) to have the motor operate normally.