Using 60hz motors in a 50hz setting

[Mike Henderson]

Hi Mike, my poor memory relates rotor current to slip, and CEMF to rotational speed.

I may well be wrong, it's been too many years since college, however if CEMF was related to slip, a VFD wouldn't have to reduce it's output voltage linearly with frequency, unless I'm missing something.....Regards, Rod.

I'll check my motors book tonight (bedtime reading) but I think the reason the VFD needs to reduce input voltage as Hz decreases is because the impedance of the stator field goes down with the Hz.

Mike

[Rod Sheridan]

Thanks Mike, I'm still wondering about the original post however.

At 83% speed at 50 Hz I think the centrifugal start switch on the single phase motor may be an issue.

Lowering the line voltage is no problem, a small buck transformer would take care of that.

Have you ever tried a 60Hz single phase motor at 50Hz?

Regards, Rod.

[Rick Christopherson]

I'll check my motors book tonight (bedtime reading) but I think the reason the VFD needs to reduce input voltage as Hz decreases is because the impedance of the stator field goes down with the Hz.

Mike

Correct. I believe (and hope) you will find that there are two interrelated equations (or one equation with two variable) that relate current, voltage, sync (line) frequency, and slip frequency.

Rod's assertion is that the sync frequency will be the more prevailing variable when relating current and voltage. (My interpretation of his words, not his words.) However, I believe that the slip frequency will be the more prevailing variable. But of course this will depend on the magnitude of each.

I believe that while operating near sync rpm (low slip) the component of the current will be predominantly dictated by the slip speed.

As a matter of fact, this should be pretty evident on the face of it, because the high current flowing during a locked rotor condition is entirely due to the resistance of the wire and the impedance component of the raw coil of wire (the sync frequency).

Conversely, the DC Braking function of a VFD is based solely on the resistance of the coil, and the back emf of the slip frequency. In other words, this function removes the impact of the sync frequency.

Mike, I look forward to hearing what you find tonight. It is an interesting discussion.

[Mike Henderson]

Thanks Mike, I'm still wondering about the original post however.

At 83% speed at 50 Hz I think the centrifugal start switch on the single phase motor may be an issue.

Lowering the line voltage is no problem, a small buck transformer would take care of that.

Have you ever tried a 60Hz single phase motor at 50Hz?

Regards, Rod.

My problem is that I spent my career in communications and not in power (and motors) so all I have to go on is my college course on motors and checking in a motors book.

But I would think the centrifugal switch will switch significantly below 83% RPM. Think about your grinder. When I shut mine off, it takes a long time, and the thing is turning fairly slow, before I hear the switch click. I'll bet it's below 50% RPM.

I have never tried a 60Hz motor at 50Hz but some people who've posted here reported that they have and it works but gets hotter.

Rick C. - I'll let you know what I find in the book.

Mike

[John Davidson]

I just step in to this post and thought I would put my 2 cents in. Most industrial AC induction motors today are designed to run on 50 or 60 hz. HP will be about the same. As has been noted current will increase and current is a direct reflection of torqure. So as RPMs will be reduced torque will increase. HP will change very little. CEMF decreases as slip increases. This produces more current in the stator and thus more torque to overcome the load. As long as the voltage ratings line up I don't think that there will be any problem. Remember that it is X of L that is the major current limiting factor in an AC induction motors and the effect of the CEMF from the rotor/stator interaction that effectively changes the X of L. I don't think that there would be any problem running these motors if they are well built and constucted. If on the other hand they cheaply built that may be a problem as they will run slightly hotter. That should be the only down side.

[Dan Friedrichs]

My problem is that I spent my career in communications and not in power (and motors) ...

I was just thinking that it's funny we appear to have about a half-dozen EE's here, none of whom seem to know anything about motors

It's interesting that most universities used to teach quite intensive machine courses in undergrad EE, but most have scrapped those in favor of other offerings they feel are more relevant. I actually work in a power electronics lab that used to house an undergrad machines lab, so there are these enormous switchboxes all over with access to all sorts of voltages, 3ph, all at several hundred kW. They're still powered, but not used any longer...

Well, that was totally off topic!

[Rick Naatz]

Thanks for all the comments and suggestions. Some of it a bit more technical than I understand but I will try to summarize the comments and my understanding. Angie comments about Nigeria power having spikes and sags is right on. Much of the time we do not even get 220v (sometimes drops as low as 180-160) so low voltage can be a real problem here. If I understand the comments right, the motor will run at 50hz but a bit slower (5/6ths) and will heat up faster. Especially with the low voltage we often have here, overheating may be even more of a problem. This may shorten the life of the motor or worst case it burns out.
I haven't contacted Grizzly specifically about getting a 50hz motor but from their web site comments about foreign orders, I don't think getting a replacement from then is possible. Nothing against Grizzly as they are selling primarily to a 60hz markets.
I am returning to the US in a couple weeks and think I will still purchase both machines from Grizzly. I have a cousin back home that has a rewinding shop so will talk to him when I get back and see what he thinks about the issue (don't have an email contact for him presently). Also, I think I will ship the machines to myself first, put them together and make sure everything is there and they are working okay as it will be easier to correct any problems while still in the US. Then disassemble them, repack them and send them on to the shipping agent that will be sending them to Nigeria in a container. If I decide to replace the motors, I can then leave the 60hz motors in the US and only ship the 50hz.
Thanks for all the comments.

[Mike Henderson]

One thing you might want to think about before you switch to 50Hz motors. When you run a 230V 60Hz motor on 50Hz, the ideal thing would be to run that motor at a lower voltage, about 190V.

Except when you get to really low voltages, theory says that you should keep the ratio V/f constant. So if we start at 230V, we get

230/60 = x/50

Solving for x gives about 190V.

So if low voltage is what you experience, you might be better to stick with the 60Hz motors.

Mike

[Note: this is what VFD's do. As they reduce the Hz to slow the motor down, they reduce the input voltage to keep the V/f ratio constant (except at really low Hz)]

[Jide Ogundipe]

Hi Rick,

I realise that this is coming long long after but how did you eventually solve this problem.

I am in the same situation you were in back then. I'm in Nigeria, I want to buy a table saw from grizzly, but they have a "disclaimer" on use of their products on anything other than 60Hz supply.

I'm interested to know if you eventually changed the motor. What motor did you change it to. Where can I order same or do the same conversion you did if a conversion was necessary.

I realise that you may no longer be in Nigeria but I'm sure you know things haven't changed that much in terms of the quality available in the market here and the knowledge level of the technicians I will need to do a conversion.

Any help would be greatly appreciated.

Jide

[Curt Harms]

<snip>
Around WWII, aircraft used 400Hz because it allowed lighter motors and generators.

Mike

[Go to Wikipedia and look up "Horsepower". Look down to "Electrical Horsepower/Relationship with torque".]

I always wondered by 400 hz. was used. "Iron gyro" avionics systems still use 400 hz.