There are a large number of interacting relationships in a motor's design. There are first order, second order and probably even third order aspects that are all balanced to produce a dependable motor with the desired characteristics.
I'll be discussing only the First Order aspects.
1) Rotational speed is a direct function of the power frequency. Very simply, if you drop the frequency, the motor will slow down. Conversely, if you raise the frequency, the motor will speed up. The speed change that results will be proportional to the frequency change.
2) Cooling is a direct function of rotational speed. The motor's fan is attached to the motor's spinning rotor so it will experience the same speed-up or slow down the motor does. If the motor slows down, its cooling will drop (and at a faster rate than the slow down). If the motor speeds up its cooling will increase rapidly.
3) The magnetic capacity of the motor's magnetic (iron) circuit is designed to the relationship: voltage/frequency (V/f). If the frequency drops the V/Hz goes up. This means that the motor needs a larger magnetic circuit. Without it, the magnetic circuit can be overloaded. This is called saturation and it leads to a rapid increase in current draw and a corresponding large increase in temperature, a motor's chief enemy.
If the frequency increases, the V/Hz drops with no issues since the magnetic circuit will remain plenty large enough. [Sneaking in a second order consideration here, the motor may have a worse power factor.]
With the above aspects in mind let's explore what it all means when applied to that unfortunate motor you have in your machine.
If the motor is a 50Hz unit and you're going to be using it in 60Hz-land it will spin 20% faster.
Horsepower(hp) is proportional to Torque times RPM. Since the motor's torque is not going to change appreciably with an increase in frequency it will now provide 20% more hp. Your 8hp motor just got promoted to being a 10hp motor. Something for almost nothing!
But wait! Spinning a load 20% faster is very likely going to increase its power demand by at least 20%! If the load cyclically accelerates or decelerates in operation it will be subject to greater mechanical forces. Too much? If the motor is driving centrifugal loads their demand may even go up by the square of the speed increase. Centrifugal pumps would be an example of this. Fans, depending on their style, can also experience a squared increase in demand.
A bright spot in this is that the motor's cooling fan is a centrifugal fan that will move much more air.
The motor's V/Hz goes down when up-frequencying a motor, informing us that the magnetic circuit will have no trouble carrying the increased load. We're good there.
If the motor is a 60Hz unit and you're going to be using it in 50Hz-land it will spin 20% s-l--o---w----e-----r.
This also translates to 20% less horsepower. On the bright side, turning the load slower usually means it will be demanding less power. That's good, because the motor was just demoted 20% of its hp too. All that and the cooling fan is providing less too. But the 800 pound gorilla here is the V/Hz ratio. It just went up 20%! Not good. This means that during parts of every power line cycle the magnetic structure of the motor will probably be overloaded.
When this happens the motor's ability to limit current via reactance is lost. This will cause excessive current to flow heating the motor via I squared R losses. The only recourse here is to correct the V/Hz with the variable that is reasonably easy to adjust - V the voltage. Lower the voltage with a transformer to correct the V/Hz ratio. I'll discuss that in moment.
Back to the load. Will it still do the job at the lower speed? A pump may no longer have the head needed to accomplish its task. A machine's throughput will likely drop 20%. Will you still process enough product in a given time?
Example – You have 60Hz power for a 50Hz machine.
Let's say you just got a great deal on a machine. As it's being wired up you realized that it has 50Hz on its nameplate and you have 60Hz power. STOP.
The machine will be running 20% faster! Is this going to be a problem? If it is, can the speed be returned to design speed by changing a pulley size to drop the speed 20% back to where it was?
Once this assessment has been done and sheaves are changed or other modifications are done to help mitigate the speed/power issues, move on to the next step. Read the nameplate to get the Full Load Amperage commonly known as the FLA rating for the motor at the voltage you'll be running it with.
Using a clamp-on ammeter, run the machine and check to see the amperage is below the FLA. If it is you can proceed with running the machine as desired. Do check to see that it's still under FLA when fully loaded. If it's over FLA you must do some sort of load mitigation.
Example - You have 50Hz power for a 60Hz machine.
You receive a machine and since you are in 50Hz land, the 60Hz label is bothering you. As well it should!
Again, realizing the machine will run 20% slower, will it get the job done? In this case you can not change pulley sizes to correct the speed because the motor just lost 20% of its horsepower nameplate rating. If you change pulleys it will likely be overloaded - seriously.
If the machine can run 20% slower there may still be hope. Even though it is going to lose cooling with its internal fan running more slowly, running the load slower and with a 20% less powerful motor will likely even out. The V/Hz increase may still get you.
At this point if your assessment shows you will probably be alright with the slower speed, again check the nameplate for the FLA. Run the machine and quickly check the running current with an ammeter. If it's below FLA proceed to load the machine while closely monitoring things. If you stay below FLA it will probably be OK.
But! Running at FLA now that the cooling fan has reduced ability is still possibly going to be a problem. You should monitor the motor's temperature and assure yourself that after extended running time, under load, it remains below the nameplate temperature rise.
If even unloaded you're seeing FLA or more you will need to reduce the voltage because the motor is probably saturating. Before going to the bother of adding buck transformers, seriously consider changing out the motor for the correct 50Hz version. Remember you may need to up the rated horsepower if you're going to change gear ratios to return the machine back to its original speed.
But wait! What about single phase motors?
A last issue that must be faced is single phase motors. Everything described above applies to them but there's a couple of added flies-in-the-ointment. Single phase motors have a start winding. Since single phase power has no inherent rotational component, as three phase does, a start winding provides the needed large torque to get the motor spinning. The start winding is a very large load and as such can usually only operate for a few seconds. More than a few seconds and smoke will start issuing forth.
A centrifugal switch is usually included on the rotor to control the power to the start winding. It remains closed so when power is applied to the motor, both windings, the run and the start, are both energized. As the motor quickly reaches speed, the centrifugal aspect of the switch opens the start winding, disconnecting it from further operation.
When a 50Hz single phase motor is brought to 60Hz the start function can be upset because the motor reaches the centrifugal switch speed 20% earlier than normal. When it does, the starting torque of the motor is suddenly reduced. It could fail to speed up further and never reach normal running speed. If that happens, smoke is on the way!
Conversely when a 60Hz single phase motor is brought down frequency the switch could well not ever reach opening speed. Given that the switch opening speed setpoint is usually set at around 80% of running speed, you can see the potential for a problem. Remember the motor is going to turn 20% slower. If it doesn't reach switch speed, smoke is definitely on the way! You'll be seeing it momentarily.
Single phase motors can often have two kinds of capacitors associated with them. The first is a run capacitor. The run capacitor increases the motor's regular running torque. The second, is a starting capacitor used to increase the starting torque. When the supply frequency is raised these capacitors increase their effects resulting in more torque. Usually this is not a problem. But if you're lowering the frequency, they lose their effects and starting and/or running torques are reduced. That can be a problem. However, if the load is being spun more slowly it may balance out.
Since single phase motors are usually smaller it's often more effective to just replace them.
So. Now you know why you got such a 'great deal' on your machine buy.
Keith Cress is a 'broad spectrum' consultant that finds himself involved in everything from embedded controller designs to passenger rail car power systems. Keith can be reached at kcress@flaminsystems.com
Keith is a member of the Engineering Writers Guild at www.eng-tips.com. He is also an MVP. Follow Keith (itsmoked) at http://www.eng-tips.com/userinfo.cfm?member=itsmoked