[This tutorial applies to induction motors, both single, and polyphase. Single phase motors have an added aspect that will be discussed at the end of this tutorial.]
AC motors are quite complex for all their functional simplicity - turn them on they spin.
There are a huge 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.
This tutorial will deal only with the First Order aspects.
These complexities include:
- Rotational speed is a direct function of the power frequency.
- Cooling is a direct function of rotational speed.
- The magnetic capacity of the motor’s magnetic(iron) circuit is designed to the relationship: voltage/frequency (V/f).
- Back-emf decreases as a motor slows down.
- Current increases with reduced back-emf.
Lets look at these complexities each in turn.
If you drop the frequency the motor will slow down.
If you raise the frequency the motor will speed up.
If the motor slows down it’s cooling will drop (and at a faster rate then the slow down).
If the motor speeds up its cooling will increase rapidly.
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 saturate. This 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. This is not a first order consideration. [The motor may have a worse power factor.]
A motor is a device that ‘wants’ to turn at its designed speed, set by its designed operating frequency. It delivers the required horsepower(hp) the load needs when the load is spun at the motor’s designed speed. Different loads change their hp demands in very different manners, depending on the type of load they are.
If a motor’s load increases for some reason the motor will be slowed, this means the back-emf drops, this causes the motor to draw more current. More current is where the motor gets more power to turn the heavier load. Here the prudent motor user makes sure that the motor is pulling less current then the motor’s full load amperage(FLA) rating.
If the load remains the same and the voltage is lowered the motor will draw more current to continue meeting the load’s hp requirement. Remember the motor will still be running at the same speed since the frequency wasn’t changed.
So looking at the case shifting a 50Hz motor to 60Hz duty.
a) It will turn 20% faster.
b) The cooling will increase dramatically.
c) The load’s horsepower requirement will increase, possibly dramatically.
d) The V/f will drop which will not cause a current draw increase.
You should discuss what the load will do when sped up 20% with someone knowledgeable with that type of load.
If the load can take the speed increase then run the load and promptly check the motor’s current draw. If it is under the FLA you should have no other problems.
Using a 60Hz motor in 50Hz duty.
w) It will turn 20% slower.
x) Cooling will drop dramatically.
y) The load’s horsepower requirements will drop, possibly dramatically.
z) V/f will increase possibly causing a large increase in current draw.
Make sure the load will still do what is required of it when running slower. For instance, a fan load would now move less air. Still enough?
The cooling will reduce dramatically. Is it enough to matter? This depends on how the load has decreased. If the hp requirement dropped because the load has decreased the current will drop and less heat will be released internally. You would want to take temperature measurements until the motor reaches a steady state temperature running in its new frequency application.
The motor’s hp will drop because hp is a function of speed x torque. The motor’s torque doesn’t change but its speed has dropped so it is now a lower hp motor. If you change pulley sizes to return the load to its original speed your motor will likely be undersized, possibly seriously. Example: A 10 hp motor is now an 8 hp.
The most serious issue is the V/f issue. The V/f will increase. Likely enough to cause a large increase in the motor’s current. This coupled with the reduced cooling may cause rapid overheating. However the V/f problem can be fully mitigated! You reduce the voltage to the motor by the amount required to return the V/f back to its original value. This removes the hazard of excess current from an increased V/f.
Example: A 60Hz 240Vac motor is going into 50Hz service.
V/f = Y Hence: 240/60 = 4.0
So if Y x f = V then by plugging in the new frequency of 50 we see:
4.0 x 50 = V
V = 200
Running the motor at 200V at 50Hz will remove the V/f problem.
Another way to think of this is:
New voltage = Old voltage x 50/60
New speed = Old speed x 50/60
New Horsepower = Old horsepower x 50/60
Note: This would instead be 60/50 for a 50 to 60 Hz conversion.
Single phase complications.
Single phase motors must be assisted because single phase power does not have an inherent rotational aspect to it. This means an additional start winding is required to provide rotational starting torque. A common method for controlling power to this temporary winding during starting is a centrifugal switch mounted on the motor’s rotating shaft. This switch is normally closed during start up. Once the motor achieves a certain speed during start up the switch opens de-powering the start winding. If a single phase motor is moved to a new frequency domain the operation of any rotational switch must be checked. A 60Hz motor in a 50Hz application turning 20% slower may not achieve a speed sufficient to open the centrifugal switch. This would likely result in an immediate burn out. Likewise a 50Hz motor placed into 60Hz service may switch off the start winding at too low a speed for the motor to make the transition to running.
Some single phase motors have start or run capacitors and some both. If the motor is being shifted from 50 to 60Hz then their effect will increase. This will usually pose no problem.
Alternatively, taking a motor from 60 to 50Hz will reduce the capacitor effects. This will result in possibly lower running and starting torques. This may mean the motor cannot successfully start its load or maintain running a load.
A failure to start may result in a rapid burnout(seconds) so this should be carefully monitored initially. Larger capacitors or additional ones may be needed.
Other points to note:
With respect to the previously mentioned V/f ratio, 460/60 motors match 380/50 motors.