Motor RPM Control?

Question

How can I control the RPM of an industrial motor of 7 horse power? I need it to have high torque and low RPM, like 100 per minute.

Answer 1

*For stepless control of AC motor, you need to have a VFD
[variable frequency drive].

Basic equation is:

RPM=120*frequency/no. of poles

here,no of poles are fixed for your motor and 120 is constant.

So, to control the RPM you have to change the frequency.

100/120=frequency/no of poles

0.83 =frequency/no of poles

check the no of poles of your motor and put in the above formula,you will get the frequency.

#Example
A 4-pole motor that is connected directly to 60 Hz utility (mains) power would have a synchronous speed of 1800 RPM:

60*120/4=1800
If the motor is an induction motor, the operating speed at full load will be about 1750 RPM.

If the motor is connected to a speed controller that provides power at 40 Hz, the synchronous speed would be 1200 RPM:

120*40/4=1200

#A variable-frequency drive (VFD) is a system for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor. A variable frequency drive is a specific type of adjustable-speed drive. Variable-frequency drives are also known as adjustable-frequency drives (AFD), variable-speed drives (VSD), AC drives, microdrives or inverter drives. Since the voltage is varied along with frequency, these are sometimes also called VVVF (variable voltage variable frequency) drives.

#Adjustable speed drive (ASD) or variable-speed drive (VSD) describes equipment used to control the speed of machinery. Many industrial processess such as assembly lines must operate at different speeds for different products. Where process conditions demand adjustment of flow from a pump or fan, varying the speed of the drive may save energy compared with other techiques for flow control.

Where speeds may be selected from several different pre-set ranges, usually the drive is said to be "adjustable" speed. If the output speed can be changed without steps over a range, the drive is usually referred to as "variable speed".

Answer 2

This is not easy to do with a standard AC motor. The motor works by generating two opossing electromagnetic fields which link between rotor and windings. Simply decreasing the current decreases the field, decreases the torque and quickly stops the motor turning completely.

The problem is overcome by changing the frequency of the AC current. As the frequency changes, the fields begin to link less often than they did at 60 cycles per second (standard AC frequency). The net effect is that the rotor turns more slowly, but because the current has not decreased, there is little change in the amount of torque.

Achieving this effect involves a multi-stage electronic circuit. In the first stage, the AC input is rectified to a stable DC voltage around 5 volts. This could probably be achieved by using a computer power supply. In the second stage, the current is turned into a rectified AC signal. This stage of the circuit splits the signal into a "stereo" output consisting of a positive and negative series of "half wave" signals. This stage also contains a frequency adjust setting and is controlled by a series of integrated circuit chips. The final stage of the circuit increases the voltage to its original value and combines both halves of the output from the second stage. It is suggested the third and second stages be isolated because it is bad engineering to directly link high and low voltage sections of a circuit. This separation is achieved with an optical isolater. It works by turning a low voltage input frequency into an infrared signal. The signal then drives the high voltage output section of the isolater chip.

This is the theory and practice behind AC motor control. The easiest solution is to just purchase a pre-built controller, but it has to be compatible with the actual type of AC motor. Most industrial kinds are linked field models, but some have permenant magnet rotors and might need a different type of controller. DC motors are very easy to control, but don't generate the torque an AC motor can.

Answer 3

I'm assuming that it's an AC motor, since most DC motors are already set up for variable speed operation.

If that's the case, you'll need a Variable Frequency Drive (VFD), also known as a Variable Speed Drive (VSD) or Adjustable Speed Drive (ASD). You can find out more about them here:

http://www.motorcontrol.com/2007HomePageLinks/acdrives.htm

Definitely do your homework before buying, though. If you just size the drive to the motor horsepower, you may end up with too small a drive and a lot of expensive smoke.

Answer 4

If you have a way to decrease the AC input frequency thats the easiest way. Depending on the application of the motor you can have it maintain a certain RPM and simply change the power to it (by changing the amps applied) to change the HP it puts out, but that is really only certain applications

Answer 5

All sealing fans are AC motors. You can use a variable resistance or stepped resistance. This is not an accurate method. The speed can also be controlled by varying the frequency. This method is accurate and expensive.

Answer 6

What El Jefe said.

I've used VFDs on a couple of large control projects in the past. They are the 'way to go' for controlling AC motors.

His link will point you in the right direction, too.

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