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HOW TO CHANGE THE SPEED OF A MOTOR | Electric Motors & Pumps

How To Change The Speed Of A Motor

Posted on: June 23rd, 2014 by admin 17 Comments

It seems that everyone these days wants to change the speed of their motor. Whether it is because they want more air from their fan on a hot day or they want to reduce the cost of operating a motor, it has become nearly standard practice to change or vary the speed of electric motors.

From the time the three phase induction motor was invented (circa 1889) it was a foregone conclusion that it would run at a speed dictated by the frequency of the power supply. If a different speed was needed a device was attached to the output shaft of the motor to accomplish the task. Pulleys, gears, gear reducers – all were used to obtain the desired speed at the driven shaft. Some of us old timers remember devices like vari-drives and eddy current clutches that would allow the speed to vary with the demands of the application.

Sometime in the ‘70s the term Variable Frequency Drive (VFD) crept into our lexicon. The early units were very crude by today’s standards but they got the job done. It was then possible to operate a motor at virtually any speed from 0 to nameplate rpm with the twist of a dial. Next came the ability to operate above name plate rpm, follow an electrical signal for “automatic” speed adjustment to maintain system settings, and set up PID loops. Then vector drives, steppers and servos. Whoa… my head’s spinning.

Speed changes today can be as simple as adding an electronic drive or as invasive as rewinding the motor for a different speed. Like anything else, each method has its up side and down side.

Some (not all) small single phase motors can be operated at various speeds through the addition of a resistor(s). It should be pointed out that the resultant speed is dependent on the load placed on the motor. What is actually being done is the weakening of the motor by reducing the voltage across the winding (the resistor acts as a voltage divider). The “lost” voltage is converted into heat which, for all practical purposes, is wasted into the atmosphere.

Most large single phase motors can’t (or, more correctly, shouldn’t) be operated at reduced speeds. These motors often utilize a centrifugal mechanism to disconnect the start winding. If the speed of the motor is reduced too far the centrifugal switch recloses, allowing the high starting current to flow in the smaller start winding. The end result is a motor winding failure.

There are a number of different types of single phase motors: universal, shaded pole, permanent split capacitor, capacitor start, capacitor start/capacitor run, repulsion-induction. If you are not sure what type of motor you have, give the motor experts at Gem State Electric a call at 208-344-5461. We’ll help you figure it out.

Three phase induction motors are a different story. All three phase induction motors can be run at variable speeds with the help of a properly sized VFD. Some three phase motors are much better suited to variable speed operation than others but all can run on a VFD. A number of factors must be considered to determine a specific motor’s suitability however, so give us a call to discuss your specifics.

As stated above, for decades the three phase motor was designed for a specific rpm based on the frequency of the incoming line. Only rarely were devices employed to change the frequency of the incoming power; many of these rotary frequency changers are still in use however.

With the growing popularity of VFD electronic drives a whole new industry was opened. Companies now marketed the devices as a great way to save energy, particularly in variable torque applications such as pumps and fans.

By using a 60 Hz. input to create an easily variable frequency output the speed of the motor is now adjustable. By changing voltage and frequency with a constant ratio the output torque is consistent from nearly zero to full line voltage.

Various types of electronic drives, utilizing a dizzying assortment of bells and whistles, are now available. Your motor can do things now that its ancestors could only dream of!

Another way to (permanently) change the speed of a motor is to redesign the winding during the winding process. The speed choices are much more limited however. There are, for example, no speed choices between 1800 and 3600 rpm when redesigning the winding.

There are some practical limitations in doing so, not the least of which is the diameter of the rotor. The centrifugal forces on the rotor increase dramatically as the rpm goes up. In most cases a manufacturer will use a smaller diameter rotor in a 2 pole (3600 rpm) rated motor than they do in their 1800 rpm (and lower rpm). If the larger rotor is now spun at twice the rpm because of a winding redesign, the rotor is suddenly required to handle 4 times the centrifugal force it was designed to experience (force is increased by the square of the increase in speed).

I had a customer back in the ‘70s that did not consider this when adjusting his brand new VFD. The drive was capable of outputting frequencies up to 300 Hz. His motor did not make it past 120 Hz before the rotor literally exploded!

Other considerations are the amount of torque required by the load at increased or decreased speeds, the bar/slot combination of the original motor, decreased cooling effect at lower speeds, core saturation at the new output levels, etc. Like many other things, just because you can change the speed of a winding design doesn’t mean you should do so. Before considering a change like this, consult a motor expert, like the experts at Gem State Electric, for example. (Now tell the truth – you were expecting that commercial weren’t you?)

If you do not have access to three phase power but need to change the speed of your motor you are still in the game. A customer recently came to us with a challenge. He needed more water to irrigate his field but the utility would only allow single phase power to be brought to his meter location. Single phase motors are, for all practical purposes, available up through 15 HP, but he needed a 20 HP motor/pump to move enough water for his crops. We were able to find a three phase motor/pump that we ran off of a VFD, and powered it from a single phase line – with the blessing of the utility!

With any such application it is always a good idea to consult the motor/pump/control experts at Gem State Electric (did you see that one coming?)

We are never further away than your phone, at 208-344-5461, to discuss your electric needs.

17 Responses

  1. sarjeraopatil says:

    plz provide some speed reducing mechanism of 3 phase motor

    • admin says:

      This is a pretty general question. Speed reduction can be done by mechanical or electronic means, as well as by rewinding/redesigning the motor. We would need more specific information regarding what your desired end result is.

  2. Mark says:

    Hello, I have a 3 phase 7.5hp Baldor electric motor that I would like to increase the rpms from 1800 to 3600 rpms for an experiment. How can this be done safety?

    • admin says:

      There are a couple of ways to increase the speed of your three phase motor. If you need only 3600 rpm (instead of variable) it could be done by rewinding the motor for operation at the higher speed. A less invasive means (and possibly less expensive) would be adding a variable frequency drive (VFD), sometimes referred to as an adjustable speed drive (ASD). This will allow for operation at a variety of speeds, up to a practical maximum of about 4,000 rpm. Keep in mind that when the speed goes up the torque goes down once you exceed the rated rpm of the motor. If you need further information please give us a call. We would be glad to assist you in determining which of these approaches is best for your application.

  3. jeff says:

    Is here a mechanism (dimmer) that can be made or available for a 120V AC , 60hz, 10A electric motor?

    It is to adjust the RPM on a Wet bridge style Tyle saw.

    Rpm is 5000 and I would like to adjust it from 5000 to between 1000 and 2000 Rpm ..

    thank you

    • admin says:

      Based on your RPM we are assuming that there are brushes in this motor. In which case, yes there is a switch that you can use to slow the speed down. If you would like to give us a call we would be more than happy to quote you one.

  4. I have a 1/6 hp Dayton split phase motor. I want to control the speed of this motor with a foot pedal.
    RPM 50 to 500 approximately.
    How do I control this, can you supply a package.

    I am ready to buy.


    • admin says:

      If your motor has a centrifugal switch in the start circuit you can not very the speed of the motor. You can REDUCE the output RPM with the use of gear reducer but it will still run at a constant, though reduced, speed. To run any motor at the type of RPM you are looking for would require an auxiliary cooling method, such as a constant speed fan.

  5. Jojo says:

    We are planning to use 25HP, 3Phase, 60Hz, 1800 Rpm TEFC Induction Motor and VFD to lower the speed/rpm of the said motor to 300 rpm. This electric motor will drive a hydraulic pump that could withstand 3000 psi. This pump will drive the existing hydraulic motor to run variably as needed. Would there be problem with electric motor to run very low at 300 rpm? I have read that the cooling will go down as the fan runs slow thus needs an external cooling blower/fan. What

    • admin says:

      Yes, running at that low an RPM will result in the inability of the motor to cool itself in most cases. Because your motor is TEFC (Totally Enclosed Fan Cooled) the manufacturer has designed the motor to stay within its temperature limits by moving a specific amount of air across the outside of the motor shell. When you reduce the RPM the fan does not move the same volume of air and therefore the motor will run hotter. Even the elevation at which you operate the motor is relevant as the air at higher elevations is less dense and therefore does not provide the same cooling effect. In mountain locations the load may need to be reduced, even at full speed, to provide adequate cooling of a motor.
      The specifics of your application would most likely require the use of a premium efficient motor and a constant speed auxiliary fan. The cost of the auxiliary fan would depend on the frame size of the motor (this can be found on the motor nameplate) and may even then require some fabrication to figure out how to mount it.

  6. Walt says:

    I have a need to reduce 1 speed tap on a furnace blower motor by about half of designed cfm. It is a PSC motor with Start/Run Capacitor. What can I do to achieve this and can I control it easily by say 10% increments?

    Thanks for any advice you can give me!

    • admin says:

      If you have both a start AND a run capacitor, you do not have a PSC (Permanent Split Capacitor) motor. A start capacitor must be removed from the circuit once the motor is near full speed and if you reduce the speed of the motor this would no longer happen with the use of a typical centrifugal switch.
      if you have a PSC motor (run capacitor only) you can reduce the RPM by reducing the strength of the winding. A multi-tap single phase motor is not actually a multiple speed motor, it is a multi-voltage motor. By using winding taps to add resistance to the circuit, the motor is weakened and the load causes the motor to slow down. Remove the load and the motor will run at the same speed regardless of which tap is selected.
      If the motor is being used as a blower motor (as opposed to using a blower motor for a different application) a rheostat can be added to the input line, adding external resistance and weakening the motor to obtain the desired speed. If you desire specific speeds instead of a range of speeds, you could add steps of resistance by trial and air. Just be aware that if your load changes (such as changing static air pressure), so will your speed.

  7. Shah says:

    Can we change the 3 Phase induction motor poles? I rewind 3 Phase induction motor with 4 poles but previously it had 2 poles but i keep same number of turns. Please tell me the consequences. Can we modify or not? if yes then what type of precautions i should keep in mind.
    380V A.C

    • admin says:

      Yes, you can redesign a winding to change the magnetic poles but by doing so it will be necessary to change several other winding characteristics. In order to increase the number of poles (reduce the speed) you will need to change the span, wire size and turns per coil to keep the back-iron and tooth magnetic flux densities within normal standards. By keeping the same number of turns you are weakening the motor, reducing its ability to produce torque.

  8. Jen says:

    Hi there – I have an old duel shaft motor. The plate gives only this information: Eberbach, Ann Arbor, Mich. 115 Volts – 60 CY. – A.C. I searched “Eberbach” and they have supplied lab equipment for many years. From what I can gather, it looks like Eberbach used Bodine motors. I looked through some old Eberbach catalogs and based on the specs, it looks like it was possibly used for a lab stirrer. The RPMs are very low – I’d guess in the low 100s at most. I’ve attached the motor to a buffer/grinder, and need to get it up to at least 1075 RPM. Is there a possibility I can increase the RPM on this motor?


  9. adam says:

    add mor capacitor to the motor input and speed increase

  10. balkrushna jani says:

    Dear Sir,

    I have a 2800rpm 3 phase motor & i want variable speed speed from 0 to 2800rpm without adjustable speed drive…………………..only mechanically with one handle like mayo engineering’s slurry coating machine how its possible……………. (Urgent requirement)……

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