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How to raise RPM of 3ph inductive motor

  1. May 30, 2013 #1
    As the title says, I need to raise the RPM of a 3ph 480VAC motor by approximately 20%. I looked into Variable speed drives but from what I have seen, they are used to drive the motor from 20-100% of input voltage.

    Any ideas on what to look for? For reference, motor power is 3HP.

    Thank you in advance

  2. jcsd
  3. May 30, 2013 #2


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    Staff: Mentor

    3ph inductive motors are typically (all?) synchronous. They rotate at an RPM that is a multiple of the input frequency. So a variable speed drive varies the speed by varying the frequency. Most also vary the voltage, but only in order to limit amperage as a byproduct, not as part of the speed contro.

    Most VFDs wil give you 0-200% rpm.
  4. May 30, 2013 #3
    Induction machines are asynchronous and typically run at 95-97% of synchronous speed (3-5% slip) at rated load in your power range, so if your motor is of decent build quality (bearings, cooling etc.), all you need to do is find a VFD that can put out its nameplate voltage and 120% of its nameplate supply frequency + maybe a little extra to make up for slip.

    In order for the motor to be able to produce its rated torque, you would need to maintain the ratio of the supply voltage to frequency given on its nameplate - a 20% increase in supply frequency should be accompanied by a 20% increase in supply voltage etc. If your motor design follows a NEMA standard, it can tolerate a 10% increase in supply voltage, if I recall correctly, but not knowing more about your motor, I wouldn't recommend messing with its supply voltage.

    If you increase the supply frequency of an induction motor without increasing its supply voltage, its effective torque rating decreases. The slip of the motor is a function of how much torque it's putting out, so if its slip under full load is much larger than when running under rated conditions, you might be near its breakdown torque, which is not good.
  5. May 30, 2013 #4
  6. May 31, 2013 #5
    Thank you for all answers. I definitely had the wrong info on the variable speed drives. Last question on this subject if I may. What would be typical maintenance on a Variable speed drive? Can it run 365 days a year without interruptions? Do they all need to be mounted on a well vented area or can they dissipate heat through surface conduction?
  7. May 31, 2013 #6
    A quality product can run continuously for many years if you keep it in a clean, low humidity environment. There'll probably be a fan you need to keep unobstructed and the ambient temperature needs to be within specifications, but it's really no different than what you would expect for any other solid state product. Check the documentation beforehand so you know if the VFD can operate in your environment or maybe just give the manufacturer a call.

    Your VFD has to be sized to handle the power requirement of your load. I hope I haven't made you think its voltage and frequency range is all that matters. Before buying anything, have a talk with the manufacturer, if possible, about your application.
    Last edited: May 31, 2013
  8. Jun 5, 2013 #7


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    Gold Member

    induction motor & synchronous motor are different machines (their stators may be similiar).

    Voltage is regulated in order to maintain constant value of flux in the core (to not exceed temperature rise).

    May I ask you to correct your post :confused:

    I agree with info provided by milesyoung ;) +1 for you Sir :)
  9. Jun 5, 2013 #8
    Typical VFD actually use a control scheme referred to as Volts per Hertz - so both the Frequency and Voltage is regulated, and this is done in Speed control ( F typically a Speed control) or Torque control where it also uses current as a feedback, but it will vary the Voltage ( basically the V/Hz ratio) to achieve or limit torque.

    For 3 HP this is the best choice, but you need to first determine what type of load is on the motor to make sure you select the proper drive. Ref : http://www.automationdirect.com/static/manuals/gs2m/ch4.pdf

    So when you want 20% more speed -what do you mean. 20% more speed at max torque is more power than the motor is rated for. If you know the load ( mechanical ) is less than the 3HP, does the torque change, or is it constant. etc. The application of the motor has more detail than jusr 20% more speed.
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