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Variable Speed Motor Drives: Boon or Bane?

I've been watching a discussion on a LinkedIn group that is addressing some writers' concerns about variable speed drives (VSD).

The discussion is titled “Are variable speed drives harmful to motor? [sic]” and part of the discussion can be found here. Note that you need to be a member of the group to read it. If you're not, the cogent part of the discussion from the original poster is summarized thusly: How will the PWM-based variable voltage/variable frequency excitation from a typical inverter-based VSD affect a typical induction motor? Several people provided thoughtful answers that gave details on what could go wrong and how it could go wrong.

VSDs are used to vary the speed of induction motors. Induction motors have traditionally been constant speed devices. They run at a speed that is synchronous (mostly) with the frequency of the applied current, although the phase relationship lags. To vary the speed, you need a power source (probably high power) with frequency (and probably voltage, too) that can be varied. Do this and you have a VSD. With a VSD controller, the applied voltage level is usually adjusted downward as the frequency is lowered: Since back EMF decreases at lower speeds, a lower applied voltage is needed (or else motor current would rise excessively).

So far, this is pretty straightforward. But how do we get a variable frequency? We only have 50Hz or 60Hz available. Easy as pie. Or possibly easy as π radians. Take the AC line voltage, full-wave rectify it, filter it, and apply that bus voltage to an H-bridge (or a 3-phase H-bridge) made from power FETs or IGBT devices. As described, this is the heart of a power inverter. So far, so good.

Control the drive signals to the H-bridge with an appropriately timed and phased arrangement of PWM signals. By varying the pulse width/duty cycle, the H-bridge outputs will produce current in the motor windings that, on average, appears sinusoidal, mostly. The motor's winding inductance and its inertia will tend to smooth things out. But this is where things can get a little troublesome.

Those pesky PWM waveforms are, on average, OK — but the fact that they are switching on-and-off rapidly means that there is a lot of high-frequency content in them. This may cause excessive heating in the motor windings. The high di/dt also means there will high voltage transients across the motor windings. If the motor is not designed for inverter duty, it may be damaged.

Damage can occur where you probably would expect it — the insulation on the motor windings may deteriorate or break down completely. Smoke and arcing are the telltale signs.

Damage can also occur where you probably don't expect it — in the bearings. Again, the high-frequency content of the PWM current is the culprit. It can capacitively couple current through the bearings to the motor housing. This can cause damage that will appear to be pitting or corrosion — bearings aren't designed to carry current, just mechanical loads.

One more concern — EMI radiating from the connections between inverter and motor; or conducted back through the incoming AC power lines. Be aware of that so you can deal with it. A good resource is a paper written by several Rockwell Automation engineers, EMI Emissions of Modern PWM AC Drives.

Adding L-C filters between the inverter and the motor can help with the heating and insulation issue cited above. Sometimes this just moves the heating due to circulating currents from the motor to the filter elements, so be aware of that, too. Enlisting applications engineering help from the inverter and motor vendors is essential when dealing with this issue.

Have you had to deal with VSD/inverter problems like this? How did you resolve the problems?

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13 comments on “Variable Speed Motor Drives: Boon or Bane?

  1. eafpres
    June 20, 2013

    I'm aware of  a government facility that installed VFDs (Variable Frequency Drives) for HVAC blower motors.  They saved a bunch of money on energy costs and did not have problems.  

    When you mention that the motors must be designed for inverters, does that simply imply heavier insulation and possibly some special grease in the bearings, or a way to electrically isolate the bearings (I've seen that done with bakelite sleeves)?

    It is possible that in the case I was aware of the motors were over-spec'd in the beginning,

  2. Brad Albing
    June 20, 2013

    It absolutely means better insulation on the wire. For the bearings, some sort of insulation is needed, but I'm not sure what is used.

  3. David Maciel Silva
    June 21, 2013

    A good solution for switching and H-bridge, and one min to release yet called PROFET, the infineon, nothing more than a FET protected and with many applications industruiais.

    Follow the link:

    http://www.infineon.com/cms/en/product/automotive-ics/smart-high-and-low-side-switches/profet-tm-smart-high-side-switches/channel.html?channel=ff80808112ab681d0112ab69e2d40357

     

    Some of our supporters have used this new technology?

  4. Brad Albing
    June 21, 2013

    @Maciel – looks like an OK part to use in some lower power applications – integrates some of what is needed in an H-bridge, so that's good. care must still be taken with the motors of course regarding some of the concerns I outlined.

  5. Brad Albing
    June 21, 2013

    Watch for a follow-up blog that talks about another workhorse in commercial, industrial, and automotive applications: the BLDC or brushless DC motor – i.e., motor with permanent magnets in the rotor – hence, don't need brushes. But still needs methods to do commutation.

    Watch for the ways and means – probably next week.

  6. D Feucht
    June 22, 2013

    @ Brad – Not only do high-frequency currents cause bearing degradation, so does the additional bearing runout causaed by radial vibration from torque ripple caused by the spurious frequency components of the drive waveform.

    Additionally, the high-frequency harmonics of the PWM cause magnetic power loss in the armature, which is usually electrical steel and though laminated, is not sliced  thinly enough to reduce the heating to a negligible amount. I was once designing a motor drive for step motors (which are essentially permanent-magnet synchronous (PMS) or “BLDC” motors) with a variable-frequency PWM up to 150 kHz, and at the high-frequency end, the dominant heating of the motor was caused by magnetic core loss. Electrical loss also increases significantly caused by the proximity effect at PWM frequencies. These motors historically have not been designed with more than 60 Hz in mind. They are like 60 Hz transformers in non-switching power supplies when what is needed are motors designed more like ferrite switching transformers.

    All in all, field-oriented control is a good idea, especially for induction motors. With it, additional control over the motor is possible that a power line does not exhibit and efficiency and (closer to a) resistive input can be maintained over a large operating region. Even at low speed, winding-sensed (or “sensorless”) control can allow for the low speed end of the VSD be almost zero speed.

  7. Brad Albing
    June 23, 2013

    @D Feucht – good point – I overlooked the issue of core material for the motor. It seems to me some companies are using ferrites now, but that's an area to which I've paid insufficient attention.

  8. jkvasan
    June 26, 2013

    @Brad,

    We used VFDs to drive medical treadmills for quite sometime. Operating noise-free, we have not faced much problems. For EMI issues, good amount of LC filtering was necessary between inverter and motor.

    I also look forward to the BLDC blog. Off late, BLDC motors are being used in treadmills.

  9. Brad Albing
    June 26, 2013

    @JK – You're right about the filtering – if the manufacturer of the drive did not put a filter on the output (or use a filter with sufficient attenuation) you would probably need to add additional stages of filtering to keep that high-frequency current circulating locally – else you'd have a nasty radiation problem.

     

  10. WKetel
    June 26, 2013

    A very useful posting here. I had not considered the hysteresis heating of the iron due to the high frequency components, that would need to be addressed in the design of the motor. But it seems that it would be a bigger problemat higher speeds, but that “intuitive” answer may be wrong. But the problem can be solved with different materials more suited to the task. Of course the insulation will need to be better to handle the higher voltage, but better insulation is easy, although not cheap. And for the bearings, how about using those ceramic balls, and a copper slip ring with a grounding brush. But the nonconductive balls should solve some part of the problem, or at least move the sparking someplace else.

  11. jkvasan
    June 27, 2013

    @Brad, In a medical device used for Electrosurgery, the concept is similar to VSD but the amplitude only is changed and there are fixed career frequencies. This device helps cut tissue and dry up the bleeding.

    The DC voltage is between 30 to 200 V and is applied to a HF transformer through FETs operating at frequencies around 500 KHz. We needed several RC filters at strategic places to bring down the radiation problem.

  12. Brad Albing
    June 30, 2013

    @JK – I'm familiar with those surgical tools. i should blog about them – intereting devices.

  13. jkvasan
    July 1, 2013

    @Brad,

    As an electronic device, the electrosurgical unit contains every technology that is used in electronics – analog, digital, electromagnetics, sensors, embedded, etc.

    Deployed almost in every hospital in the world, this is a tool a surgeon would not want to miss.

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