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Brad Albing

Variable Speed Motor Drives: Boon or Bane?

Brad Albing
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JAYARAMAN KIRUTHI VASAN
JAYARAMAN KIRUTHI VASAN
7/1/2013 2:00:34 AM
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Master
Re: Treadmills
@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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Brad Albing
Brad Albing
6/30/2013 11:07:33 PM
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Re: Treadmills
@JK - I'm familiar with those surgical tools. i should blog about them - intereting devices.

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JAYARAMAN KIRUTHI VASAN
JAYARAMAN KIRUTHI VASAN
6/27/2013 5:06:26 AM
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Master
Re: Treadmills
@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.

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WKetel
WKetel
6/26/2013 8:28:49 PM
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VSD motor operating problems.
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.

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Brad Albing
Brad Albing
6/26/2013 2:36:43 PM
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Re: Treadmills
@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.

 

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JAYARAMAN KIRUTHI VASAN
JAYARAMAN KIRUTHI VASAN
6/26/2013 12:24:03 PM
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Master
Treadmills
@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.

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Brad Albing
Brad Albing
6/23/2013 10:19:24 PM
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Re: BLDC - related toVSD for induction motors
@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.

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D Feucht
D Feucht
6/22/2013 9:21:00 PM
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Re: BLDC - related toVSD for induction motors
@ 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.

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Brad Albing
Brad Albing
6/21/2013 11:22:21 AM
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BLDC - related toVSD for induction motors
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.

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Brad Albing
Brad Albing
6/21/2013 11:17:22 AM
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Re: PROFET
@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.

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