3 PHASE INDUCTION MOTOR DESIGNING FOR LOW FREQUENCY OPERATION

On 15 Aug 2004 21:33:35 -0700, NASIR put forth the notion that...

Synchronous speed is 120 X frequency, divided by the number of poles. I'm not sure how slip enters into the equation for a motor turning that slow, but I suspect that aspect of it will be hard to control at such a slow speed. Why not use a gear reduction motor instead, or possibly even a stepper?

As far as I know the slip and the motor terminal voltage will determine the motor run torque, whether you're running at 20RPM or 2000. The only real difference is that at 20RPM most of your input frequency will be contributing to slip.

If you're going to get this to work at all you're going to have to build your driver to achieve a commanded motor torque, then wrap that with a motion control loop to achieve whatever slow speed you want. You'll probably need to have a sensor of some sort on the motor (since I know nothing about your application I'll suggest an encoder, but other choices may be better).

I wish I could recommend a book, but I don't know any that specifically address this issue.

One item often overlooked for motors that will be run at a significantly lower RPM than design is cooling.

Regardless of what other design factors you look at, be sure to consider the cooling needs of the motor when operated at low RPM for any length of time. Many motors intended for frequent low speed operation have an additional motor designed specifically to drive the cooling fan at full speed regardless of the main motor speed.

Louis--

********************************************* Remove the two fish in address to respond

in article snipped-for-privacy@posting.google.com, NASIR at snipped-for-privacy@hotmail.com wrote on 8/15/04 9:33 PM:

You should mention your application. If you want any significant power from this motor, it will be at high torque and low speed. This is what we call an impedance mismatch.

Impedance mismatches can be mechanical as well as electrical. The matching transformer is usually a gear train.

Bill

Just build a 720-pole stator. :-)

Cheers! Rich

Impedance mismatch is not a factor considered in the design of electric machines. It can be a factor in matching a given motor to a given load as you indicate-i.e. gears as matching transformers as has been done for over

100 years. Certainly if you have a machine that has a design speed of 1000 rpm, a good gear train is necessary if you want to couple this to a 20 rpm load. If the motor is designed for low speeed high torque operation then such matching becomes moot. As Rich indicates - all one needs is a lot of poles if at 60 Hz. If at a lower frequency such as 5Hz, then the magnetising considerations become paramount and the resultant machine will be due to the amount of iron needed . (a 5Hz 20 rpm motor will have 30 poles and flux density is limited- all affecting design and voltage rating) Such a machine will inherently be high torque at low speed-set the power and speed desired and the torque follows accordingly. In the desired speed range- it actually might be best to forget an induction motor and consider a DC machine -preferrably with a gear train as you indicate.

In electric power applications, in general, impedance matching is not a particularly useful concept- No power line, for example is operated anywhere near the impedance matched conditions- too inefficient and asking for instability. The "source" impedance is generally kept much lower than the load impedance. Communications and electronics march to a different drummer-mainly because lines are generally multiple wavelengths and different demands..

-- Don Kelly snipped-for-privacy@peeshaw.ca remove the urine to answer

Sincere thanks to all of you who have commented on my question.

My application is such that which requires low torque(1 Nm) at this low speed(20 rpm) and use of gears is not possible.

Mr. Tim could you elaborate your controller concept, because i have tried this but i have not succeeded in it( i.e motor designed at 50 Hz and run at 20 rpm by employing V/f control). If you know a book that touches "low speed designig" then it can be of great help.

Mr Kelly, You have presented a very good approach about increasing the number of poles but i have seen a SIEMENS motor that was 4 poles (without gear) and still operating for 0-50 Hz( 0-1500rpm. I wonder how they would have achieved that thing.

If you start with a 50 Hz motor that is rated to output 75 Nm @ 1500 RPM and the controller is matched to the motor it should be possible to get your requested 1 Nm @ 20 RPM. The lower the frequency, the greater the error in speed control however. If you require good regulation on the speed, you should consider a closed loop system with feedback. A vector control with an encoder which will add to the cost of the system but should have better performance at lower speeds.

The manufacturer of the motor should be consulted regarding the question of cooling. For continued operation at this low speed additional cooling may be required through use of a constant velocity fan unless the motor is rated for a turndown ratio this low.

You should also consider a motor with 6 or motor poles to raise the frequency to the motor @ 20 RPM.

Ed

Have you considered an ordinary motor and Variable-Frequency Drive?

Good Luck! Rich

What will be the source frequency? Synchronous speed is RPM = 120 * f / N where N is the number of poles. For a 60 Hz source, you need 360 poles for

20 RPM (300 poles for 50 Hz). Slip is typically 2 to 7 per cent of base speed at rated torque for the motors with which I'm familiar (900 RPM to 10,000 RPM). Your slip mileage may vary.

For starters, see "Electric Machinery" by Fitzgerald, Kinglsey, and Kusko.

You could look into a VFD (Variable Frequency Drive) as the source. These days they are claiming full-rated torque from zero speed on up to base speed. Some can do it without feedback by relaxing regulation requirements. Perhaps you will settle on a combination of many motor poles with a VFD.

You may find that you cannot buy the laminations you will need for this monster motor and, as another poster mentioned, you have cooling requirements to consider.

I do not envy you. Good luck.

John

Sincere thanks to all of you who have commented on my question.

My application is such that which requires low torque(1 Nm) at this low speed(20 rpm) and use of gears is not possible.

Mr. Tim could you elaborate your controller concept, because i have tried this but i have not succeeded in it( i.e motor designed at 50 Hz and run at 20 rpm by employing V/f control). If you know a book that touches "low speed designig" then it can be of great help.

Mr Kelly, You have presented a very good approach about increasing the number of poles but i have seen a SIEMENS motor that was 4 poles (without gear) and still operating for 0-50 Hz( 0-1500rpm. I wonder how they would have achieved that thing.

in article snipped-for-privacy@posting.google.com, NASIR at snipped-for-privacy@hotmail.com wrote on 8/18/04 12:47 AM:

Why an induction motor? Why not one of these appliance motors if you can find on to run that slowly? I have seen a piezo electric motor using a bimorph twister. That however uses a pawl and ratchet arrangement so that is some kind of gear. The gear would have 3600/20 = 180 teeth assuming that it steps once per cycle. You might also consider some kind of Selsyn system where you motor tracks a small master motor.

My guess is that you are trying to use an elephant gun to kill a fly.

Have fun.

Bill

Probably uses flux vector or some similar field oriented control method. More complex than a simple V/f but the controllers are readily available.

Robert

Hmm. What happened? Were you modulating the voltage on your variable frequency drive? How large of a motor were you using? If all you ever wanted to do was go at 20rpm you should be able to use a fixed 5Hz or so and adjust the torque by adjusting the drive voltage - 20rpm is slow enough that you're essentially not moving the motor, you're just exerting torque.

While you're pondering that, why aren't you using a DC motor? The drive is certainly easier -- if you're doing a 1-off project the savings in engineering time will more than offset the more expensive motor.

If I have not erred in the conversion, 1 Nm is 0.7376 lbf*ft. The horsepower represented by this torque at 20 RPM is T*RPM/5252 or less than .003 horsepower. (This assumes that the motor is designed to operate at a base speed of 20 RPM.) However, using a 4-pole motor, your motor's horsepower rating will need to be .7376 * 1800/5252 or somewhere around .25 hp. You would need only a tiny motor and VFD with a feedback setup.

Or get a .25 hp, 230 V, 60 Hz, 3-phase, 4-pole motor and design a 3-phase PWM bridge to drive the motor at 60*20/1800 or .666 Hz. Use a bridge DC supply voltage of 230*1.414*20/1800 or about 3.6 V plus a little more for the semi drops. (PWM is needed here because of the cogging you'd get with a

6-step drive.)

Remember that the slip is somewhere around 5% or so with an itty-bitty motor (IIRC). That's maybe 90 RPM. So you have a motor doing 20 RPM at no load and it has to drop 90 RPM to supply full load. It can't. But you can manually compensate for a fixed load by adjusting the frequency and bridge voltage higher (keep the same V/f ratio, approximately).

This sounds like a career in the making.

Good luck.

My comment on the number of poles was partly to indicate an impractibility (theoretically possible) approach. ED has given a good answer and Rich has given the name that such a scheme goes by.

Thanks to all of you who have replied.

My application is:

--airgap larger than 6mm

--torque 1 Nm

--speed range 20 to 80rpm

--low current

--220V supply

--also have dimension constraints

I have been successful in designing this motor for 50Hz operation with four pole stator( synchronous speed = 1500 rpm). It was satisfying all the parameters. Now when I tried to control the motor speed by applying V/f control the results were horrible. Current rose to 16 amperes( four times of orignal value), maximum torque achieved was very lower than the required one and also there was saturation problem.

I have tried every thing in my knowledge but to no avail. I have not tried increasing the number of poles and vector control yet.

What are your kind auggestions.

regards, Nasir.

Thanks to all of you who have replied.

My application is:

--airgap larger than 6mm

--torque 1 Nm

--speed range 20 to 80rpm

--low current

--220V supply

--also have dimension constraints

I have been successful in designing this motor for 50Hz operation with four pole stator( synchronous speed = 1500 rpm). It was satisfying all the parameters. Now when I tried to control the motor speed by applying V/f control the results were horrible. Current rose to 16 amperes( four times of orignal value), maximum torque achieved was very lower than the required one and also there was saturation problem.

I have tried every thing in my knowledge but to no avail. I have not tried increasing the number of poles and vector control yet.

What are your kind auggestions.

regards, Nasir.

in article snipped-for-privacy@posting.google.com, NASIR at snipped-for-privacy@hotmail.com wrote on 8/19/04 1:18 AM:

I think that you are just rediscovering why motors ar not built to operate at such low speeds.

Bill