3 PHASE INDUCTION MOTOR DESIGNING FOR LOW FREQUENCY OPERATION

[NASIR]

I am working on the designing of a 3 phase induction motor for
operation at 20 rpmm. But i am not finding the exact
relationships/foumulas which hold true at low frequencies(i.e below 5
Hz). Could anyone guide me about a "booK" which deals with the abouve
mentioned subject or can give me some guidelines which should be
observed while designing at low frequencies.

 

[Checkmate]

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

I am working on the designing of a 3 phase induction motor for
operation at 20 rpmm. But i am not finding the exact
relationships/foumulas which hold true at low frequencies(i.e below 5
Hz). Could anyone guide me about a "booK" which deals with the abouve
mentioned subject or can give me some guidelines which should be
observed while designing at low frequencies.

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?

 

[Tim Wescott]

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.

 

[Louis Bybee]

I am working on the designing of a 3 phase induction motor for
operation at 20 rpmm. But i am not finding the exact
relationships/foumulas which hold true at low frequencies(i.e below 5
Hz). Could anyone guide me about a "booK" which deals with the abouve
mentioned subject or can give me some guidelines which should be
observed while designing at low frequencies.

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

 

[Repeating Rifle]

I am working on the designing of a 3 phase induction motor for
operation at 20 rpmm. But i am not finding the exact
relationships/foumulas which hold true at low frequencies(i.e below 5
Hz). Could anyone guide me about a "booK" which deals with the abouve
mentioned subject or can give me some guidelines which should be
observed while designing at low frequencies.

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

 

[Rich Grise]

I am working on the designing of a 3 phase induction motor for
operation at 20 rpmm. But i am not finding the exact
relationships/foumulas which hold true at low frequencies(i.e below 5
Hz). Could anyone guide me about a "booK" which deals with the abouve
mentioned subject or can give me some guidelines which should be
observed while designing at low frequencies.

Just build a 720-pole stator.

Cheers!
Rich

 

[Don Kelly]

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

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 <big> 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..

 

[NASIR]

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 <big> 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..

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.

 

[Rich Grise]

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.

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

Good Luck!
Rich

 

[The other John Smith]

I am working on the designing of a 3 phase induction motor for
operation at 20 rpmm. But i am not finding the exact
relationships/foumulas which hold true at low frequencies(i.e below 5
Hz). Could anyone guide me about a "booK" which deals with the abouve
mentioned subject or can give me some guidelines which should be
observed while designing at low frequencies.

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

 

[NASIR]

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.

 

[Repeating Rifle]

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.

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

 

[R Adsett]

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.

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

 

[Tim Wescott]

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.

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.

 

[The other John Smith]

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.

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.

 

[Don Kelly]

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.

 

[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.

 

[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.

 

[Repeating Rifle]

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

Bill

 

[John Woodgate]

I read in sci.electronics.design that Repeating Rifle
net>) about '3 PHASE INDUCTION MOTOR DESIGNING FOR LOW FREQUENCY
OPERATION', on Thu, 19 Aug 2004:

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

Wouldn't a stepper motor be an option?