one of our customer need a cheap speed control for some 3/4 horse grinders. Were trying to say away from VFD's they get expensive. they have 8 motors to control. keeping the torque high is a big requirement. These motors grind hard metal all day long. We contacted Allen Bradley with no luck. If anyone out there has some idea's it would be much appreciated.
You are using an out of date browser. It may not display this or other websites correctly.
You should upgrade or use an alternative browser.
You should upgrade or use an alternative browser.
need cheap motor speed control
- Thread starter mrwoo1981
- Start date
- Status
- Not open for further replies.
Re: need cheap motor speed control
I think if you could develop a simple and cheep drive to accomplish this you would have to work the rest of your life.
If they are induction motors, which they probably are, the speed is based upon the frequency, i.e. in very basic terms the synchronous speed of a 2p motor at 60hz is 3600rpm where the actual slip speed is approximately 3450rpm slip and at 50hz the same motor sync speed is 3000rpm, 4p = 1800rpm syc. and 1725rpm slip and 1500rpm sync at 50hz, etc. So the motor rpm is related to the frequency which = a vfd or afd.
You also are asking for the torque to be kept high. Again, the drive will have to control the voltage and current to the motor at the given frequency to realize the desired torque.
I think if you could develop a simple and cheep drive to accomplish this you would have to work the rest of your life.
If they are induction motors, which they probably are, the speed is based upon the frequency, i.e. in very basic terms the synchronous speed of a 2p motor at 60hz is 3600rpm where the actual slip speed is approximately 3450rpm slip and at 50hz the same motor sync speed is 3000rpm, 4p = 1800rpm syc. and 1725rpm slip and 1500rpm sync at 50hz, etc. So the motor rpm is related to the frequency which = a vfd or afd.
You also are asking for the torque to be kept high. Again, the drive will have to control the voltage and current to the motor at the given frequency to realize the desired torque.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Re: need cheap motor speed control
I have actuly used heavy duty fan controls to do just what your asking. Econ makes one that will even control up to 2hp@240vac there alot cheaper than soft start or VFD's. Now would it be code compliant well that would be another thread.
I have actuly used heavy duty fan controls to do just what your asking. Econ makes one that will even control up to 2hp@240vac there alot cheaper than soft start or VFD's. Now would it be code compliant well that would be another thread.
Re: need cheap motor speed control
A HD speed control, interesting. Is it able to "keeping the torque high?" It's not uncommon to control the speed of a motor by reducing the voltage against a load and causing it to slip. Soft starts control voltage and current. But, how is the torque requirement addressed? Although mrwoo1981 gave sketchy information with regards too his application, I would assume that it is mot a starting or stopping issue, it's a run issue.
In addition, a fan load is often constant so the speed may possibly be controlled by voltage, again, causing the motor to slip, thus reducing the RPM. A grinding load is more often than not a duty cycle from an unloaded condition to a loaded condition. A motor with a reduced voltage applied to it will seek its synchronous speed while drawing a given current. As soon as a load is applied to the motor the soft start or other controlling means must react fast enough with voltage and current to provide enough torque to prevent the motor from bogging down. To monitor the variations in load by electronic means is not a cheep issue. This allocation issue has proven to be problematic when soft start technology is also applied to the economizing energy saving applications.
A HD speed control, interesting. Is it able to "keeping the torque high?" It's not uncommon to control the speed of a motor by reducing the voltage against a load and causing it to slip. Soft starts control voltage and current. But, how is the torque requirement addressed? Although mrwoo1981 gave sketchy information with regards too his application, I would assume that it is mot a starting or stopping issue, it's a run issue.
In addition, a fan load is often constant so the speed may possibly be controlled by voltage, again, causing the motor to slip, thus reducing the RPM. A grinding load is more often than not a duty cycle from an unloaded condition to a loaded condition. A motor with a reduced voltage applied to it will seek its synchronous speed while drawing a given current. As soon as a load is applied to the motor the soft start or other controlling means must react fast enough with voltage and current to provide enough torque to prevent the motor from bogging down. To monitor the variations in load by electronic means is not a cheep issue. This allocation issue has proven to be problematic when soft start technology is also applied to the economizing energy saving applications.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Re: need cheap motor speed control
I was told that fan controls controled the speed by frequency and since a motor controled by frequency would cause a motor to try to stay in sync the torque would not be a problem.
templdl:By: templdl A fan load is often constant so the speed may possibly be controlled by voltage,
I was told that fan controls controled the speed by frequency and since a motor controled by frequency would cause a motor to try to stay in sync the torque would not be a problem.
Re: need cheap motor speed control
hurk27, Then it's an AFD which I wouldn't think would be cheep. But, then again, cheep is a relative term. A fan is most likely a fairly constant load where a grinding operation with almost all certainty in a cycling load.
hurk27, Then it's an AFD which I wouldn't think would be cheep. But, then again, cheep is a relative term. A fan is most likely a fairly constant load where a grinding operation with almost all certainty in a cycling load.
tony_psuee
Senior Member
- Location
- PA/MD
Re: need cheap motor speed control
hurk,
As templdl states a fan is a fairly constant load. A VFD, or AFD, maintains the constant torque output by maintaining the volts/hertz ratio equal to nameplate volts/nameplate hertz, ex. 460/60=7.67v/Hz. For a VFD on a constant torque load check anywhere between minimum speed and full speed and the volts/Hertz will equal 7.67, or close to it. Hope this helps.
Tony
hurk,
As templdl states a fan is a fairly constant load. A VFD, or AFD, maintains the constant torque output by maintaining the volts/hertz ratio equal to nameplate volts/nameplate hertz, ex. 460/60=7.67v/Hz. For a VFD on a constant torque load check anywhere between minimum speed and full speed and the volts/Hertz will equal 7.67, or close to it. Hope this helps.
Tony
charlie tuna
Senior Member
- Location
- Florida
Re: need cheap motor speed control
to do the job right you will have to go with vfd to maintain constant torque at reduced speed. any other arrangement and it will get very expensive replacing motors. this is an expensive design and i have seen people try short cuts and waste alot ot time and money before realizing the correct avenue to solve the problem. one was in an injection operation - they made boat fuel tanks out of foam!!! this is big bucks they get for these tanks - and had a series of injection pumps to pump the liquid into the mold at the same time and volume.... and when they finally went to vfd's they undersized them!!!!!! they had alot of money invested before they got it right!
to do the job right you will have to go with vfd to maintain constant torque at reduced speed. any other arrangement and it will get very expensive replacing motors. this is an expensive design and i have seen people try short cuts and waste alot ot time and money before realizing the correct avenue to solve the problem. one was in an injection operation - they made boat fuel tanks out of foam!!! this is big bucks they get for these tanks - and had a series of injection pumps to pump the liquid into the mold at the same time and volume.... and when they finally went to vfd's they undersized them!!!!!! they had alot of money invested before they got it right!
G
Guest
Guest
Re: need cheap motor speed control
aqre these grinders single phase or three phase? makes a huge difference in any type of control. Single phase grinders in that hp range are cpacitor start motors and connot be speed controlledat all. So which do you have ?
aqre these grinders single phase or three phase? makes a huge difference in any type of control. Single phase grinders in that hp range are cpacitor start motors and connot be speed controlledat all. So which do you have ?
- Status
- Not open for further replies.