Slow down speed on two single phase ac torque motors

Thread Starter

maninthemoon

Joined Jan 23, 2019
1
i have not posted on forums previously please bear that in mind.

I have a PLC that controls two 230v single phase torque motors on a set of gates that run with 5uF capacitors. i need to slow the motor down in its final 30 degrees in order for the gate to gain safety compliance when force tested
The motors are:
Single Phase.
Voltage - 220VAC
Torque - 50Nm
5hp
Speed - 7.5 rpm
50hz

I have seen another similar gate with the same motors with four capacitors, one to each motor and an additional one to each motor switched via relay as a form of speed control. is this the best method?

I was told i could use a VFD with a single phase output as long as the voltage and HZ were adjusted simultaneously to prevent damaging the motor, but once again would this be OK? Could it damage the motor? it was a motor manufacturer offered this information.

Any advice would be greatly appreciated, thank you
 

MaxHeadRoom

Joined Jul 18, 2013
19,029
The availability of VFD's for single phase motors are rare, this is due to the often experienced drop out of run at low RPM's and/or a load at low rpm. Especially for 5hp.
I am surprised a motor manf would offer this suggestion.
If 5hp they are most likely capacitive start, and cap run with the start cap switched out when up to speed via a centrifugal sw. Assuming they are run-of-the-mill Induction motors.
The bottom line is that single phase induction motors above 1/2hp do not do well using either phase control or frequency control.
Max.
 

LesJones

Joined Jan 8, 2017
2,319
Hi Max,
50Nm and 7.5 rpm does not seem to tie up with 5 HP. If my calculation is correct it is only about 0.026 HP.
Edit. Correction it should be 0.052 HP.

Les.
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
19,029
If in fact it is a fractional HP motor as Les surmises, then it may be possible to use a SCR control if it is a simple PSC motor. Maybe 0.5HP?
With gearing of some kind evidentally.
Max.
 

MisterBill2

Joined Jan 23, 2018
3,869
A series capacitor is one way to reduce the speed of a motor, but for a 5HP motor they would be big and expensive. So I am wondering what is the current of the motors?And with that slow speed of 7.5 RPM I am presuming there is a gearbox involved.Using a capacitor is a reasonable approach to slow the speed and reduce the power as well. Unfortunately it may not be acceptable for safety purposes.
 

LesJones

Joined Jan 8, 2017
2,319
Hi Max,
It was the 5uF capacitor that first made me think that it did not seem right to go with 5 HP. I have no sense of how large a torque of 50 Nm is so I used an online converter to convert it to pound feet. This was 36.9 lb ft so this would be 36.9 x 2 x Pi foot pounds per revolution. (I initially forgot the 2 in the calculation.)

Les.
 

MaxHeadRoom

Joined Jul 18, 2013
19,029
@maninthemoon At 7.5rpm, I would assume a reduction of some kind, what is the rpm of the motor, if 4 pole it would be in the region of 1450rpm, so if you final rpm is 7.5 that would mean you have a reduction of around 200:1.
Where is the 50Nm measured?
Max.
 

Alec_t

Joined Sep 17, 2013
10,361
I agree that 0.5HP is much more likely than 5HP. Perhaps the TS could have another look at the rating plate? That speck of dirt is a decimal point :).
 

MisterBill2

Joined Jan 23, 2018
3,869
Of course I have seen elevator motors that did run that slowly. They were very old elevator motors, with the entire control systems built on open panels in the elevator level of the building. Those were the manually operated ones, with that crank thing on the wall. Another good question will be the physical size of the motors, 6 or 7 inches diameter and a foot long points to 1/2 HP while 10 inches diameter and 24 inches long points more towards 5HP.
 

MisterBill2

Joined Jan 23, 2018
3,869
Easy answer is to simply read the motor plate info!:rolleyes:
Max.
I have come across motors having "motor plate" information that is totally illegible, either through wear and abuse or, more often, because there was no quality control on the printing process. And for one industrial lathe, because all of the characters, including all numbers, were in Chinese, a language and alphabet that I am not able to read.
 

MisterBill2

Joined Jan 23, 2018
3,869
The profile lists him as an engineering manager. But not what kind of engineering.
Does an engineering manager have to be an engineer??
I know that at Methode we lost our manager who was a good engineer and in pace we got a manager whose main focus was keeping cubicles neat. So the job went from one that I looked forward to every morning to one that was a whole lot less wonderful. Worse yet, that manager initiated a staff reduction, and then added that "if we find that we have removed too many engineers, we will just hire some more", indicating that he thought all engineers were equally qualified to step into any job. That might be true for bean counters, and certainly true for MBA types, but it is not even close to sort of correct for experienced engineers.

Sorry for being a bit off-topic for this thread.

I see the explanation for why there was a need to slow the motors but I do not believe that torque motors would be used to open and close gates. Probably LIMITED torque motors, but those are different. I once designed a machine that required a limited torque motor that had to stall if overloaded. That was easy to do.
 
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