Motor starts at full torque. Does not ramp up. Why? - Electrician Talk - Professional Electrical Contractors Forum
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Old 08-19-2019, 03:51 PM   #1
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Question Motor starts at full torque. Does not ramp up. Why?

Good day folks. I know just a touch about motors. I know how to wire them, but to diagnose issues, I'm not so good.

The motor on a client's woodworking machine (3HP, 240AC, single-phase) seems to start up with full torque instead of ramping up to speed over a second or two. The start-up stresses the gears and chains of the machine and I think something may break if the situation isn't corrected.

Can somebody point me in the direction of what to check. As I said, I know little about motor troubleshooting. I don't have a clue what to check.

Any information or suggestions would be greatly appreciated!
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Old 08-19-2019, 03:59 PM   #2
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I would start by testing the Caps, then the cap switch.
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Old 08-19-2019, 04:03 PM   #3
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By "cap switch" you are referring to the centrifugal switch?
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Old 08-19-2019, 04:31 PM   #4
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Quote:
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By "cap switch" you are referring to the centrifugal switch?
yup. Spaced the name 😉
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Old 08-19-2019, 05:16 PM   #5
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It sound like it has a mechanic issue. A single phase motor does not have a startup curve, it just tries to get up to speed ASAP. I've never seen it designed into a machine because it is unpredictable due to the load on the motor. Look for worn sprocket or coupling.

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Old 08-19-2019, 06:55 PM   #6
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Motor is probably fine. If your stressing mechanical parts to the point where the fear of damage is near, then its a mechanical problem. How can a motor starting fast be a new symptom?

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Old 08-19-2019, 08:44 PM   #7
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What kind of controller does it have? Across the line starter? Unless it has a soft start, it's doing what motors do.
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Old 08-20-2019, 09:40 PM   #8
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99%* of AC motors start with MORE than full rated torque when energized, about 160% of rated Full Load Torque, UNLESS you use some artificial means to dampen that, such as a "soft start" or other form of "reduced voltage starter" or VFD. But those devices don't work well with 95% of single phase motors.


So if you are under the impression that a motor normally "ramps" up to speed, that's the thing that is incorrect. Sometimes it APPEARS to take a while to get to full speed because of high inertia in the load or high friction, but the motor itself will be at full starting torque within 1 second or less.


*The exception are Shaded Pole motors, which are primarily sub-fraction HP size and used on little appliances like fans. Think "muffin fans".
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Old 08-21-2019, 08:13 AM   #9
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Electric motors have nearly instantaneous max torque. This is the advantage of electric motors, and sometimes a disadvantage. It's why the Tesla Model S does 0-60 in 2.8 seconds.
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Old 08-21-2019, 04:52 PM   #10
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Quote:
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Electric motors have nearly instantaneous max torque. This is the advantage of electric motors, and sometimes a disadvantage. It's why the Tesla Model S does 0-60 in 2.8 seconds.
hi gr.weldon
i agree with Jraef and coolwill this motor will start as if it has no load FAST because it is a woodworking machine and will normally don't have wood stuck in the blade or some ware in the process (don't know what machine you are referring to)
what you can do is install a rubber coupling between the motor and the mechanical drive to lessen the direct torque on the system if it is possible
otherwise get your money and go ASAP (and don't give a warranty on the mechanical part hahaha)
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Old 08-21-2019, 07:26 PM   #11
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I've never seen one but Allen Bradley (and presumably other manufacturers as well) makes an in-line soft-start for single phase motors up to 3 HP @ 230 Volts.

The one you'd need is a 154-SP1C25NAD.

It goes between the switch and the motor and it needs a constant 120 - 240 Volt control source. This can come from the line side of the switch.

I've used these on 3Ø motors with good results but not on a single phase one......yet......

You can set the initial torque anywhere from 0 - 80% and the ramp time from 0.5 - 5 seconds.
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Old 08-26-2019, 05:21 PM   #12
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Quote:
Originally Posted by micromind View Post
I've never seen one but Allen Bradley (and presumably other manufacturers as well) makes an in-line soft-start for single phase motors up to 3 HP @ 230 Volts.

The one you'd need is a 154-SP1C25NAD.

It goes between the switch and the motor and it needs a constant 120 - 240 Volt control source. This can come from the line side of the switch.

I've used these on 3Ø motors with good results but not on a single phase one......yet......

You can set the initial torque anywhere from 0 - 80% and the ramp time from 0.5 - 5 seconds.

On single phase motors, you can't use those on any motor that has a starting capacitor or a centrifugal switch. So that's about 95% of single phase motors.
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Old 08-26-2019, 08:59 PM   #13
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Quote:
Originally Posted by JRaef View Post
On single phase motors, you can't use those on any motor that has a starting capacitor or a centrifugal switch. So that's about 95% of single phase motors.
I thought the same thing but if you look that part number up on Rockwells site, it plainly states, in the product overview part, that it can be used on PSC (obviously......) and also on capacitor-start and capacitor -run as well.

I don't know if it's a mis-print or if ramping the voltage on a CSIR or CSCR motor would be ok or not. In a certain sense, a small long extension cord would be a form of reduced-voltage starting......not that anything like that ever actually happens.......lol.

I suspect if the motor were driving a centrifugal pulp or blower, it's start ok but if it were driving something that starts hard, it'd have a tough time transitioning from start to run. Might burn up the start capacitor and/or the centrifugal switch fairly fast.......

But, as I stated, I've never used a single phase model, only 3Ø ones. They work great, by the way.
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Old 08-26-2019, 09:59 PM   #14
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You're just phase shifting the voltage with the caps, no reason it can't work if the soft start takes the place of the potential relay and doesn't ramp up too long. 5-10 seconds would be fine, not 30.


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Old 08-26-2019, 10:28 PM   #15
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To the SCRs in the soft start, the capacitor charging current looks like a short circuit, so high dI/dt (rate of change in current) which can cause them to self-commutate, meaning turn on when you are not telling them to. So if your SCRs fire at the wrong time, they can damage each other.

At the same time, the chopped waveform of the SCR Phase Angle firing that comprises a "soft start" has a very high harmonic content, which gets absorbed by the capacitors. But the capacitors are not designed / selected for that. On top of that, starting caps are selected to be in the circuit for a second, maybe two, not for a longer period encountered by soft starting.

So what happens is a race to see what fails first; the SCRs in the soft starter or the caps in the motor.

PSC motors can survive better because the caps are designed to be in the circuit all of the time and they are in series with one set of motor windings, so there is effectively an inductor ahead of them which slows down the rise time of the capacitor charging current seen by the SCRs. Not ideal, but less risky of failure on both parts. However PSC motors are low starting torque to begin with and are typically relegated to pumps and fans, not machine tools.

Most soft starter mfrs have pulled their single phase soft starters off the market because there were so may failures in the field. But it's usually the motor caps that fail, not the starters. I see however that A-B redesigned these to only allow up to 5 seconds of ramp time max., that's probably why it can work. When I was in the Soft Start industry and we experienced all of those problems, people were setting the ramp times out to 30 seconds or more.
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Last edited by JRaef; 08-26-2019 at 10:31 PM.
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