Add a start capacitor to a 1 HP 120vac induction motor

Thread Starter

sdowney717

Joined Jul 18, 2012
648
This motor has a start and run winding with a centrifugal switch with no capacitors.
I would like to reduce the startup amp surge. What about adding a start capacitor in series with the start windings?
And what mfd to choose?
 
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GetDeviceInfo

Joined Jun 7, 2009
1,624
What do the manufacturers spec call for? Any deviation will likely increase draw. Reducing the inrush reduces torque, and subsequently speed, which is required to reduce the inrush.
 

Thread Starter

sdowney717

Joined Jul 18, 2012
648
This is a Dynamo pool pump, pumps water.
Motor is a Magnetech.
It did not originally come with any caps.
It does have a centrifugal switch and start winding. I know for sure, I took it apart and cleaned and painted it.
Here is that motor, this one in the ad says 'century', but it looks just like mine.
http://www.inyopools.com/Products/03001195000407.htm

Link shows various motors, some have no capacitors.
http://www.inyopools.com/motors_pentair.aspx

And there may be room for a cap under the plastic end cover, if it was not a big one. Otherwise, maybe secured into the motor housing with a clamp or tied on etc...
 
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Thread Starter

sdowney717

Joined Jul 18, 2012
648
20180612_063919.jpg 20180612_063903.jpg https://www.amazon.com/Motor-Start-Capacitor-189-227-110-125/dp/B00C0VYC8I/ref=pd_sbs_60_5?_encoding=UTF8&pd_rd_i=B00C0VYC8I&pd_rd_r=SXX4VZN5KNQ14WT3GE61&pd_rd_w=1ZWYL&pd_rd_wg=HCwq9&psc=1&refRID=SXX4VZN5KNQ14WT3GE61

This one looks interesting? 1 3/8 wide. 2 3/4 long.
Someone used it for their pool pump motor.
yes it will fit under there, I made a mock up that is 3 inch long by 1.5 inch wide. It even has molded in standoffs for a capacitor on the end bell. Do you think they saved money by not using a start cap since it can start ok without one?
 
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MaxHeadRoom

Joined Jul 18, 2013
19,058
. Sure a start capacitor increases starting torque, but may increase initial amp draw, makes sense as the cap needs to charge up, so maybe just making a bigger electrical hole to dump current into.
The reason for the start cap is to create a close as possible 90° phase shift (L - C) in the start winding, if just a start winding, I doubt it is close to 90° but probably enough to get the initial spin & direction.
Max.
 
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Thread Starter

sdowney717

Joined Jul 18, 2012
648
The reason for the start cap is to create a close as possible 90° phase shift (L - C) in the start winding, if just a start winding, I doubt it is close to 90° but probably enough to get the initial spin & direction.
Max.
Do you think adding any value of start capacitor would lower current inrush?
I could imagine a start cap lowering total current used to get to full motor speed if it was not optimal.
I can imagine if at 90* then it can't get any better. Motor does spin to full speed fast. ALthough why some pump motors are equipped with them, must make a difference.

Could you add a resistor in series with start windings to lower startup current?

I have this pool pump on our boat to pump salt water. If I have a lamp on the same circuit, it basically goes out or very dim momentarily as the pump comes on. Marina power is a 30 amp feed into the boat. I have a 60 amp gen and it starts with less dimming. Of course a 3000 watt inverter can not start this motor, unfortunately.

And I have pump working in the slip at marina on gen and grid utility power. We take boat out, then with gen running turn on pump. It trips the AFCI-GFCI breaker. I tried it 3 times, . First time on, it ran for a second, then trips. Every subsequent attempt instant trip. We get back to dock, I plug in grid power, it starts every time no trip. I unplug shore power, start gen, it starts every time no trip. so makes me wonder what happened that day.

So I am buying a new Square-D AFCI-GFCI breaker and going to run a dedicated line to the pump, as to me none of what happened makes sense. The Square-D combo breaker I found out can tell you the fault reason, so if it happens again I will know what it thinks it is doing. The AFCI-GFCI circuit I was plugging into for the pump has about 10 outlets all strung out on each side of the boat. Putting this pump on its own line, is a good idea.

I did find this about DC starting resistors.
http://electriciantraining.tpub.com/14177/css/Starting-Resistors-61.htm
2-15 ARMATURE REACTION is the distortion of the main field in a motor by the armature field. This causes the neutral plane to be shifted in the direction opposite to that of armature rotation. Interpoles and compensating windings are used to reduce the effect of armature reaction on motor operation.

STARTING RESISTORS are necessary since the dc resistance of a motor armature is very low. Excessive current will flow when dc voltage is first applied unless current is limited in some way. Adding resistance in series with the armature windings reduces initial current. It may then be removed after counter emf has been built up.
 
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MaxHeadRoom

Joined Jul 18, 2013
19,058
Generally you want to get a induction motor up to run mode ASAP, the longer in start mode the longer/higher the current.
The run winding is the much lower in resistance so the start current is not going to be reduced that much by restricting the higher resistance start winding.
DC motor are slightly different due to the generated BEMF of the armature.
Max.
 

Thread Starter

sdowney717

Joined Jul 18, 2012
648
Well, IT WORKS, adding in the start capacitor of about 150 mfd.
I had an RC0810 Supco hard start which I no longer use. It has a capacitor. And it fits like it was made for it, which it sort of was.

The red wire is for the start winding into the motor, so pull that from the lug, and put the capacitor in series between lug and red wire.
It now starts up fine on my inverter. It seems to be about the same start up time, but somehow the MSW 3000 watt inverter now likes the motor. It easily starts just like if it was on utility or gen power.

Now why is that? does the capacitor allow the MSW to ramp its power output due to it has to fill the cap with power before it can get the motor spinning? Versus it just being this sudden extreme shorting type load? This motor draws abut 9.5 amps when running according to my digital current meter in my boat AC panel.
20180615_162759.jpg
 

MaxHeadRoom

Joined Jul 18, 2013
19,058
The reason is the sooner it comes up to speed, the lower the current, so the closer the phase shift angle in the start winding is to 90° the more efficient the start cycle/period.
The LC combination of the start winding and the cap value provides this phase angle difference.
Max.
 
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Thread Starter

sdowney717

Joined Jul 18, 2012
648
The reason is the sooner it comes up to speed, the lower the current, so the closer the phase shift angle in the start winding is to 90° the more efficient the start cycle/period.
The LC combination of the start winding and the cap value provides this phase angle difference.
Max.
Pump maker left it off to save money, as it is not really needed, but it was not optimal for every situation, like running off an inverter, I mean who does that, hah! Me.

I almost did not bother trying the cap, but it was like, I have it, it cost me zero money to try, and if I did not try, I would always be wondering if it would work.
 
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