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40hp Pony start RPC blowing 150amp breaker

simonswb6

Aluminum
Joined
Feb 11, 2013
Location
ny
I wanted to post a solution to a pony start rpc problem that has gone on for weeks, hopefully it will save someone some time should they choose to go this route. There are many success stories using a pony start rpc and few reported problems which was one of the frustrating things about this project. I was at the point where i was going to throw in the towel and go the capacitor start, i even bought what i needed.

The pieces:
40hp rpc
1.5hp pony
50hp contactor
150amp breaker
50hp disconnect
200amp service

Problem:
1) Motor would blow the 150amp breaker with violent impact, sometimes, and this is the one of the main points of this post. Why would it start perfectly sometimes, but blow the breaker other times? There were times that it appeared to spin the rpc backward when the single phase was thrown.
2) in setting this all up, i blew the secondary fuse on the transformer.
3) i burned up one of the pony motors that i had trying to configure all of this.

Solution:
For a larger (40hp and up) pony start rpc certain conditions need to be met in order for a smooth startup without a violent breaker throw. First, bringing a pony start rpc online with grid power is similar to paralleling 2 generators. If you have two 5000w generators, when you parallel you effectively have a 10kw system. However, the manner in which you connect both generators is critical. Here is a list of requirements that need to be met to parallel, in 2 generators:
Frequency: the frequencies must be the same
Voltage: The generators should produce or be set to produce the same voltage
Phase number: The two systems should have the same number of phases, either three or single phase.
Phase rotation: For three phase systems, each of the three phases must be matched. This prevents excessive mechanical and electrical stresses as well as avoiding power surges.
Voltage Phase angle: The wave-forms should be matched such that they rise and fall together. There should be no angle difference and the potential difference between the phases should be zero.

This list of requirements is the same for a rpc/pony start
Frequency: the frequencies must be the same, we try and get rpc as close a match to the rpm rated for the motor as possible.
Voltage: the incoming voltage is the coming from the grid, so that is a match.
Phase number: we are trying to match the single phase.
Phase rotation: single
Voltage Phase angle: The wave-forms should be matched such that they rise and fall together. There should be no angle difference and the potential difference between the phases should be zero. I believe this the most important aspect of a large pony start rpc. When an attempt is made to try to synchronize an rpc and is out of phase with a grid , the magnetic fields of the rpc are in opposition (depending on the amount of "asynchronism") to those induced by the grid. So, the rpc motor has to speed up or slow down, sometimes by as much as 180 degrees, in a very short period of time. That's where the damage comes into the picture; as fast as it has to speed up or slow down to get into synchronism with the grid field it has to suddenly stop when synchronism is achieved. And, the damage can be caused by either or both of the sudden speed change and/or the sudden speed stop. I have noted that if i started when the there is a significant difference in the phase angle that the rpc will appear to turn in the opposite direction as the pony motor. This introduces a tremendous amount of current into the system which is why the breaker was getting thrown so violently. when the things were in-sync, there was very little in-rush current, and things would start smoothly.

Bring a rpc motor in phase with the grid power
This is the key to the entire setup. If you look at the drawing of the phase angle where they 2 sine waves are out of sync, you see the point where they cross the x axis, the current will be zero. If you connect a 240v light between the incoming single phase power l1 and the rpc l1, you will set the light turning on and off. the light is on when the sine waves are negative or positive, the light goes out when both sine waves cross zero. When they both cross zero axis, and you turn on the grid power to the rpc, you will force both waves to start at the same point, forcing them to be in sync. see video below:
Pony Start RPC - YouTube

Smaller pony start RPC.
There is little talk of problems with smaller RPC pony start motors. In fact, scouring the forum there was very little information on large RPC pony start motors. I believe larger RPC have tremendous inertia created by a significant mass of steel of bigger motors. Once going in one direction, it is very difficult to right the phase of the grid unless timing is correct. With smaller motors, i believe this can be easier to correct and the phase angle can be forced it to be in sync w/ the grid, even when the timing is off.

Things to watch for.
1) the second time i threw the switch to try and start the pony rpc motor, with out knowing about things being out of phase, i blew the secondary fuse on the main grid connected transformer. I had a 25kva transformer, when the power company came to take a look, i mentioned that i had some purchased some newer milling equipment and i believed that the transformer i had was a little undersized. They agreed and replaced it with a 50kva transformer.
2) i had a 100amp breaker on this setup. Even after i figured out the how to start the rpc in phase w/ the grid, it was still blowing breakers. Not nearly as violently when things were out of phase, but still blowing. I ended up putting a 150amp breaker and things worked fine.
3) I had an older pony motor setup. The violent shifting that occurred when the rpc was out of phase, eventually burned it up.
 
Did you consider using a 3 phase pony motor with a VFD for a really slow ramp up and then disconnecting the VFD/pony once up to speed?
 
There's a guy in the UK did a 4-vid youtube series on RPC's who touched on this subject. He got around the massive inrush by putting the balancing caps on a breaker and switching them in AFTER switching over from pony motor to idler. He said that he got some pretty big spikes with the caps connected but they went away by switching them in after the idler had sync'ed.

https://youtu.be/lydbIbj1EGo

Description at 2.25 though he attributes the surge to a speed loss and charging the caps.
 
An induction motor has no "phase" until power is applied *, because aside from some residual magnetism, there is no field (unless balance caps are connected). The field comes from the rotor current, which does not exist until there is a magnetic field in the stator established by the mains current. That rotor field is automatically in the correct phase, and cannot be opposite..

Your issues seem to be from having balance caps connected, and possibly from the speed to which the pony gets the main motor at startup. It would seem that if you disconnect the "balance" capacitors during startup, and perhaps insert a resistance in series during startup to cut the remaining surges, you should have no further problems.

Any residual magnetic field will only generate a few volts, and is not a major issue (unless "balance" caps are connected)**

There IS a considerable inrush because there is a fair bit of iron in a 40 HP motor, plus the rotor current has to be established. Until back EMF is present, the motor is nearly a short across the line. But your 150A breaker is actually bigger than should be needed for that size motor, and would be expected to be "enough".

I have no idea what speed your motor is brought up to by the pony motor. The closer that is to formal no-load rpm, the better, because there is also a current surge due to acceleration, which can be considerable by itself.

*
The rotor has no preferred position of the field. The field is established ny the currents in the "squirrel cage" windings, and can be at ANY angle around the rotor, there is no special position. And, there is no current until it is established from outside the motor, by the mains voltage, or by "excitation capacitors". The position of the rotor field is established "as needed" by the currents in the stator, and will be in correct position. So there should be no need for any adjustment of the rotor position to bring it into "correct phase" as long as there are no capacitors connected.

**
If "balance" caps are attached, then there can be a "phase angle" if the caps are of sufficient size to allow the motor to "build up" as an induction generator. They would also be a considerable load on the pony motor, as I have found when experimenting with smaller motors in my lab. It's a good idea to connect them later, after the motor is running.

With a residual field, the balance caps have some voltage applied by the small "generation" due to that rotating field. They draw current, that current strengthens the field and can "build up" by feedback until the motor is generating at least a full rated mains voltage (it can even go higher). The capacitors are said to "supply exciting current" to the motor.
 
How about connect the motor online with a resistor in series, then short out the resistor. 1 ohm perhaps for a 40hp motor.

There was someone else on this forum who had the same problems with a large motor. his was mounted vertically.

edit: https://www.practicalmachinist.com/...d-vfd/big-rpc-stalling-out-pony-start-357415/

Hey, that's me.

I did use 1 ohm on each leg in the form of stove coils. (A bit less when they are cold).

I made the panel I just haven't wired it in yet.
 
JST has nailed this but just let me offer something to cut down the switch over current spike. Drive the RPC from the pony through a belt, clutched if you don't want the pony running normal operation. Adjust the pulley ratios so that The RPC is just below synchronous. Connect the RPC through a resistor until the motor is switched over, then short it out. Look up primary resistor reduced inrush starter. Also look up closed transition wye-delta starters.

It is well known and documented and I have experienced it that when a motor (or transformer) is de-energized, that depending on the phase angle that the iron can be near saturation in one direction (high Hc). When the power is re-established if the phase angle of the power is near 180* of the remnant Hc, there will be a pulse of current to reset the core flux angle. This is often large enough to trip large breakers.

The primary resistors avoid this problem by refluxing the core and maintaining the phase angle when the full power is applied.

Tom
 
> idea what speed your motor is brought up to by the pony motor

its close, not perfect put close to the 1750 required by the 40hp motor. i used an arduino and Hall-Effect Sensor to see where i was. im using the same size pulley on both the pony motor and main motor.

I think there are a couple of options here that people who want to build bigger pony start rpc's have. My sync method with a $2 pilot light to find a Zero crossing point of the AC wave is both simple and cheap. I thought about using a TRIAC and another SSR to switch on the contactor and even have the circuit made, but never installed. Taking the balance caps out and adding some resistors with another contactor is another way to go.
 








 
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