Another motor question.

This is for a friend.

This setup worked before they replaced one of the motors.

Setup as follows: Two single-phase 230V 7.5Hp 1725rpm motors are belted on to one shaft. The first is started and then the second. The first runs over rated current for as second or two before the second motor is started. The power company won't let them start them both at the same time. Again this used to work.

They replaced one of the motors. Same ratings but a different frame size. The pulleys are the same diameter. Now, when the first motor is started it draws about 41 amps (the FLA is about 31). When the second is started the amps in the first motor jump to 48. The second draws in the mid-40's. Soon thermal overloads trip.

I'm not as up to speed on single phase as three phase. They were wondering if one motor is running faster than the other would make them fight each other. I thought that maybe one would not give the full 7.5 hp of help if it is not a full slip, but they wouldn't fight. I am not sure of this.

I suggested reversing the power leads and keeping the starting leads at the same polarity, wondering it the phase difference had an effect. No report on that, yet.

The behavior is the same no matter which motor is started first.

Any ideas on what is happening? TIA

Just a thought,

Try starting them in the normal order with the load disconnected.

Try starting them TOGETHER with the load disconnected.

Measure the actual motors shaft speed with no load / belt.

Take current / voltage readings as you perform tests, this could give some good info back on the problem.

Other thought,

You may consider changing out the whole system for a 1, single phase inverters driving two, 3 phase motors.(granted, this isn't cheap)then both could start at the same time.

Actually you could use one inverter to drive those motors.

My first thought would be to verify that the motors match...rpm...current etc. The RPM rating wouldnt have to be far off to make them "fight" each other but if someone slipped up and used a 3600 when an 1800 was needed would definitely create a problem. Even if both are 1800 rpm but there load rated speed is 1750 and 1725 that could cause issues.

As paradym suggested, test the motors without load and compare readings.

By the way the principles behind motors arent different when using single phase compared to 3 phase. Naturally the current draw is approximately 2 times for a single phase...rule of thumb: a 3ph 230 motor will pull approximately 2.5A per HP, a 1PH 230v motor will pull approximately 5A per HP. In this case the full load should be about 7.5 x 5 = 37.5. This is an approximation, the motor plate should offer more precise info.

check the shaft dia.

Rick, you said

They replaced one of the motors. Same ratings but a different frame size. The pulleys are the same diameter.

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If the pulleys are the same size but the shaft dia changed on the new motor due to the frame size change, then the rpm difference between the two motors may be great enough to make them work against each other instead of helping. As a side thought, what utility has a problem with across the line starting two 7.5 hp motors? I mean 75hp or 750hp I could see, but 7.5hp? They must be on that outdated flimsy northeast power grid we are all hearing about since yesterday. How flug dung anyway? I personally would recommend getting rid of the two motors and putting one 3ph motor there large enough to do the job, assuming there is 3ph power available. You know how that assuming thing works. It will be interesting to see what you find to be the problem.

This is on a farm. No three phase is available.

Remember, this worked before they changed the motor.

If the pulley on the shaft is the same diameter, just a smaller hub, the rpm is the same.

The checking of rpm's, etc is happening right now. Voltage holds steady.

Motor shaft size and frame size isn't the problem. I suspect that the new motor has slightly different slip characteristics, and they are bucking each other. Remember, the nominal speed of the motor may be 1800, but it will run slower as the load requires torque. 1725 was the approximate full load speed of the motor. The "missing" 75 rpm is slip. The new motor undoubtedly has different slip vs. load characteristics.

I figured that. I was wondering exactly what different slips would mean. I thought that the one with less slip would just develop less torque, but not actively fight the other.

What happens under load is that one motor may run at 1750 and the other at 1725..this causes one motor to "pull" or drag the other motor so it can maintain its speed, this in turn will cause more load to the faster motor and possibly more to the other motor in an attempt to mainatin.

Did the guys have a motor shop match the motor with the other or did they just go ask for a 7.5hp 230v motor somewhere?

I would check to see if enough reactive power is being supplied by power factor correction capacitors (run capacitors). Its possible that when the second motor is on that it is using the reactive power available to the first motor. A slight rpm differance between the two motors should only unload the slower motor. At least that what I think.

rsdoran said:

Did the guys have a motor shop match the motor with the other or did they just go ask for a 7.5hp 230v motor somewhere?

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Just bought one.

I am currently out of touch with them. I won't be back in communication until tues.

A guy at work's son owns a small electrical contracting business in western Wis. He ends up doing a lot of farm work.

Not to change the subject, but I believe paradym was pointing out that single phase in, three phase out inverters are commonly available. On a side note, a single motor or even both could provide a speed ramp, preventing high current on startup.

Telling a customer to scrap everything and start over probably won't get you invited back, though.

I am treating this as more of an intellectual excercise. I want to know why this behavior.

Further help I hope!

This site on motors may lend some insight to the problem. I find it preferable to consult experts on subjects I am not up to speed on and this is certainly one. You will have to watch out for the typo’s and miss-spellings in the document at this address. I can make understanding the intent a bit of a challenge.

Rick, I also apologize for my comment about your friends power utility, I understand better now that I know he is a farmer on a rural single phase feed. The neighbors probably appreciate his staggered starting more than the power company.

That idea about a single phase in 3 phase out inverter for say a 15 or 20 hp 3ph induction motor sure sounds like a plan. If the guy is like a lot of our farmers though, he is not going to want to lay out any more money than he has to. Good luck, this is interesting.

Like several of the others, I suspect that the two motors have different enough slip characteristics so that they are fighting each other.

There might be a mechanical solution. If they're using V-belts for the power transmission between the motor and the common shaft, try putting a variable pitch pulley on either of the motors. Then adjust the pitch diameter so that both motors turn the shaft at the same RPM. It will take a little tweaking to get the RPMs matched under load.

If they are fighting each other, why?