Large 8-pole induction motor for slow speed application
Large 8-pole induction motor for slow speed application
(OP)
All,
I am investigating the upgrade of an old 5,000Hp 150rpm DC motor to AC for a pump application. Am curious about an "off-the-shelf" induction machine operating off PWM versus synchronous due to up-front price. Appears that an 8-pole induction machine is not stretching this too far but would like to avoid a reduction gear if possible. I assume a PWM could be operated only in the 0-10Hz range to effectively operate an 8-pole 900rpm machine as a 150rpm motor? But, what will be the downsides? Low end speed regulation, how the torque curve may be affected, upstream harmonics...? Does an AFE affect the equation?
Any information including reference materials or whitepapers that might help would be great appreciated.
Thanks!
I am investigating the upgrade of an old 5,000Hp 150rpm DC motor to AC for a pump application. Am curious about an "off-the-shelf" induction machine operating off PWM versus synchronous due to up-front price. Appears that an 8-pole induction machine is not stretching this too far but would like to avoid a reduction gear if possible. I assume a PWM could be operated only in the 0-10Hz range to effectively operate an 8-pole 900rpm machine as a 150rpm motor? But, what will be the downsides? Low end speed regulation, how the torque curve may be affected, upstream harmonics...? Does an AFE affect the equation?
Any information including reference materials or whitepapers that might help would be great appreciated.
Thanks!





RE: Large 8-pole induction motor for slow speed application
Sleeve bearings may require higher viscosity to maintain oil film at the low speeds (rolling element bearings won't care).
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(2B)+(2B)' ?
RE: Large 8-pole induction motor for slow speed application
You may want to re-think the case for a reduction gear.
Bill
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"Why not the best?"
Jimmy Carter
RE: Large 8-pole induction motor for slow speed application
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(2B)+(2B)' ?
RE: Large 8-pole induction motor for slow speed application
Maybe I need to go back and hit my old textbooks, but I thought that a 5000Hp motor fed from a PWM VSI with the V/Hz curve adjusted to give full voltage at 10 Hz rather than 60 Hz, and with a direct torque type control that would output rated current at 10Hz, could acheive 5000Hp at the reduced 10Hz PWM output?
RE: Large 8-pole induction motor for slow speed application
One more issue relates to slip, if you have 1% of slip in 900 RPM and rated power, which means 9 RPM, then at 120 RPM you would have 7.5% slip if you apply rated torque. This is because slip in rpm stays constant regardless of speed, if the torque remains constant. In practice this means, that 7.5% of your input power turns into heat in the rotor winding instead of 1%. I would guess that this will cause problems with heating.
RE: Large 8-pole induction motor for slow speed application
You need to re-think the gear drive or the motor selection.
RE: Large 8-pole induction motor for slow speed application
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RE: Large 8-pole induction motor for slow speed application
So to further that thought and tie it to Bill's observation, if you want full torque of a 5000HP 900RPM motor at 10Hz, no problem. But if what you want is the torque of a 5000Hp 150RPM motor at 10 Hz, then you have to start off with that much higher torque value at the motor design point, or a lot more poles in the motor (in this case, 48 poles). Remember, HP is a shorthand expression of "x torque at y speed".
Do your torque calcs first, then determine how you want to get there.
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— Kilgore Trout (via Kurt Vonnegut)
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RE: Large 8-pole induction motor for slow speed application
I agree with the constant slip in rpm as we decrease frequency.
I don't think this results in increased rotor heating. It would be a higher fraction of input power, but input power is decreasing (for constant torque with decreased supply frequency and proportionately decreased voltage). At constant torque, rotor I^2*R should stay relatively constant when decreasing frequency and voltage proportionately.
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(2B)+(2B)' ?
RE: Large 8-pole induction motor for slow speed application
But this seems like an obvious case where a synchronous machine is a better choice, at least this would have been the case in the past.
RE: Large 8-pole induction motor for slow speed application
I don't think this results in increased rotor heating. It would be a higher fraction of input power, but input power is decreasing (for constant torque with decreased supply frequency and proportionately decreased voltage). At constant torque, rotor I^2*R should stay relatively constant when decreasing frequency and voltage proportionately."
Yes, you are right of course on the heating, rotor copper losses are still about the same as in rated point despite of the high (relative) slip. Well, at least efficiency will be quite poor due to this.
RE: Large 8-pole induction motor for slow speed application
The same losses expressed as a percentage of a 30,000 HP motor will not be as large as when expressed as a percentage of a 5000 HP motor.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Large 8-pole induction motor for slow speed application
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(2B)+(2B)' ?
RE: Large 8-pole induction motor for slow speed application
RE: Large 8-pole induction motor for slow speed application
RE: Large 8-pole induction motor for slow speed application
Why do you need to replace the DC machine? Is a variable speed required? Have you considered a thyristor type drive for the existing machine (or is it already?)
Is this is actually a compressor (not a pump)?
A lot more info is needed for anyone on the forum to give any sensible advice.
If it is in fact a centrifugal pump, a custom built synchronous machine is the best solution, assuming that the pump is started up dewatered. There would be also a power factor advantage as the synchronous machine can supply VARS to the system if needed.
Also what is the operating environment? Is a brushless machine desired? Are there problems with the brushes/commutator of the existing DC unit?
rasevskii
RE: Large 8-pole induction motor for slow speed application
Barry
RE: Large 8-pole induction motor for slow speed application
I think that after some net investigation that this has to be a floating dredge or dredge ship slurry pump. A very strange piece of machinery indeed due the slow speed and DC drive. Therefore a possible requirement for a heavy start and variable speed.
The suggestions above by myself and others are in that case possibly totally unusable.
Possibly it is some sort of Ward-Leonard drive system from an onboard DC generator...
How about it, HenryOhm, don't keep us guessing. What is this exotic piece of equipment...?
rasevskii
RE: Large 8-pole induction motor for slow speed application
However for some special applictions also direct drive systems with inverter are used. But this is usually not done to save moeny, but to fulfill special requirements. You may find such designs for example in direct driven wind turbines - of course working as generator is this case.
RE: Large 8-pole induction motor for slow speed application