I'm trying to solve some puzzles around the electronics of an amusement ride. It's basicly an old carousel with a wave-like pattern (3 tops/downs along the circle) that i'm restoring. (So that you know the application) I've been researching some while now and would like to have answers instead of doubting myself over and over, spending too much time on it when I can just ask.

I've got a 3 phase slip ring (wound rotor) motor with the following plate:

  • HP: 2
  • V st. 220 / 380
  • A st. 5.9 / 3.4
  • V rot. 53
  • A rot. 19
  • RPM: 1385
  • Freq.: 50 Hz

(ofcourse I'm in Western Europa and using the 230V now)

I found out that I could make this a normal ac motor by shorting the slip rings (just connecting them together) so that I can use a VFD and connect it in delta. (Please correct me if i'm wrong on anything)

1) Is just shorting it ok? I read something about putting little resistors between? Why would that be better?

2) Do I need a dv/dt filter? (it sounds like overkill to me?) The length of the cable from vfd to the motor will be max 10 meters.

3) Risk here is that the insulation could fail. How do I measure this and what values should I get? (ofcourse all of them giving the same value)

4) Now the plate gives me that it's actually a 1.3kW motor. Now to select an VFD I assumed I would need a 1,5kW VFD. But I got told that I would need a 2,2kW VFD due to that it is a slip ring motor, is this true? And what ac motor should i buy when i decide to replace the old one? Would 1,5kW VFD still be sufficient?

5) Would a VFD with built-in brake unit (resistor) of 50% nominal current be good enough?

6) If I calculate the slip % it is 1500 - 1385 = 115, thus slip = 115/1500 * 100 = 7,67%. In this article they say this is a high slip motor and therefor I should use sensorless vector instead of a V/F VFD?

7) I don't know how I came up with this info, but somewhere I read I should ground the rotor in some special case. I was shocked and now i'm just becoming insecure. Am I true that this is weird? And you should just ground the stator.

8) Some company told me I should use a linear curve instead of an S-curve in my application. I thought the S-curve was the way to go.. So which is it and why?

If anyone can answer one or more of these questions, please do so. Even if you're not sure, just give your opinion so that I can see how many people think what. Thanks a lot!


closed as too broad by Andy aka, Voltage Spike, Sparky256, Kevin Reid, Bimpelrekkie Feb 13 '18 at 12:13

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.


1) Is just shorting it ok?

Just short the rings, there is no reason to add resistors. The best sorting method is to remove the brush assembly and braze a piece of copper between the rings. If you are concerned about rotor balance, use two pieces on opposite sides of the rotor.

Do I need a dv/dt filter?

The winding insulation in an old wound-rotor motor may not be as good as modern motor insulation. Even with 10 meter leads, there could be high voltage peaks. However the motor is wound for 380 V and you are only using 220 V, so that will help. My guess is that a dv/dt filter is not necessary.

4) I got told that I would need a 2,2kW VFD due to that it is a slip ring motor, is this true?

You should select the VFD based on the motor nameplate current. You may need extra current for high starting current re 6).

5) Would a VFD with built-in brake unit (resistor) of 50% nominal current be good enough?

Unless the existing motor has electrical braking, don't worry about it. The gearbox and other friction will probably provide fast enough stopping.

6) ...use sensorless vector instead of a V/F VFD?

Yes. A wound rotor motor provides high starting torque. A sensorlesss vector drive may help. However the VFD may have difficulty tuning for the wound rotor motor. It is difficult to predict how much torque you will be able to get out of it. You may need to oversize the VFD quite a bit.

7) I don't know how I came up with this info, but somewhere I read I should ground the rotor in some special case. I was shocked and now i'm just becoming insecure. Am I true that this is weird? And you should just ground the stator.

Do not ground either winding. Sometimes there can be bearing failures due to high frequency current leaking through the bearings to ground. I don't think that is often a problem, but I don't know much about it.

8)...linear curve instead of an S-curve..?

S curve acceleration provides a softer start, less of a jerk. You probably don't need S curve. You should not be using a very fast acceleration and will probably not have a problem with linear acceleration.

  • \$\begingroup\$ 1) Thanks for confirming. I was already gonna put 2 or more pieces on for redundancy and balance just in case. \$\endgroup\$ – Matthijs van Hest Feb 8 '18 at 16:56
  • \$\begingroup\$ 2) I use delta, that's 400/380 right? So would this filter still be obsolete? 4) The VFD I'm thinking of has a 1 min overload of 150%. That would do right? But do you suggest a 2,2 kW anyways (in case of motor replacement) 5) mhm okay. I guess so. 6) oversize by how much? 2,2kW sufficient? How much does this sensorless vector drive help? 7) There's already a ground attached on the old setup, just to the case of the motor (not a winding). This should be remained right? 8) Ok so that's just luxury i suppose.. \$\endgroup\$ – Matthijs van Hest Feb 8 '18 at 17:04
  • \$\begingroup\$ the issue of leaving resistance in the circuit has to do with replacing the WRIM controller with a soft starter, because a soft starter doesn't have the same current limiting capability of a VFD. But a VFD has more finite control, so you can indeed short the rotor bars as suggested. I too suggest over sizing the VFD, you will regret it if you don't. WRIMs have atypical torque / speed / current curves, you don't want to starve it when you want to accelerate. yes, get the Sensorless Vector Control, same issue. Yes, leave the case ground in place. \$\endgroup\$ – J. Raefield Feb 8 '18 at 20:06
  • \$\begingroup\$ The delta connection is the lower voltage connection. If you are intending to operate at 400 volts, I think you should get the dv/dt filter. I would buy a VFD that is oversized so that the intermittent current rating is effectively at least 200% of the motor's rated current. Yes. The motor frame should be grounded. \$\endgroup\$ – Charles Cowie Feb 8 '18 at 21:05
  • \$\begingroup\$ So to round things up: get a 2.2kw? And how can i measure the insulation to check if the motor is still ok? What values should i get? \$\endgroup\$ – Matthijs van Hest Feb 9 '18 at 10:05

You can't really "measure" the insulation capability, it's something that was integral to the design criteria of the motor; i.e. a decision that was made. Your motor manufacturer SHOULD be able to tell you what was used.

If the motor is old, the decisions would have been to use insulation for the windings that was at least 2x the RMS voltage of the expected supply, so a 400V motor would have been made with magnet wire using a minimum of 800V insulation. More likely though, it would have been 1000V; that was more common in the days before VFDs were commonplace.

Now, the MINIMUM for a 400-480V motor would need to be 1200V, I would not use a motor with less than 1488V now (an new industry "standard" for what's called "Inverter Spike Resistant" magnet wire; aka "ISR"). Some mfrs are even promoting 1600V, 2000V and last year, I saw someone saying 2200V insulation was recommended.

Given that hardly anyone is making Wound Rotor motors that small any more, the chances that your motor has anything greater than 1000V rated insulation are slim to none. But again, if you are using the motor / drive at 230V, then even the 1000V insulation is well over the minimum we like to recommend. So you only need to worry about this if you INTEND on using 400V.

Even if you do end up having to feed it with 400V, you could run the motor as is until the insulation fails, then have it re-wound by a motor shop with new ISR insulation. But keep in mind the cost of down-time while you wait for that.

  • \$\begingroup\$ Thanks, this clears things up. The manufacturer doesn't excist anymore. You and Charles helped me out big time. Hard to pick an answer right now. I also found that there's a national guideline stating that the resistence must be at least 500k ohm. I'll check for that later this week. \$\endgroup\$ – Matthijs van Hest Feb 13 '18 at 11:16
  • \$\begingroup\$ That has to do with determining if your insulation has ALREADY failed, not what it is capable of. \$\endgroup\$ – J. Raefield Feb 13 '18 at 18:02

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